Understanding neutron radiography post exam reading x a

Understanding Neutron Radiography Reading X 27th August 2016 Post Exam Reading (Passed!)

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The “X”

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The 10th & 11th, Acoustic Emission & Neutron Radiography Testing 25th August 2016- Hurray Exam Passed Results released today!

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Spallation Source

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Spallation Source

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Applications

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Collimator

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Neutron generator NG-150MďźšThe most common is the generator NG-150,

which is a compact accelerator of deuterium ions with a beam current of 3 mA and energy of 150 keV deuterons. In the capacity of generator there are used titanium-tritium targets with a diameter of 45 mm. The neutron yield is 2*1011 neutrons / sec.

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Generator NG-11-IďźšIn recent years there were developed generators NG-11 NG-11

equipped with electromagnetic mass separator to clean the beam from molecular components. These generators can operate in both continuous and pulsed modes in a wide range of durations and pulse repetition rate. The neutron yield is 2*1011 neutrons/sec.

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Generator NG-12-2ďźšFor wider application, including neutron activation

analysis, there were designed neutron generators up to 2*1012 neutrons /sec. In these generators, the beam of atomic ions with the current up to 15 mA and energy up to 250 keV irradiates a rotating titanium-tritium target with a diameter of 230 mm. There is a possibility of additional channels installation for the beam transportation (0 and ¹ 45°), as well as target devices. Installation provides both continuous and pulsed neutron fluxes

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The Magical Book of Neutron Radiography

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数字签名者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang@ qq.com, c=CN 日期：2016.08.31 01:01:27 +08'00' Charlie Chong/ Fion Zhang

ASNT Certification Guide NDT Level III / PdM Level III NR - Neutron Radiographic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of penetrating radiation • Interaction between penetrating radiation and matter • Neutron radiography imaging • Radiometry 2. Equipment/Materials • Sources of neutrons • Radiation detectors • Non-imaging devices

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3. Techniques/Calibrations

• Electron emission radiography

• Blocking and filtering

• Micro-radiography

• Multifilm technique

• Laminography (tomography)

• Enlargement and projection

• Control of diffraction effects

• Stereoradiography

• Panoramic exposures

• Triangulation methods

• Gaging

• Autoradiography

• Real time imaging

• Flash Radiography

• Image analysis techniques

• In-motion radiography • Fluoroscopy

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4. Interpretation/Evaluation • Image-object relationships • Material considerations • Codes, standards, and specifications 5. Procedures • Imaging considerations • Film processing • Viewing of radiographs • Judging radiographic quality 6. Safety and Health • Exposure hazards • Methods of controlling radiation exposure • Operation and emergency procedures Reference Catalog Number NDT Handbook, Third Edition: Volume 4, Radiographic Testing 144 ASM Handbook Vol. 17, NDE and QC 105 Charlie Chong/ Fion Zhang

菲帆教授 Dr. Fion Zhang at Shanghai 27th August 2016

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一分耕耘一分收获

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SME- Subject Matter Expert http://cn.bing.com/videos/search?q=Walter+Lewin&FORM=HDRSC3 https://www.youtube.com/channel/UCiEHVhv0SBMpP75JbzJShqw

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Gamma- Radiography TABLE 1. Characteristics of three isotope sources commonly used for radiography.

Source

T½

Energy

HVL Pb

HVL Fe

Specific Activity

Dose rate*

Co60

5.3 year

1.17, 1.33 MeV

12.5mm

22.1mm

50 Cig-1

1.37011

Cs137

30 years

0.66 MeV

6.4mm

17.2mm

25 Cig-1

0.38184

Ir192

75 days

0.14 ~ 1.2 MeV (Aver. 0.34 MeV)

4.8mm

?

350 Cig-1

0.59163

Th232

Dose rate* Rem/hr at one meter per curie

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0.068376

八千里路云和月

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闭门练功

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http://greekhouseoffonts.com/

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Spallation The Wiki

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https://en.wikipedia.org/wiki/Spallation

Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection or vaporization of material from a target during impact by a projectile. In planetary physics, spallation describes meteoritic impacts on a planetary surface and the effects of a stellar wind on a planetary atmosphere. In the context of mining or geology, spallation can refer to pieces of rock breaking off a rock face due to the internal stresses in the rock; it commonly occurs on mine shaft walls. In the context of anthropology, spallation is a process used to make stone tools such as arrowheads by knapping.

In nuclear physics, spallation is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic weight.

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https://en.wikipedia.org/wiki/Spallation

Spallation in solid mechanics Spallation can occur when a tensile stress wave propagates through a material and can be observed in flat plate impact tests. It is caused by an internal cavitation due to stresses, which are generated by the interaction of stress waves, exceeding the local tensile strength of materials. A fragment or multiple fragments will be created on the free end of the plate. This fragment known as "spall" acts as a secondary projectile with velocities that can be as high as one third of the stress wave speed on the material. This type of failure is typically an effect of high explosive squash head (HESH) charges.

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https://en.wikipedia.org/wiki/Spallation

Laser spallation Laser induced spallation is a recent experimental technique developed to understand the adhesion of thin films with substrates. A high energy pulsed laser (typically Nd:YAG) is used to create a compressive stress pulse in the substrate wherein it propagates and reflects as a tensile wave at the free boundary. This tensile pulse spalls/peels the thin film while propagating towards the substrate. Using theory of wave propagation in solids it is possible to extract the interface strength. The stress pulse created in this fashion is usually around 3~8 nanoseconds in duration while its magnitude varies as a function of laser fluence. Due to the non-contact application of load, this technique is very well suited to spall ultra-thin films (1 micrometre in thickness or less). It is also possible to mode convert a longitudinal stress.

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https://en.wikipedia.org/wiki/Spallation

Nuclear spallation

See also Cosmic ray spallation

Nuclear spallation occurs naturally in Earth's atmosphere owing to the impacts of cosmic rays, and also on the surfaces of bodies in space such as meteorites and the Moon. Evidence of cosmic ray spallation (also known as "spoliation") is evidence that the material in question has been exposed on the surface of the body of which it is part, and gives a means of measuring the length of time of exposure. The composition of the cosmic rays themselves also indicates that they have suffered spallation before reaching Earth, because the proportion of light elements such as Li, B,and Be in them exceeds average cosmic abundances; these elements in the cosmic rays were evidently formed from spallation of oxygen, nitrogen, carbon and perhaps silicon in the cosmic ray sources or during their lengthy travel here.

Cosmogenic isotopes of aluminium, beryllium, chlorine, iodine and neon, formed by spallation of terrestrial elements under cosmic ray bombardment, have been detected on Earth.

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Cosmic Spoliation

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Cosmic Spoliation

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Cosmic Spoliation

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Cosmic Spoliation

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Cosmic Spoliation

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Cosmic Spoliation

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Nuclear spallation is one of the processes by which a particle accelerator may be used to produce a beam of neutrons. A mercury (Hg) , tantalum, lead or other heavy metal target is used, and 20 to 30 neutrons are expelled after each impact. Although this is a far more expensive way of producing neutron beams than by a chain reaction of nuclear fission in a nuclear reactor, it has the advantage that the beam can be pulsed with relative ease. The concept of nuclear spallation was first coined by Nobelist Glenn T. Seaborg in his doctoral thesis on the inelastic scattering of neutrons in 1937.[2]

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https://en.wikipedia.org/wiki/Spallation

Nobelist Dr. Glenn T. Seaborg

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https://seaborg.llnl.gov/seaborg.php

Nobelist Glenn T. Seaborg

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https://seaborg.llnl.gov/seaborg.php

Nobelist Glenn T. Seaborg 尊师重教为我中华美德

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https://seaborg.llnl.gov/seaborg.php

Production of neutrons at a spallation neutron source Generally the production of neutrons at a spallation source begins with a high-powered proton accelerator. The accelerator may consist of a linac only (as in the European Spallation Source) or a combination of linac and synchrotron (e.g. ISIS neutron source) or a cyclotron (e.g PSI) . As an example, the ISIS neutron source is based on some components of the former Nimrod synchrotron. Nimrod was uncompetitive for particle physics so it was replaced with a new synchrotron, initially using the original injectors, but which produces a highly intense pulsed beam of protons. Whereas Nimrod would produce around 2 ÂľA at 7 GeV, ISIS produces 200 ÂľA at 0.8 GeV. This is pulsed at the rate of 50 Hz, and this intense beam of protons is focused onto a target. Experiments have been done with depleted uranium targets but although these produce the most intense neutron beams, they also have the shortest lives. Generally, therefore, tantalum or tungsten targets have been used (mercury?) .

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https://en.wikipedia.org/wiki/Spallation

Spallation processes in the target produce the neutrons, initially at very high energies—a good fraction of the proton energy. These neutrons are then slowed in moderators filled with liquid hydrogen or liquid methane to the energies that are needed for the scattering instruments. Whilst protons can be focused since they have charge, chargeless neutrons cannot be, so in this arrangement the instruments are arranged around the moderators.

See also: ISIS neutron source, Spallation Neutron Source, and SINQ Inertial confinement fusion has the potential to produce orders of magnitude more neutrons than spallation.[3] This could be useful for Neutron radiography which can be used to locate hydrogen atoms in structures, resolve atomic thermal motion and study collective excitations of photons more effectively than X-rays.

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https://en.wikipedia.org/wiki/Spallation

The Spallation Neutron Source (SNS) The Wiki

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https://en.wikipedia.org/wiki/Spallation_Neutron_Source

The Spallation Neutron Source (SNS) is an accelerator-based neutron source facility that provides the most intense pulsed neutron beams in the world for scientific research and industrial development. Each year, this facility hosts hundreds of researchers from universities, national laboratories, and industry, who conduct basic and applied research and technology development using neutrons. SNS is part of Oak Ridge National Laboratory, which is managed by UT-Battelle for the United States Department of Energy (DOE). SNS is a DOE Office of Science user facility, and it is open to scientists and researchers from all over the world.

Neutron scattering research Neutron scattering allows scientists to count scattered neutrons, measure their energies and the angles at which they scatter, and map their final positions. This information can reveal the molecular and magnetic structure and behavior of materials, such as high-temperature superconductors, polymers, metals, and biological samples. In addition to studies focused on fundamental physics, neutron scattering research has applications in structural biology and biotechnology, magnetism and superconductivity, chemical and engineering materials, nanotechnology, complex fluids, and others.

