M. Carson, University of Sheffield IDM 2004, University of Edinburgh Veto performance for a large xenon detector

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Sources Neutrons and gammas from U/Th decay (α, n), SF. Gammas from Co 60 (1.17 MeV, 1.33 MeV) and K 40 (1.46 MeV). External sources: rock, CR muons … Internal sources: PMTs, copper, steel, target … Neutron spectra from Sources 4A package (LANL). U and Th gammas from reference spectra (Lewin & Smith, 1990).

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Detector configuration Detector is 250 kg of liquid Xe viewed by array of R8778 PMTs contained in copper vessel. Surrounded by CH 2 veto in stainless steel container 0.5 cm thick. 10 cm lead shielding outside. Neutrons and gammas generated in copper vessel and propagated isotropically through the detector. Internal neutrons only, lead shielding reduces external neutron flux (later).

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Contamination levels U (ppb)Th (ppb)KCo 60 (ppb) PMT ppb1.9×10 -9 Cu Vessel ppbN/A NaCl ppmN/A

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Source neutron spectrum

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Efficiency for neutrons Graph shows veto efficiency as a function of veto threshold energy for 5, 10, 20, 30 and 40 g cm -2 (CH 2 ρ = 1 g cm -3 ). Xenon recoils are between keV (2- 10 keV ee ). Proton recoils only. Quenching factor for protons is 0.2×E 1.53 (E in MeV). Efficiency = Addition of Gd to CH 2 can help improve the efficiency by detecting gamma from neutron capture on Gd.

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Neutron capture Neutrons can be captured anywhere in the set-up and subsequent gamma may deposit energy in veto. Efficiency increases from 65% to 82% with increasing Gd loading. Counting either proton recoils or neutron capture efficiency can increase to 89%. Xe n Photon 0.2 % Gd NC only NC & || PR

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Reality Have assumed full 4π veto coverage and infinite time window to detect gammas from neutron capture (not very likely). Capture time (e τ ) inversely proportional to Gd loading: τ = 30μs for 0.1% Gd and 6μs for 0.5% Gd. For protons τ = 200 μs. If time window is reduced to 100 μs then efficiency drops to 82%. For more realistic geometry get 82% efficiency (and 70% with 100μs time window). One possibility is for a modular veto design. This means less coverage and more gamma/neutron emitting material. Passive CH 2 with Gd

M. Carson, University of Sheffield IDM 2004, University of Edinburgh External gammas Gammas due to U and Th decay from trace elements in rock, copper PMTs…everything. Lead shielding can reduce external gamma flux. Need to have enough shielding in order to reduce background down to levels below that due to internal contamination.

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Internal gammas Spectrum of gammas entering target from Cu vessel

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Energy deposition in target 0.9 kg -1 day kg -1 day kg -1 day kg -1 day cm CH keV

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Gammas Veto configuration optimised for neutrons – 40 g cm -2, 0.2 % Gd. For gammas from copper vessel or PMTs get 40% efficiency between keV ee, above 100 keV in veto. Get absorbtion on the Cu vessel walls, veto container and PMTs. Increasing the energy range causes efficiency to drop. Compton Photoelectric

M. Carson, University of Sheffield IDM 2004, University of Edinburgh Conclusions 70% - 80% veto efficiency for internal neutrons. 40% efficiency for internal gammas. Of course, numbers depend upon your detector configuration.