Liquid Xenon Gamma Screening Luiz de Viveiros Brown University

A simulation is set up to analyze the level of radiation in a Liquid Xenon detector caused by contaminants in Photomultiplier Tubes. The objective is to determine the efficiency of Xenon in screening gammas from the PMTs and to determine whether there are sufficiently large regions in the detector with background radiation below acceptable levels. The target background is 6 x counts/keV/kg/day. The target is a cylinder of Liquid Xenon, 30cm high and 30cm in diameter (63 kg). The 20 PMTs are represented by a single isotropic point source of gamma-rays. The source is located 5cm above the liquid surface. All simulations are run with 1 million emitted photons. The depth of a event is defined as the energy scaled average position of all the interactions in that event, given by: Simulation Settings

Multiple scatter events can be filtered out by identifying the events with a large deposition spread, measured by calculating the energy weighted standard deviation of interactions in an event. Estimating a resolution of 1cm in a future Liquid Xenon detector, the criteria for a single scatter event is a standard deviation smaller than 1cm (Sigma <= 10mm) from the average position. Multiple and Single Scatter Events

The simulation is run with a source that simulates the emission lines of the 20 physical PMTs. For this simulated PMT, we used the Hammamatsu R8778 tube. The radiation is due to 3 contaminants in each PMT: Potassium (13 mBq), Uranium (5 mBq) and Thorium (13 mBq). 2D Hitogram – Energy vs. Depth Potassium / Uranium / Thorium Source – 31 mBq Total Activity On the rate scale, is the smallest detected rate and is the target background rate

Simulated PMT Source Potassium / Uranium / Thorium – 31 mBq Total Activity Energy HistogramDepth Histogram

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each 2D Histogram – Energy vs. Depth On the rate scale, is the smallest detected rate and is the target background rate

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Energy Histogram

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Depth Histogram

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Radial Histograms Depth: 10 cm – 20 cmDepth: 20 cm – 30 cm

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Copper Cryostat 1cm thick 2D Histogram – Energy vs. Depth On the rate scale, is the smallest detected rate and is the target background rate

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Effects of Copper Cryostat Radial Histograms Depth: 10 cm – 20 cm Middle of Detector With CryostatWithout Cryostat

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Effects of Copper Cryostat Radial Histograms Depth: 20 cm – 30 cm Bottom of Detector With CryostatWithout Cryostat

Background Analysis – 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Effects of Copper Cryostat Depth Histograms With CryostatWithout Cryostat

Background Analysis – 2.5MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each 2D Histogram – Energy vs. Depth On the rate scale, is the smallest detected rate and is the target background rate

Background Analysis Background Rates for Sources of Different Energies Each source emitted 1 Million Photons, And has rated activity of 20 Low-Activity PMTs, 6 mBq each Number of Detected Photons for Sources of Different Energies Each source emitted 1 Million Photons

Background Analysis 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Detectors of Different Depths 20cm, 30cm and 40cm 2D Histogram – Energy vs. Depth

Background Analysis 1MeV Gamma Source 20 Low-Activity PMTs, 6 mBq each Detectors of Different Depths 20cm, 30cm and 40cm Depth Histograms

Conclusions The top 10 cm of the detector present high background rates and can be eliminated to bring the overall background below the target rate. The outer layers of the detector have higher background due to the backscattering of particles off the cryostat. Two courses of action can reduce this background – increasing the radius of the detector and/or filtering out the events in the region closer to the cryostat. The thickness of the detector does not significantly affect the background rate at all depths in the detector. The advantage of adding more Liquid Xenon to the bottom of the detector is the increase in volume of the fiducial region with very low background.