Characterisation of a Composite LEPS

0.34 MB·English

by  Moumita Roy Basu

PreprinttypesetinJINSTstyle-HYPERVERSION
5
1
0 Characterisation of a Composite LEPS
2

n
u
J

5

t] MoumitaRoy Basua, SudattaRayb∗, AbhijitBisoib†, M.SahaSarkarb‡
e
d aUniversityofCalcutta,
-
s Kolkata-700073,INDIA
n
bSahaInstituteofNuclearPhysics,
i
.
s Bidhannagar,Kolkata-700064,INDIA
c
E-mail: [email protected]
i
s
y
h
p ABSTRACT: A low energy photon spectrometer (LEPS), which is a composite planar HPGe, has
[
beencharacterisedexperimentally. Ithasbeenshownthatbeyond200keV,effectofimagecharges

2
deteriorates theefficiencyofthedetector initsaddback mode. Datahavebeencorrected onevent-
v
0 by-event basisresulting inimprovementoftheperformance.
4
8
5 KEYWORDS: : Gamma detectors (scintillators, CZT,HPG,HgIetc);X-raydetectors;
0
Interaction of radiation with matter; Dataprocessing methods.
.
1
0
5
1
:
v
i
X
r
a
∗PresentlyatAmityUniversity,Noida-201303,INDIA
†PresentlyatIndianInstituteofEngineeringScienceandTechnology,Howrah-711103,INDIA
‡Correspondingauthor.
Contents
1. Introduction 1
2. ExperimentalDetails 1
2.1 Thedetector 1
2.2 Modesofdetection inacomposite detector 2
2.3 TheExperimentandDataAnalysis 3
3. ResultsandDiscussion 5
3.1 Relativeefficiency 5
3.1.1 Causes 5
3.1.2 Imagechargeformation 6
3.1.3 Correction 7
3.1.4 AddbackFactor 8
4. Conclusion 8
1. Introduction
Apart from afew medium sized array [1], gammadetector arrays usually have very low detection
efficiencyforphotonswithenergieslessthan50keVduetotheabsenceofsuitabledetectorswhich
areefficientattheselowenergies. However,thisenergyrangehasspecialimportancefordetection
of(i)X-raysforidentificationoffission-fragments,(ii)highlyconvertedlowenergygammasemit-
tedfromisomersinmediumandheavynuclei,(iii)gammatransitions connecting theloweststates
in heavy nuclei, etc. The low energy photon spectrometers (LEPS) having reasonable resolution
at this energy range have very low efficiency. Soit is desirable to use acomposite detector which
includes more than one planar LEPS having good resolution to get higher efficiency. Electrically
segmented planar LEPShas been used in some of the arrays as a solution to this problem. In this
paper, asegmentedplanarLEPSwillbecharacterized.
2. Experimental Details
2.1 Thedetector
In the present work; a composite planar HPGe low energy photon detector (designated as "Com-
posite" along the text) [3] with four segments mounted on a common cryostat (Table 1) similarly
toa Clover detector has been used. The detector has been fabricated by DSG Detector Systems
–1–
Figure1. PreamplifieroutputsofoneofthesegmentsoftheCompositedetector,(a)withoutand(b)with
bias.
Company1, Germany. The crystal in this planar detector is of p-type. The volume of each crystal
(segment) issuchthateachofthemhasaresolution ofaround 500eVfor122keV.Theefficiency
ofthetotalsystemisaround4timesthatofasingledetector. Theactiveareais4×80mm2. Ithas
athin Bewindow (Table 1), to maximize thelow energy efficiency ofthe detector. Thenecessary
reverse bias for this composite LEPSfor operation is very low, only -300 V. Bias always depends
on the material, crystal quality and design. This low bias for optimum performance indicates that
thecrystalqualityisgood. TheCompositedetectoruseasinglebiassupply. Thedetectorassembly
isdesigned suchthatthesupplyprovides reversevoltage tofoursegments.
Thenoise levelisquitelowevenwithoutbias. Fig. 1showsthepreamplifier output from one
ofthecrystalswithoutbiasaswellasthesamepreamplifier outputwith-300Vdetectorbias.
In this work the performance of this detector with a normal planar HPGe detector (ORTEC
GLP10180/07SHGLP:designated as"OldLEPS")willbecompared. Specificationsofthesetwo
detectors areshowninTable1.
2.2 Modesofdetectioninacompositedetector
The total full energy peak efficiency of any composite detector with more than one crystal (seg-
ment)includes theeffectoftwocomplimentary processes [2]. Theyare
• singlefoldevents,whenfullgammaenergyisdepositedinanyoneoftheindividualcrystals.
Thisisrelatedtothedirectfullenergy peakdetection efficiencies ofindividual crystals,
• the full gammaenergy resulting from the partial absorption intwoor morecrystals through
Compton effect (and/or pair production process followed by escape of one/both of the 511
keV gamma rays for high energy gammas). This is related to the coincidence detection
efficiency.
Direct detection efficiency of a composite detector is obtained by treating the signals from
individual crystals separately. It is obtained from the uncorrelated sum of spectra from each of
1DSGDetectorSystemsGmbH,RobertBoschStrasse38,55129Mainz,Telephone: [phone], Fax: 06131
507540,email:info[at]detectorsystems.de.Thecompanyhaspulledbackfromoperations.
–2–
y 0.10
y (a) enc
nc 1 Effici 0.08
e e
v
ci ati 0.06
i el
f R
f 0.04
E (b)

