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