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Gamma backgrounds, shielding and veto performance for dark matter detectors

Gamma backgrounds, shielding and veto performance for dark matter detectors. M. Carson, University of Sheffield. Sources of radioactivity. Gammas/neutrons from Uranium and Thorium decay chains. Gammas from 60 Co (1.17 MeV, 1.33 MeV) and 40 K (1.46 MeV). Radon and 85 Kr (in Xe).

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Gamma backgrounds, shielding and veto performance for dark matter detectors

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  1. Gamma backgrounds, shielding and veto performance for dark matter detectors M. Carson, University of Sheffield ILIAS-Valencia-April 15 2005

  2. Sources of radioactivity • Gammas/neutrons from Uranium and Thorium decay chains. • Gammas from 60Co (1.17 MeV, 1.33 MeV) and 40K(1.46 MeV). • Radon and 85Kr(in Xe). • External sources: rock, laboratory … • Internal sources: readout (PMTs), copper, steel, target … • Aim is to figure out the main contributions to background signal in the target and develop techniques to remove them: shielding, active veto, muon veto … ILIAS-Valencia-April 15 2005

  3. ILIAS-Valencia-April 15 2005

  4. Radon and Kr • 222Rn from decay chain of 238U. • Rn decay in air produces alpha, beta and gamma radiation. Detector vessel can shield against alpha and beta radiation but gammas may deposit energy in target. • Beta decay of 214Pb and 214 Bi gives main contribution to gammas from Rn. • In liquid Xenon, 85Kr beta-decay can also deposit energy in target. ILIAS-Valencia-April 15 2005

  5. Model detector PMTs 250 kg liquid xenon Cu (1m diameter) CH2 Pb ILIAS-Valencia-April 15 2005

  6. Contamination levels ILIAS-Valencia-April 15 2005

  7. Model detector NaCl rock ILIAS-Valencia-April 15 2005

  8. Gammas from rock • A is spectrum of gammas from rock (input). Total rate 0.09 cm -2 s-1. • B, C, D, E after 5, 10, 20, 30 cm of lead shielding. • F shows gamma spectrum after 20 cm Pb + 40 g cm-2 CH2. ILIAS-Valencia-April 15 2005

  9. Energy deposition in target 2-10 keV 222Rn (10 Bq/m3) 0.9 kg-1 day-1 PMTs 85Kr (5 ppb) 0.03 kg-1 day-1 Copper vessel 0.004 kg-1 day-1 + 40 cm CH2 10 cm Pb + 40 cm CH2 + 40 cm CH2 20 cm Pb + 40 cm CH2 0.00002 kg-1 day-1 ILIAS-Valencia-April 15 2005

  10. Veto performance • Detector is 250 kg of liquid Xe viewed by array of R8778 PMTs contained in copper vessel. • Surrounded by CH2 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. ILIAS-Valencia-April 15 2005

  11. Source neutron spectrum (Copper vessel) ILIAS-Valencia-April 15 2005

  12. Efficiency for neutrons • Graph shows veto efficiency as a function of veto threshold energy for 5, 10, 20, 30 and 40 g cm-2 (CH2ρ = 1 g cm-3). • Xenon recoils are between 10-50 keV (2-10 keVee). • Proton recoils only. • Quenching factor for protons is 0.2×E1.53 (E in MeV). • Efficiency = • Addition of Gd to CH2 can help improve the efficiency by detecting gamma from neutron capture on Gd. ILIAS-Valencia-April 15 2005

  13. NC & || PR NC only Xe n 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%. NC only 0.2 % Gd ILIAS-Valencia-April 15 2005

  14. Reality • Have assumed full 4π veto coverage and infinite time window to detect gammas from neutron capture (not very likely). • Capture time (eτ) 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 CH2with Gd ILIAS-Valencia-April 15 2005

  15. Internal gammas Spectrum of gammas entering target from Cu vessel ILIAS-Valencia-April 15 2005

  16. Gammas • Veto configuration optimised for neutrons – 40 g cm-2, 0.2 % Gd. • For gammas from copper vessel or PMTs get 40% efficiency between 2-10 keVee, 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 ILIAS-Valencia-April 15 2005

  17. Conclusions • 70% - 80% veto efficiency for internal neutrons. • 40% efficiency for internal gammas. • Precise numbers depend on detector configuration. • 10 cm Pb is enough to shield this model detector (of course, depends upon internal contamination). • Radon gas within the shielding may present a problem (ventilation?). ILIAS-Valencia-April 15 2005

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