KRATTA, a versatile triple telescope array for charged reaction products

2.3 MB·English

by  J. Łukasik

KRATTA, a versatile triple telescope array for charged reaction products
J.Łukasik∗,P.Pawłowski,A.Budzanowski1,B.Czech,I.Skwirczyn´ska
InstituteofNuclearPhysics,IFJ-PAN,31-342Krako´w,Poland
J.Brzychczyk,M.Adamczyk,S.Kupny,P.Lasko,Z.Sosin,A.Wieloch
InstituteofPhysics,JagiellonianUniversity,30-059Krako´w,Poland
M.Kisˇ,Y.Leifels,W.Trautmann
GSI,D-64291Darmstadt,Germany
3
1
0
2Abstract

n
A new detection system KRATTA, Krako´w Triple Telescope Array, is presented. This versatile, low threshold, broad energy
a
Jrangesystemhasbeenbuilttomeasuretheenergy,emissionangle,andisotopiccompositionoflightchargedreactionproducts. It
consistsof38independentmoduleswhichcanbearrangedinanarbitraryconfiguration. Asinglemodule,coveringactivelyabout
0
14.5 msr of the solid angle at the optimal distance of 40 cm from the target, consists of three identical, 500 µm thick, large area
photodiodes,usedalsofordirectdetection,andoftwoCsI(1500ppmTl)crystalsof2.5and12.5cmlength,respectively. Allthe

]signalsaredigitallyprocessed. Theloweridentificationthreshold,duetothethicknessofthefirstphotodiode,hasbeenreducedto
t
eabout2.5MeVforprotons(∼65µmofSiequivalent)byapplyingapulseshapeanalysis. Thepulseshapeanalysisallowedalso
d
to decompose the complex signals from the middle photodiode into their ionization and scintillation components and to obtain a
-
ssatisfactory isotopic resolution with a single readout channel. The upper energy limit for protons is about 260 MeV. The whole
n
setupiseasilyportable. ItperformedverywellduringtheASY-EOSexperiment,conductedinMay2011atGSI.Thestructureand
i
.performanceofthearrayaredescribedusingtheresultsofAu+Aucollisionsat400MeV/nucleonobtainedinthisexperiment.
s
c
iKeywords: chargedparticledetection,tripletelescope,CsI(Tl)scintillator,largeareaPINphotodiode,lownoisepreamplifier,
s
pulseshapeanalysis
y
h
p
[1. Introduction possibleidentificationthreshold,thefirstlayerofthetelescope

is usually made of a gas chamber (e.g. DELF [6], MULTICS
1
v Charged-particle detection and identification with isotopic [7], FASA [8], INDRA [9], ISiS [10], GARFIELD [11], FI-
7resolution over a large dynamic range in particle type and en- ASCO[12])orofathinSidetector(e.g. FAUST[13],INDRA
2ergy, is mandatory for studies of isotopic effects in heavy-ion [9],LASSA[14],CHIMERA[15],HiRA[16],NIMROD[17],
1reactions. Phenomena related to the isotopic composition of FAZIA [18]). The first, ∆E, layer is then followed by one or
2
.thereactionsystemandtheemittedproductshavebeenshown two,thicker,Sidetectorsorscintillators.
1
tobeusefulforexploringthepropertiesofneutron-richnuclear
0 Theoptionwiththesilicon∆Elayer,hastheadvantageofa
matteras,e.g.,encounteredinneutronstars[1,2]. Theirimpor-
3 better resolution and is easier to handle, but usually results in
1tancewillincreasewiththeavailabilityofsecondarybeamsof
higherthresholdsandiscostly. ThepresentedKRATTAmod-
:highintensity. Alsostudiesofnuclearstructurenearthelimits
v ulesbelongtothisclassoftelescopes,however,theyhavebeen
ofstabilityrequireincreasinglysophisticatedandprecisedetec-
i optimizedtobebudgetfriendly,withoutloosingthequalityof
Xtionsystems. Acomprehensiveoverviewoftherecentandfu-
detection. Instead of using the Si detectors of different thick-
rturedevelopmentsinthisfieldofinstrumentationcanbefound
a ness, they are using three identical, catalog size, photodiodes
inRef. [3].
