1. Progress of CMOS CSA for Point Contact HPGe Detectors
Xuezhou Zhu, Zhi Deng*
Department of Engineering Physics, Tsinghua University
04/10/2013
Symposium of the Sino-German GDT Cooperation
Tubingen, Germany, 4/8~4/12, 2013

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Outlines
Introduction
Objectives
What’s the noise limit of a CMOS CSA
To what level it could be competing with JFET
Methods
Results and Discussion
Summary
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Introduction
ULE HPGe Detector for Dark Matter Search
The key technologies for lowering the energy threshold is using small capacitance detector and ultra low noise readout
Point-contact HPGe, ~1pF
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Objectives
Noise components for CSA
tm, ns
As the detector capacitance goes down, the contribution from 1/f noise become non-dominant anymore!
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What is the limit for 1/f noise
Borrowed from Paul O’Connor’s talk in FEE2006
e.g., Kf = 10-25J, Cd=1pF  ENCf ~ 5 e-
Why not CMOS then?
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Methods
Noise Optimization for 1pF Detectors
Pulse reset
PMOS for lower 1/f noise, with optimized size
Adjustable bias current for noise study
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Circuit Design and Implementation
10ns rise time for negative signal for 200pF // 1kOhm load (A. Pullia and F. Zocca, 2009 IEEE NSS)
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Test Setup
Shaper
Power Supply
Signal Generator
Oscilloscope
Bump
Temperature sensor
Vacuum chamber
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Mount with detector
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Results
Electronics low temperature
Cin=2pF
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Noise capacitor room temperature
RC(ms)
Noise ratio (tst, e/pF) 2
18.0 4
14.4 8
12.8 12
9.58
Very close to simulation results for small input capacitance, still some room to improve
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Detector leakage current
Best Id ： 1000mV*0.125pF/15.2ms = 8.25pA
Typical Id :
80-120pA, very sensitive to detector mounting
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Veto fake signals by reset
Using a wider version of reset signal to veto the fake signals
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Cs-137 Spectrum
HV=230V, Id~100pA, RC=2ms
662keV peak
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Discussion
Energy resolution: keV
Fano limit: 1033 eV (F = 0.1)
Electronics noise:
100pA leak current ~ 347 eV (50 e in rms)
CMOS CSA ~ 174 eV (25 e in rms)
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Electronics noise when connected to the detector
RC (ms)
sV (mV)
0.5
33.3 1 20 2 10 4 6
13.3 12
The shaper gain is estimated to be 38 from the MCA address of 662keV peak. Hence the CSA noise is estimated to be:
206 e in rms or 1428 eV in FWHM
But still…
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Peak Value Drift
Peak area
(cnts)
Resolution
(% FWHM)
(eV FWHM)
550
0.39%
2582
1600
0.48%
3178
3600
0.597%
3952
1763 eV
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Non-Gaussian Distribution
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Summary
The noise performance of the CMOS CSA keeps improving, down to RC=12ms, Cd=2pF. But there is some room for shorter RC time, e.g. 2ms.
The test with detector is very challenging and still far from the optimized conditions. Energy resolution of 0.4% to 0.68% for 662keV full-energy-peak can be achieved for short and long term measurements.
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