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

2. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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). 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 …

3. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005

4. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Radon and Kr 222 Rn from decay chain of 238 U. 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 214 Pb and 214 Bi gives main contribution to gammas from Rn. In liquid Xenon, 85 Kr beta-decay can also deposit energy in target.

5. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Model detector 250 kg liquid xenon CH 2 Pb PMTs Cu (1m diameter)

6. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Contamination levels U (ppb)Th (ppb)KCo 60 (ppb) PMT (R8778)440.31 ppb1.9×10 -9 Cu Vessel0.02 1 ppbN/A NaCl603001300 ppmN/A

7. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Model detector NaCl rock

8. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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 CH 2.

9. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Energy deposition in target 0.9 kg -1 day -1 0.03 kg -1 day -1 0.004 kg -1 day -1 0.00002 kg -1 day -1 + 40 cm CH 2 2-10 keV 222 Rn (10 Bq/m 3 ) PMTs 85 Kr (5 ppb) Copper vessel 10 cm Pb + 40 cm CH 2 20 cm Pb + 40 cm CH 2

10. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Veto performance 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.

11. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Source neutron spectrum (Copper vessel)

12. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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 10-50 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.

13. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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 0.2 % Gd NC only NC & || PR

14. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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

15. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 Internal gammas Spectrum of gammas entering target from Cu vessel

16. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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 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

17. M. Carson, University of Sheffield, UKDMC ILIAS-Valencia-April 15 2005 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?).