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https://en.wikipedia.org/wiki/Spallation_Neutron_Source

How SNS works The spallation process at SNS begins with negatively charged hydrogen ions (H-) that are produced by an ion source. Each ion consists of a proton orbited by two electrons. The ions are injected into a linear particle accelerator, or linac, which accelerates them to very high energies (eventually to 90% the speed of light). The ions pass through a foil, which strips off each ion's two electrons, converting it to a proton. The protons pass into a ring-shaped structure, a proton accumulator ring, where they spin around at very high speeds and accumulate in “bunches.� Each bunch of protons is released from the ring as a pulse, at a rate of 60 times per second (60 hertz). The highenergy proton pulses strike a target of liquid mercury, where spallation occurs.

The spalled neutrons are then slowed down in a moderator and guided through beam lines to areas containing special instruments where they are used in a wide variety of experiments.

Note that the previous reference is now stale; the Internet Archive has the original citation

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https://en.wikipedia.org/wiki/Spallation_Neutron_Source

The Spallation Neutron Source delivered its first neutrons in 2006. Since then, 15 one-of-a-kind scientific instruments have been completed for use by researchers from all over the world. The instrument suite will eventually include up to 25 instruments.

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https://en.wikipedia.org/wiki/Spallation_Neutron_Source

History Most of the world's neutron sources were built decades ago, and although the uses and demand for neutrons have increased throughout the years, few new sources have been built. To fill that need for a new, improved neutron source, the DOE Office of Basic Energy Sciences funded the construction of SNS, which would provide the most intense pulsed neutron beams in the world for scientific research and industrial development.

The construction of SNS was a partnership of six DOE national laboratories: Argonne, Brookhaven, Lawrence Berkeley, Los Alamos, Oak Ridge, and Jefferson. This collaboration was one of the largest of its kind in U.S. scientific history and was used to bring together the best minds and experience from many different fields.

After more than five years of construction and a cost of $1.4 billion, SNS was completed in April 2006.[5] The first three instruments began commissioning and were available to the scientific community in August 2007. As of 2011, a total of 15 instruments have been completed, and SNS is hosting about 700 researchers per year Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Spallation_Neutron_Source

Three-dimensional rendering of the Spallation Neutron Source facility layout indicating the national laboratory responsible for each primary part of the facility. The areas in red are constructed underground.

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https://en.wikipedia.org/wiki/Spallation_Neutron_Source

Mercury Targets

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Mercury circulation for spallation neutron source in J-PARC has been succeeded - Small induction pump for liquid metal has been developed

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http://www.j-parc.jp/en/topics/2007/Mercury.html

Point The permanent magnet rotating type induction pump (PM pump)3) has been developed and installed in the mercury circulation system2) of neutron spallation source in J-PARC (Japan Proton Accelerator Research Complex)1) and it could supply a great deal of mercury (20 tons) to a mercury target5).

This newly developed PM pump can be small and can provide sufficient mercury flowing by using the strong permanent magnet to apply the induction force into mercury. The PM pump is expected to use for general industrial field as a pump for molten metals (ex. Pump for molten aluminum alloy forging to make automobile engine.)

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

Summary J-PARC center which is managed by JAEA and KEK are proceeding to construct and operate J-PARC facilities. 6 times intense neutron beam will be produced by spallation neutron source facility in J-PARC and the neutron beam4) will be used for experiments in various fields. R & D on the neutron source with a high power target is being carried out in JAEA. The neutrons are produced by the spallation reaction6) in the spallation neutron source due to bombarding accelerated proton beam to a mercury target with a heat generation in the target. In conventional spallation neutron sources, the system in which heavy metal solid-target was cooled by water was adopted. However it is difficult to cool the solid target by water in the high power target aimed in J-PARC. Therefore, the liquid metal target system using the mercury l was adopted because mercury had both functions of neutron spallation source and cooling material.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

In the mercury target system, mercury must be circulated by a mercury circulation system. A mechanical pump could be small however it was worried about mercury leak from the seal around the drive shaft of the pump. On other hand, an induction pump has no seal but efficiency was worse so that it was difficult to achieve the enough mercury flow to cool the target.

Then, JAEA developed the PM pump in corporate with Sukegawa denki Co. This newly developed PM pump can be small and can provide sufficient mercury flowing by using the strong permanent magnet to apply the induction force into mercury.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

[Term] 1) Japan Proton Accelerator Research Complex (J-PARC) J-PARC is generic name of the Proton Accelerators and Experimental facilities which are being constructed in Tokai-mura, Japan by cooperation between Japan Atomic Energy Research Agency (JAEA) and KEK. In J-PARC, innovative research and industrial application research experiments will be carried on by using the particles such as neutrons, muons and neutrinos which are produced by collision of the accelerated protons into target materials.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

2) Mercury circulation system (*Footnote1) Mercury circulation system provides mercury for mercury target and circulates mercury in the mercury target and the system. The system is set on the back of the mercury target toward proton beam going direction. Main components of the system are pump, heat exchanger and surge tank. These components are connected by pipes of 150 mm in diameter to circulate the radioactive mercury without leak.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

â– [Footnote 1] In neutron spallation source, the proton beam bombards the heavy metal to produce neutron beam. Mercury is used as the target material in J-PARC, the mercury circulation system is set on the back of the mercury target toward proton beam going direction to circulate mercury into the mercury target. Main components of the system are PM pump, heat exchanger and surge tank.

These components are connected by pipes of 150 mm in diameter to circulate the radioactive mercury. Mercury of 1.2 m3 in total volume is circulated with 41 m3/h in flow rate to and heat generated in the mercury target of about 500 kW is removed by the heat exchanger.

Since mercury is radioactivated by spallation reaction, the components must be exchanged by remote handling. The components is set on the narrow space of 5.4 m in length and 2.6 mm in width on a target trolley which can be moved from an operation position to produce neutrons to maintenance position where the components is exchanged.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

[Footnote 1]

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

3) Permanent magnet rotating type induction pump (PM pump) (*Footnote2) In the PM pump, mercury flowing channel is set around the magnet rotor which is cylindrical shape. The N and S poles of permanent magnets are alternatively arranged on the surface of the magnet rotor. Mercury is drove by the Lorenz force generated by rotation of the magnet rotor.

[Footnote 2] A conventional induction pump drive the fluid due to the Lorentze force generated by the moving magnetic fields perpendicular to the fluid flowing direction which is produced by current in coils set around flowing channel.

Since the driving force for fluid flowing is proportional to the length applying the magnetic field, the length becomes long to get large driving force.

In the PM pump, mercury flowing channel is set around the magnet rotor which is cylindrical shape.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

The N and S poles of permanent magnets are alternatively arranged on the surface of the magnet rotor. Mercury is drove by the Lorentze force generated by rotation of the magnet rotor. Then, JAEA developed the PM pump in corporate with Sukegawa denki Co. This newly developed PM pump can be small and can provide sufficient mercury flowing by using the strong permanent magnet to apply the induction force into mercury. On the other hand, because loss of the Lorentze force generate heat in induction pump, motor of 90 kW is set in to rotate the magnet rotor of PM pump from the viewpoint of capacity of heat exchanger. And the configuration of flowing channel duct is optimized to satisfy the conflicting characteristics; structural integrity and reduction of the heat generation. Consequently, PM pump can drive mercury continuously with sufficient flow rate to remove the heat generated by the spallation reaction. Additionally, the PM pump can be operated with low noise and low vibration.

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

â– [Footnote 2]

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http://www.j-parc.jp/en/topics/2007/Mercury.html#F1

3) Permanent magnet rotating type induction pump (PM pump) (*Footnote2) In the PM pump, mercury flowing channel is set around the magnet rotor which is cylindrical shape. The N and S poles of permanent magnets are alternatively arranged on the surface of the magnet rotor. Mercury is drove by the Lorenz force generated by rotation of the magnet rotor.

4) Neutron beam Neutrons and secondary particles radiate due to the spallation reaction by the accelerated protons incident in the spallation target. The neutrons moderated in the optimized energy and extracted in the uniform direction are called neutron beam. This neutron beam will be used for the neutron scattering experiments etc. The intensity of neutron beam in J-PARC is more than 6 times higher than that in ISIS (UK, Proton beam power: 160 kW) which has been the most powerful spallation neutron source in the world yet.

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5) Target The secondary particles such as neutrons are produced by the spallation reaction when the accelerated protons incident high atomic number materials such as heavy metal. The high atomic number material called target material.

The system consists of the target material and vessel to contain the target material and the coolant to remove heat generate in the spallation reaction is called target. In the conventional target system, solid heavy metals such as tungsten or tantalum have been used with water coolant. However it is difficult to keep both of high neutron yield and solid target cooling when the proton beam power becomes high.

6) Spallation reaction When high-energy protons incident atoms, the atoms break up to yield secondary particles such as neutrons and to generate heat. This reaction is called spallation reaction.

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Neutron Detection The Wiki

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https://en.wikipedia.org/wiki/Neutron_detection

Neutron Star

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Neutron detection is the effective detection of neutrons entering a well-positioned detector.

There are two key aspects to effective neutron detection: (1) hardware and (2) software.

Detection hardware refers to the kind of neutron detector used (the most common today is the scintillation detector) and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.

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https://en.wikipedia.org/wiki/Neutron_detection

Scintillation Fluid Neutron Detector

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Scintillation Fluid Neutron Detector

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http://www.bunkerofdoom.com/nuclear/detect/index.html

Scintillation Detector

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Scintillation Detector DIY Scintillation Probe for Ludlum Ratemeters Using Surplus XP3312/SQ PMT

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http://www.diyphysics.com/2013/03/22/diy-scintillation-probe-for-ludlum-ratemeters-using-surplus-xp3312sq-pmt/

Plastid Scintillation Detector under UV irradiations

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Gamma Scintillation Detector

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http://rmdinc.com/clyc/

Scintillation Detector Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.