andtwoCsI(Tl)crystals. Thankstothedigitalsignalprocess-
Most of the existing charged particle detectors for the in- ing and the off-line pulse shape analysis, the obtained mass
termediate energy range (up to a few tens or hundreds of
resolutions for light charged particles are very satisfactory in
MeV/nucleon) base their identification on the two- or three-
a broad energy range. On one hand, the pulse shape analysis
fold telescope method [4, 5]. In order to provide the lowest
allowedtoreducetheenergythreshold, resultingfromtherel-
atively thick first layer, by a factor of 3. On the other hand,
itallowedtoeffectivelydoublethethicknessofthesilicon∆E
∗Correspondingauthor.
layer, by combining the ionization components from the first
Emailaddress:[email protected](J.Łukasik)
1Deceased. two photodiodes, and consequently, to improve the resolution
PreprintsubmittedtoNuclearInstrumentsandMethodsA January11,2013
ofthefirstsegmentofthetelescope. Photodiodes[23]
ActiveAreaa 28×28mm2
Thicknessa 500±15µm
2. Motivationandrequirements
Thicknessnon-uniformitya <3µm
DeadLayersa 1.5/20µmfront/rear
ThemainparametersoftheKRATTAarrayhavebeenmoti- SurfaceOrientationa (111)
vatedbytheneedsoftheASY-EOSexperiment[19]. Thisex- FullDepletionVoltageb(FD) 120-135V
perimenthasbeendesignedtostudythedensitydependenceof DarkCurrent@FDb 6-16nA,typ. 9nA
the nuclearsymmetry energy bymeasuring flowsand isotopic TerminalCapacitance@FDb 190±3pF
compositions of the reaction products from the 197Au+197Au, RiseTime(Laserδpulse)a 40ns
96Ru+96Ru,and96Zr+96Zrreactionsat400MeV/nucleon.Dur-
ing the experiment the most relevant products, neutrons and CsI(Tl)Crystals[24]
Z=1 and 2 particles, have been measured by the LAND [20] Tlconcentrationc 1500ppm
detector and the direction and magnitude of the impact vector Lightoutputnon-uniformityc <7%
were estimated using the CHIMERA [15] and ALADIN ToF- Shape Truncatedpyramids
Wall[21]detectors. TheKRATTAarrayhasbeendesignedto Tolerance ±0.1mm
complementtheneutronandhydrogendetectionwithLANDby Wrapping[26]
measuring the isotopic composition and flow of light charged Reflectancea >98%
reaction products up to atomic number Z (cid:46) 7, and specially, Thicknessa 65µm
toidentifythehydrogenandheliumisotopeswitharesolution
muchbetterthanachievablewithLAND.Thearraywasplaced aValuesfromtechnicalnote.
on the opposite side of the beam with respect to LAND, and bValuesfrommanufacturer’sInspectionSheet.
covered approximately the same solid angle (160 msr). It has cNominalvalues.
been designed to detect energetic particles emerging from the
“mid-rapidity”regionofthe400MeV/nucleonreactions.Mod- Table1: Maincharacteristicsoftheactiveelements.
ulardesign,portability,lowthresholds(below3MeV/nucleon)
and high maximum energy (∼260 MeV/nucleon for p and α),
a “Single Chip Telescope”, SCT [25], configuration and pro-
allowthearraytobeusedinvariousconfigurationsandexperi-
videsacompositesignalcombinedofadirect(ionization)com-
ments.Inparticular,itwillverywellsuittheneedsofthefuture
ponent and of a scintillation component coming from the thin
cyclotron facility at IFJ-PAN in Krako´w, planned for proton
crystal (CsI1). The third photodiode (PD2) reads out the light
beams from 70 to 250 MeV. Last, but not least, the KRATTA
fromthethickcrystal(CsI2)and,inaddition,providesanion-
modules are also compatible with the design of the existing
izationsignalforparticlesthatpunchthroughthecrystalwithin
KrakowForwardWallDetector[22]andcanbeusedforreach-
itsactivearea.
ingcompletecoverageofthe2πazimuthalangle.
3. Activeelementsandgeometry
The modules of KRATTA are composed of three large area
HAMAMATSUPINphotodiodesfordirectdetection[23]and
oftwoCsI(Tl)crystals[24].Thelayoutanddimensionsofthese
activeelementsarepresentedinFig. 1andtheirmaincharac-
teristicsaresummarizedinTable1.
m]
transverse dimension [c--20211 PDP0D128.00 mm2C.5s cI1m29.67 mm32.77 mm 12C.5s Ic2m 38.50 mmPD2
40 42 44 46 48 50 52 54 56 58
distance from the target [cm]
Figure1: Schematiclayoutoftheactiveelements. Figure2: Singlemodulecontent.
