precision metrology research program

Nodes involved: ANU, UWA

Chief Investigators: M. Goryachev, S. Tims, M. Tobar

Postdocs: M. Froehlich, Z. Slavkovska

RHD Students: W. Campbell, F. Dastgiri, G. Flower, I. Goel, R. Limina, C. Millar, A. Quiskamp, B. Roughan, C. Thomson

Associate Investigators: E.Ivanov

Partner Investigators: A. Wallner, M. Hotchkis

Characterisation of the materials used in dark matter detectors is proceeding at the ANU node. This is to ensure the intrinsic radioactive background present in the detector components is minimised, particularly where it would contribute to the energy region of interest for dark matter detection. Significant progress has been made towards improving detection sensitivity for a number of the uranium decay series radionuclides at ANU, and for 210Pb at ANSTO.

The UWA node continues the development of low phase noise oscillators as detectors in the sub-eV range, and their potential for improving bolometers to detect axions as described in the AXION section.

Nuclear metrology

AMS capability

During 2022 we have been developing new AMS techniques and chemical separation and purification methods to improve AMS sensitivity for the important isotope 210Pb. A major focus this year has been towards identifying a suitable lead carrier material to quantify the fraction of Pb atoms chemically extracted from materials where they will only be present at trace levels. It is imperative that the carrier has a 210Pb content that is low enough that it will not interfere with the 210Pb measurements of the material to be measured.

The relative abundance of 210Pb to the other Pb isotopes in the carrier material needs to be orders of magnitude lower that what is normally present in most cases. Consequently, chemical processing alone cannot produce suitable carrier material and, the proportion of 210Pb in the carrier needs to be close to, or below, the current detection limits achievable with AMS.

In parallel with the search for a suitable carrier material, and, in collaboration with our PIs from ANSTO and HZDR, we have been progressing the development of new AMS techniques to improve 210Pb measurement sensitivity. The VEGA accelerator at ANSTO was specifically designed to measure isotopes of similar mass to 210Pb, and potentially could have the required sensitivity. In 2021 we identified PbF2 as the optimal sample material for extraction of lead from the accelerator ion source (as PbF3 negative ions) and subsequent AMS measurement of Pb atoms. Significant progress was made in 2022, with substantial improvements in the yield, stability and intensity of lead tri-fluoride beams (see Figure below). Extracted beam currents are up to a factor of ~30 larger than reported at other AMS facilities [Nucl. Instrum. Methods Phys. Res. B 529 (2022) 18], which should translate to a similar improvement in sensitivity. The experimental endeavour to improve the sensitivity will continue in 2023. In parallel with this, there is an effort to understand the effect of trace-level contaminants in materials.

Modifications to the time-of-flight detector have improved the noise levels and detector resolution, and allowed first tests of integration of the detector equipment with the fast-isotope switching system. These initial tests were very successful and led to subsequent preliminary tests of automated measurements of 236U with the timeof-flight detector equipment. Preliminary results from these recent tests indicate a range of uranium decay series isotopes could be measured with the greater precision.

ICP-MS

Tests with dilutions of certified reference materials at the ANU ICP-MS facility appear to have improved the quality of the results reported for 40K analysis. The sensitivity is now close to what is needed, however there still appears to be an issue with accuracy. Further discussions and measurement tests are in progress. The ANU facility and a local commercial ICP-MS laboratory both seem capable of making satisfactory measurements to 40K levels of ~10 ppb, but this remains at least an order of magnitude above what is desired.

New low background Ge detector

A third low background germanium detector has been installed in newly constructed lead castle at ANU. The new detector is currently being commissioned and will initially be used to, assess the internal radioactivity in sapphire crystal oscillators used by the UWA node.