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https://www.psi.ch/niag/neutron-imaging-detectors

Basic physics of neutron detection Signatures by which a neutron may be detected Atomic and subatomic particles are detected by the signature they produce through interaction with their surroundings. The interactions result from the particles' fundamental characteristics.  Charge: Neutrons are neutral particles and do not ionize directly; hence they are harder than charged particles to detect directly. Further, their paths of motion are only weakly affected by electric and magnetic fields.  Mass: The neutron mass of 1.0086649156(6) u. is not directly detectable, but does influence reactions through which it can be detected.  Reactions: Neutrons react with a number of materials through (1) elastic scattering producing a recoiling nucleus, (2) inelastic scattering producing an excited nucleus, or (3) absorption with transmutation of the resulting (unstable?) nucleus.

Most detection approaches rely on detecting the various reaction products.

Total Attenuation σtotal = σscattering + σAbsorption Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Neutron_detection

 Magnetic moment: Although neutrons have a magnetic moment of −1.9130427(5) μN, techniques for detection of the magnetic moment are too insensitive to use for neutron detection.  Electric dipole moment: The neutron is predicted to have only a tiny electric dipole moment, which has not yet been detected. Hence it is not a viable detection signature.  Decay: Outside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 s (about 14 minutes, 46 seconds).[1] Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay: n0 → p+ + e− + Ṽe.

Although the p+ and e− produced by neutron decay are detectable, the decay rate is too low to serve as the basis for a practical detector system.

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https://en.wikipedia.org/wiki/Neutron_detection

Classic neutron detection options As a result of these properties, detection of neutrons fall into several major categories: 1. Absorptive reactions with prompt reactions - low energy neutrons are typically detected indirectly through absorption reactions. Typical absorber materials used have high cross sections for absorption of neutrons and include helium-3, lithium-6, boron-10, and uranium-235. Each of these reacts by emission of high energy ionized particles, the ionization track of which can be detected by a number of means. Commonly used reactions include： 3 He(n,p) 3H, 2 6 Li(n,α) 3H, 3 10 B(n,α) 7 Li and 5 3 the fission of uranium.[3]

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https://en.wikipedia.org/wiki/Neutron_detection

Helium-3

P P

N

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http://hubpages.com/technology/China-is-Now-on-the-Moon-But-Why

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Uranium-235

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Uranium-235

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Uranium-235

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https://online.science.psu.edu/phys010_archive_suwd/node/8285

Uranium-235

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https://online.science.psu.edu/phys010_archive_suwd/node/8285

Uranium-235

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https://online.science.psu.edu/phys010_archive_suwd/node/8285

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Uranium-235

Uranium-235

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https://online.science.psu.edu/phys010_archive_suwd/node/8285

Plutonium-239

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https://online.science.psu.edu/phys010_archive_suwd/node/8285

Plutonium-239

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https://online.science.psu.edu/phys010_archive_suwd/node/8285

 Activation processes - Neutrons may be detected by reacting with absorbers in a radiactive capture, spallation or similar reaction, producing reaction products that then decay at some later time, releasing beta particles or gammas. Selected materials (e.g., indium, gold, rhodium, iron (56Fe(n,p) 56Mn), aluminum (27Al(n,α)24Na), niobium (93Nb(n,2n) 92mNb), & silicon (28Si(n,p) 28Al)) have extremely large cross sections for the capture of neutrons within a very narrow band of energy. Use of multiple absorber samples allows characterization of the neutron energy spectrum.

Activation also allows recreation of an historic neutron exposure (e.g., forensic recreation of neutron exposures during an accidental criticality).[3]  Elastic scattering reactions (also referred to as proton-recoil) - High energy neutrons are typically detected indirectly through elastic scattering reactions. Neutron collide with the nucleus of atoms in the detector, transferring energy to that nucleus and creating an ion, which is detected.

Since the maximum transfer of energy occurs when the mass of the atom with which the neutron collides is comparable to the neutron mass, hydrogenous[4] materials are often the preferred medium for such detectors.[3] Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Neutron_detection

Scattering Scattering is a physical process which involves the deviation of some forms of radiation, like light, sound, or moving particles from their straight trajectory.

For example; a beam of light get scattered due to change in medium or irregularities on surface and deviate from its straight path.

A beam of sunlight gets scattered by dust particles present in atmosphere and rays of sunlight redirected in a new direction after hitting a dust particle.

There will be no change in wave length and intensity of light during scattering process. (?)

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http://chemistry.tutorvista.com/nuclear-chemistry/nuclear-chain-reaction.html

1. Elastic Scattering In an elastic collision there is no lose of energy during collision, hence the incident and target particles remain intact same before and after collision. The projectile particle shares its kinetic energy with the target particle after the collision; momentum is of course always conserved.

Elastic scattering is usually takes place with a point like target particle. For example; there is an elastic scattering of an neutron strike to the nucleus which involve the conservation of momentum as well as kinetic energy.

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http://chemistry.tutorvista.com/nuclear-chemistry/nuclear-chain-reaction.html

Conservation of Momentum & Kinetic Energy Elastic scattering is usually takes place with a point like target particle. For example; there is an elastic scattering of an neutron strike to the nucleus which involve the conservation of momentum as well as kinetic energy.

By the law of conservation of momentum the two objects must move off in opposite directions.

Let the kinetic energy of the shell, mass m, be E and that of the cannon, mass M, be E'.

Now: E = ½ mu2 and E' = ½ Mv2 Charlie Chong/ Fion Zhang

http://www.schoolphysics.co.uk/age16-19/Mechanics/Dynamics/text/Explosions_/index.html

E = ½ mu2 and E' = ½ Mv2 By the conservation of momentum mu = -Mv and therefore: mu = Mv Mv2 = muv v = muv/Mv =mu/M E’ = ½ Mv2 = ½ mu(mu/M) E' = ½ [m2u2]/M E= ½ mu2 and E' = ½ [m2u2]/M Kinetic energy of shell/Kinetic energy of cannon = E/E' = M/m nd so the fragment with the smaller mass has the larger kinetic energy.

A bullet with a mass one hundredth that of the gun that fires it will have one hundred times the kinetic energy of the gun.

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http://www.schoolphysics.co.uk/age16-19/Mechanics/Dynamics/text/Explosions_/index.html

Example problems Q1. A rifle of mass 3 kg fires a bullet of mass 0.025 kg at 100 ms-1. Calculate the kinetic energies of the rifle and bullet.

Kinetic energy of bullet = ½ mu2 = 0.5x0.025x100x100 = 125J Kinetic energy of rifle = ½ m2u2/M = 0.5·0.0252·1002/3 = 1.04J Q2. Uranium-235 is an alpha-emitter the resulting nucleus having a mass of 231 units.

If a stationary uranium -235 nucleus emits an alpha particle (mass 4 units) what is the ratio of the kinetic energy of the alpha-particle to that of the residual nucleus?

Kinetic energy of alpha/Kinetic energy of nucleus = E/E' = M/m = 231/4 = 57.75

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2. Inelastic Scattering Inelastic scattering not follows the conservation of momentum and kinetic energy. The incident particles lost some of its kinetic energy inside the target through some internal process and only a fraction of it goes into moving the whole target.

Generally inelastic scattering will occur with that target which consists of smaller components. There will be no change in target during elastic scattering, while the target can break up into new forms during inelastic scattering. For example, when a neutron involve in inelastic scattering; it is absorbed by the target nucleus and form a compound nucleus.

This compound nucleus finally emits a neutron of lower kinetic energy and leaves the target nucleus in its excited state. This excited nucleus emits the excess energy in the form of one or more gamma radiation and reaches to its ground state. If the incident particle is an electron than the probability of inelastic scattering is usually less than that of elastic scattering and depends upon the energy of the incident electron. However in Raman scattering a photon acts as incident particle.

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In Raman scattering, the incident photon interacts with target and the frequency of the photon gets changed due to transfer of some part of the energy of the photon to the interacting matter or vice-versa. Another type of scattering involves the interaction between an electron and a photon and process is called Compton scattering.

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NEUTRON RADIATION Neutrons are one type of secondary cosmic ray particle produced when primary cosmic rays interact with matter. Neutrons are particles found in the nucleus of atoms and, unlike protons and electrons, neutrons have no net electric charge and a mass slightly greater than that of a proton. Neutron radiation is a type of indirectly ionizing radiation that consists of free neutrons (neutrons that are released from atoms). Free neutrons are unstable and will disintegrate in about 10.6 minutes by beta minus decay to a proton and electron if they do not interact with matter.

When free neutrons do come into contact with matter, they do not interact with the electrons as do charged particles, but instead interact only with the nuclei of atoms. When this happens, several results are possible depending on the energy of the neutron and the mass of the nucleus; however, all interactions are governed by the laws of conservation of momentum and energy. Since neutrons interact primarily with the small atomic nuclei rather than the atomic electrons, they can penetrate very deeply into matter.

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NEUTRON RADIATION When free neutrons do come into contact with matter, they do not interact with the electrons as do charged particles, but instead interact only with the nuclei of atoms.

Electron Cloud Nucleus Travelling Neitron

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1. Elastic Scattering Elastic collisions are billiard ball-like collisions which result in a sharing of kinetic energy between the target nucleus and the impacting neutron. If the sum of the kinetic energies of the neutron and nucleus following collision is equal to the sum of these quantities before collision, the collision is said to be elastic (i.e., kinetic energy is conserved). Maximum energy transfer (about half of the total energy) occurs when the neutron collides with a nucleus of equal mass, namely the hydrogen atom.

When a neutron strikes a hydrogen nucleus, the protons themselves become ionizing because their energy level and charge enables them to interact with the electrons in matter. Neutrons tend to bounce and get slowed down by light nuclei due to elastic scattering, which is why hydrogen-rich materials, such as water, polyethylene and concrete, make good shielding against neutron radiation Note: some of the energy was used for ionizations of interacted matters.

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NEUTRON RADIATION

Ionized?

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2. Inelastic Scattering When a neutron collides with a heavier nucleus, it can ricochet off 空间弹球.

When this happens, the neutron can transfer some of its energy to the nuclei and in turn lose some energy itself. When part of the kinetic energy is converted into excitation energy of the struck nucleus, the collision is said to be inelastic (i.e., kinetic energy is not conserved). The additional energy acquired by the nuclei is released as gamma ray photons.

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3.