Thefirstphotodiode(PD0inFig. 1)servesasaSi∆Edetec- The larger front face of the longer crystal with respect to
tor providing the ionization signal alone. It has been “reverse the rear face of the smaller one (see Fig. 1) permits its use
mount”,i.e.theohmicsidetowardstheincomingparticles.The at an about 2 times larger distance from the target, in config-
secondphotodiode(PD1),naturally“reversemount”,worksin uration that completes the missing half of the FWD phoswich
2
oftheconsecutiveactivelayersaresummarizedinTable2. As
willbeshownlateron,thelowthresholdcanbefurtherreduced
by applying a pulse shape analysis to the ionization signals of
PD0. ThethresholdsfromTable2donottakeintoaccountthe
target, the air and the entrance window present in front of the
active layers. In the case of the ASY-EOS experiment, these
layers caused increase of the lower threshold, E , by about
low
12.9MeV/nucleonforprotonsandαparticles(8.6,1.8and2.5
MeV/nucleon energy losses due to the 350 µm Au target, 40
cmofairand100µmofCu,respectively). Forlowenergyex-
periments, the effects of air and the entrance window can be
minimizedbyperformingthemeasurementinvacuumandre-
movingtheCufoilormakingitthinner.
4. Electronicsanddataacquisition
Figure3: KRATTAina7×5configuration. Themainelectronicsanddataacquisitionfunctionsarepre-
sentedschematicallyinFig.4.Thephotodiodes(3permodule)
have been reverse biased at 120 V, using the in-house made
Fragment E E E
low int up
120-channel, remote controlled, high voltage power supplies.
1H 8.3 89.6 254.4
The signal from each photodiode has been integrated with the
4He 8.3 89.4 253.9
own-design,lownoise,chargepreamplifier[28].
7Li 9.5 103.6 296.5
20Ne 19.9 231.3 719.0
43Ca 26.7 339.7 1134.2
91Zr 34.0 513.9 1911.8
197Au 38.6 775.8 3550.9
Table2: Lower,E andintermediate,E ,thresholdsandup-
low int
perlimits,E ,forselectedspecies(inMeV/u). Thethresholds
up
correspondtotheenergylossesin500µmofSi,in1000µmof
Si + 2.5 cm of CsI and in 1000 µm of Si + 15 cm of CsI, re-
spectively. TheyhavebeencalculatedusingtheATIMAtables
[27].
array[22]. Thecrystalshavebeenpolishedandwrappedwith
ahighlyreflectiveESR[26]foil,exceptforthefrontandback
windows. Thewindowshavebeenprotectedwith6µmMylar
foils. The crystals were optically decoupled. The photodiode Figure4: Analog,logicalanddigitalsignalflowchart. HVPS
chipshavebeengluedontocustom-madePCBframesandput -highvoltagepowersupply. PA-chargesensitivepreamplifier.
incloseopticalcontactwiththecrystalwindows.Theactiveel- V1724 - CAEN digitizers. TRIVA5 - VME Trigger Synchro-
ementshavebeenplacedinsidealuminumboxestogetherwith nizationModule[30]. MT-mastertrigger. FIFO-logicalFan-
the charge preamplifiers (see Fig. 2). The photodiode frames InFan-Outmodule.RIO4-VMEcontrollerboard[31]running
andthealuminumhousingreducedthegeometricacceptanceof LynxOS.MBS-MultiBranchSystem,aGSIacquisitionstan-
asinglemoduletoabout54%. Theactivesolidangleofamod- dard[32]. SMS-SharedMemorySegment.
