1. Study of PC-HPGe detector for dark matter search CDEX collaboration Application of Germanium Detector in Fundamental Research Apr. 7-13,2013

2. Outline Brief review: PC-HPGe detector HPGe detector development Conclusion 2 of 22

3. Point-contact HPGe detector 3 of 22 First developed in the late 1980s as large volume, low noise HPGe detectors  1 pF capacitance ~ 300 eV noise threshold Recently “rediscovered” for neutrino detection, dark matter search, etc. MAJORANA, GERDA, CoGeNT, CDEX,…

4. Charge Collection &Signal Induction 4 of 22 Charge collection and signal induction characteristics can be used to separate single- and multi-site events Hole Drift(mm/ns) Strong field exists in front of the point contact Significant signal induced only in close proximity to point contact - significant contribution typically only made by holes - relatively insensitive to electron trapping Strong field exists in front of the point contact Significant signal induced only in close proximity to point contact - significant contribution typically only made by holes - relatively insensitive to electron trapping Drift Time Electric Field Distribution

5. Pulse Shape 5 of 22 Current pulse Charge pulse Very useful for background suppression: WIMP interaction is eminently single-site type. A large fraction of background is not. Very useful for background suppression: WIMP interaction is eminently single-site type. A large fraction of background is not.

6. Detector Development Laboratory Crystal processing Pre-amplifier design –JFET based –CMOS based Simulation study 6 of 22

7. Laboratory devices Clean room Chemical Lab 7 of 22 Vacuum Coating Machine Sputtering Device Mechanical-room

8. Crystals Processed 8 of 22

9. Detector Performance Planar Configuration 9 of 22 @132 K Recycled crystal ~10 g  E = 2.24 KeV  E = 2.13 KeV

10. Detector Performance Point-contact Configuration-7# (17mm diameter) Test Result with 2-outer grooves Recycled crystal ~10 g  E = 1.33 KeV  E = 880 eV

11. 11 of 22 Cs-137, 550V,6  s,  =0.273% Am-241, 550V,4  s,  =1.69% Detector Performance Point-contact Configuration-11# (25mm diameter)

12. 13# Crystal (50mm diameter) 12 of 22 First try, ER = 1.34% @(1600V, 662keV) No improvement after many times of try Why? crystal? Capacitance analysis Leakage Current analysis  See next pages Capacitance analysis Leakage Current analysis  See next pages surface after etching More flaw founded, see following page

13. Capacitance Analysis 13 of 22 Conclusion ： –Match the results from others’ study and the simulation values; –The cryostat and the electronics design meet the requirement of low capacitance. 2 pF

14. Leakage Current Analysis 14 of 22 7# detector 11# detector Theory Model 13# detector 13#crystal, the tendency is OK, but the value is too big

15. More flaws were discovered: 15 of 22 Hole diameter: 10 mm Hole surface: groove Hole diameter: 10 mm Hole surface: groove Enlarge the hole Hole diameter: 13 mm Hole surface: smooth Hole diameter: 13 mm Hole surface: smooth

16. Pre-amplifier Study: J-FET based Feedback methods –Resistor feedback –Pulse-reset feedback Test Cryostat for Pre- amplifier

17. Performance: J-FET based Resistor Feedback Pulse-reset feedback ENC=14 e @-112  C 17 of 22 ENC=55 e @ -100  C With Low noise resistor With unpackaged JFET die

18. 18 of 22 In order to reduce the radioactivity, several bonding materials for J-FET were studied Teflon Quartz PCB based on PTFE J-FET bonding methods

19. CMOS-ASIC Pre-amp 19 of 22 No on-chip reset 6.5e

20. CMOS-ASIC Pre-amp （ 2 ） Second version chip – with on-chip reset and much higher driver capability 20 of 22 10ns rise time for negative signal for 200pF // 1kOhm load

21. CMOS-ASIC Pre-amp （ 3 ） 21 of 22 Noise capacitor ratio @ room temperature Noise @ low temperature 10.6e @ Cin=2pF

22. Framework of Simulation Study Geometric model Electric field simulation Geant4: energy deposition Simulation of carriers trajectory Calculation of induced current

23. Simulation Study on MSE/SSE Event Discrimination E/A method LE method LT method Amplitude of charge pulse / amplitude of current pulse method

24. MSE current pulse E/A method LE method LT method Simulation Study on MSE/SSE Event Discrimination

25. Conclusion We are making progress, though slow… Some preliminary results are achieved; Next step: –Further understanding –Larger size –Better performance New processing technology –Passivation: Amorphous germanium sputtering Amorphous-Ge R f Digital signal processing  –Low radiation background material selection. 25 of 22