Neutron Capture

Slower neutrons can interact directly with a nucleus in a process called neutron capture. In this case, the nucleus ‘captures’ the neutron and a new nucleus is produced (nucleosynthesis). The new, heavier nucleus enters an excited state (becomes a radioactive isotope) and emits a particle and electromagnetic radiation (gamma ray photon). The resulting nucleus itself may also be unstable and decay, emitting various type of ionizing radiation.

Boron Neutron Capture Therapy is an example of the use of neutron capture to kill cancer cells in the head and neck (see below). Boron is used because it can strongly absorb neutrons to produce ionizing radiation.

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Types of neutron detectors â– Gas proportional detectors Gas proportional detectors can be adapted to detect neutrons. While neutrons do not typically cause ionization, the addition of a nuclide with high neutron cross-section allows the detector to respond to neutrons. Nuclides commonly used for this purpose are helium-3, lithium-6, boron-10 and uranium-235.

Since these materials are most likely to react with thermal neutrons (i.e., neutrons that have slowed to equilibrium with their surroundings), they are typically surrounded by moderating materials to reduce their energy and increase the likelihood of detection.

Further refinements are usually necessary to differentiate the neutron signal from the effects of other types of radiation. Since the energy of a thermal neutron is relatively low, charged particle reactions are discrete (i.e., essentially monoenergetic and lie within a narrow bandwidth of energies) while other reactions such as gamma reactions will span a broad energy range, it is possible to discriminate among the sources.

As a class, gas ionization detectors measure the number (count rate), and not the energy of neutrons.

Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Neutron_detection

Discussion Subject: Gas proportional detectors As a class, gas ionization detectors measure the number (count rate), and not the energy of neutrons.

Discussed on the above subject.

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https://en.wikipedia.org/wiki/Neutron_detection

â– 3He gas-filled proportional detectors An isotope of Helium, 32He provides for an effective neutron detector material because 3He reacts by absorbing thermal neutrons, producing a 1H and 3H ion. Its sensitivity to gamma rays is negligible, providing a very useful neutron detector. Unfortunately the supply of 3He is limited to production as a byproduct from the decay of tritium (which has a 12.3 year half-life); tritium is produced either as part of weapons programs as a booster for nuclear weapons or as a byproduct of reactor operation.

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Tritium production A third important nuclear reaction is the one that creates tritium, essential to the type of fusion used in weapons. Tritium, or hydrogen-3, is made by bombarding lithium-6 (6Li) with a neutron (n). This neutron bombardment will cause the lithium-6 nucleus to fission, producing helium-4 (4He) plus tritium (3T) and energy: 6

3Li

+ 10n → 42He + 31H + 5MeV

A nuclear reactor is necessary to provide the neutrons if the tritium is to be provided before the weapon is used. The industrial-scale conversion of lithium-6 to tritium is very similar to the conversion of uranium-238 into plutonium-239. In both cases the feed material is placed inside a nuclear reactor and removed for processing after a period of time.

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https://en.wikipedia.org/wiki/Nuclear_weapon_design#Tritium_production

Alternatively, neutrons from earlier stage fusion reactions can be used to fission lithium-6 (in the form of lithium deuteride for example) and form tritium during detonation. This approach reduces the amount of tritium-based fuel in a weapon. The fission of one plutonium atom releases ten times more total energy than the fusion of one tritium atom. For this reason, tritium is included in nuclear weapon components only when it causes more fission than its production sacrifices, namely in the case of fusion-boosted fission.

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https://en.wikipedia.org/wiki/Nuclear_weapon_design#Tritium_production

Flash X-Ray images of the converging shock waves formed during a test of the high explosive lens system.

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■ BF3 gas-filled proportional detectors As elemental boron is not gaseous, neutron detectors containing boron may alternately use boron trifluoride (BF3) enriched to 96% boron-10 (natural boron is 20% 10B, 80% 11B).[5] It should be noted that boron trifluoride is highly toxic.

10

5B

+ n →73Li + 42α

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https://en.wikipedia.org/wiki/Neutron_detection

BF3 Detector

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http://www.rtftechnologies.org/physics/fusor-mark3-neutron-detector.htm

BF3 Neutron Detector Tube: N Wood enriched BF3 (99% Boron 10). Tube detects thermal neutrons by the reaction B10 + n => Li7 + a. The charge on the (Li7 / a) produces a 1-3mV signal across the input of the counter that is amplified and recorded.

• Model G-10-2A • Serial: G43092 • Active Length: 60mm • Diameter: 2.54cm • Plateau: 1700-2000 V

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http://www.rtftechnologies.org/physics/fusor-mark3-neutron-detector.htm

■ Boron lined proportional detectors Alternately, boron-lined gas-filled proportional counters react similarly to BF3 gas-filled proportional detectors, with the exception that the walls are coated with 10B. In this design, since the reaction takes place on the surface, only one of the two particles will escape into the proportional counter.

10

5B

+ n →73Li +

Charlie Chong/ Fion Zhang

4

2α

https://en.wikipedia.org/wiki/Neutron_detection

Boron Trifluoride (BF3) Neutron Detectors Paul Frame, Oak Ridge Associated Universities

GENERAL A typical BF3 detector consists of a cylindrical aluminum (brass or copper) tube filled with a BF3 fill gas at a pressure of 0.5 to 1.0 atmospheres. The boron trifluoride gas accomplishes two things: ■ it functions as the proportional fill gas.

■ it undergoes an n alpha interaction with thermal neutrons: 10

5B

+ n →73Li + 42α

To improve the detection efficiency, the BF3 is enriched in B-10. Typical enrichments increase the B-10 component to 96% (ordinary boron is 20% B10 and 80% B-11). Aluminum is typically used as the detector (cathode) wall (?) because of its small cross section for neutrons. The anode is almost always a single thin wire running down the axis of the tube.

Charlie Chong/ Fion Zhang

http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

Boron Trifluoride (BF3) Neutron Detectors .

This neutron detector was produced by 20th Century Electronics in England. The company began manufacturing BF3 counters in the early 1950s. It is approximately 16 ½ inches long, 2 inches in diameter, copper walled and filled with BF3. One end (towards the right in the above photo) has a threaded cap to protect the fragile glass insulator.

The model number, marked on the wall of the tube, is 15EB70/50/G/UA0539. The EB refers to "enriched boron trifluoride." The 50 refers to the tube diameter, i.e., 50 mm.

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http://www.orau.org/PTP/collection/proportional%20counters/Proportionalcounters.htm

This rather substantial neutron detector came from the Deep River Neutron Monitoring Station where it was used in cosmic ray investigations. It is 183 cm long, 14.5 cm in diameter, copper walled and filled with BF3. The construction of the tube is described in the Chalk River publications 1961 Solar Geophysical Data, Part B, CRPL-F-204 and 205 by Steljes, J.F. and Carmichael, H. In use, it would have been encased in paraffin to moderate the fast neutron component of the cosmic rays.

This is the predecessor to what became known as the NM-64 (or IQSY) detector. NM-64 refers to "neutron monitor 1964."

The year 1964 was designated by cosmic ray investigators as the international quiet year of the sun, hence IQSY. The NM-64 detector was designed by Hugh Carmichael at Chalk River Laboratories and they became the worldwide standard neutron detector for cosmic ray studies.

The modifications were as follows.

1. The NM-64 counter was surrounded with polyethylene instead of paraffin wax.

2. The heavy copper tubing that served as the cathode in this older design was replaced by thin stainless steel in the NM-64.

3 The NM-64 was 10% longer.

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http://www.orau.org/PTP/collection/proportional%20counters/Proportionalcounters.htm

PULSE FORMATION BY NEUTRONS When a neutron is absorbed by the B-10 component of the gas, an alpha particle and a recoil Li-7 nucleus are produced that travel off in opposite directions. The movement of the alpha particle and Li-7 nucleus create primary ion pairs in the gas.

The size of the resulting pulse depends on whether the lithium nucleus was left in the ground state or an excited state. When the lithium nucleus is left in the ground state (about 6% of the time), the pulse is larger than if the nucleus were left in an excited state (about 94% of the time) because the alpha particle and Li-7 nucleus have more kinetic energy (2.792 MeV vs 2.310 MeV) with which to create ion pairs.

Charlie Chong/ Fion Zhang

http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

Charlie Chong/ Fion Zhang

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THE BF3 SPECTRUM AND THE WALL EFFECT In a large diameter detector, all the kinetic energy of the alpha particle and recoil Li-7 nucleus is deposited in the detector gas. The pulse height spectrum therefore shows two peaks: a large one at 2.31 MeV (the lithium nuclei were left in an excited state) and a small one at 2.792 MeV (the lithium nuclei were left in the ground state).

For typical sized tubes (e.g., 2 - 5 cm diameter), smaller pulses are often produced because either the alpha particle or Li-7 nucleus deposit some of its energy in the detector wall rather than the gas. Only rarely would the alpha particle and Li-7 nucleus both strike the detector wall. If the neutron interaction takes place in the gas close enough to one side of the tube for either the alpha particle or lithium nucleus to strike the wall, the distance to the other side of the tube would be greater than the range of the particle heading towards it.

Charlie Chong/ Fion Zhang

http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

Charlie Chong/ Fion Zhang

http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

Charlie Chong/ Fion Zhang

http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

The resulting "wall effect" creates two steps on the left side of the 2.31 MeV peak. The lower step on the left is produced as a result of the alpha particle striking the wall and the Li-7 depositing all its energy (e.g., 0.84 MeV) in the gas. The higher step to the right results from the Li-7 nucleus striking the wall and the alpha particle depositing all its energy (e.g., 1.47 MeV) in the gas.

The following is a "textbook" version of the pulse height spectrum of a BF3 detector. A "real" spectrum is not quite as well defined. In particular, the two steps to the left of the 2.31 MeV peak are more difficult to distinguish than shown here.

Charlie Chong/ Fion Zhang

http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

It is important to remember that this spectrum is unrelated to neutron energy it is simply a function of the detector construction. In almost all cases, it is the count rate from the detector that carries useful information, not the pulse height.

Note that gamma rays produce relatively small pulses and these can be easily rejected by properly setting the threshold. Increasing the size of the detector reduces the wall effect seen in the above spectrum and this makes it easier to distinguish gamma pulses from those produced by neutrons.

