m. carson, university of sheffield, ukdmc ilias-valencia-april 15 2005 gamma backgrounds, shielding...

M. Carson, University of Sheffield, UKDMC

ILIAS-Valencia-April 15 2005

Gamma backgrounds, shielding Gamma backgrounds, shielding and veto performance for dark and veto performance for dark

matter detectorsmatter detectors

M. Carson, University of Sheffield



Sources of radioactivitySources 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 …



Th chain decaysb1: with; b0: without Coulomb correction

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

25 1025 2025 3025 4025 5025 6025 7025 8025 9025

E (keV)

DN

(/5

0 k

eV b

in/d

ecay

)

a

b1

b0

g

c.e.

U chain decaysb1: with; b0: without Coulomb correction

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

25 1025 2025 3025 4025 5025 6025 7025 8025 9025

E (keV)

DN

(/5

0 k

eV b

in/d

ecay

)

a

b1

b0

g

c.e.



Radon and KrRadon 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.



Model detectorModel detector

250 kg liquid xenon

CH2

Pb

PMTs

Cu (1m diameter)



Contamination levelsContamination levels

U (ppb) Th (ppb) K Co60 (ppb)

PMT (R8778) 4 4 0.31 ppb 1.9×10-9

Cu Vessel 0.02 0.02 1 ppb N/A

NaCl 60 300 1300 ppm N/A



Model detectorModel detector

NaCl rock



Gammas from rockGammas 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.



Energy deposition in targetEnergy 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 CH2

+ 40 cm CH2

2-10 keV 222Rn (10 Bq/m3)

PMTs

85Kr (5 ppb)

Copper vessel

10 cm Pb + 40 cm CH2

20 cm Pb + 40 cm CH2



Veto performanceVeto 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.



Source neutron spectrum Source neutron spectrum (Copper vessel)(Copper vessel)



Efficiency for neutronsEfficiency for neutrons

• Graph shows veto efficiency as a function of veto threshold energy for 5, 10, 20, 30 and 40 g cm-2 (CH2 ρ = 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.

recoilsescoincidenc

##



Neutron captureNeutron 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 only

NC & || PR



RealityReality• 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



Internal gammasInternal gammas

Spectrum of gammasentering target fromCu vessel



GammasGammas

• 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



ConclusionsConclusions

• 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?).

m. carson, university of sheffield, ukdmc ilias-valencia-april 15 2005 gamma backgrounds, shielding...

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