e
v 0.02250 300 350 400
i
t Energy (keV)
a0.1
l
e
R
Composite: Crystal:1
Composite: Crystal:2
Composite: Crystal:3
Composite: Crystal:4
0.01 Old LEPS
Add-Composite or Uncorrected Addback
Corrected Addback-Composite
10 100 Energy (keV)
Figure 2. (a) Comparison of relative efficiencies of individual crystals of Composite LEPS in singles,
add,addbackandcorrectedaddbackmodeswiththatoftheOldLEPS.(b)(Inset)Thedifferencesbetween
relative efficiencies of Old LEPS and Composite LEPS, in add and corrected addback modes have been
displayedinanexpandedscaleforclarity.Theefficiencieshavebeennormalisedto1at81keV.
these crystals. Ontheotherhand, incoincidence detection mode, twoormorecrystals areintem-
poral coincidence, mainly due to Compton scattering from one crystal to its neighbouring ones.
The energy signals of individual crystals fired simultaneously, are recorded event by event in the
list mode. The coincidence detection efficiency is determined from the spectra (addback spectra)
generatedfromtimecorrelatedsum(addback)ofthesesignals. Sothetotalfullenergypeakdetec-
tion efficiency of this composite detector is e =e +e . Therefore, at relatively higher
total direct addbk
energy, where Compton processes become important, the addback contribution is responsible for
theenhancementofthetotalfullenergypeakdetectionefficiencycomparedtothedirectmode. As
ameasure ofthis enhancement, the addback factor Fisdefined asthe ratio ofthe total fullenergy
peakefficiencytothedirectdetection efficiency;i.e.,F=e /e .
total direct
2.3 TheExperimentandDataAnalysis
Wehaveused152Eu,133Baand241Amradioactivesourcestodeterminetherelativeefficiencyofthe
detectors. The efficiency of the composite detector has been determined from the sum spectrum
generated by addition of independent singles spectra of four segments. We have designated this
sumspectrumasAdd spectrumandthismodeasAdd mode. InCloverdetectors,ataround1MeV,
the addback improves the absolute efficiency by about 1.5 times [2]. It means that the efficiency
–3–
Table1. Specificationsofthetwodetectorsusedinthepresentwork
Detector Dimensions Be-Window
Make Type High Dia. Height Area Dist. 2 Thick
Voltage(V) (mm) (mm) (mm2) (mm) (mm)
ORTEC GLP -1000 10 7 80 5 0.127
Planar: n-type
single
DSG PGP4seg80-7 -300 24 7 4×80 5 0.127
Planar: p-type
4segments
2Distancebetweenthecrystalandtheendcap
Crystal 1 +Crystal 2
Crystal 1 +Crystal 3
Crystal 1 +Crystal 4
1 2
Crystal 3 +Crystal 2
Crystal 2 +Crystal 4
1000 4 3 Addback
Add
Addback
s Add
t 100
n
u
o 23
C 14
12
10
24
13
1
Channel
1700 1720 1740 1760 1780 1800
Figure 3. The comparison of add, addback and different two-fold spectra (12,14,23,24and 13) for the
CompositeLEPS.Thetwo-foldspectra12(say,)isgeneratedfromsimultaneouseventsinsegments1and
2. Thesegmentsnumbersareindicatedintheinsetfigure.
in the Addback mode is 1.5 times more than that in the Add mode. However, this ratio is energy
dependent. Atlowenergies, whereComptoneffectisnotdominant, addbackprocessisnotadvan-
tageous. To take advantage of the addback mode (as discussed earlier in Section 2.2) at energies
above 200 keV where the efficiency of a small area planar detector falls sharply, correlated total
spectrum ofthefourcrystals(similartoaddbackspectrum ofaClover)hasbeengenerated. Inthis
case,theAddbackspectrumisgeneratedbyeventwisesummingofcorrelateddataobtainedinfour
–4–
segmentsofthecompositedetector. WehavedesignatedthismodeasAddback(uncorrected)inthe
figuresandtext. WehavecollectedthedatafromthecompositedetectorintheLISTMode. Inthis
work,therelativeefficiencyofthecomposite detectorinAdd andcorrelated Addback modeshave
beencomparedwiththerelativeefficienciesofindividual crystalsandthatofOldLEPS)(Fig.2a).
3. Results andDiscussion
3.1 Relativeefficiency
It has been found that any single crystal of the composite LEPS (Fig.2a) or the summed spectra
of the four crystals show almost similar relative efficiencies at low energies (≤ 100 keV) when
compared withtheOldLEPS.Itisobserved thatforthecomposite detector, therelativeefficiency
intheAddbackisexactlysimilartothatintheAddmode. SotheyareindicatedasAdd−Composite
(uncorrected Addback) in Fig. 2. No improvement is noted beyond 200 keV. It is even worse
than the Old LEPS (Fig. 2b). At higher energies the efficiency of the Old LEPS improves and is
betterthantheAdd(Addback-uncorrected) modesofthecompositedetector. Thisisanunexpected
observation andneedscloserinspection todetermineitscause.
133Ba
V
300 e
k
6
5
V 3
e
k
2 Shadow peak
0
3
200
s
t
n
u
o
C
100 Single fold
Two fold
0
1500 1600 1700 Chann1e80l0