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http://www.orau.org/PTP/collection/proportional%20counters/bf3info.htm

OPERATING VOLTAGE AND THE CHARACTERISTIC CURVE The best way to determine the operating voltage of a BF3 detector is to generate a characteristic curve (count rate versus high voltage). The curve is similar to that of a Geiger Mueller detector in that they both have long flat plateaus. As was the case with the GM detector, the operating voltage of the BF3 detector should be selected on the plateau just above the knee. If the operating voltage is set too high, electronic noise and pulses due to background gamma rays can exceed the threshold setting and generate spurious counts. A typical voltage for a BF3 detector might be 1500 to 2000 volts.

DETECTION OF THERMAL AND FAST NEUTRONS "Bare" BF3 detectors almost exclusively respond to slow (low energy) neutrons - the probability that a fast (high energy) neutron would be absorbed by boron-10 is very small. To be able to detect fast neutrons, the BF3 tube can be surrounded by a suitable moderator. The thickness of the moderator (e.g., polyethylene) might range from 1 to 6 inches depending on the neutron energy spectrum and other constraints.

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â– Scintillation neutron detectors Scintillation neutron detectors include liquid organic scintillators, crystals, plastics, glass and scintillation fibers.

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https://en.wikipedia.org/wiki/Neutron_detection

■ Neutron-sensitive scintillating glass fiber detectors Scintillating 6Li glass for neutron detection was first reported in the scientific literature in 1957 and key advances were made in the 1960s and 1970s.

Scintillating fiber was demonstrated by Atkinson M. et al. in 1987 and major advances were made in the late 1980s and early 1990s at Pacific Northwest National Laboratory where it was developed as a classified technology. It was declassified in 1994 and first licensed by Oxford Instruments in 1997, followed by a transfer to Nucsafe in 1999. The fiber and fiber detectors are now manufactured and sold commercially by Nucsafe, Inc. http://www.nucsafe.com/cms/Our+Products/3.html

6Li 6

+ n → 31H (2:05MeV) + 42α (2:73MeV):

3Li(n,

α)

Charlie Chong/ Fion Zhang

3

1H

https://en.wikipedia.org/wiki/Neutron_detection

The scintillating glass fibers work by incorporating 6Li and Ce3+ into the glass bulk composition. The 6Li has a high cross-section for thermal neutron absorption through the 6Li(n,Îą) reaction. Neutron absorption produces a tritium ion, an alpha particle, and kinetic energy. The alpha particle and triton interact with the glass matrix to produce ionization, which transfers energy to Ce3+ ions and results in the emission of photons with wavelength 390 nm 600 nm as the excited state Ce3+ ions return to the ground state. The event results in a flash of light of several thousand photons for each neutron absorbed. A portion of the scintillation light propagates through the glass fiber, which acts as a waveguide. The fibers ends are optically coupled to a pair of photomultiplier tubes (PMTs) to detect photon bursts. The detectors can be used to detect both neutrons and gamma rays, which are typically distinguished using pulse-height discrimination. Substantial effort and progress in reducing fiber detector sensitivity to gamma radiation has been made. Original detectors suffered from false neutrons in a 0.02 mR gamma field. Design, process, and algorithm improvements now enable operation in gamma fields up to 20 mR/h (60Co).

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The Scintillating Glass Fibers

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The Scintillating Glass Fibers

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http://www2.uni-wuppertal.de/FB8/groups/Teilchenphysik/atlas/dosi.english.html

The Scintillating Glass Fibers

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http://www2.uni-wuppertal.de/FB8/groups/Teilchenphysik/atlas/dosi.english.html

The scintillating fiber detectors have excellent sensitivity, they are rugged, and have fast timing (~60 ns) so that a large dynamic range in counting rates is possible. The detectors have the advantage that they can be formed into any desired shape, and can be made very large or very small for use in a variety of applications.[24] Further, they do not rely on 3He or any raw material that has limited availability, nor do they contain toxic or regulated materials. Their performance matches or exceeds that of 3He tubes for gross neutron counting due to the higher density of neutron absorbing species in the solid glass compared to high-pressure gaseous 3He. Even though the thermal neutron cross section of 6Li is low compared to 3He (940 barns vs.

5330 barns), the atom density of 6Li in the fiber is fifty times greater, resulting in an advantage in effective capture density ratio of approximately 10:1. Ṅ = N·ρ/A Ṅ =number of nuclei per cm3 Na = Alvarado's number A = atomic weight ρ = density

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Neutron cross sections Neutron cross sections are defined in Part 1 of this Section. Values for thermal neutrons for many materials (elements) are given in Table 9 (see Bibliography item 8 for a more extensive compilation). Generally, neutron cross sections decrease with increasing neutron energy; exceptions include resonances, as mentioned earlier. Cross section values can be used to calculate the attenuation coefficients and the neutron transmission as shown in eqs. 1 and 2. For compound inspection materials, the method for calculating the linear attenuation coeffici ent is shown following Table 9.

If the material under inspection contains only one element, then the linear attenuation coefficient is: μn = ρ∙Nσ/ A

or

μn = Ṅσ

Eq.7

Where: μn is the linear attenuation coefficient (cm-1 ) ; ρ is the material density (g/cm3); N is Avogadro's number (6.023 X 1023 atoms/gram-molecular weight) ; σ is the total cross section in barns (cm2 ) ; and A is the gram atomic weight of material.

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Ṅ is the number of atoms per cubic centimeter Ṅ=

ρ∙N A

μn = Ṅ∙σ Where: μn is the linear attenuation coefficient (cm-1 ) ; ρ is the material density (g/cm3); N is Avogadro's number (6.023 X 1023 atoms/gram-molecular weight) ; σ is the total cross section in barns (cm2 ) ; and A is the gram atomic weight of material.

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TABLE 9. Thermal Neutron Linear Attenuation Coefficients Using Average Scattering and 2200m/s Absorption Cross Sections for the Naturally Occurring Elements

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If on the other hand, the material under inspection contains several elements, or is in the form of a compound, then the linear absorption coefficient is: μ = ρ ∙ N∙(ѵ1σ1 + ѵ2σ2 + ѵ3σ3 +..... ѵiσi )

Eq. 8

M Where: μ is the linear attenuation coefficients of the compound (cm-1) ; ρ is the compound density (g/cm3 ) ; N is Avogradro's number (6.023 X 1023 atoms/gram-molecular weight) ; M is the gram molecular weight of the compound; ѵi is the number of absorbing atoms of ѵi kind per compound molecule; and, σ; is the total cross section of the ith atom (cm2).

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As an example, consider the calculations of the linear attenuation coefficient, p.., for the compound polyethylene (CH2)N : μ = ρ∙N (ѵ1σ1 + ѵ2σ2 + ѵ3σ3 +..... ѵiσi ) /M μ = ρ∙N (ѵCσC + ѵHσH) /M for:

ρ = 0.91 g/cm3 N = 6.023 X 1023 atoms/g-mol M= 14.0268 g ѵC = 1 σC = 4.803 X 10-24 cm2 ѵH= 2 σH = 38.332 X 10-24 cm2

μ = 0.91 x (6.023 x 1023) x (1x 4.803+ 2x 38.332) x 10-24 14.0268 μ = 3.18329 cm-1

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Eq. 8

σH

σC

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â– LiCaAlF6 LiCaAlF6 is a neutron sensitive inorganic scintillator crystal which like neutron-sensitive scintillating glass fiber detectors makes use of neutron capture by 6Li. Unlike scintillating glass fiber detectors however the 6Li is part of the crystalline structure of the scintillator giving it a naturally high 6Li density. A doping agent is added to provide the crystal with its scintillating properties, two common doping agents are cesium and europium. Europium doped LiCaAlF6 has the advantage over other materials that the number of optical photons produced per neutron capture is around 30,000 which is 5 times higher than for example in neutron-sensitive scintillating glass fibers.[25] This property makes neutron photon discrimination easier. Due to its high 6Li density this material is suitable for producing light weight compact neutron detectors, as a result LiCaAlF6 has been used for neutron detection at high altitudes on balloon missions.[26] The long decay time of Europium doped LiCaAlF6 makes it less suitable for measurements in high radiation environments, the cesium doped variant has a shorter decay time but suffers from a lower light-yield.

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https://en.wikipedia.org/wiki/Neutron_detection

■ Semiconductor neutron detectors Semiconductors have been used for neutron detection.[27] ■ Neutron activation detectors Activation samples may be placed in a neutron field to characterize the energy spectrum and intensity of the neutrons. Activation reactions that have differing energy thresholds can be used including 56Fe(n,p) 56Mn, 27Al(n,α)24Na, 93Nb(n,2n) 92mNb, & 28Si(n,p)28Al.[28]

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https://en.wikipedia.org/wiki/Neutron_detection

■ Fast neutron detectors Fast neutrons are often detected by first moderating (slowing) them to thermal energies. However, during that process the information on the original energy of the neutron, its direction of travel, and the time of emission is lost. For many applications, the detection of “fret” 担忧,过去 neutrons that retain this information is highly desirable.[29] Typical fast neutron detectors are liquid scintillators,[30] 4He based noble gas detectors [31] and plastic detectors. Fast neutron detectors differentiate themselves from one another by their 1.) capability of neutron/gamma discrimination (through pulse shape discrimination) and 2.) sensitivity. The capability to distinguish between neutrons and gammas is excellent in noble gas based 4-He detectors due to their low electron density and excellent pulse shape discrimination property.

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Detection of fast neutrons poses a range of special problems. A directional fast-neutron detector has been developed using multiple proton recoils in separated planes of plastic scintillator material. The paths of the recoil nuclei created by neutron collision are recorded; determination of the energy and momentum of two recoil nuclei allow calculation of the direction of travel and energy of the neutron that underwent elastic scattering with them.[32]

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https://en.wikipedia.org/wiki/Neutron_detection

Fast Neutron Detector This is a fast neutron detector produced by Radiation Counter Laboratories (RCL) of Skokie, Illinois. The tube is approximately 8 1/4 inches long and 1 7/8 inches in diameter. A brass evacuation tube can be seen projecting to the right from the brass chamber. The actual proportional counter chamber is 1.2 inches long, lined with 1/16 inch of polyethylene, and filled with methane at a pressure of 150 cm.

