Charge migration study of a massive cryogenic Ge
detector for the detection of light dark matter using
the Neganov-Luke effect
Nadine Foerster, Institute of Experimental Nuclear Physics (IEKP), Karlsruhe Institute for Technology
KIT – The Research University in the Helmholtz Association
www.kit.edu
EDELWEISS-III detectors
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High purity germanium crystals
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24 detectors a 800g
Two measuring channels
1. Heat (phonons) with NTD
(Neutron Transmutation Doped sensor):
Erecoil ≈ Eheat
2. Ionization yield for particle identification,
Q = Eion/Erecoil
Q = 1 for electron recoils
Q ≈ 0,3 for nuclear recoils
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
EDELWEISS-III: Surface event discrimination
Interleaved electrode structure
with collecting and veto electrode
→ fiducial volume (FID800)
fiducial
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
EDELWEISS-III: Surface event discrimination
Interleaved electrode structure
with collecting and veto electrode
→ fiducial volume (FID800)
fiducial
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Current dark matter experiments
Explore low WIMP
mass region with
Erecoil < 1 keV
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Actual performance of EDELWEISS-III
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Threshold for analysis of good detectors around 1 keV e.e.
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Baseline resolution ionization: ΔEFWHM = 500 eV e.e.
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Baseline resolution heat:
ΔEFWHM = 300 eV e.e.
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Improve electronics for ionization signals
(High-electron-mobility transistors HEMTs)
→ Analysis as before
→ Limited by number of created electron hole pairs
∆V = 10 V
Use Neganov-Luke effect to amplify heat signals
→ Only heat measurement
→ No discrimination of electron and nuclear recoils
Eheat = Erecoil (1+ΔV/3)
S/N: 40
Time [ms]
Amp. [mV]
●
Amp. [mV]
How to lower energy threshold of the detector?
∆V = 90 V
S/N: 250
T
Time [ms]
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Neganov-Luke amplification for EDELWEISS-III
Neganov-Luke boost / detector performance depends on charge migration
→
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Needed: simulation of hot carrier transport in Ge
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Simulation of charge migration
Calculation of electric potential and electric field maps
→ drift velocities and induced charges
Voltage [V]
First step:
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Simulation of charge migration
Energy E [eV]
Conduction
band
Band gap
Valence
band
Electron transport anisotropy
+ Intervalley scattering
T = 19 mK
Wave vector
E
[100]
[111]
In close collaboration with
Alexandre Broniatowski,
(Orsay, University Paris Sud)
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Simulation of charge migration
Trajectories:
• holes
• electrons
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Calibration experiment with prototype detector
Planar mode: VA = VB = -VC = VD
241
●
Am
109
Cd
T = 23 mK
Am source:
Emission energy for
γ-rays: 59.5keV
Absorption length: 0.1 cm
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241
→
single energy deposits in
the near surface region
Inside cryostat
Advantage 59.5 keV: Ionization signals easy to measure (4 channel read-out)
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Event locations created by the
241
241
Am source
Am
Hole collection
Electron collection
Monte Carlo Simulation
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Neganov-Luke amplification planar mode
A
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●
●
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Expected amplitude
calculated from ΔV = 6 V ± 0.5V
(Amplification: 1+ΔV/3)
Voltage configuration:
1-3: VA > 0 V
4: VA < 0 V
VA = VB = -VC = -VD
Errors from uncertainty
of voltage configuration
Institute of Experimental Nuclear Physics (IEKP)
Reduction of incomplete charge collection with
increasing field
Low field
High
High field
field
VA = VB = -VC = VD = +3V
VA = VB = -VC = VD = -60V
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Reduction of incomplete charge collection with
increasing field
Low field
High
High field
field
Reduction factor of 130
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Heat Spectrum: VA = VB = -VC = -VD = - 60 V
Event categories
by ionization amplitude
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FWHM All: 5.920± 0.003 keV e.e.
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FWHM BD: 2.033±0.166 keV e.e.
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Event categories for electron collection
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Locations of categorized events
Red: Full charge collection
Blue: Incomplete charge collection
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Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Event categories for hole collection
Discrepancy
for shared events
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7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Summary and Outlook
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Low field → high field: Reduction of incomplete charge collection by factor 10²
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Experiment is well described by the simulation
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Study of discrepancy for holes is ongoing
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How behaves charge collection at the lateral surface?
7/18/16
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Backup
Projection for Neganov-Luke amplified detection
Goal for 2017: 350kg days
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7/18/16
Institute of Experimental Nuclear Physics (IEKP)
Event categories for electron and hole collection
Electrons
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Holes
Nadine Foerster – Charge migration study of a massive cryogenic Ge detector
Institute of Experimental Nuclear Physics (IEKP)
Backup
Event locations in the crystal:
VB = -2.0V
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VB = +2.0V
Institute of Experimental Nuclear Physics (IEKP)
Planar mode: VA = VB = -VC = -VD = +50 V
Heat amplitude spectrum
uncalibrated
Zoom 59.5keV
peak
Important tail
contribution
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Focus analysis
on 59.5 keV peak
Define event categories
via ionization signals
First results from Neganov-Luke amplified detection in Orsay
Institute of Experimental Nuclear Physics (IEKP)
Planar mode: VA = VB = -VC = -VD = +50 V
Heat spectrum
uncalibrated
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Event categories defined
via ionization amplitudes
Focus on full charge collection in
ionization ((A+B-C-D)/2)
for 59.5 keV events
Heat spectrum
uncalibrated
Not AC
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5 % shift in position between
blue and red distributions
→ different Neganov-Luke
amplification factors
FWHM blue: 6.043 ± 0.278 keV e.e.
FWHM red: 4.541 ± 0.100 keV e.e.
First results from Neganov-Luke amplified detection in Orsay
Institute of Experimental Nuclear Physics (IEKP)
Planar mode: VA = VB = -VC = -VD = +50 V
Calibrated heat spectrum
Independent calibration
of different event categories
(discrimination via ionization signal)
Zoom
59.5 keV
peak
Resolution:
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59.5 keV line: σ = 1.871 ± 0.001 keV e.e.
→ FWHM: 4.400 ± 0.003 keV e.e.
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Baseline: σ = 0.096 ± 0.003 keV e.e.
→ FWH: 0.227 ± 0.007 keV e.e.
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7/18/16
First results from Neganov-Luke amplified detection in Orsay
Institute of Experimental Nuclear Physics (IEKP)
Explanation for different amplification factors
Difference of voltages at electrodes due to imperfect insulation to ground.
V = V0 R' / (RL+R')
V0
RL = 150 M
R'
V
R' = leakage resistance to ground
(e.g. through leaky connector)
order of magnitude:
R' = 3GΩ , V0 = 50 V
→ V0 - V ~ 2.5 volts
→ 5% drop from the
nominal voltage V0
→ Check by simulation and by direct inspection of the electrical cabling
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First results from Neganov-Luke amplified detection in Orsay
Institute of Experimental Nuclear Physics (IEKP)
Comparison with simulation
VA = +50V, VB = +50V,
VC = -50V, VD = -50V
Voltage configuration
VA = +50V, VB = +45V,
VC = -50V, VD = -50V
5%
No noise included in simulation
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7/18/16
First results from Neganov-Luke amplified detection in Orsay
Institute of Experimental Nuclear Physics (IEKP)
Electron transport at low and high field
Intervalley transition rate increases
Low field: 106 s-1
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High field: 109 s-1
Institute of Experimental Nuclear Physics (IEKP)