Recent status of dark matter search with ULE-HPGe detector

Tsinghua University

Qian Yue

2007.11.23

2nd Korea-China Joint Seminar on Dark Matter

Search

Contents ：

1. Physics goal

2. Detector system

3. The first physics result

4. Neutron background Study

5. QF measurement preparation

The recent results and our goal region

Our detector systemOur detector system 5g Ge prototype detector5g Ge prototype detector

Detector @ Y2L detector simulation Detector @ Y2L detector simulation

Y2L environment parameter

1~2 pCi/liter空气中 222Rn含量

8×10-7/cm2·sμ子通量

2.7×10-7/cm2·s中子通量

74m2×5m内部空间

4.1%K2O

5.6±2.6 ppm232Th

< 0.5 ppm238U岩石成分

35 ~ 60%湿度

20 ~ 25℃温度

2 km入口通道长度

~700 m最小岩石厚度

Y2L实验室名称

Setup and DAQ system

HPGe High Gain (0~9keV)

HPGe Low Gain (0~100keV)

CsI(Tl) anti-Compton detector

(Ta, Ca, Cs, Ti, Mn, Fe, Cu X-ray)

2

113.13

12.9617

offset eV

ndf

HPGe High Gain (0~9keV) HPGe Low Gain (0~100keV)

(Np, Ag ( X-ray), Am (gamma))

2

0.0694

20.67

offset keV

ndf

HPGe detector calibration

i ii

ii

I tt

IPSD:

CsI(Tl) energy threshold ~ 10keV

( ) 131.419

( ) 160.646

ns

n ns

Veto condition

CsI(Tl) signalTime relation between

CsI(Tl) and HPGe signals

Blue ： γ eventGreen： n eventRed: Bg

Energy Threshold: 300eV

Event rate @ threshold

~103cpd

Energy range ： 300eV ~ 100keV

Veto efficiency ： 60%

Mean background rate :~ 40 cpd

Threshold Background Event Rate

max

min ( )

( , ) ( ) ( )R

vT

W W RR Rv E

NdR df m vf v vE dv

dE m dE

Physical result

0 2 ( ) (for coherent cross-section)c RR c

dF E

dE

2max 2

20 0

min

10

0 0

2

0 1( ) ( ) ( )

escR

RRc v vc E

E r vT R T

v

kdRdRm E rmdE kdE

N vf v F E dv N v c e e

0 2

2 2c n

W nA

r

A r

max

2 21

1 1

( ) ( , )

k

k km E m

Parameter ：Eth ： 300eVBG rate @ Eth ： 900cpdEnergy ： 300eV~10keVm×T ： 0.7kg•day

2 30.3GeV c cm

0 230 /v km s

232 /sunv km s

650 /escv km s

Physical result

What should be considered?What should be considered? Background level is still high than Background level is still high than

expected.expected.

What’s the source of background?What’s the source of background?

Veto efficiency improve?Veto efficiency improve?

Energy range ： (300eV) ~ 100keV (1keV/bin)

Possible main Background source: neutron

Veto efficiency of CsI detector

Red ： BgBlue ： γGreen： n

Neutron flux and spectrumNeutron Spectrum in CPL was used for neutron simulation

Neutron flux in Y2L: 8 x 10-7 /cm2/s (1.5MeV < En < 6MeV)

Neutron simulationN = 1E6 After Veto

Shielding EfficiencyShielding Efficiency

N=1E6 , Ge range: 0~100 keV:N=1E6 , Ge range: 0~100 keV: 216(130) events before(after) veto216(130) events before(after) veto

Flux: Flux: 8 x 10-7 /cm8 x 10-7 /cm22/s/s Area: 90*60*4 + 60*60*2 cmArea: 90*60*4 + 60*60*2 cm22

Event rate in Ge:Event rate in Ge: 0.43(0.26) counts/day0.43(0.26) counts/day 0.86(0.52) counts/(day*Kg*keV)0.86(0.52) counts/(day*Kg*keV) CsI Veto Eff. : 40%CsI Veto Eff. : 40%

New detector for neutron background measurement: 1. fast neutron 2. thermal neutron 3. reject low energy gamma background 4. physical match the room of Ge detector