Figure4. Thesingle-andtwo-foldspectraarecomparedfora133Basource.
3.1.1 Causes
Theadd,addback,single-foldandtwo-foldspectra(Figs.3,4)havebeencomparedtounderstand
this feature. It isobserved that the addback spectrum of the composite LEPScontains arelatively
low intensity shadow peak corresponding to each higher energy gamma peak (Fig. 3). While
comparing the spectra generated from single- fold and two- fold events only, it is observed that
–5–
these peaks are solely generated by the two fold events as evident from Fig.4. The two -fold
spectrum (Fig.4) does not contain the photopeak, it only includes the shadow peak corresponding
toit.
To identify the origin of these shadow peaks more accurately, two- fold spectra generated
from neighbouring crystals (12,14 and 23) as well as diagonally opposite crystals (13,24) have
beencompared inFig.3. Itisclearlyseentheneighbouring crystals arecontributing nearly double
of that contributed by the diagonally opposite ones in the shadow peak. The peak-shifts vary as a
functionofsecondcrystalpositionwithrespecttothatofthefirstcrystaldependingontheposition
ofthesource.
Figure5. Amplifier(negative)outputsinChannel2correspondingtoImagepulses(negative)generatedin
the neighbouringsegment(say,2)areobservedsimultaneouslyifpositiveamplifieroutputscorresponding
totherealpulsesinthesegmentnumber1(say,)areusedasthetriggersintheoscilloscope.
Earlierauthorshavediscussed[4,5],thatdoublepeakingisobservedwhenaddbackspectraare
reconstructed fromthetwo-foldeventsinacompositeplanardetector. However,intheirwork[4],
thesecondpeakoriginatedfromcross-talkbetweentwoneighbouringsegments. Crosstalkinduced
second peak usually appears at higher energy compared to the original one, due to its positive
nature. However,inthepresentcase,thesecondpeakisatlowerenergies(Fig. 3),indicatingeffect
ofimagechargeformation.
3.1.2 Imagechargeformation
Forsegmenteddetectors,alongwithanetchargesignalfromthecollectingelectrode,onealsogets
transientsignalsofneighboringsegmentswhereimagechargesareinduced. Thesetransientsignals
are utilised to get position information of the interaction point of the gamma in the detector. The
transient pulses corresponding to image charge formation, are of opposite polarity to the original
pulses. Theformation of the pulses corresponding to image charges isevident when the pulses in
–6–
twosegmentsoftheLEPSdetectorareobservedsimultaneouslyifoneistriggeringtheother,inthe
oscilloscope. Such characteristic of a segmented planar detector has also been discussed by R.J.
Cooperetal.. [4,5]. ForComptonscatteredeventswhicharedetectedintwoneighbouringcrystals,
their sum peak may have peak height lower than the actual gamma energy due to the presence of
simultaneous negative pulses originated from image charges. The variation of the heights of the
image pulses with change in the initial interaction position of the gammain the detector has been
important indeveloping imagingandortracking detectors [5].
Figure6. Agamma-gammatwodimensionalspectrumshowingthesimultaneouseventsrecordedbytwo
crystalsofthedetectorfor662keVgammafroma137Cssource.
Atwodimensionalmatrixhavebeengeneratedfromthecorrelatedeventswitha137Cssource,
and is plotted in Fig.6. It shows an intense diagonal line running from the top left to the bottom
right corresponding to true Compton scattered correlated gamma events detected in two adjacent
segmentswhichadduptoatotalenergyof662keV.
3.1.3 Correction
The magnitude of theshift ofthe shadow peak originated due toimage charges wascalculated by
plotting the energy of the second peak (in keV) as a function of gamma ray energy. This result is
