Measurement of the dead layer thickness in a p-type point contact germanium detector

0.89 MB·

by  H. Jiang

Chinese Physics C Vol. xx, No. x (2014) xxxxxx
Measurement of the dead layer thickness in a p-type point contact
germanium detector
H. Jiang1,2 Q. Yue1,2;1) Y.L. Li1,2;2) K.J. Kang1,2 Y.J. Li1,2 J. LI1,2 S.T. Lin3 S.K. Liu1,2
H. Ma1,2 J.L. Ma1,2 J. Su1,2 H.T. Wong4 L.T. Yang1,2 W. Zhao1,2 Z. Zeng1,2
1 Dept. ofEngineeringPhysics,TsinghuaUniversity,Beijing100084,China
6
2 KeyLaboratoryofParticle&RadiationImaging(TsinghuaUniversity),MinistryofEducation,Beijing100084,China
1
3 CollegeofPhysicalScienceandTechnology,SichuanUniversity,Chengdu610064,China
0 4 InstituteofPhysics,AcademiaSinica,Taipei11529,China
2

n Abstract: A994gmassp-typePCGedetectorwasdeployedbythefirstphaseoftheChinaDarkmatterEXperiment
a
aimingatthedirectsearchesoflightweaklyinteractingmassiveparticles. Measuringthethicknessofthedeadlayerof
J
ap-typegermaniumdetectorisanissueofmajorimportancesinceitdeterminesthefiducialmassofthedetector. This
7
workreportsamethodusinganuncollimated133Basourcetodeterminethedeadlayerthickness. Theexperimental
1
design, data analysis and Monte Carlo simulation processes, as well as the statistical and systematic errors are

] described. Anagreementbetweentheexperimentaldataandsimulationresultswasachievedtoderivethethickness
t
e of the dead layer of 1.02 mm.
d
- Key words: dead layer, CDEX, PPCGe, Monte Carlo
s
n PACS: 95.35.+d; 29.40.-n
i
.
s
c
1 Introduction anincompletechargecollectionbecauseoftheveryweak
i
s electricfieldinthisregion[5–7]. Burnsetal. foundthat
y
the dead layer could be considered as two layers: an in-
h
The China Dark matter EXperiment (CDEX) aims
p active layer where no charge could be collected by the
atthedirectsearchesoflightWeaklyInteractingMassive
[ electrodes, and a transition layer where the charge col-
Particles (WIMPs) employing point-contact germanium
lection efficiency increased from zero to one [8]. As a
1
detector(PCGe)attheChinaJinpingUndergroundLab-
v result, the events in the dead layer cannot provide us
oratory (CJPL), which has about 2400 m of rock over-
5 the primary energies that deposited in the detector and
0 burden. Asthefirststep,CDEXhasreportedtheresults should be discriminated from the bulk events with effi-
3 fromtheCDEXphaseIexperiment(CDEX-1)byusinga
ciency correction. At the same time, the fiducial volume
4 p-typePCGe(PPCGe)detector(CDEX-1A)ofmass994
and mass should also be calculated based on the thick-
0
g [1]. PPCGe detector has very excellent properties for
. ness of the dead layer.
1 the dark matter search experiment. A small area of the
The characteristics of the dead layer have been in-
0 point contact electrode, which can significantly reduce
vestigated in some literatures. In 1998, Clouvas used
6
the capacitance, results in the low electronic noise and
1 a formula to describe the charge collection in the dead
energy threshold [2]. The CDEX-1A detector can reach
: layer. There was a good agreement between the sim-
v athresholdof∼400eVee(eedenoteselectron-equivalent
i ulation spectrum and experimental spectrum based on
X energy) [3]. Besides that, the localized weighting poten-
this formula. But they got a dead layer thickness of 2.5
tial resulting in distinct current pulses from individual
r mm which had a huge disagreement with the manufac-
a interaction charge clouds provides the ability to distin-
tory’s result of 0.5 mm [9]. Studies of N. Q. Huy et
guishbetweenSingleSiteEvents(SSE)andMultipleSite
al. showed that the dead layer thickness increased over
Events (MSE) which can be used to reduce the signals
timebecausethelithiumdiffusionwashappeningallthe
from the background [4].
