Wednesday 06 June 2018 Speaker: Chamkaur Ghag (UCL) Title: "Low Background Techniques for Rare Event Searches” Abstract: Rare event search experiments, such as those seeking scattering of galactic dark matter particles or evidence of neutrino-less double beta decay, must deploy extremely radio-quiet targets to expose any hints of signal. This necessitates operating the detectors in deep underground laboratories, passive and active shielding and, crucially, construction from exceptionally radio-pure materials. The LZ dark matter experiment requires an unprecedented low background rate within its fiducial volume, defining strict constraints on radioactivity from construction materials. An aggressive screening campaign with cutting-edge instrumentation including underground gamma spectroscopy, mass spectrometry, radon emanation measurements, and neutron activation has been performed to measure the trace levels of radioisotopes in materials to ensure the experiment reaches a sensitivity to spin-independent WIMP-nucleon scattering below 1.6x10-48 cm2. These assays, together with measurements of exposure to airborne contaminants during experiment construction, inform the high-precision LZ Background Model against which the statistical significance of any potential signal detected once operations begin in 2020 will be evaluated. This talk will present the LZ experiment’s Background Model and the key radio-assay techniques developed and deployed in the UK to complete the radio-purity campaign for LZ. All future rare event search experiments, irrespective of the chosen technology, will require these techniques again and in some cases significant assay sensitivity improvements and new capability will be necessary. The talk will also outline the research already underway to meet these future challenges.