Radio-impurity measurements for a Dark Matter Sodium Iodide Detector
Ms Ferdos Dastgiri1, Dr Zuzana Slavkovska1, Dr M.B. Froehlich1, Dr M. A. C. Hotchkis2, Mr Dominik Koll1, S. Merchel1,3,4, Dr Stefan Pavetich1, Dr S. G. Tims1, Professor Keith Fifield1, Dr Anton Wallner3,1
1Australian National University, Canberra, Australia, 2Australian Nuclear Science and Technology Organisation, Sydney, Australia, 3Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany, 4Faculty of Physics, University of Vienna, Vienna, Austria
The first dark matter detector is being built in the Stawell gold mine in south-eastern Australia, as the southern hemisphere arm of an international collaboration SABRE (Sodium Iodide with Active Background Rejection). This experiment employs ultra-low background sodium iodide (NaI) detectors placed in highly shielded vessels across both hemispheres. The aim is to confirm or refute annual modulation claims attributed to dark matter particles by the DAMA/LIBRA collaboration at the Laboratori Nazionali del Gran Sasso in Italy. This requires the lowest possible concentration of radio-contaminants that can be achieved, to minimise the potential for radiation signals that can mimic dark matter particles signals.
We report on the techniques employed for the detection of potentially problematic contaminants in the NaI material from which the crystals will be grown. We focus on the establishment of the measurement techniques of ⁴⁰K and ²¹⁰Pb at the Australian National University and ANSTO.
For the measurement of ⁴⁰K, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure the concentration of ³⁹K, and from the well-known natural abundance ratios of ³⁹K/⁴⁰K, the concentration of ⁴⁰K was inferred. The challenges associated with measuring ultra-precise levels of ³⁹K, and the techniques of minimising the introduction of potassium in the sample preparation will be discussed.
210-Lead was measured using AMS. The ²¹⁰Pb concentration in the NaI powder is very low, which necessitates that large amounts (~ 1kg) of the powder need to be processed to result in sufficient atoms for an AMS measurement. This low concentration requires the additions of a Pb-carrier (~ 1mg), which itself needs to contain minimal ²¹⁰Pb. Several lead materials have been investigated and will be reported. In addition, we will discuss the different lead compounds and cathode materials used to optimise the beam current and minimise the background.
Other contaminants of potential interest such as ³H, ²³²Th and ²³⁸U; especially those identified in DAMA/LIBRA and other NaI detectors will be presented.
My name is Ferdos Dastgiri and I am a first year PhD candidate at the ANU.
My thesis is related to employing advanced metrology techniques in dark matter direct detection. I work with the Dark Matter Centre of Excellence in Canberra in both AMS and direct detection. Our broad goals are to develop sensitive measurement techniques for the ultra-sensitive SABRE dark matter detection.