time [10]. Thus, it is not reliable to use the dead layer
However, PPCGe detector has a dead layer at the
thicknessgivenbymanufactory. Inordertocalculatethe
surface caused by lithium diffusion. Events generating
fiducialvolumeandmass,thethicknessofthedeadlayer
in the dead layer will lead to a slow rise time pulse and
1)E-mail:[email protected]
2)E-mail:[email protected]
(cid:13)c2013 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of
ModernPhysicsoftheChineseAcademyofSciencesandIOPPublishingLtd
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Chinese Physics C Vol. xx, No. x (2014) xxxxxx
should be measured. low to high energy as shown in Table 1. Lower energy
This article reports a measurement of the dead layer photonsaremorelikelytointeractinthesurfacecompar-
thickness of the PPCGe detector from CDEX-1A exper- ing to the higher energy photons and due to the partial
iment with an uncollimated 133Ba source. In the follow- chargecollection,thesurfaceeventsdonotcontributeto
ing sections, the experimental design, data analysis and the photoelectron peaks at all. It means that different
MoteCarlosimulationprocesses,aswellasthestatistical deadlayerthicknesseswillcausedifferentintensityratios
and systematic errors are discussed. ofphotoelectronpeaksforthegammarayswithdifferent
energies.
2 Detector geometry
Table 1. Gamma ray intensities of a 133Ba source [11]
The CDEX-1A detector was fabricated by Canberra
Company. Thegermaniumcrystalhasadiameterof62.2 Energy/keV Intensity/%
±0.1mmandaheightof62.3±0.1mmandisencapsu- 53.16 2.14
lated in a Oxygen Free High Conductivity (OFHC) cop- 79.61 2.65
81.00 32.95
per cryostat. The copper thickness at the endcap side
160.61 0.64
oppositeofthepointcontactis1.5±0.1mmandatthe
223.24 0.45
side is 2.0 ± 0.1 mm. The distance between the endcap
276.40 7.16
andthegermaniumcrystalis4.5±0.1mmandnoother
302.85 18.34
material but vacuum between them. At the side, there
356.01 62.05
are some supports, foils and screws made of OFHC cop-
383.85 8.94
per, lead and brass. The point contact is at the bottom
of the crystal and connected to a brass pin to take out Thestepstomeasurethedeadlayerthicknessarefol-
the signals. lowing: Firstly,anuncollimated133Basourcewasusedto
The PPCGe detector can be distinguished into two get the ratios of different photoelectron peaks in exper-
parts: adeadlayerwherenochargeorpartofthecharges iment. Secondly, as all the parameters except the dead
is collected by electrodes, a bulk where all the charges layer thickness were known, a series of dead layer thick-
deposited in it can be collected by electrodes. Signals nesses were assumed to get the simulation results of the
generatinginthedeadlayerhaveamuchslowerrisetime ratios,respectively. Atlast,thedeadlayerthicknesswas
and an incomplete charge collection as shown in Fig. 1. derivedbycomparingtheexperimentaldataandsimula-
tion data. This method has also been used by GERDA
and MAJORANA Collaborations to get the dead layer
thickness of their germanium detectors [12].
133Ba source
Dead layer
Bulk
Bulk
Cryostat

P o i n t
contact
Fig.1. Typicalsignalsat∼10keVfromdeadlayer Dead
(top)andbulk(bottom). Signalsfromdeadlayer l ayer
have a much slower rise time and an incomplete Fig.2. ThelayoutoftheCDEX-1APPCGedetec-
charge collection, e.g. the actual deposit energy torandsourceposition. Thepointanddeadlayer
is larger than 10 keV. The size of the point and are not shown in scale.
thethickofthedeadlayerarenotshowninscale.
In the experiment, an uncollimated 133Ba source was
3 Experimental process put right above the detector endcap, as shown in Fig. 2.
A height gauge was used to fix the 133Ba source and
A 133Ba source is an excellent source to measure the measure the distance between the source and the end-
dead layer, which has several photoelectron peaks from cap. Signals from the point contact electrode were fed
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Chinese Physics C Vol. xx, No. x (2014) xxxxxx
into a pulsed reset preamplifier. Then, a shaping am- time difference between the event and its nearest prior
plifier at 6us shaping time and a 14 bit 100 MHz flash reset inhibit signal.
analog-to-digital convertor (FADC) were used to shape, If the electronic level difference between the baseline
amplify and digitize the signals. level to the reset point is less than the voltage drop pro-
duced by a large induced current from an incident par-
4 Results and discussion ticle, the output signal just represents part of the total
energy deposited in the detector. This effect decreases
4.1 Experimental data analysis thedetectionefficiencyfortheeventsofthesingle-energy
incident gamma-rays. And the higher energy particles
Astheexperimentisbasedonthecontrastofexperi-
are more easier to reset the preamplifier. That means
mentalresultsandsimulationresults,itisverynecessary
the detection efficiency is energy dependent which will
to make sure that the experimental conditions are quite
change the ratios of different photoelectron peaks.
the same with the simulation conditions. In order to
The events selection from preamplifier reset period
achieve this goal, several events selections and rejections
was derived from the parameter T distribution, aim-
should be done in the experimental data before we can −
ing at choosing a region where the detection efficiency
get the experimental spectrum of a 133Ba source.
was energy independent to make sure that the ratios of
4.1.1 Events selection from preamplifier reset period different photoelectron peaks were not affected. Fig. 4
A pulsed reset preamplifier is used by CDEX-1A shows the relationship between the event counts of a
PPCGe detector to achieve ultra-low noise level. The 133Ba source and T at the source position of 73 mm.
