Strong evidence shows most of the Universe’s mass is made of dark matter - non-luminous, non-baryonic material that interacts very weakly with normal matter. It is expected to be 5 times more abundant than standard matter, but its interaction with normal matter is extremely weak.

The SABRE (Sodium iodide with Active Background REjection) experiments aim to detect annual modulation signals from dark matter using ultra-pure NaI(Tl) crystals. SABRE South, located at the Stawell Underground Physics Laboratory (SUPL) in Australia is the first deep underground lab in the Southern Hemisphere. It will help distinguish seasonal or site-related effects from the dark matter-like signal observed by DAMA/LIBRA in the Northern Hemisphere. 
 
Radioimpurities such as ⁴⁰K, ²³⁸U, ²¹⁰Pb and ²³²Th, either intrinsic to the detector material or surface contamination, provide a fundamental limit to the sensitivity of SABRE.  Therefore, it is crucial to characterise and minimise this background to allow unambiguous detection of the signals that will indicate detection of dark matter particles. 

Here, the student will participate in the chemical extraction, Accelerator Mass Spectrometry (AMS) measurements to determine the ultra-low radioactivity levels and analysis of potential detector materials. AMS is a single atom counting technique that measures extraordinarily low concentrations of rare (radioactive) isotopes of natural or anthropogenic origin. It will involve the use of Australia's 15-million-volt accelerator at ANU as well as a complementary accelerator at ANSTO in Sydney. 

Furthermore, to improve the sensitivity, the crystals are surrounded by a large liquid scintillator veto that rejects background radiation. This veto consists of Linear Alkyl Benzene (LAB) solvent with 3.5 g/L of 2,6-diphenyl oxazole (PPO) as the primary fluorophore and 15 mg/L of bis-MSB as the secondary fluorophore. To ensure reliable operation, it is essential to characterise the optical properties of this scintillator mixture. 

Here, the student will

• Measure optical absorption spectra with UV-VIS spectrophotometry.
• Determine the light yield of the scintillator using a photomultiplier tube (PMT).
• Analyse results in Python to extract quantitative performance parameters.