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

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

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,…

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

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.

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

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

Crystals Processed 8 of 22

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

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

13# Crystal (50mm diameter) 12 of 22 First try, ER = 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

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

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

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

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

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

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

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

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

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

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

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

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

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