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Status of Ultra-low Energy HPGe Detector for low-mass WIMP search

Status of Ultra-low Energy HPGe Detector for low-mass WIMP search. Li Xin (Tsinghua University) KIMS collaboration. Oct.22nd, 2005. Index. Motivation Previous status Current system setup Calibration Background data analysis Future plan. 5g Ge. 1cpd. Motivation.

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Status of Ultra-low Energy HPGe Detector for low-mass WIMP search

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

  2. Index • Motivation • Previous status • Current system setup • Calibration • Background data analysis • Future plan

  3. 5g Ge 1cpd Motivation • 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. 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 CsI calibration • Source : • Na-22 (0.511 & 1.275MeV) • Mn-54 (0.835MeV) Source: Fe-55 (5.9, 6.5 keV) Target: Ti (4.5, 4.9 keV)

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

  8. Background level and veto efficiency by He Dao High gain channel Low gain channel 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% (22.1 days data)

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

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

  14. 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)

  15. HPGe high gain channel calibration Source: X-ray generator (internal peaks) *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-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)

  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 channel Low gain channel ( 0 ~ 9 keV ) ( 0 ~ 100 keV )

  21. Background data analysis HPGe threshold Threshold: 260eV

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

  23. 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

  24. Future plan • 1. PSD of HPGe high gain channel for noise reduction • — to reduce the threshold • Time coincidence relation between HPGe and CsI • — improve the discrimination for Compton veto events • Simulation and shielding for neutron • — to reduce the background level

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