Study of PC-HPGe detector for dark matter search

Transcription

1 Study of PC-HPGe detector for dark matter search Yulan Li CDEX collaboration Research March 23-30, 2011, Beijing, China

2 Outline What s a PC-HPGe detector? What we have done? Conclusion 2of 22

3 Point-contact HPGe detector 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, 3of 22

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

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

6 What We Have Done Laboratory set-up Crystal processing Pre-amplifier design JFET based CMOS based Cryostats design Simulation study 6of 22

7 Clean room Laboratory Set-up Vacuum Coating Machine Wet Lab Machine-shop Magnetron Sputtering Device 7of 22

8 Crystal Processing Typical Processing Technology is used (so far) Mechanical Preparatio n Lithium Diffusion Wet Lab Boron Implant Accelerator Boron Implant 8of

9 Detector Performance(1) Planar Configuration Recycled crystal ~10 g ΔE = 2.13 K ΔE = 2.24 KeV 9of 22

10 Detector Performance(2) Point-contact Configuration Recycled crystal ~10 g Test Result with 2-outer grooves Cs 7# HV=350V t=6us ΔE = 1.33 KeV 137 Cs 7# HV=350V t=6us Peak= %@662KeV Low Chan= Am 7# HV=350V t=4us Counts Low Chan=15 Counts Counts ΔE = 880 ev Peak= %@59.5KeV Channel Peak=50 ~10KeV Channel Channel

11 Pre-amplifier Study: J-FET based Feedback methods Resistor feedback Pulse-reset feedback Test Cryostat for Pre-amplifier Teflon substrate for J-FET die 11 of 22

12 Performance: J-FET based Resistor Feedback Pulse-reset feedback ENC= C ENC= C For detail, please see Zhu Weibin s talk this afternoon 12 of 22

13 Performance: CMOS ASIC based For detail, please see Deng Zhi s talk this afternoon 13 of 22

14 Cryostat Design Traditional design: not optimized for pointcontact configuration New design: Point-contact probe Scalable for different sizes of crystal Low background material: Quartz substrate for J- FET bonding Quartz substrate with silk-printed circuit Teflo n Cu frame 14 of 22

15 Simulation Study Simulation items Software tools used Electric field Real field Weighting field Capacitance Depletion characteristics Depletion profile Full depletion voltage Energy Deposit & Energy spectrum Charge collection & Signal induction Maxwell 3D GEANT4 + MaGe Many thanks to Dr. Liu Xiang and Dr. Liu Jing for their help concerning the use of MaGe 15 of 22

16 Simulation: Electric Field 50 y unit:m m φ: φ: mm mm H: H: mm mm P + + :: φ = 1mm, D = 1mm ρ(bottom) = 0.8E10/cm³ ρ(top) = 1.5E10/cm³ V(N V(N + + )) = 3000 V V(P V(P + + )) = 0V 0V x unit:mm 16 of 22

17 Simulation: Capacitance Detector Configuration CapacitancepF By Theory Calculation Planar (φ=5mm, H=8.5mm Open-ended Co-axial (R i =25mm, R o =50mm, H=50mm) By Maxwell simulation Point- Contact Detector Crystal Size φ=50 mm, H=50mm φ=40 mm, H=40mm φ=50 mm, H=50mm Point Contact Size (Depth = 1mm) φ=1 mm 0.72 φ=2 mm 1.11 φ=1 mm 0.72 φ=2 mm 1.11 φ=1 mm 0.73 φ=2 mm of 22

18 Simulation: Depletion characteristics Depletio n Profile XY Plot 1 Maxwell3DDesign1 ANSOFT Curve Info 0.00 Voltage Setup1 : LastAdaptive Full Depletion Voltage Voltage [kv] of Distance [mm] -2.50

19 Simulation: Charge Collection Drift Trajectory Drift Time P N+ hole electron Z unit:mm carriers drift line energy deposit position X unit:mm 19 of 22

20 Simulation: Signal Induction Charge Pulse Current Pulse Application of Germaninum Detector in Fundamental 20 of 22

21 Conclusion We are at the very beginning; 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. 21 of 22

22 Ends Many Thanks for Your Attention!! Thanks the authors from whom I stole slices/pictures for this talk. 22 of 22

23 CDEX & PC-HPGe CJPL proposed to use to Point Contact HPGe detector to detect WIMP directly, because of its: Low capacitance > low threshold Pulse shape analysis > discrimination between SSE and MSE High purity material> low background Module availability > good for manufacture, test, installation, maintenance, ready for extension to larger volume High density, small volume > good for shielding 23 of 23