−
charge and discharge procedures are shown in Fig. 3(a). It is shown that the event counts were dropped rapidly
The baseline level of the preamplifier decreases with the at T > 0.22 s which meant some gamma rays were lost
−
time due to the continuous leakage current of the de- as the preamplifier was reset by them. In order to avoid
tector itself and the induced current (signal) from the the efficiency problem, we set T cut at 0.2 s to reject
−
incident particles. This is the so called charge proce- the events with T > 0.2 s.
−
dure. Whenthebaselinelevelreachestotheresetpoint,
the preamplifier would be reset immediately to make it
work again. This is the so called discharge procedure. 1.2
(a) 1
34 nit) 0.8
U
volt (V)-- 21012 signal ounts(Arb. 00..46 Accept Reject
- 3 C
- 4
0.2
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
timing (s)
(b)
0
1.0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
T_(s)
0.8
nit0.6 inhibit signal inhibit Fig.4. RelationshipbetweeneventcountsandT−
y u at source position 73 mm. The black line is the
arbitrar00..24 … T- T+ … eisvetnhteceovuennttscoofutnhtestooftathlesp8e1ctkrueVm.phTohteoerleedctlrionne
0.0 peak. The green line is the event counts of the
- 0.2 356keVphotoelectronpeak. Thecountsforpho-
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
timing (s) toelectronpeaksof81keV,356keVandthetotal
spectrumhavebeennormalizedtothesamescale
Fig. 3. (a) Baseline level of the preamplifier. (b)
in this figure.
Eachtimethepreamplifierisreset,aresetinhibit
signalisobtainedandT meansthetimebetween
−
the event to its nearest prior reset inhibit signal.
4.1.2 Accidental coincidence events rejection
There are some accidental coincidence events in the
Each time the preamplifier is reset, a reset inhibit time window of our DAQ system. Since the amplitude
signal is obtained as shown in Fig. 3(b) and the typical wasusedtodotheenergycalibration,somelowerenergy
averageresetperiodisabout0.4switha133Basourceir- events could be masked by higher energy events which
radiationforCDEX-1APPCGedetector. T meansthe would also change the ratios of different energy peaks.
−
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Chinese Physics C Vol. xx, No. x (2014) xxxxxx
The rejection criterion of accidental coincidence events
is based on the correlation between the maximal ampli-
tude(Amp) and the integral of the signal pulse(Q).
103
Fig. 5 shows the relationship between Q and Amp.
The main distribution band are normal events because
the Q-Amp distribution are linear correlation, while the Counts102
accidentalcoincidenceeventsdisplaydifferentbehaviors.
10
The signal selection region is defined at 3σ of the linear
events distribution band.
1
0 50 100 150 200 250 300 350 400
Energy(keV)
Fig. 6. An experimental spectrum of the 133Ba
source by CDEX-1A PPCGe detector
The fitting result of 81 keV photoelectron peak is
shown in Fig. 7(a). As the photoelectron peaks of 79.61
keV and 81 keV are too close to each other, two Gaus-
sian functions and a linear function are applied to fit
the two peaks and the background. The green line is
the Gaussian function of 79.61 keV photoelectron peak,
the blue line is the Gaussian function of 81 keV photo-
Fig.5. Alltheeventsoutsidethetworedlinesare electron peak, the black line is the linear function of the
rejected. Theinsetfigureshowsthe356keVpho- background and the red line is the combined results.
toelectron peak and the 3σ cut lines. The fitting result of 356 keV photoelectron peak is
showninFig.7(b). AGaussianfunctionandlinearfunc-
tion are used to describe the peak region.
The relationship between the real rate and measured
rate of one certain energy photoelectron peak is shown
as the formula below:
R =R −2τR R =R (1−2τR ) (1)
m r r tot r tot
Where R refers to the measured rate of the cer-
m
tain energy photoelectron peak after the accidental co-
incidence events rejection, R refers to the real rate of
r
the certain energy photoelectron peak detected by the
detector, τ refers to the time window of the accidental
coincidence,R referstotherealeventrateofthewhole
tot
spectrum detected by the detector.
