1. Status of Ultra-low Energy HPGe Detector for low-mass WIMP search Li Xin (Tsinghua University) KIMS collaboration Oct.22nd, 2005

2. Index 1.Motivation 2.Previous status 3.Current system setup 4.Calibration 5.Background data analysis 6.Future plan

3. 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

4. DepthMinimum 700 m Temperature20 ~ 25 o C Humidity35 ~ 60 % Rock contents 238 U less than 0.5 ppm 232 Th 5.6 +/- 2.6 ppm K 2 O 4.1 % Muon flux 4.4 x 10 -7 /cm 2 /s Neutron flux 8 x 10 -7 /cm 2 /s 222 Rn in air 2 ~ 4 pCi/liter Y2L Underground Lab

5. 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)

6. 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)

7. HPGe detector threshold Energy threshold by He Dao CsI (Tl) detector threshold HPGe Threshold: 265eVCsI Threshold: 50keV

8. 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 channelLow gain channel (22.1 days data)

9. PSD for HPGe noise reduction Time region 400 ~ 2000 (40ns/bin) (the best time range for discrimination) Total window: 80us, 2000bin Blue: calibration data Red: background data

10. 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

11. 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)

12. HPGe high gain channel calibration Gain shift: Date: Sep.6th~13th Source: Fe-55 5.9keV peak Equation: For stabilization: 10 days Amplitude of gain shift ~ 2.5% (7 days)

13. HPGe high gain channel calibration The carbon window will stop the particles whose energy is lower than about 2keV. Structure of HPGe detector

14. 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).

15. HPGe high gain channel calibration Source: X-ray generator (internal peaks) peakEnergy (keV) σ (keV) Expected element Expected energy (keV) ΔE/ σ A1.680±0.01390.074Ta (Ma)1.7020.2973 B2.7519± 0.00360.0586Ru (L)2.717850.5815 *red: we cannot explain the source of the element polyelectric crystal (LiTaO3)

16. HPGe high gain channel calibration Peaks: Ta, Ca, Cs, Ti, Mn, Fe, Cu X-ray After gain correction

17. HPGe low gain channel calibration Source: Am-241Source: 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)

18. HPGe low gain channel calibration Peaks: Np (L X-ray), Ag (K X-ray), Am (alpha decay gamma)

19. 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

20. Background data analysis Only 5.33 days ’ data HPGe energy spectrum High gain channelLow gain channel ( 0 ~ 9 keV )( 0 ~ 100 keV )

21. Background data analysis HPGe threshold Threshold: 260eV

22. CsI (Tl) PSD for noise reduction PanoramaDetail Blue: calibration file (U-238) Red: background file

23. Background data analysis Background level and veto efficiency Veto efficiency: 191/436=43.81% High gain channelLow gain channel Counting rate: (436-191)/100/0.005/5.326≈92cpd

24. 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