1. Recent status of dark matter search with ULE-HPGe detector Tsinghua University Qian Yue 2007.11.23 2 nd Korea-China Joint Seminar on Dark Matter Search

2. Contents ： 1. Physics goal 2. Detector system 3. The first physics result 4. Neutron background Study 5. QF measurement preparation

4. The recent results and our goal region

5. Our detector system 5g Ge prototype detector 5g Ge prototype detector

6. Detector @ Y2L detector simulation

7. Y2L environment parameter 1~2 pCi/liter 空气中 222 Rn 含量 8×10 -7 /cm 2 · s μ 子通量 2.7×10 -7 /cm 2 · s 中子通量 74m 2 ×5m 内部空间 4.1%K2OK2O 5.6 ± 2.6 ppm 232 Th < 0.5 ppm 238 U 岩石成分 35 ~ 60% 湿度 20 ~ 25 ℃温度 2 km 入口通道长度 ~700 m 最小岩石厚度 Y2L 实验室名称

8. Setup and DAQ system HPGe High Gain (0~9keV) HPGe Low Gain (0~100keV) CsI(Tl) anti-Compton detector

9. (Ta, Ca, Cs, Ti, Mn, Fe, Cu X-ray) HPGe High Gain (0~9keV) HPGe Low Gain (0~100keV) (Np, Ag ( X-ray), Am (gamma)) HPGe detector calibration

10. PSD: CsI(Tl) energy threshold ~ 10keV Veto condition CsI(Tl) signal Time relation between CsI(Tl) and HPGe signals Blue ： γ event Green ： n event Red: Bg

11. Energy Threshold : 300eV Event rate @ threshold ~10 3 cpd Energy range ： 300eV ~ 100keV Veto efficiency ： 60% Mean background rate : ~ 40 cpd Threshold Background Event Rate

12. Physical result

13. Parameter ： Eth ： 300eV BG rate @ Eth ： 900cpd Energy ： 300eV~10keV m×T ： 0.7kg day Physical result

14. What should be considered? Background level is still high than expected. Background level is still high than expected. What ’ s the source of background? What ’ s the source of background? Veto efficiency improve? Veto efficiency improve?

15. Energy range ： (300eV) ~ 100keV (1keV/bin) Possible main Background source: neutron Veto efficiency of CsI detector Red ： Bg Blue ： γ Green ： n

16. Neutron flux and spectrum Neutron Spectrum in CPL was used for neutron simulation Neutron flux in Y2L: 8 x 10-7 /cm2/s (1.5MeV < En < 6MeV)

17. Neutron simulation N = 1E6 After Veto

18. Shielding Efficiency N=1E6, Ge range: 0~100 keV: N=1E6, Ge range: 0~100 keV: 216(130) events before(after) veto 216(130) events before(after) veto Flux: 8 x 10-7 /cm 2 /s Flux: 8 x 10-7 /cm 2 /s Area: 90*60*4 + 60*60*2 cm 2 Area: 90*60*4 + 60*60*2 cm 2 Event rate in Ge: Event rate in Ge: 0.43(0.26) counts/day 0.43(0.26) counts/day 0.86(0.52) counts/(day*Kg*keV) 0.86(0.52) counts/(day*Kg*keV) CsI Veto Eff. : 40% CsI Veto Eff. : 40%

19. New detector for neutron background measurement: 1. fast neutron 2. thermal neutron 3. reject low energy gamma background 4. physical match the room of Ge detector Detector selection : 1,Stilbene to measure fast neutron and reject gamma background based on PSD 2, BC702 scintillation counter to measure thermal neutron Neutron background measurement

20. Physical Properties of Stilbene(C14H12) Molecular weight (g/mol) 180 Density (g/cm3) 1.22 H/C - ratio 0.857 Melting point (°C) 124 Wavelength of emission (nm) 390 Refractive index 1.64 Light output ： 1.4 ×10 4 photons/MeV Decay time (3.5ns ) Energy resolution <10%, ( 137 Cs conversion electrons) Radiation degradation (Mrad) 4.0 Working temperature -40 to 60°C Quench factor 0.1-0.2 ? Two components 3ns fast & 400ns slow

21. Energy resolution ~160KeV for 2.5Mev n

22. Neutron Gamma Identification stilbene volume: 4cm *  4cm FADC: 12bit; 200 MHz; dt F =25 ns ; dt s =125ns AmBe ( americium-beryllium ) r and n source

23. Energy range of n /  identification Energy range for n/r identification: E e = 100KeV Neutron energy is 0.5 MeV - 1.0MeV AmBe (americium-beryllium) Source spectrum

24. Merit of stilbene with energy & sampling (M= 1, Ee/  = 100KeV in 12bit) 200Mhz Samples M=deltaP/(W  +Wn)

25. BC 702 thermal neutron detector Light out : Comparable to NaI(Tl) Light out : Comparable to NaI(Tl) Decay time : 0.2  s Decay time : 0.2  s Wavelength : 450nm Wavelength : 450nm High efficiency for detect neutron High efficiency for detect neutron Against gamma Against gamma Neutron energy efficiency 0.01eV 60% 0.025eV 55% 0.1eV 30% 1eV 10%

26. Ge detector structure

27. BC702 Light out : Comparable to NaI(Tl) Decay time : 0.2  s Wavelength : 450nm Neutron energy efficiency 0.01eV 60% 0.025eV 55% 0.1eV 30% Neutron Detector Base 200 BC702 PMT R6233 50 6.4 43 100 ? Stilbene   OFHC copper 2mm Cable 25 10 Stilbene Refractive index 1.64 Light output (10 4 photons/MeV) 1.4 Decay time (ns) 3.5 Wavelength 390 nm PMT R6233 3” K free

28. Parameters of detector PMT : efficiency and rise time better than RCA 8575 FADC : 200MHz and 12bit Cable : shorter and 50 ohm BC701/stilbene arrangement ( different light reflector design & light collection) Double crystal detector performance ? Neutron energy recoil proton energy quench factor ? Energy range of n /r identification ? Detection efficiency of neutron ?

29. Our setup for simulation (Contributed by Prof. Chunxu Yu @ nankai university) Detector: Stilbene, Source: neutron, Source position: center of the tube Tube size: h = 5 cm, d = 5cm

30. Monte Carlo study:Number of recoil proton

31. Monte Carlo study:Energy of recoil proton

32. Energy deposited in crystal random & central

33. 1, The detector will be ready in Dec. 2, We need to study the detector using source in Beijing 3, The detector will be sent to measure neutron background at Y2L. Things to do for neutron BG measurement:

34. QF measurement for ULE-HPGe Experimental setup

35. The minimum neutron energy can reach down to about 30 keV, but the neutron energy spread is large near the threshold. For example, 1 keV target thickness can lead to ~ 20 keV energy spread for the neutrons. So, it is better to choose higher neutron energy to reduce the neutron energy spread.

38. Summary ULE-HPGe RUN Period I have finished: ~40cpd,veto efficiency 60% ULE-HPGe RUN Period I have finished: ~40cpd,veto efficiency 60% Background source should be studied: neutron or other Background source should be studied: neutron or other New detector for neutron BG measurement under construction New detector for neutron BG measurement under construction QF measurement of ULE-HPGe QF measurement of ULE-HPGe Detector upgrade to 4*5g Array Detector upgrade to 4*5g Array