It operated at 2100 volts. Fast neutrons knock protons off the polyethylene lining. The protons then ionize the methane fill gas to produce the signal.

The RCL detector designation is the Mark 2, Model 201, Serial 127. The "1" at the end of the model number (201) refers to the number of chambers housed in the unit.

The Models 202 and 203 used two and three chambers respectively.

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Applications Neutron detection is used for varying purposes. Each application has different requirements for the detection system.  Reactor instrumentation: Since reactor power is essentially linearly proportional to the Neutron flux, neutron detectors provide an important measure of power in nuclear power and research reactors. Boiling water reactors may have dozens of neutron detectors, one per fuel assembly.

Most neutron detectors used in thermal-spectrum nuclear reactors are optimized to detect thermal neutrons.  Particle physics: Neutron detection has been proposed as a method of enhancing neutrino detectors.[33]  Materials science: Elastic and inelastic neutron scattering enables experimentalists to characterize the morphology of materials from scales ranging from ångströms to about one micrometer.

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 Radiation safety: Neutron radiation is a hazard associated with neutron sources, space travel, accelerators and nuclear reactors. Neutron detectors used for radiation safety must take into account the relative biological effectiveness (i.e., the way damage caused by neutrons varies with energy).  Cosmic ray detection: Secondary neutrons are one component of particle showers produced in Earth's atmosphere by cosmic rays. Dedicated ground-level neutron detectors, namely neutron monitors, are employed to monitor variations in cosmic ray flux.  Special nuclear material detection: Special nuclear materials (SNM) such as uranium-233(?) and plutonium-239 decay by spontaneous fission, yielding neutrons. Neutrons detectors can be used for monitor for SNM in commerce.

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Uranium-233 is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel.[2] It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of only 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a half-life of 27 days and beta decays into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur.

233U usually fissions on neutron absorption but sometimes retains the neutron, becoming uranium-234. The capture-to-fission ratio is smaller than the other two major fissile fuels uranium-235 and plutonium-239.

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https://en.wikipedia.org/wiki/Uranium-233

Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in thermal spectrum nuclear reactors, along with uranium-235 and uranium-233.

Plutonium-239 has a half-life of 24,110 yearsThe nuclear properties of plutonium-239, as well as the ability to produce large amounts of nearly pure Pu-239 more cheaply than highly enriched weapons-grade uranium-235, led to its use in nuclear weapons and nuclear power stations. The fissioning of an atom of uranium-235 in the reactor of a nuclear power plant produces two to three neutrons, and these neutrons can be absorbed by uranium-238 to produce plutonium-239 and other isotopes. Plutonium239 can also absorb neutrons and fission along with the uranium-235 in a reactor.

Of all the common nuclear fuels, Pu-239 has the smallest critical mass. A spherical untamped critical mass is about 11 kg (24.2 lbs),[2] 10.2 cm (4") in diameter. Using appropriate triggers, neutron reflectors, implosion geometry and tampers, this critical mass can be reduced by more than twofold. This optimization usually requires a large nuclear development organization supported by a sovereign nation.

The fission of one atom of Pu-239 generates 207.1 MeV = 3.318 Ă— 10−11 J, i.e. 19.98 TJ/mol = 83.61 TJ/kg,[3] or about 2 322 719 kilowatt hours/kg.

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https://en.wikipedia.org/wiki/Plutonium-239

A 99.96% pure ring of plutonium

A weapons-grade ring of electrorefined plutonium, typical of the rings refined at Los Alamos and sent to Rocky Flats for fabrication. The ring has a purity of 99.96%, weighs 5.3 kg, and is approx 11 cm in diameter. It is enough plutonium for one bomb core. The ring shape helps with criticality safety (less concentrated material).

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https://en.wikipedia.org/wiki/Plutonium-239

Experimental neutron detection Experiments that make use of this science include scattering experiments in which neutrons directed and then scattered from a sample are to be detected.

Facilities include the ISIS neutron source at the Rutherford Appleton Laboratory, the Spallation Neutron Source at the Oak Ridge National Laboratory, and the Spallation Neutron Source (SINQ) at the Paul Scherrer Institute, in which the neutrons are produced by spallation reaction, and the traditional research reactor facilities in which neutrons are produced during fission of uranium isotopes. Noteworthy among the various neutron detection experiments is the trademark experiment of the European Muon Collaboration, first performed at CERN and now termed the "EMC experiment." The same experiment is performed today with more sophisticated equipment to obtain more definite results related to the original EMC effect.

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https://en.wikipedia.org/wiki/Neutron_detection

Challenges in neutron detection in an experimental environment Neutron detection in an experimental environment is not an easy science.

The major challenges faced by modern-day neutron detection include background noise, high detection rates, neutron neutrality, and low neutron energies. â– Background noise The main components of background noise in neutron detection are highenergy photons, which aren't easily eliminated by physical barriers. The other sources of noise, such as alpha and beta particles, can be eliminated by various shielding materials, such as lead, plastic, thermo-coal, etc. Thus, photons cause major interference in neutron detection, since it is uncertain if neutrons or photons are being detected by the neutron detector. Both register similar energies after scattering into the detector from the target or ambient light, and are thus hard to distinguish. Coincidence detection can also be used to discriminate real neutron events from photons and other radiation.

In physics, a coincidence circuit is an electronic device with one output and two (or more) inputs. The output is activated only when signals are received within a time window accepted as at the same time and in parallel at both inputs. Coincidence circuits are widely used in particle physics experiments and in other areas of science and technology.

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■ high detection rates If the detector lies in a region of high beam activity, it is hit continuously by neutrons and background noise at overwhelmingly high rates. This obfuscates 使模糊 collected data, since there is extreme overlap in measurement, and separate events are not easily distinguished from each other. Thus, part of the challenge lies in keeping detection rates as low as possible and in designing a detector that can keep up with the high rates to yield coherent data.

■ Neutrality of neutrons Neutrons are neutral and thus do not respond to electric fields. This makes it hard to direct their course towards a detector to facilitate detection. Neutrons also do not ionize atoms except by direct collision, so gaseous ionization detectors are ineffective.

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â– Varying behavior with energy Detectors relying on neutron absorption are generally more sensitive to lowenergy thermal neutrons, and are orders of magnitude less sensitive to highenergy neutrons. Scintillation detectors, on the other hand, have trouble registering the impacts of low-energy neutrons.

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â– Experimental setup and method Figure 1 shows the typical main components of the setup of a neutron detection unit. In principle, the diagram shows the setup as it would be in any modern particle physics lab, but the specifics describe the setup in Jefferson Lab (Newport News, Virginia). In this setup, the incoming particles, comprising neutrons and photons, strike the neutron detector; this is typically a scintillation detector consisting of scintillating material, a waveguide, and a photomultiplier tube (PMT), and will be connected to a data acquisition (DAQ) system to register detection details.

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https://en.wikipedia.org/wiki/Neutron_detection

Figure 1: The experimental setup

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https://en.wikipedia.org/wiki/Neutron_detection

The detection signal from the neutron detector is connected to the scaler unit, gated delay unit, trigger unit and the oscilloscope. The scaler unit is merely used to count the number of incoming particles or events. It does so by incrementing its tally of particles every time it detects a surge in the detector signal from the zero-point. There is very little dead time in this unit, implying that no matter how fast particles are coming in, it is very unlikely for this unit to fail to count an event (e.g. incoming particle). The low dead time is due to sophisticated electronics in this unit, which take little time to recover from the relatively easy task of registering a logical high every time an event occurs.

The trigger unit coordinates all the electronics of the system and gives a logical high to these units when the whole setup is ready to record an event run.

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The oscilloscope registers a current pulse with every event. The pulse is merely the ionization current in the detector caused by this event plotted against time. The total energy of the incident particle can be found by integrating this current pulse with respect to time to yield the total charge deposited at the end of the PMT. This integration is carried out in the analogdigital converter (ADC). The total deposited charge is a direct measure of the energy of the ionizing particle (neutron or photon) entering the neutron detector. This signal integration technique is an established method for measuring ionization in the detector in nuclear physics.[34] The ADC has a higher dead time than the oscilloscope, which has limited memory and needs to transfer events quickly to the ADC. Thus, the ADC samples out approximately one in every 30 events from the oscilloscope for analysis.

Since the typical event rate is around 106 neutrons every second,[35] this sampling will still accumulate thousands of events every second.

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- Separating neutrons from photons The ADC sends its data to a DAQ unit that sorts the data in presentable form for analysis. The key to further analysis lies in the difference between the shape of the photon ionization-current pulse and that of the neutron. The photon pulse is longer at the ends (or "tails") whereas the neutron pulse is well-centered.[35] This fact can be used to identify incoming neutrons and to count the total rate of incoming neutrons. The steps leading to this separation (those that are usually performed at leading national laboratories, Jefferson Lab specifically among them) are gated pulse extraction and plotting-thedifference.

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- Gated pulse extraction Ionization current signals are all pulses with a local peak in between. Using a logical AND gate in continuous time (having a stream of "1" and "0" pulses as one input and the current signal as the other), the tail portion of every current pulse signal is extracted. This gated discrimination method is used on a regular basis on liquid scintillators.[36] The gated delay unit is precisely to this end, and makes a delayed copy of the original signal in such a way that its tail section is seen alongside its main section on the oscilloscope screen.

After extracting the tail, the usual current integration is carried out on both the tail section and the complete signal. This yields two ionization values for each event, which are stored in the event table in the DAQ system.

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- Plotting the difference In this step lies the crucial point of the analysis: the extracted ionization values are plotted. Specifically, the graph plots energy deposition in the tail against energy deposition in the entire signal for a range of neutron energies.

Typically, for a given energy, there are many events with the same tail-energy value. In this case, plotted points are simply made denser with more overlapping dots on the two-dimensional plot, and can thus be used to eyeball the number of events corresponding to each energy-deposition. A considerable random fraction (1/30) of all events is plotted on the graph.

If the tail size extracted is a fixed proportion of the total pulse, then there will be two lines on the plot, having different slopes. The line with the greater slope will correspond to photon events and the line with the lesser slope to neutron events. This is precisely because the photon energy deposition current, plotted against time, leaves a longer "tail" than does the neutron deposition plot, giving the photon tail more proportion of the total energy than neutron tails.