ule amounts to 4.5 msr. The entrance window has been made
ofa100µmthickcopperfoil. Duringtheexperiment,35mod- The preamps were supplied with ±6 V and their dynamic
ules have been arranged in a 7×5 array (Fig. 3), all placed at rangespannedabout3.6V.Threenominalchargegainsofthe
aradialdistanceof40cmfromthetarget, andoperatedinair. preamplifiers have been used, depending on the azimuthal an-
In a spherical coordinate system, with the origin at the target gleofthemodule: 44.5,22.2and13.5mV/MeV,with1,2and
and the beam axis defining the reference equatorial plane, the 3.3 pF feedback capacitors, respectively. After optional am-
columnsfollowthemeridiansfromthebeamleveluptoabout plification, the signals have been digitized with the 100 MHz,
27◦ elevation (latitude) and the rows span the azimuth angles 14 bit digitizers [29] and stored for the off line analysis. All
(longitude) between 21◦ and 64◦ with respect to the beam di- logical and digital electronics modules shown in Fig. 4 have
rection. The energy thresholds resulting from the thicknesses beencontrolledwiththeRIO4board[31]withinasingleVME
3
crate. During the experiment, the 14 Flash ADC boards have (cid:88)2 (cid:32) e−∆t/RCRC
V (t)=RC Q
beentriggeredwithanexternaltriggersplitanddeliveredinto S Sk (RC−τ )(RC−τ )
each FADC module. The stored waveforms spanned 5.12 or k=1 RS Fk
10.24 µs (512 or 1024 time bins), with a 2 µs pre-trigger en- + e−∆t/τRS τRS + e−∆t/τFkτFk (cid:33) (5)
ablingaprecisebaselineestimation. Theshortersampleshave (τRS −RC)(τRS −τFk) (τFk−τRS)(τFk−RC)
been sufficient for the first photodiode supplying the fast ion-
where, for generalization, V is the baseline and ∆t = t −t ,
ization signal alone. The expected data throughput amounted 0 0
witht beingthebeginningofthepulse.
to about 5 MB/s, assuming 1 kHz single hit rate. The actual 0
The two scintillation components (5) have been introduced
dataratedidnotgobeyondthisestimateduringtheexperiment.
to account for the fast and slow decay modes of the CsI(Tl)
The digitizers have been remotely set up and monitored using
crystals. Therisetimes,τ andτ ,accountforthephotodi-
a self-developed software. The data flow has been controlled RD RS
ode,scintillatorandthepreamprisetimes. Overall,themodel
usingthestandardGSIMBSsystem[32].
dependson11parameterslistedinTable3.
5. Pulseshapeanalysis Parameter Value
Ionization
Thepulseshapeanalysishastwomainpurposesinthecase
Q Amplitude fitted
ofKRATTAdata.Firstofall,ithastoenablethedecomposition D
τ Risetime ∼90ns fixed/fitted
ofthesignalsfromthemiddlephotodiode,PD1(SCT),intothe RD
τ Falltime <300ns fixed/fitted
ionization and scintillation components. Secondly, it is used FD
to resolve masses of particles stopped in the first photodiode, Scintillation
PD0, utilizing the relation between the range of a particle and Q Fastcomponentamplitude fitted
S1
thetimecharacteristicsoftheinducedcurrentsignal[33]. Q Slowcomponentamplitude fitted
S2
The following assumptions have been made to accomplish τF1 Fastfalltime ∼650ns fitted
these two goals. The preamplifier response has been mod- τF2 Slowfalltime ∼3.2µs fixed
eled using a simple parallel RC circuit approximation [34]: τRS Risetime ∼140ns fixed
Common
i(t) dV(t) V(t)
= + (1) RC Preampfalltimeconstant ∼6µs fixed
C dt RC t Timeoffset ∼2µs fitted
0
V Baseline fitted
where,RCisthefeedbackcouplingtimeconstant,i(t)isthein- 0
ducedcurrentduetothecarriermotioninthephotodiode,and
V(t) is the measured voltage pulse. This relation assumes an
Table3: 11parametersofthemodelwaveforms.
infinite open loop gain, small detector capacitance and a zero
risetimeofthechargeintegrator, whichisanidealization, but
ThepreampfalltimeconstantparameterRC hasbeendeter-
enables an analytical approach. The induced current has been
mined individually for each chip by selecting the pulses with
approximatedwithonedirect(ionization), i (t),andtwoscin-
D
the fast ionization component alone. The resulting RC con-
tillation,i (t),components,allofthesameform:
Sk
stants were smaller than the nominal ones by a few µs due
e−t/τRD −e−t/τFD to small leakage currents. In order to describe precisely the
i (t)= Q , (2)
D D τ −τ shapes due to particles stopped in the first photodiode, PD0,
RD FD
e−t/τRS −e−t/τFk boththetimeconstants,τRD andτFD,werefittedandthescin-
iSk(t)= QSk τ −τ , k=1,2 (3) tillationcomponentswereobviouslynotused. IncaseofPD1
RS Fk and PD2, the rise and fall times τ and τ were fixed. The
RD FD
where Q’s are the induced charges and the τ ,τ and the fitsweredoneusingtheFUMILI[35]minimizationpackage,a
RD RS
τ ,τ aretheriseandfalltimesfortherespectivecomponent. relativelyfastandpreciseimplementationoftheGauss-Newton
FD Fk
The assumed shapes attempt to account for both, the compli- algorithm. Constrainingsomeoftheparameterswasfoundin-
cated actual current pulse shape induced by the electrons and evitable,notonlytoobtainagooddescriptionofthewaveforms
holes drifting in the photodiode [33], and for the instrumen- (χ2) but, at the same time, to maintain the global agreement
talrisetimeofthepreamp. Withtheassumedpreamplifierre- betweenthereconstructedamplitudesofthethreecomponents
sponse(1)andthecurrentshapes(2)and(3),itwaspossibleto and the predictions of the range-energy tables (see discussion
obtainthecorrespondinganalyticalmodelshapesofthewave- ofFigs. 16,17).
