Transcript Document

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
Low mass Dark Matter candidate search
- Low energy threshold necessary
- Use 5g of prototype Ge detector ( plan to upgrade up to 1 kg )
5g Ge
1cpd
Expected threshold: ~100eV
Y2L Underground Lab
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
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
HPGe calibration
Source: Fe-55 (5.9, 6.5 keV)
Target: Ti (4.5, 4.9 keV)
by He Dao
CsI calibration
Source :
Na-22 (0.511 & 1.275MeV)
Mn-54 (0.835MeV)
Energy threshold
by He Dao
HPGe detector threshold
CsI (Tl) detector threshold
HPGe Threshold: 265eV
CsI Threshold: 50keV
Background level and veto efficiency
High gain channel
by He Dao
Low gain channel
Ge signal beyond threshold vetoed by CsI signal:
(22.1 days data)
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%
PSD for HPGe noise reduction
Blue: calibration data
Red: background data
Time region
400 ~ 2000 (40ns/bin)
 At

A
i i
t
i
i
i
Total window: 80us, 2000bin
(the best time range for discrimination)
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
Equation:
height 
e p 0 p1time( hour )  p2
p0  4.20955e  00, p0  4.40426e  03
p1  1.98411e  02, p1  1.98459e  04
p 2  2.23065e  03, p2  2.20358e  01
  50hours
For stabilization: 10 days
Amplitude of gain shift ~ 2.5% (7 days)
HPGe high gain channel calibration
Structure of HPGe detector
The carbon window
will stop the particles
whose energy is
lower than about
2keV.
HPGe high gain channel calibration
Source: X-ray generator (AMPTEK INC.)
Polyelectric crystal (LiTaO3) is used to
generate electrons that produce X-ray
in the target material (Cu).
Target: Ti (4.5, 4.9 keV)
Target: CsI (4.3, 4.6, 5.3 keV)
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
offset  117.7eV
2
ndf
 12.23
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)
offset  0.0694keV
2
ndf
 20.67
CsI (Tl) channel calibration
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
offset  1.397keV
2
ndf
 1.99972
Background data analysis
Only 5.33 days’ data
HPGe 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 t
t
I
i i
i
vs energy (keV )
i
i
Blue: calibration file (U-238) Red: background file
Background data analysis
Background level and veto efficiency
High gain channel
Low gain channel
Veto efficiency: 191/436=43.81%
Counting rate: (436-191)/100/0.005/5.326≈92cpd
Future plan
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