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https://en.wikipedia.org/wiki/Neutron_detection

The effectiveness of any detection analysis can be seen by its ability to accurately count and separate the number of neutrons and photons striking the detector. Also, the effectiveness of the second and third steps reveals whether event rates in the experiment are manageable. If clear plots can be obtained in the above steps, allowing for easy neutron-photon separation, the detection can be termed effective and the rates manageable. On the other hand, smudging and indistinguishability of data points will not allow for easy separation of events.

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- Rate control Detection rates can be kept low in many ways. Sampling of events can be used to choose only a few events for analysis. If the rates are so high that one event cannot be distinguished from another, physical experimental parameters (shielding, detector-target distance, solid-angle, etc.) can be manipulated to give the lowest rates possible and thus distinguishable events.

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https://en.wikipedia.org/wiki/Neutron_detection

- Finer detection points It is important here to observe precisely those variables that matter, since there may be false indicators along the way. For example, ionization currents might get periodic high surges, which do not imply high rates but just high energy depositions for stray events. These surges will be tabulated and viewed with cynicism if unjustifiable, especially since there is so much background noise in the setup.

One might ask how experimenters can be sure that every current pulse in the oscilloscope corresponds to exactly one event. This is true because the pulse lasts about 50 ns, allowing for a maximum of 2Ă—107 events every second.

This number is much higher than the actual typical rate, which is usually an order of magnitude less, as mentioned above.[35] This means that is it highly unlikely for there to be two particles generating one current pulse. The current pulses last 50 ns each, and start to register the next event after a gap from the previous event.

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https://en.wikipedia.org/wiki/Neutron_detection

Although sometimes facilitated by higher incoming neutron energies, neutron detection is generally a difficult task, for all the reasons stated earlier. Thus, better scintillator design is also in the foreground and has been the topic of pursuit ever since the invention of scintillation detectors. Scintillation detectors were invented in 1903 by Crookes but were not very efficient until the PMT (photomultiplier tube) was developed by Curran and Baker in 1944.[34] The PMT gives a reliable and efficient method of detection since it multiplies the detection signal tenfold. Even so, scintillation design has room for improvement as do other options for neutron detection besides scintillation.

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Figure 2: Expected plot of tail energy against energy in the complete pulse plotted for all event energies. Dots represent number densities of events.

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Neutron Imaging Detectors Paul Scherrer Institut https://www.psi.ch/niag/neutron-imaging-detectors

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https://www.psi.ch/niag/neutron-imaging-detectors

Paul Scherrer Institut

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Paul Scherrer Institut

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What is Neutron Imaging ?

Neutron Imaging (NI) is a radiographic testing method using neutrons. As such it is quite similar to X-ray, which is well known from medical applications.

Neutrons are able to transmit material layers of certain thickness which depends on the specific attenuation properties of that material. This enables to establish neutron imaging as a non-destructive inspection method which can favorably be applied for research and practical, industrial related problems. A similarity and complementarities to the conventional X-ray imaging are given. Because different materials have different attenuation behavior the neutron beam passing through a sample can be interpreted as signal carrying information about the composition and structure of the sample.

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Figure 1: Radiograph of an analog camera: by neutrons (top)

by X-rays (bottom).

While X-rays are attenuated more effectively by heavier materials like metals, neutrons make it possible to image some light materials such as hydrogenous substances with high contrast: in the X-ray image, the metal parts of the photo apparatus are seen clearly, while the neutron radiograph shows details of the plastic parts.

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Neutron Imaging Setup The neutron generating source can be a reactor (like FRM-2 in Garching near Munich), the target of an accelerator (like the spallation source SINQ) or a neutron emitting isotope. The collimator is a beam forming assembly which determines the geometric properties of the beam and may also contain filters to modify the energy spectrum of the beam or to reduce the content in gamma rays of the beam. The image resolution achievable with the beam depends much on the collimator geometry and is expressed by the L/D ratio, where L is the collimator length and D is the diameter of the inlet aperture of the collimator on the side facing the source. The beam is transmitted through the object and recorded by a plane position sensitive detector, i.e. the detector records a two dimensional image that is a projection of the object on the detector plane.

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By combining images from measurements at different angles tomographic reconstruction may be carried out. Dynamic processes can be studied by detector systems with fast imaging capacity. This is the field of neutron realtime imaging. Various detector systems are employed in NR : combinations of film and a neutron-sensitive converter foil, combinations of a light-emitting scintillator screen with a CCD camera , neutron sensitive imaging plates, track etch foils or recently amorphous silicon flat panel arrays.

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Figure 2: Schematic basic neutron imaging setup.

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Neutron Physics The neutron is one of the constituent nucleons of the atomic nucleus (the other is the proton). It has zero electric charge, but a magnetic moment and its mass is about 1840 times that of the electron. Outside the nucleus a free neutron will decay into a proton, electron, and antineutrino with a lifetime of about 15 minutes. The neutron can be described as a classical particle with mass m but it shows wave character too, which can be described with the deBroglie wave-length Îť. Let m=1.6749 10-27 kg be the neutron mass, v its velocity and h Planck's constant. Below are the relations for neutron energy E given in meV (i.e. 10-3eV), wave-length Îť in Ă…ngstrom and velocity v in m/s.

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Neutrons can be classified according to their kinetic energy as shown in the table below. For neutron imaging thermal and cold neutrons are preferred due to their favorable detection reactions and due to their very useful contrast behavior.

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Neutron Imaging Techniques and Methods There are several possibilities how neutron imaging can be used for nondestructive evaluation. Single radiographs or 3D data delivering acquisitions using neutron tomography are applied to rigid objects or almost stationary processes. Dynamic processes e.g. humidity uptake or evaporation, investigations of running motors, etc. are investigated as time series of radiographs providing movies of the process. Contrast enhanced radiographs need neutron energy selective imaging methods and/or measuring techniques exploiting the neutrons wave properties as in differential phase contrast imaging. This wide range of imaging modalities requires different characteristics for neutron detector systems. Further information is given in the sections: ■ Neutron Imaging Detectors ■ Neutron Tomography ■ Dynamic Neutron Imaging ■ Advanced Imaging Techniques

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Neutron Imaging Detectors Detectors in use for Neutron Imaging (NI) purposes are those, which are able to measure the neutron field in two dimensions perpendicular to the beam direction. Therefore, the detector area should be in the order or larger than the beam cross-section. Further boundary conditions are the spatial and time dependent resolution of the detector, which can be very different among the existing detectors systems. An overview about these parameters is given for the most common systems in figure 8. The inherent detectors properties are mainly given by the detection process, which is a nuclear reaction initiated by the neutrons. The primary detection reactions for thermal neutrons are mainly neutron capture by an absorbing material emitting secondary radiation, which can be used as the real neutrons proof. This is why neutrons have no electric charge and cannot make ionization directly which is needed for the detection.

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Spatial resolution (mm)

Figure 8: Overview of neutron detectors

Time Resolution (s) Charlie Chong/ Fion Zhang

Figure 9: Standard combination of a CCD camera and a neutron scintillator for neutron imaging

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The most important detection reactions are (for thermal and cold neutrons): 3He + 1n → 3H + 1p + 0.77 MeV 6Li + 1n → 3H + 4He + 4.79 MeV 10B + 1n → ( 7%) 7Li + 4He + 2.78 MeV 10B + 1n → (93%) 7Li* + 4He + 2.30 MeV + γ (0.48 MeV) 155Gd + 1n → 156Gd + γ + conversion electrons (7.9 MeV) 157Gd + 1n → 158Gd + γ + conversion electrons (8.5 MeV) 113Cd + 1n → 114Cd + γ + conversion electrons Internal conversion (electron) is a radioactive decay process wherein an excited nucleus interacts electromagnetically with one of the orbital electrons of the atom. This causes the electron to be emitted (ejected) from the atom.[1][2] Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but not from the nucleus. For this reason, the high-speed electrons resulting from internal conversion are not called beta particles, since the latter come from beta decay, where they are newly created in the nuclear decay process.

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The further process for imaging in radiography based on the previous reactions is possible in different ways:  by light excitation in a scintillator  by blackening of a suited film  by excitation of electronic (metastabile) states in a crystal (imaging plates)  by creation of micro-traces in special foils (track-etch method)  by charge separation in a semiconductor material

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The following detector systems for radiography have been developed on the basis of the mentioned processes and are in use for different applications: â– X-ray film in connection with converter foils from Gd, Dy, In and others. The excitation and blackening of the film is caused by gamma and beta radiation as well as by conversion electrons.

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â– Highly light sensitive CCD camera detectors (cooled in most cases) looking onto the weak light emission from a neutron sensitive scintillator (Li-6 or Gd as neutron absorber).

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â– By the use of image intensifiers, the light intensity can importantly be increased (as intensifier tubes or micro-channel plates). In this way, either less sensitive cameras can be applied or higher frame rates becomes possible.

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â– Imaging Plates contain Gd as neutron absorber and BaFBr:Eu 2+ as the agent which provides the photoluminescence. A imaging plate scanner is extracting the latent image information as digitised data file from the plates by de-excitation caused by a laser signal.

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â– Track-etch-foils are "scratched" by Îą-particles created in a capture reaction of B-10 with thermal neutrons. These very small tracks can be enlarged so much by chemical treatment (etching in an alkaline bath) that a macroscopic image occurs, which can be digitalized or optical enlarged by optical means.

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â– Flat panels based on amorphous silicon can provide digital information directly and an optical magnification (as with cameras) is not necessary.

However, they have to be placed into the direct beam, which can cause some problems for long term use.

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A summary of important properties of radiography detectors is given below:

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Neutron Tomography Computed tomography is a method to acquire three dimensional information about the structure inside a sample. The method applies to neutron as well as the more known X-ray imaging. It uses radiographic projection images from many views to reconstruct the distribution of materials in the sample. Mostly, the projections are acquired with equiangular steps over either 180o or 360o to cover the whole sample. Figure 9 shows an experiment setup used for neutron tomography (NT). In contrast to medical imaging the samples is rotated instead of the beamline. The projection images are acquired using a combination of a scintillator to convert the neutrons to visible light and a CCD camera. The transform of the projection data into a three dimensional image is a computationally intensive task handled by special reconstruction software.

