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18-fold segmented HPGe, prototype for GERDA PhaseII
Xiang Liu for the MPI Munich group and the GERDA Collaboration
(now at Shanghai Jiaotong University)18-20, May, 2010
workshop on germanium-based detectors and technologies, UC Berkeley
Segmented detector for 0νββ search segmentation operation in cryoliquid pulse shape simulation and analysis
Characterization (input for PSS) e/h drift velocity crystal axis orientation impurity mirror pulse from neighbor segments
Other applications neutron interaction
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18-fold segmented detector
n-type true coaxialinner Ф 10mmouter Ф 75mmheight 69.8mm1.632kgBias 3kV
18 segments3-fold along z6-fold along Ф
“snap” contactswith Kapton cableand PTFE button
19g Cu,7g PTFE,2.5g Kapton per detector
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Relatively straightforward:2 persons1 hour
Replacing kapton cable
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Test stands for HPGe in vacuum and in cryoliquid
can handle 3 segmented HPGe in LN2 or LArnot optimized for resolution
JINST 4(2009)p11008
dewar
detector with Kapton cable
IR shield
detector operated in vacuum for characterization
Characterization of 18-fold detectorNIM A 577 (2007) 574
readout cables
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Performance in LN2 and LAr
FWHM at 1332keV:core 4.1keV segs 3.6-5.7keV (all with warm FETs)(2.6-4.0keV in vacuum)
leakage current 35±5pAstable for 5 months in LN2and 3 months in LAr
operation of an 18-fold segmented n-type HPGe detector in liquid nitrogenI Abt et al 2009 JINST 4 P11008
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Active suppression of photon-induced background
1st. Detector and segment anti-coincidence 2nd. pulse shape analysis
single-site event
multi-site event0νββ mostly single-site 2 electron in Ge <1mm
2MeV Photonmostly multi-sitemean free path ~4cm
segmentation size optimized with: signal efficiency & bg rejection extra electronic readout
PSA:many systematic uncertainties involved
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Suppression of photon background by segmentation
Vacuum LN2
Suppression factor
sample Data (vacuum) MC (vacuum) Data (LN2)
DEP 1.09 ± 0.02 1.09 ± 0.04 1.10 ± 0.01
1620keV 2.85 ± 0.01 2.84 ± 0.13 3.38 ± 0.03
Sources at different position have different suppression factor.
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Suppression of photon background by segmentation
suppression by segmentationrobust & well-understood.
SF at different photon linessource 10cm above detector ambient source
sample data MC
Co60 14.2 ± 2.1 12.5 ± 2.1
Th228 1.68 ± 0.02 1.66 ± 0.05
Suppression at RoI (2039keV) NIM A583(2007)332-340
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Preliminary pulse shape analysis without simulation
PSA only on core pulse,optimized with DEP vs. 1620keV,
Fraction of events passed PSA:DEP: 89%, 1620.5: 54%2614.5: 44%, RoI: 81% EPJ C52(2007) 19-27
many systematic uncertainties: DEP samples not “clean” (with Compton bg.) DEP biased (energy deposit close to surface)
need pulse shape simulation
10-30% (left)and10-90% (right)rise time
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PSS package developed at MPI Munich
1.electric field calculation 2.e(left) and h(right) drifting trajectory
3.weighting field calculation 4.induced pulses and currents
segments A,C mirror pulse
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PSS package developed at MPI Munich
taken into account: detector deadlayer, electronic effects (bandwidth, noise..)
input parameters: crystal axis impurity density (affects E field) parameters for e/h mobility (e/h drift velocity)
5.add electronic effects etc.
arXiv:1004.0862, accepted by EPJC
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Measure electron and hole mobility
Cd109 in core, measure hole mobility
T[ns]
averaged pulse
Eu152 from outside,measure e mobility,
simulated pulse 10%shorter than observed,indicate in simulation: e mobility too highor impurity too low
Cd109
Eu152 collimated
segment pulse of 88keV events
Core pulse of121keV events
arXiv:1004.0862, accepted by EPJC
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Use PSA & occupancy to locate crystal axis
e/h drifting trajectories bendeddue to crystal axis.
Crystal anisotropy >>different # of eventsin segments with the same size.
Occupancy can be used to locate crystal axis orientation.
