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Mid Term Review

Pulse shape discrimination for particle identification in NaI:Tl crystals for the SABRE dark matter experiment

Mr Nathan Spinks
Department of Nuclear Physics

One of the greatest unsolved mysteries of modern physics is the nature of dark matter. Weakly Interacting Massive Particles (WIMPs) are a strongly motivated dark matter candidate. Direct detection experiments search for WIMP signals by measuring WIMP interactions in excess of known backgrounds. The majority of WIMP direct-detection searches have produced null results, with the exception of DAMA. A key difference between this experiment and experiments that produced null results is the use of NaI:Tl as a detection medium. This motivates other NaI:Tl-based direct detection experiments and highlights the necessity of an unambiguous test of the DAMA result. SABRE (Sodium-Iodide Active Background REjection) is a direct detection experiment aiming to measure the annual modulation of the dark matter, with a higher sensitivity than the DAMA experiment and is expected to confirm or reject the DAMA result within a 3-year window.


A key characteristic of WIMPs is that they are expected to scatter off nuclei producing measurable nuclear recoils, while most background events such as gamma rays will scatter off atomic electrons to produce electron recoil events. With SABRE still under construction, we conducted an experiment at the ANU Tandem Accelerator to study nuclear recoil and electron recoil events, produced in a NaI:Tl crystal through interactions with neutron and gamma events, respectively. These interactions produce measurable photons through scintillation mechanisms in the crystal, which are recorded as pulses by a digital acquisition system. The interpretation of direct detection signals is improved by understanding the scintillation mechanism in NaI:Tl crystals, with different pulse shapes produced, dependent on particle type. Current Pulse Shape Discrimination (PSD) analysis of NaI:Tl crystals is limited to considering pulse decay times. A new PSD approach is developed using Bayes' theorem and, in conjunction with a conventional decay time metric, the discrimination power is improved from previous NaI:Tl-based PSD approaches. The application of this improved PSD approach is expected to increase the sensitivity of the SABRE experiment to WIMP events.

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