During the reconstruction process, slices perpendicular to the rotation axis are produced. When these slices are stacked in a sequence they form a three-dimensional volume image of the sample. The reconstructed volume data can be visualized using three-dimensional rendering graphics software.

Using such tools, regions can be segmented based on their attenuation coefficients and geometry. This can be used to reveal details inside the sample in three dimensions.

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Figure 10: Experimental setup for neutron tomography at NEUTRA

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Figure 11: Tomography data acquisition

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Figure 12: Computer tomography reconstruction

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Figure 13: Visualization of 3D neutron tomography data of a horsefly

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Time Dependent Neutron Radio- and Tomography Neutron radiography and tomography can trace the propagation of small amounts of hydrogenous substances within metallic casings (e.g. a combustion engine) or rock and soil matrices. Exactly triggered radiographs with short exposures can take snapshots of rapid periodic processes. Many short exposures must be usually summed in order to achieve sufficient image contrast. Non-periodic processes may also be observed if the neutron flux intensity is high and the neutron detector permits acquiring images at sufficiently high frame rate.

Dynamic processes pose an additional challenge to the acquisition of tomographies. Normally, the sample is not allowed to change during the scan.

Otherwise, motion artifacts will appear. By changing the order in which the projections are acquired, it is still possible to obtain useful spatio-temporal data even though the sample is changing during the scan.

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Figure 2: Humidity exchange between small spheres followed in 3D over time.

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Advanced Neutron Imaging Techniques Neutron Grating Interferometry with Neutrons Measured neutron images of two metallic rods with 6 mm diameter made of copper and titanium. (Left) Conventional absorption image. (right) phase contrast image.

Conventional absorption image Charlie Chong/ Fion Zhang

phase contrast image

Instead of measuring the direct attenuation of the neutron beam, there are measuring techniques exploiting the (de-Broglie) wave properties of the neutron. They provide additional contrast mechanisms due to neutron wave interference and small angle neutron scattering.

More on: Phase contrast and dark-field imaging with neutrons

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More on: Phase contrast and dark-field imaging with neutrons Phase contrast and dark-field images with visible light are indispensable tools for the modern microscopy technology. PSI had succeeded to develop the corresponding imaging techniques for neutrons. Hereby quantum mechanical interaction of neutrons with matter can be made visible in two-dimensional and three-dimensional images.

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Particle physicists consider neutrons as small particle. Due to the waveparticle dualism neutrons can be described by matter waves, with a certain wave length. In difference to the conventional absorption contrast, where the contrast differences arise from the different attenuation of the materials, the image information in the phase contrast and dark-field images originate from the change of the wavelength in the material.

In the case of the phase contrast method one uses the fact, that waves which transverse an object having a different velocity than the waves which do not transverse the object. They therefore have a different wavelength. The resulting displacement of the wave maxima leads to a change of the propagation direction and therefore to an angular change. An example from the classic geometrical optics clarifies how a phase sensitive image can be obtained. Considering a beam path through a collection lens, it follows from the law of refraction that initially parallel light rays are refracted towards the optical axis. In a wave-optical description this corresponds to lens induced angular change of the light rays, and namely a spatially dependent phase shift of the wave front.

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In order to obtain phase contrast imaging, we measure the local angular variation of the neutron beam caused by the object. The experimental difficulty is, therefore, to measure efficiently for a variety of image points such small diffraction angles which are in the range of 10-4 degrees. Therefore one uses two grids (G1 and G2) which are composed of lines with lattice constants of a few micrometers. Such a grating is shown in Figure 1. The gratings together with a spatially resolving neutron detector then form the socalled neutron grating interferometer as depicted in Figure 2. The local angular change can be determined by analyzing pixel wise the measured intensity.

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Figure 1: One of the gratings with fine lines in the micrometer range manufactured at PSI as used in the neutrons grating interferometer. The wafer has a diameter of 100 mm. The grid area is 64 x 64 mm2. The rainbow is caused by the refraction of light at the fine structures of the grating.

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Figure 2: Schematical setup of neutron grating interferometer consisting of a phase and an absorption grating and an imaging detector. With the help of this setup neutrons can be detected, which are deflected or scattered at an angle of 10-4 degrees.

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Results Measurements of the phase shift of neutrons in interferometry experiments have been used so far to experimentally verify quantum mechanical predictions. The work at PSI aims to combine the available information about the phase shift with a space-resolved imaging technique, to image quantum mechanical interaction of neutrons with matter and to offer new contrast possibilities.

Figure 3 shows the results obtained for test samples. The conventional neutron absorption image (left) shows no measurable difference in the attenuation behavior of the two metals comprising of copper and titanium. The image of the measured phase shift (right), however, clearly shows a difference. Particularly interesting is the opposite sign of the phase shift of copper (black in the image against the gray background) and titanium (white to gray). This is a consequence of the different signs of the refractive indices of the materials.

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Figure 3: Measured neutron images of two metallic rods with 6 mm diameter made of copper and titanium. (Left) Conventional absorption image. (right) phase contrast image.

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For the dark-field imaging method the scattering properties in the in interior of materials is considered. Neutrons experience during the passage of the sample, which consist of materials with different refractive indices, as well a small angular change. This can be used for example to visualize magnetic domains in ferromagnetic materials, as shown in Figure 4. Neutrons are deflected due to the interaction of the neutron spin with the local magnetic fields, because magnetic domains with different orientations have different refractive indices. Therefore, neutrons are scattered in the transition from one domain to the next at the domain walls. The domain walls are seen in the dark-field image as white lines.

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Figure 4: Measured neutron images of a magnetic sample (silicon iron) in the form of a disk with a thickness of 300 microns and 10 mm in diameter. (Left) Conventional absorption image. (right) Dark-field image. The white lines which form a rhombus are magnetic domain walls.

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Another Example is the investigation of electrical steel sheet, used in nearly every type of electrical machines. The magnetic properties of electrical steel in magnetic cores such as stator and rotor laminations of electrical machines are known to be influenced by manufacturing conditions such as punching, interlocking or laser cutting. The effect of cutting is shown in figure 5. Two mechanical cutting techniques are compared with two laser cutting techniques. The mechanical techniques show a strongly reduced dark-field signal near the cutting edge, interpreted as a higher density of magnetic domain walls that is undesired and increases magnetic core losses.

Furthermore it is not only magnetic scattering that can be detected like figure 6 shows. Two brass step wedges are shown here. The only difference is 3% of lead that is forming small precipitations in the left wedge. Since lead has a different refractive index for the traversing neutron the neutrons are scattered by the lead inclusions resulting in a measurable signal in the dark field image.

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Figure 5: (Left) Dark-field image of 4 differently cut non-oriented FeSi electrical steel laminations. (Right) Profile along colored line. Mechanical cut samples show a decreased dark-field signal at the edge (punch press is only cut on the left hand side), while laser-cut sample don’t show this edge effect.

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Figure 6: (Left) The transmission image of two brass step wedges. The left wedge is containing 3% of lead but no difference is reveal in this image.

(Right) The dark-field image however clearly highlights the ability of detecting scattering, occurring from lead precipitations in the brass wedge

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Energy Selective Neutron Imaging Steel weld: (left) taken with full spectrum, (right) texture effects due to Bragg scattering become visible at limited energy bandwith.

Usually neutron radiographs are taken with the full neutron energy spectrum of the beamline. Cold neutrons provide additional imaging contrast using a limited energy/wavelength bandwith in those materials showing Bragg scattering.

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Spectrum of the ICON cold neutron imaging beamline and NEUTRA thermal imaging beamline, together with the theoretical microscopic cross-section for the iron bcc and fcc phases. Energy-selective neutron imaging exploits the wavelength-dependent behaviour of the materials cross-section σ(λ) as a new source of image contrast.

Many polycrystalline materials’ cross section exhibit sharp edges, so-called Bragg edges. Imaging around these Bragg edges offers a.o.: Increased contrast between materials or phases, as they exhibit different Bragg edge positions Increased material penetration, by selecting wavelengths of low material cross-section.

Increased material content quantification, by imaging in the long wavelength absorption range where little scattering contributions are present.

Qualitative texture mapping, as different textures emphasize different Bragg edges.

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A prerequisite for energy-selective imaging is a cold neutron spectrum (λ ≥ 2.5Å) - as this is where the most dominant Bragg edges appear – and a monochromator for energy-resolution. Both are available at the ICON beamline, with the beam port looking at the 25K cold neutron source and an integrated neutron velocity selector offering Δλ/λ = 15% monochromaticity.

Development of a second monochromator for reaching Δλ/λ = 5% is ongoing.

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Question 1,2,3

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Q14: D1-D3?

Q26: D4-D1?

Q85: D5-D1? a) b) c) d)

effective thermal neutron content, NC effective scattered neutron content, S effective gamma content, Îł effective pair production content, P,

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Gadolinium-157 has the highest thermal neutron capture cross-section among any stable nuclides: 259,000 barns. Only xenon-135 has a higher cross section, 2 million barns, but that isotope is unstable. https://en.wikipedia.org/wiki/Gadolinium

Charlie Chong/ Fion Zhang

Gadolinium

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

TABLE 4. Average Characteristics of Thermal-Sources Type of Source

Typical Radiographic Intensity*

Resolution**

Exposure

Characteristics

Time

Radioisotope

101 to 104

Poor to Medium

Long

Stable operation. medium investment cost. possibly portable.

Accelerator

103 to 106

Medium

Average

On-off operation. medium cost. possibly mobile.

Subcritical Assembly

104 to 106

Good

Average

Stable operation, medium to high investment cost, mobility difficult

Nuclear reactor

105 to 108

Excellent

Short

Stable operation, medium to high investment cost. mobility difficult

*Neutrons per square centimeter per second. n/cm2∙s **These classifications are relative Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

More Reading http://www.scielo.mec.pt/scielo.php?pid=S0870-83122010000200005&script=sci_arttext http://nptel.ac.in/courses/112107144/welding/lecture11&12.htm

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Good Luck!

Charlie Chong/ Fion Zhang