(data taken with source positioned above detector and in the center position)
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Use rise time to determine crystal impurity density
segment
core
crystal impurity density > electric field > e/h drifting velocity > pulse rise time
simulated pulses with different density fit to data
fitted impurity density 0.6 1010 cm-3 , agrees well with 0.62 provided by Canberraextra information about impurity ingredientneed to evaluation systematic uncertainties: parameters of e/h, preamp transfer function.
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Mirror pulse simulation and analysis
left-right asymmetry gives Φ information
mirror pulse polarity gives radius information
mirror pulses and charge pulses together: identify single-segment multi-site events identify background source (gamma tracking, surface etc.) collect back single-site two-segment events analysis on going
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Study neutron interaction with segments
Segmented HPGe
detector exposed to AmBe
measure directly recoil energyby tagging de-excitation photon
EPJ A36, 139-149 (2008)
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Study neutron interaction with segments
693.4 keV72Ge(n,n’)
596 keV74Ge(n,n’)
596keV line: n+74Ge 74Ge*+n’ 74Ge+γ
693.4keV line: n+72Ge 72Ge*+n’ 72Ge+e
distinguish the two processes>> unique power of segmentation
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Summary
18-fold segmented n-type HPGe, prototype suitable for GERDA-II neutrinoless double beta decay search - stable operation in liquid Nitrogen and liquid Argon - bg. rejection with segmentation well understood and robust - PSA provides further rejection power
Full characterization ongoing - crystal axis, impurity, electron/hole mobility
Applications other than double beta decay - neutron inelastic scattering
Not mention here: - surface channel effect - measure capacity vs. Bias voltage and get impurity density - temperature dependence of electron mobility - new test stand
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Back up slides
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Back up slides
Surface effect with “negative energy”
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Deadlayers at the surface of p-i-n detector,R.J. Dinger, IEEE, vol NS-22 1975
core outer surface
Voltage along surface
0Vshould be
is actually
electrons “trapped” half-way,inducing “negative energy” events,effectively inactive layer along surface.
Study surface channel effect with segments
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Observation of “negative energy” events
2-3% events have “negative energy”
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18-fold segmented detector: surface channel effect?
2-3% events with negative pulse.
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18-fold segmented detector: surface channel effect?
Ecore
Eseg
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Study surface-channel induced inactive layer
assume segments in middle layer is 100% efficient,estimate inactive layer thickness with double-ratio:# number of events under certain photon peak
(#top/#middle)_data / (#top/#middle)_MC (#bottom/#middle)_data / (#bottom/#middle)_MC
total volume 301cm3, active volume 289±2cm3
study on going
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Back up slides
New test stand
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3D scan with γ, α and laser.
vacuum tank
LN2 feedthrough
LN2 tank
stage 2
stage 1
stage 3
source & collimator
detector (inside IR shield)
New test stand under construction
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dewar withauto-fill system
tank
filling line
New test stand
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BEGe vs. 18-fold segmented HPGe
Segmented HPGe: - segmentation bg-rejection robust - suppression power depends on photon position - further information helps indentify background bg type, incoming direction... - pulse shape analysis gives extra position information
BEGePSA
18-foldsegmentation & PSA
DEP 89.2% 81.9%
1.62MeV 10.1% 19.0%
2.6MeV 9.8% 14.6%
Q region 40.2% 48.1%
fraction of events surviving PSA (BEGe) or segmentation+PSA (18-fold) cut
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Back up slides
Mirror pulse event display
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Event display with mirror pulses
DEP eventT(μs)
char
ge (
a.u.
)
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Back up slides
Leakage current
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18-fold prototype detector: leakage current in LAr
98 days in LAr with stable leakage current(increase of LC observed with p-type, due to passivation layer).
~4.5pA in segment 13
~ 6pA in segment 14
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Back up slides
Measure Capacity
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Measure capacity vs. bias voltage to determine impurity
r1 : radius of the depleted volumer2 : outer radiusρ: impurity densitiy
(another prototype detector) 8x109/cm3 from the fit, agrees well with Canberra. impurity gradient along z observed as well
core capacity [pF]
bias
V [
V]
capacitance impurity
special thanks to Peter Reiter Bart Bruyneel George Pascovicifrom AGATA Cologne group
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18-fold prototype detector capacity
courtesy of Bart Bruyneel, Peter Reiter, Gheorghe Pascovici
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18-fold prototype detector capacity
R: resistanceρ: resistivityμ: mobility
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Back up slides
Temperature dependence of electron mobility
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Back up slides
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Back up slides
PSS with prototype detector in LN2
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Back up slides
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Back up slides