status of ultra-low energy hpge detector for low-mass wimp search li xin (tsinghua university) kims...
TRANSCRIPT
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Status of Ultra-low Energy HPGe Detector
for low-mass WIMP search
Li Xin (Tsinghua University)
KIMS collaboration
Oct.22nd, 2005
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Index
1. Motivation
2. Previous status
3. Current system setup
4. Calibration
5. Background data analysis
6. Future plan
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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
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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
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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)
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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)
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HPGe detector threshold
Energy threshold by He Dao
CsI (Tl) detector threshold
HPGe Threshold: 265eV CsI Threshold: 50keV
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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)
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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
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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
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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)
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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)
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HPGe high gain channel calibration
The carbon window will stop the particles whose energy is lower than about 2keV.
Structure of HPGe detector
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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).
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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)
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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
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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)
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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
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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
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Background data analysis
Only 5.33 days’ dataHPGe energy spectrum
High gain channel Low gain channel
( 0 ~ 9 keV ) ( 0 ~ 100 keV )
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Background data analysis
HPGe threshold
Threshold: 260eV
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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
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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
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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