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 system

5g Ge prototype detector
Detector @ Y2L
detector simulation
Y2L environment parameter
实验室名称
Y2L
最小岩石厚度
~700 m
入口通道长度
2 km
温度
20 ~ 25℃
湿度
35 ~ 60%
岩石成分
238U
< 0.5 ppm
232Th
5.6±2.6 ppm
K2O
4.1%
内部空间
74m2×5m
中子通量
2.7×10-7/cm2·s
μ子通量
8×10-7/cm2·s
空气中222Rn含量
1~2 pCi/liter
Setup and DAQ system
HPGe High Gain
(0~9keV)
HPGe Low Gain
(0~100keV)
CsI(Tl) anti-Compton
detector
HPGe detector calibration
HPGe High Gain (0~9keV)
HPGe Low Gain (0~100keV)
(Ta, Ca, Cs, Ti, Mn, Fe, Cu X-ray)
(Np, Ag ( X-ray), Am (gamma))
offset  113.13eV

2
ndf
 12.9617
offset  0.0694keV
2
ndf
 20.67
Time relation between
CsI(Tl) and HPGe signals
CsI(Tl) signal
PSD:
t 
I t
I
i i
i
i
i
CsI(Tl) energy threshold ~ 10keV
 ( )  131.419ns
 (n)  160.646ns
Veto condition
Blue： γ event
Green：n event
Red:
Bg
Threshold
Energy Threshold: 300eV
Event rate @ threshold
~103cpd
Background Event Rate
Energy range：300eV ~ 100keV
Veto efficiency：60%
Mean background rate :
~ 40 cpd
Physical result
NT  
dR
 f ( W , mW ) 
dER
m
d
vf (v)
(vER )dv

dER
vmin ( ER )
vmax
 d 
0 2




c F ( ER )  (for coherent cross-section)
 dER c
dR k1
dR
m
E0 r
m
dER k0
dER
0
c 

vmax
 v2
NT  

2
vf (v) F ( ER )dv
NT   v0 (c1e

c2 ER
E0 r
e
vmin
 W n 
 c0 rn2
A2 rA2
kmax
1
1
 2
2
 (m ) k 1  ( Ek , m )
esc
v02
)
Physical result
Parameter：
Eth：300eV
BG rate @ Eth：900cpd
Energy ：300eV~10keV
m×T：0.7kg•day
  0.3GeV c2 cm3
v0  230km / s
vsun  232km / s
vesc  650km / s
What should be considered?

Background level is still high than expected.

What’s the source of background?

Veto efficiency improve?
Veto efficiency of CsI detector
Energy range：(300eV) ~ 100keV
(1keV/bin)
Possible main Background source: neutron
Red： Bg
Blue： γ
Green：n
Neutron flux and spectrum
Neutron Spectrum in CPL was used for neutron simulation
Neutron flux in Y2L: 8 x 10-7 /cm2/s (1.5MeV < En < 6MeV)
Neutron simulation
N = 1E6 After Veto
Shielding Efficiency

N=1E6 , Ge range: 0~100 keV:




216(130) events before(after) veto
Flux: 8 x 10-7 /cm2/s
Area: 90*60*4 + 60*60*2 cm2
Event rate in Ge:



0.43(0.26) counts/day
0.86(0.52) counts/(day*Kg*keV)
CsI Veto Eff. : 40%
Neutron background measurement
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
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 * 4cm
FADC: 12bit; 200 MHz;
dtF =25 ns ; dts =125ns
AmBe (americium-beryllium) r and n source
Energy range of n / identification
AmBe (americium-beryllium) Source spectrum
Energy range for n/r identification: Ee= 100KeV
Neutron energy is 0.5 MeV - 1.0MeV
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)
Decay time : 0.2 ms
Wavelength : 450nm
High efficiency for detect neutron
Neutron energy
0.01eV
0.025eV
0.1eV
1eV

Against gamma
efficiency
60%
55%
30%
10%
Ge detector structure
Neutron Detector
F 84
F76
BC702
OFHC
copper
2mm
6.4
Stilbene
PMT R6233
3” K free
200
Cable
25
10
PMT
R6233
100
Base
43
?








BC702
Light out : Comparable to NaI(Tl)
Decay time : 0.2 ms
Wavelength : 450nm
Neutron energy
efficiency
0.01eV
60%
0.025eV
55%
0.1eV
30%
•Stilbene
•Refractive index 1.64
•Light output (104 photons/MeV) 1.4
•Decay time (ns) 3.5
•Wavelength 390 nm
Parameters of detector

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
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

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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 tube
Tube size: h = 5 cm, d = 5cm
Monte Carlo study:Number of recoil proton
Monte Carlo study:Energy of recoil proton
Energy deposited in crystal
random & central
Things to do for neutron BG measurement:
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.
QF measurement for ULE-HPGe
Experimental 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.
Summary




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ULE-HPGe RUN Period I have finished:
~40cpd,veto efficiency 60%
Background source should be studied:
neutron or other
New detector for neutron BG
measurement under construction
QF measurement of ULE-HPGe
Detector upgrade to 4*5g Array