LIDINE CONFERENCE 2017 SLAC NATIONAL ACCELERATOR LABORATORY RADON BACKGROUND IN LIQUID XENON DETECTORS NATASCHA RUPP MPIK HEIDELBERG RADON AS BACKGROUND SOURCE • part of primordial decay chains • noble gas and chemically from Uranium and Thorium inert • mother nuclei present in • no external shielding detector materials possible enters LXe in two ways: • no stable isotope in nature • long half-live of 222Rn with 3.8 days (main focus in this talk) !2 RADON AS BACKGROUND SOURCE Beta decays: • due to continuous energy spectrum constant background source 214 • neutrinoless double beta decay : Bi 214 • dark matter search: Pb Alpha decays: • usually no direct background due to high energies in MeV region • used to set limits on actual radon induced background beta decays • can be measured with dedicated alpha detectors (e.g. proportional counters, radon monitors) Diploma Thesis S. Brünner (2014) !3 RADON SCREENING • allows careful selection of only radio-pure materials • gamma spectroscopy only not sufficient collecting radon atoms: sample purification: • separating radon atoms from impurities with gas chromatography !4 ALPHA DETECTORS miniaturized proportional counters: radon monitors : • originally developed for • positively charged decay solar neutrino experiment products drifted to PIN diode GALLEX/GNO • sensitivity: < 1mBq • low intrinsic background: • online measurement of large 1cpd gas samples possible • high sensitivity: 20 μBq (10 radon atoms) • extensive procedure of counter filling Phd Thesis S. Brünner (2017) !5 EMANATION MEASUREMENT RESULTS • investigation of SS tubes no surface treatment: • (0.09 ± 0.1)μBq/cm2 after welding: welded • (0.12 ± 0.1)μBq/cm2 after additional electropolishing: • (0.04 ± 0.1)μBq/cm2 reduction by factor 3 electropolished !6 RADON REMOVAL SYSTEM (RRS) • xenon and radon is continuously flushed through RRS during detector operations • radon is retained in RRS until its disintegration • challenging due to similar physical properties of xenon and radon methods for radon removal: • adsorption • cryogenic distillation !7 RADON REMOVAL - ADSORPTION • based on van-der-Waals forces and polarizability of the atoms challenges: • high adsorption rate of xenon (1.3 g/g at 1bar and T = -80°C) • radon self-emanation of adsorbent Rn • tested by XMASS collaboration K. Abe et al. (XMASS Collaboration). Radon removal from gaseous xenon with Xe activate charcoal. Nuclear Instruments and Methods in Physics Research A, 2012. • including self-emanation of charcoal: 40% reduction expected for XMASS, but no published data yet • studies performed by MPIK in framework of XENON Diploma Thesis S. Brünner (2014) • for XENON adsorption is not the best option • research on similar RRS ongoing by LZ !8 RADON REMOVAL - DISTILLATION • technique successfully applied xenon-krypton -15 separation down to 10 mol/mol level Rn XMASS, Astropart. Phys. 31, 290–296 (2009) Z. Wang et al,Rev. Sci. Instrum. 85, 015116 (2014) XENON1T , Eur. Phys. J. C 77, 275 (2017) Xe • radon is enriched in liquid phase due to lower vapor pressure • single stage distillation shown at MPIK with reduction factor R > 4 S. Brünner et al. Eur. Phys. J. C 77: 143 (2017) • integration of distillation column and external radon source in gas system of XENON100 • successful demonstration of radon reduction by factor R > 27 XENON100 Eur. Phys. J. C 77: 358 (2017) • reduction of radon level in XENON1T by ~ 20% by taking small fraction of recirculation flow XENON1T: arXiv:1705.06655 (2017) • design studies for dedicated radon distillation column handling higher flow are ongoing Phd Thesis S. Brünner (2017) !9 SUMMARY • radon is one of the highest risk contaminants for low background experiments • no shielding possible possibilities to mitigate radon: radon background in LXe experiments : • material selection with radon screening (preliminary) • surface cleaning methods references on next slide • online radon removal systems !10 XENON100: ! • XENON COLLABORATION: „Intrinsic backgrounds from Rn and Kr in the XENON100 experiment“ arXiv:1708.03617 LUX: ! • LUX COLLABORATION “Radon-Related Backgrounds in the LUX Dark Matter Search“, Physics Procedia 61 (2015) 658 XENON1T: ! • internal communication, preliminary result PANDAX: ! • PANDAX COLLABORATION: „Krypton and radon background in the PandaX-I dark matter experiment“, Journal of Instrumentation, Volume 12, 2017 XMASS: ! • XMASS COLLABORATION: „XMASS detector“ arXiv:1301.2815 EXO: ! • EXO COLLABORATION: „Investigation of radioactivity-induced backgrounds in EXO-200“ PHYSICAL REVIEW C 92, 015503 (2015) BACKUP physical properties of noble gases energy spectrum of Rn220 and Rn222 progenies decay chain 222Rn RADON REMOVAL SYSTEM (RRS) • xenon and radon is continuously flushed through RRS during detector operations • radon is retained in RRS until its disintegration • challenging due to similar physical properties of xenon and radon type I source: type II source: • located behind RRS • located in front of RRS • removal requires high recirculation speed • in principle completely removable Phd Thesis S. Lindemann (2013) !15 RADON REMOVAL - ADSORPTION • based on van-der-Waals forces and polarizability of the atoms challenges: • high adsorption rate of xenon (1.3 g/g at 1bar and T = -80°C) • radon self-emanation of adsorbent K. Abe et al. (XMASS Collaboration). Radon removal from gaseous xenon with example: XMASS experiment activate charcoal. Nuclear Instruments and Methods in Physics Research A, 2012. • test set-up with 5.5kg of activated charcoal at -85°C • measured radon retention time of ~ 4 x T222Rn Rn • including self-emanation of charcoal: Xe 40% reduction expected for XMASS • no published data yet • research on similar RRS ongoing for LZ !16 .