As the parameter 1−2τR in formula (1) is a con-
tot
stant term which is energy independent, it would not
change the value of ratios of different photoelectron
peaks. That means the ratios of measured rates after
accidental coincidence events rejection are equal to the
ratios of real rates. We can use the measured rates to
derivetheratiosofdifferentphotoelectronpeakswithout
any correction.
4.1.3 Experimental results
After doing all the selections and rejections, an ex-
perimentalspectrumofa133BasourceisshowninFig.6.
Fig. 7. The fitting results of the 133Ba source. (a)
The distance between the source and endcap is 73 mm.
The fitting result of 79.6 keV and 81 keV photo-
In this spectrum, there were five explicit photoelectron
electron peaks. (b) The fitting result of 356 keV
peaks: 81keV, 276 keV, 303 keV, 356 keV and 384 keV
photoelectron peak.
which were used to obtain the dead layer thickness.
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Chinese Physics C Vol. xx, No. x (2014) xxxxxx
The peak area is calculated by the formula below:
106
√
A = 2πσH/W (2) 105
104
Where A refers to the peak area of the photoelectron s
nt
peak, σ refers to the σ of the fitting result of Gaussian ou103
C
function,HreferstothepeakheightandWreferstothe 102
width of each bin in energy.
Using the parameters derived from the fitting result 10
ofFig.7,theratiosofdifferentphotoelectronpeakswere 1
obtained as below: 0 50 100 150 200 250 300 350 400
Energy (keV)
Fig. 8. The simulation spectrum of the 133Ba
A(356keV) H σ source. Thedistancebetweenthesourceandend-
R(81keV,356keV)= = 356keV 356keV cap is 73 mm, and the dead layer thickness is as-
A(81keV) H σ
81keV 81keV sumed to be 1.0 mm.
As the dead layer is at the surface of germanium de-
As depicted in Fig. 9, the points for different dead
tector,lowerenergyphotonsaremoresensitivetoitthan
layerthicknesseswithstatisticalerrorbarsfromthesim-
higher energy photons. The denominator of the ratio
ulation show the ratio of the number of events in the 81
was fixed with 81 keV peak area and the numerator was
keVphotoelectronpeaktothatinthe356keVphotoelec-
changed from 276 keV peak area to 384 keV peak area.
tron peak. A quadratic fitting function provides a good
Other experimental results were obtained by the same
description to the simulation data. The horizontal band
method and shown in Table 2.
with 1σ error bar shows the ratio that was measured
in the experimental data and the vertical band with
Table 2. Different energy peak ratios with a dis-
1σ error bar determines the thickness of the dead layer
tanceof73mmbetweenthesourcetothedetector
by comparing the experimental ratio and the simulation
endcap
dataimplementingtheinterpolationmethod. Usingthis
method, a series of results were obtained as shown in
Energypeak/81keV Energypeakratio
Table 3.
276keV 0.393±0.005
303keV 0.953±0.009
356keV 2.853±0.023
384keV 0.390±0.005 3.4
3.2
4.2 Simulation data analysis 3
o
Geant4 [13] was used to simulate the initial interac- k rati 2.8
tion of a 133Ba source in the PPCGe detector in CDEX- pea 2.6
y
1AexperimentandconstructallthestructuresofCDEX- nerg 2.4
1A detector and shieldings into the simulation program. E
2.2
In the simulation, the dead layer thickness was scanned
2
from 0 mm to 1.4 mm to get different simulation results
1.8
of the 133Ba source. A simulation spectrum of 133Ba 0 0.2 0.4 0.6 0.8 1 1.2 1.4
Dead layer thickness(mm)
source is shown in Fig. 8. As ratios are only concerned
Fig.9. Determinationofthethicknessofthedead
about the photoelectron peak events deposited all their
layer at the source position of 73 mm. The black
energyinthebulk,surfaceeventsdonotcontributeatall pointsarefromthesimulation,providingtheratio
to the photoelectron peaks no matter partial charge col- of the number of events in the 81 keV photoelec-
lected events or no charge collected events. The events tron peak to that in the 356 keV photoelectron
inthedeadlayerofpartialchargecollectionwhichwould peak. Thehorizontalbandisfromtheexperimen-
contribute to the low energy part of the spectrum were tal data. The vertical band determines the dead
simplifiedasnochargecollectedeventsinthesimulation. layerthicknessbycomparingtheexperimentalra-
tio and simulation fitting line. The error bars for
This is why some small photoelectron peaks can be seen
the simulation points are smaller than the data
in the simulation spectrum while in the experimental is
point size and invisible in the plot.
not.