Detector selection :

1,Stilbene to measure fast neutron and reject gamma background based on PSD

2, BC702 scintillation counter to measure thermal neutron

Neutron background measurement

Physical Properties of Stilbene(C14H12)

• Molecular weight (g/mol) 180• Density (g/cm3) 1.22• H/C - ratio 0.857 • Melting point (°C) 124 • Wavelength of emission (nm) 390 • Refractive index 1.64 • Light output ： 1.4 ×104 photons/MeV• Decay time (3.5ns ) • Energy resolution <10%, (137Cs conversion electrons)• Radiation degradation (Mrad) 4.0 • Working temperature -40 to 60°C • Quench factor 0.1-0.2 ?• Two components 3ns fast & 400ns slow

Energy resolution

~160KeV for 2.5Mev n

Neutron Gamma Identification

stilbene volume: 4cm * 4cmFADC: 12bit; 200 MHz; dtF =25 ns ; dts =125ns AmBe (americium-beryllium) r and n source

Energy range of n / identification

Energy range for n/r identification: Ee= 100KeV Neutron energy is 0.5 MeV - 1.0MeV

AmBe (americium-beryllium) Source spectrum

Merit of stilbene with energy & sampling (M= 1 , Ee/ = 100KeV in 12bit)

200Mhz Samples

M=deltaP/(W+Wn)

BC 702 thermal neutron detector

Light out : Comparable to NaI(Tl)Light out : Comparable to NaI(Tl) Decay time : 0.2 Decay time : 0.2 ss Wavelength : 450nmWavelength : 450nm High efficiency for detect neutronHigh efficiency for detect neutron

Against gamma Against gamma

Neutron energy efficiency 0.01eV 60% 0.025eV 55% 0.1eV 30% 1eV 10%

Ge detector structureGe detector structure

BC702 Light out : Comparable to NaI(Tl) Decay time : 0.2 s Wavelength : 450nm Neutron energy efficiency 0.01eV 60% 0.025eV 55% 0.1eV 30%

Neutron DetectorNeutron Detector

Base

200

BC702

PMTR6233

50

6.4

43

100

?

Stilbene

OFHC copper 2mm

Cable

2510

•Stilbene•Refractive index 1.64•Light output (104 photons/MeV) 1.4•Decay time (ns) 3.5 •Wavelength 390 nm

PMT R62333” K free

Parameters of detector

PMT : efficiency and rise time better than RCA 8575

FADC : 200MHz and 12bit Cable : shorter and 50 ohm

BC701/stilbene arrangement ( different light reflector design & light collection) Double crystal detector performance ?

Neutron energy recoil proton energy quench factor ?

Energy range of n /r identification ? Detection efficiency of neutron ?

Our setup for simulation(Contributed by Prof. Chunxu Yu @ nankai university)

Detector: Stilbene, Source: neutron, Source position: center of the tubeTube size: h = 5 cm, d = 5cm

Monte Carlo study:Number of recoil protonMonte Carlo study:Number of recoil proton

Monte Carlo study:Energy of recoil protonMonte Carlo study:Energy of recoil proton

Energy deposited in crystal Energy deposited in crystal random & centralrandom & central

1, The detector will be ready in Dec.

2, We need to study the detector using source in Beijing

3, The detector will be sent to measure neutron background at Y2L.

Things to do for neutron BG measurement:

QF measurement for ULE-HPGe QF measurement for ULE-HPGe Experimental setupExperimental setup

The minimum neutron energy can reach down to about 30 keV, but the neutron energy spread is large near the threshold. For example, 1 keV target thickness can lead to ~ 20 keV energy spread for the neutrons. So, it is better to choose higher neutron energy to reduce the neutron energy spread.

SummarySummary

ULE-HPGe RUN Period I have ULE-HPGe RUN Period I have finished: ~40cpd,veto efficiency finished: ~40cpd,veto efficiency 60%60%

Background source should be Background source should be studied: neutron or otherstudied: neutron or other

New detector for neutron BG New detector for neutron BG measurement under constructionmeasurement under construction

QF measurement of ULE-HPGe QF measurement of ULE-HPGe Detector upgrade to 4*5g ArrayDetector upgrade to 4*5g Array