case study 2

Scavenger hunt: Searching for the optimal target material for low-level 210Pb accelerator mass spectrometry

Centre postdocs Michaela Froehlich and Zuzana Slavkovska, PhD student Ferdos Dastgiri and Chief Investigator Steve Tims (all from ANU) have been collaborating with Associate Investigator Dominic Koll (HZDR) and Partner Investigators Michael Hotchkis (ANSTO) and Anton Wallner (HZDR) with the aim of measuring 210Pb in 1 kg of NaI via accelerator mass spectrometry (AMS) for the SABRE South experiment.

Radionuclides including 238U, 232Th, 210Pb and 40K, which are intrinsically present in the detector materials or arise because of surface contamination, will provide a fundamental limit to the sensitivity of the SABRE experiment. Therefore, the characterisation of these radioimpurities is essential for the identification of any additional signal above this background. They focused on 210Pb, since radiation associated with its beta decay overlaps with the low-energy region in which the dark matter signal is expected. Lead-210 measurements are usually performed by decay counting or Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) depending on the sample size, matrix and concentration. However, the past ~20 years have seen significant developments in heavyion detection by accelerator mass spectrometry (AMS). Studies were performed at the 1 MV VEGA accelerator at ANSTO to test the abundance sensitivity and measurement background of 210Pb-AMS with a state-of-the-art facility specifically designed for heavy-ion detection. However, most ion source output tests for different sample compositions were performed at the ANU.

The first step is to find the optimal target material (chemical compound and binder) to produce the highest and most stable negative ion output. The group initially studied the outputs from Pb3O4, PbO and PbF2 mixed 1:2 with Ag. The 208PbO2− and 208PbF3− currents were 0.5–1.2 μA with the procedural PbF2 compound performing slightly better. Based on these results, they explored the performance of PbF2 mixed with fluorinating agents such as AgF, AgF2 and SbF3 at different ratios. The 1:1 mixture was the best for all additives.

The results were published in Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 529 (2022) https://doi.org/10.1016/j.nimb.2022.08.015

Associate Professor Steve Tims said the collaboration had expanded to include groups at the universities of Vienna, Austria, and Ottawa, Canada, and offered a range of benefits to the researchers involved.

“The collaboration has provided frank, open and highly fruitful discussions on how to achieve the required sensitivity for the 210Pb measurements, and given me personal insight into the capabilities of the specialised infrastructure unique to the associated facilities.

“It has strengthened the relationships between the research groups involved and is having a positive impact on other collaborative projects with these groups that are outside of the scope of Centre activities. A highlight of the activities has been the desire and enthusiasm shown by younger members in the collaboration to engage in the discussions and participate in development of the cross-disciplinary techniques being developed.”

Dr Froehlich said she was proud of the achievements of the collaboration, along with the opportunity to work with local and international researchers.

“A highlight of our collaborations is certainly our diversity: it has brought together scientists from across the globe with different backgrounds of expertise and various levels of experience (from PhD students to senior scientists).”

Dr Slavkovska said the opportunity to collaborate had provided her with new knowledge and experience.

“I am a physicist involved not only in the physics but also with chemistry for this project and the learning curve was huge for me. I learned several chemical procedures. Furthermore, I had the opportunity to chat to experienced international chemists and collaborate with them. The highlight of this project for me was to run the AMS machine VEGA at ANSTO when measuring 210Pb samples together with our Partner Investigator Mike Hotchkis.”