Status of Ultra-low Energy HPGe Detector for low-mass WIMP search Li Xin (Tsinghua University) KIMS collaboration Oct.22nd, 2005
Jan 23, 2016
Status of Ultra-low Energy HPGe Detector
for low-mass WIMP search
Li Xin (Tsinghua University)
KIMS collaboration
Oct.22nd, 2005
Index
1. Motivation
2. Previous status
3. Current system setup
4. Calibration
5. Background data analysis
6. Future plan
Motivation
5g Ge
1cpd
Low mass Dark Matter candidate search - Low energy threshold necessary - Use 5g of prototype Ge detector ( plan to upgrade up to 1 kg )
Expected threshold: ~100eV
Depth Minimum 700 m
Temperature 20 ~ 25 oC
Humidity 35 ~ 60 %
Rock contents 238U less than 0.5 ppm 232Th 5.6 +/- 2.6 ppm
K2O 4.1 %
Muon flux 4.4 x 10-7 /cm2/s
Neutron flux 8 x 10-7 /cm2/s
222Rn in air 2 ~ 4 pCi/liter
Y2L Underground Lab
Previous DAQ Setup by He Dao
• DAQ:
4 channels
SR=25MHz, 8bit
100 us window
GPIB interface
Three typical signal:
HPGe High gain(0~7keV)
HPGe Low gain(0~50keV)
CsI(Tl) channel(charge signal)
HPGe & CsI Calibration by He Dao
HPGe calibration
Source: Fe-55 (5.9, 6.5 keV)Target: Ti (4.5, 4.9 keV)
CsI calibration
Source :Na-22 (0.511 & 1.275MeV)Mn-54 (0.835MeV)
HPGe detector threshold
Energy threshold by He Dao
CsI (Tl) detector threshold
HPGe Threshold: 265eV CsI Threshold: 50keV
Ge signal beyond threshold vetoed by CsI signal:
Originally: 416+764 = 1180 events
After veto: 357+456 = 813 events (270 events in 10.29keV peak)
Background level:
813/(1909350/3600/24)/0.005/55 = 133 counts/(day*Kg*keV)
Efficiency = 1 - 813/1180 = 31.1%
Background level and veto efficiency by He Dao
High gain channel Low gain channel
(22.1 days data)
PSD for HPGe noise reduction
i ii
ii
Att
A
Time region
400 ~ 2000 (40ns/bin)
(the best time range for discrimination)Total window: 80us, 2000bin
Blue: calibration data Red: background data
Current system setup
• ULE-Ge detector:– H.V.: -500V– Gain: 20x– Shaping time: 6 us– Range: 0~100keV
• CsI detector:– H.V.: -1300V– Gain: 100x
• N2 flow: 1 liter/min
New DAQ system
• DAQ device:
4-channel FADC
SR=64MHz, 12bit
64 us window
USB2.0 interface
Typical signals:
HPGe High gain(0~9keV)
HPGe Low gain(0~100keV)
CsI(Tl) channel(current signal)
HPGe high gain channel calibration
Gain shift:
Date: Sep.6th~13th
Source: Fe-55 5.9keV peak
0 4.20955 00, 0 4.40426 03
, 1 1.98459 04
2 2.23065 03, 2 2
1 1.9
.203
8411 0
8
2
5 01
p e p e
p e
p e p e
p e
0 1 ( ) 2p p time hour
height
e p
Equation:
50hours For stabilization: 10 days
Amplitude of gain shift ~ 2.5% (7 days)
HPGe high gain channel calibration
The carbon window will stop the particles whose energy is lower than about 2keV.
Structure of HPGe detector
HPGe high gain channel calibration
Source: X-ray generator (AMPTEK INC.)
Target: Ti (4.5, 4.9 keV) Target: CsI (4.3, 4.6, 5.3 keV)
Polyelectric crystal (LiTaO3) is used to
generate electrons that produce X-ray
in the target material (Cu).
HPGe high gain channel calibration
Source: X-ray generator (internal peaks)
peak Energy (keV) σ
(keV)
Expected element
Expected energy (keV)
ΔE/ σ
A 1.680±0.0139 0.074 Ta (Ma) 1.702 0.2973
B 2.7519± 0.0036 0.0586 Ru (L) 2.71785 0.5815
*red: we cannot explain the source of the element polyelectric crystal (LiTaO3)
HPGe high gain channel calibration
Peaks: Ta, Ca, Cs, Ti, Mn, Fe, Cu X-ray
After gain correction
2
117.7
12.23
offset eV
ndf
HPGe low gain channel calibration
Source: Am-241 Source: Cd-109
Np L-series X-ray:
13.9257, 16.8400, 17.7502, 20.7848 (keV)
Am alpha decay: 59.5412 (keV)
Ag K-series X-ray:
21.9903, 22.16292, 24.9424, 25.463 (keV)
HPGe low gain channel calibration
Peaks:
Np (L X-ray), Ag (K X-ray), Am (alpha decay gamma)
2
0.0694
20.67
offset keV
ndf
CsI (Tl) channel calibration
2
1.397
1.99972
offset keV
ndf
Gamma energy:
Cd-109 (Ag X-ray): 22.577 keV
Am-241: 59.5412 keV
U-238 (Th-234): 92.6 keV
Co-57: 123.66 keV
Background data analysis
Only 5.33 days’ dataHPGe energy spectrum
High gain channel Low gain channel
( 0 ~ 9 keV ) ( 0 ~ 100 keV )
Background data analysis
HPGe threshold
Threshold: 260eV
CsI (Tl) PSD for noise reduction
Panorama Detail
( )i i
i
ii
I tt vs energy keV
I
Blue: calibration file (U-238) Red: background file
Background data analysis
Background level and veto efficiency
Veto efficiency: 191/436=43.81%
High gain channel Low gain channel
Counting rate: (436-191)/100/0.005/5.326≈92cpd
1. PSD of HPGe high gain channel for noise reduction
— to reduce the threshold
2. Time coincidence relation between HPGe and CsI
— improve the discrimination for Compton veto events
3. Simulation and shielding for neutron
— to reduce the background level
Future plan