displayed in Fig. 7, showing the proportionality nature of the shift. The gradient of the fitted line
indicates thatthemagnitude ofthisshiftisonly≃2%ofthedeposited energy.
Thisenergyshiftinthepeakgeneratedfromthetwofoldeventscanbecompensatedbyapply-
inganeventbyeventcorrectioninthecalculated addbackenergy. Theeffectofthiscorrection can
–7–
beseeninFig. 8. Itisseenthatthecorrectionremovestheeventsfromtheshadowpeaktoimprove
the count of the full energy peak. The full energy peak for 356 keV gamma from a 133Ba source
hasbeencomparedwiththesamephotopeakafterreconstructionincludingthecorrectedtwo-fold
events(Fig. 8). However,athigherenergies, graduallythebaseofthepeakwidensandtheFWTM
(full width at one-tenth of maximum) deteriorates after correction. This is due to the shift varia-
tion according to the relative location of the two segments of the detectors fired simultaneously.
Thus, the corrections need to be different depending on this relative position. In the present case,
showninFig. 8,different pairswerenotdistinguished andthesamecorrection factorwasusedfor
any two -fold event depending on the incident energy. The variation of the heights of the image
pulseswithchangeintheinitialinteractionpositionofthegammaneedstobeconsideredforbetter
reconstruction ofthespectrum.
3.1.4 AddbackFactor
This correction factor improved the performance of the composite LEPSdetector which has been
estimatedbytheaddbackfactorasusuallydefinedincompositedetectors[2]. Anestimationofthe
variation oftheaddback factorwithincrease inincident gammaenergyisshowninFig. 9.
4. Conclusion
Inthepresentstudy,thecompositeLEPShasbeencharacterisedusingradioactivesourcesupto1.4
MeV. Formation of image charges in neighbouring segments deteriorates the performance of the
detector. Event-by-eventcorrectionhasbeenimplemented. Thecorrectedresultsindicatethatthese
detectorscanalsobeusedinaddbackmodewithanincreaseinefficiencyby≃20%at1.408MeV.
1500 133Ba
)
V
e 152Eu
k
k ( 137Cs
a
e
p 1000
d
n
o
c
e
s
e
h
t 500
f Equation y = a + b*x
o
y
g
r -- Value Standard Error
e
n Intercept 0.38404 0.52959
E Slope 0.98106 8.30683E-4
0
0 500 1000 1500
Energy of the incident photon (keV)
Figure 7. Plot showing the energy of the second shifted (shadow) peak as a function of energy of the
originalpeak.
–8–
700 356 keV
600
500 Addback
400 Addback-corrected
Corrected
150
s
t
n
u
o
C 100
uncorrected
350 keV
50
0
345 350 355
Energy (keV)
Figure8. Spectrashowingthe 356keV photopeakfrom133Ba inthe addbackspectrabefore(containsa
secondshadowpeakatlowerenergies)andaftershiftcorrection.
However, to improve the peak shape of the reconstructed peak, one may use different correction
factors fordifferent relativelocations ofthetwosegmentsofthedetectors firedsimultaneously.
Acknowledgments
Theauthorssincerely thankMs. JonakiPanjafortechnical helpduringtheexperiment.
References
[1] http://www.tlabs.ac.za/nuclear-physics-afrodite/;MuralitharSetal.,IndianNationalGammaArrayat
InterUniversityAcceleratorCentre,NewDelhi2010Nucl.Instr.andMeth.A622281.
[2] SahaSarkarMetal.,CharacteristicsofaComptonsuppressedCloverdetectorupto5MeV 2002
Nucl.Instr.andMeth.A491113;CharacterisationofaComptonsuppressedCloverdetectorforhigh
energygammarays≤11MeV,2006ibid.,556266andreferencestherein.
[3] SahaSarkarM,RaySudatta,BhattacharyaPurba,RoyBasuMoumita,BisoiAbhijit,
CharacterisationofaNovelCompositeLEPS,2009Proc.DAE-BRNSInt.Symp.Nucl.Phys.(India)
54656;http://www.sympnp.org/proceedings/54/H9.pdf
[4] CooperRJetal.,Chargecollectionperformanceofasegmentedplanarhigh-puritygermanium
detector,2008Nucl.Instr.andMeth.A595401andreferencestherein.
–9–