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Chinese Physics C Vol. xx, No. x (2014) xxxxxx
Table 3. The estimated dead layer thicknesses The systematic error caused by accidental coinci-
with statistical errors for different energy ratios dence events rejection was derived by changing the
of photoelectron peaks normal signals selection region from 3σ region to 5σ
region and it was 0.003 mm.
Enegypeaks Deadlayerthickness Stat. error
/81keV /mm /mm (2) Theaccuracyofsourcelocationanddetectordimen-
276keV 0.994 0.033 sion
303keV 1.035 0.025
Other systematic errors were caused from the accu-
356keV 1.027 0.020
racyofsourcelocation, endcapdimension, andcrys-
384keV 1.010 0.032
tal dimensions. These systematic errors could be
obtained by Monte Carlo simulations. We consid-
4.3 Statistical and systematic errors
ered the error of endcap thickness to be 0.1 mm.
4.3.1 Statistical error Therefore, the endcap thickness was set as 1.4 mm,
1.5 mm and 1.6 mm in the simulation program, re-
In Table 3, several results of the dead layer thick-
spectively. Then new dead layer thicknesses were
nesseswereobtainedbychoosingdifferentphotoelectron
obtained by comparing new Monte Carlo simulation
peaks of 133Ba source. As the peak areas were achieved
resultswithexperimentalresults. Thedifferencebe-
by the formula (2), the statistical errors of these results
tween the dead layer thickness results we got from
can be achieved by the following steps:
1.4mmand1.6mmthickendcapsimulationmodels
Step1: Using the formula below to get the statistical
to the 1.5 mm thick endcap simulation model was
errors of different photoelectron peak ratios:
the systematic error caused by accuracy of endcap
dimension. The same method was used to get the
(cid:115) systematic errors caused by the accuracy of source
A H σ σ2 σ2 σ2 σ2
σ =σ( 1)= 1 1 H1 + σ1 + H2 + σ2 (3) positions, and the accuracy of crystal dimensions.
R A H σ H2 σ2 H2 σ2
2 2 2 1 1 2 2
All the systematic errors were listed in Table 4.
Whereσ referstothestatisticalerrorsofpeakratios,A
R i
referstothepeakareaofphotoelectronpeaki, H refers
i Table 4. Systematic errors
to the peak height of photoelectron peak i, σ refers to
i
thesigmaofphotoelectronpeaki(i=1,2). Alltheerrors Eventsselectionfrompreamplifierresetperiod 0.004mm
of peak ratios were also shown in Table 2. Accidentalcoincidenceeventsrejection 0.003mm
Step2: The statistical errors of energy peak ratios Sourcelocationaccuracy 0.005mm
were added to the experimental result as the blue band Endcapdimensionaccuracy 0.135mm
shown in Fig. 9. The blue band has two intersections Crystaldimensionaccuracy 0.022mm
with the black line. A red band was derived by the two Totalsystematicerror 0.137mm
intersections in Fig. 9 which was the statistical error of
the dead layer thickness. All the statistical errors were Afteralltheerrorsincludingstatisticalerrorandsys-
obtainedbythismethodandwerealsoshowninTable3. tematic errors were considered, the dead layer thickness
Step3: From all the results in Table 3, the central oftheCDEX-1APPCGedetectorwasderivedtobe1.02
value and statistical error of dead layer thickness was ±0.14mm. Thisresultwasalsocross-checkedbychang-
derived with weighted average method which was 1.021 ing the 133Ba source position from 73 mm to other three
± 0.013 mm. positionsof42mm,113mmand159mm. Agoodagree-
mentwasderivedamongthesedifferentsourcepositions.
4.3.2 Systematic errors
The systematic errors arise from:
5 Conclusion
(1) Events selections and rejections
This paper presents the process of dead layer mea-
Thesystematicerrorcausedbyeventsselectionfrom surementexperimentonCEDX-1A.Inthisstudy,a133Ba
preamplifier reset period was derived in three steps: source was used to measure the dead layer thickness.
Firstly, the T cut was scanned from 0.18 s to 0.22 After all the errors were taken into account including
−
s. Secondly, by using the same method as T = 0.2 thestatisticalerrorandsystematicerrors,thedeadlayer
−
s, new results from the new T cut were obtained. thickness was measured to be 1.02 ± 0.14 mm. Statisti-
−
Finally, by comparing the new results to the old re- cal and systematic errors were studied in detail and the
sult, the systematic error caused by events selection endcap dimension accuracy contributed more than 90%
from preamplifier reset period was 0.004 mm. of the total systematic error.
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