forms,V(t)=V +V (t)+V (t),with: Anadditionaladvantageofusingthedigitizationofthesig-
0 D S
(cid:32) e−∆t/RCRC nals and the fitting method, was the knowledge of the actual
V (t)=RCQ chargesQforeachcomponent,irrespectivelyofthesubstantial
D D (RC−τRD)(RC−τFD) ballisticdeficit(reductionoftheamplitude)duetoarelatively
+ (τRD−e−R∆Ct/)τ(RDτRτDRD−τFD) + (τFD−e−τ∆Rt/Dτ)F(DττFFDD−RC)(cid:33) (4) sthheorstudmiscQhSar1g+etQimSe2 orefptrheesepnretsamthpest(oRtaCl (cid:39)lig6htµsp)r.oFdourceindsitnantchee,
4
2000
scintillators. For typical values of the parameters presented in A PD0 D PD1
Table 3, the ballistic deficit amounts to about 5-15% for the 40 1500 (SCT)
ionizationsignalandtoabout25and50%forthefastandslow
1000
CsI(Tl) components, respectively. Usually, the fall time con- 20
500
stantofthepreampisacompromisebetweenthelevelofpile-
0
ups, the baseline variation, and the ballistic deficit. However, 0
since ballistic deficit is not an issue in our approach, the short V] 0 100 200 300 400 500 0 200 400 600 800 1000
m1000 B PD0 E PD1
preampfalltimes,oftheorderofthetimespanofthewaveform s/ 1500 (SCT)
el
andoftheslowCsI(Tl)decaytime,havetheadditionaladvan- n
n 1000
tage of making the maximum and the tail of the pulse visible ha 500
c
withinthedigitizedsample. 7 500
todWioadvee,foPrDm0s,ohfavloewbeeennerfigtytepdawrtiitchleasssitnogplpeedcoimnpthoenefinrtstofphthoe- V(t) [ 00 100 200 300 400 500 00 200 400 600 800 1000
form(4),withthetimeconstantparameterstreatedasfreefitpa- C PD2 F PD1
2000 (SCT)
rameters. Amoreprecisetimecharacteristicsoftheassociated 1000
current pulse (2) was needed to perform a pulse shape based
1000
500
identificationoftheseparticles(seediscussionofFig. 13).
ThequalityofthefitsisdemonstratedinFig.5.Theordinate
0 0
representsthemeasuredvoltage,intheFADCchannels,which 0 200 400 600 800 1000 0 200 400 600 800 1000
isabouttwicesmallerthantheactualoneduetothematching Time [10 ns/bin]
of the 50 Ω FADC input impedance. The hits corresponding
Figure 5: Example waveforms and their decomposition. Full
totheselectedwaveformsarealsomarkedintheidentification
time scale corresponds to 5 (panels A-B) or 10 (panels C-
mapspresentedinthenextsection.
F) µs. The ordinate represents the measured voltage in the
PanelAofFig. 5showsawaveformforalowenergyelec-
FADC channels. Histogram (when visible): measured wave-
tron,particleorgamma,registeredbarelyabovetheacquisition
form. Solid line: sum of all fit components. Dashed line:
thresholdandstoppedinPD0(seealsoFig. 13). Herethemea-
ionization component. Hatched areas: fast and slow CsI(Tl)
sured histogram is visible and the deviations from the smooth
scintillation components. A: low-energy electron, particle or
fit visualize the level of the total noise. The resultant signal
gamma ray, barely above the acquisition threshold, stopped in
distortions, includingthephotodiode, preamplifier, FADCand
PD0 (see corresponding hit in Fig. 13). B: α particle stopped
pickup noise sources, have been observed on the level of 0.4
inPD0(seeFig. 13). C:αparticlestoppedinCsI2(seeFig. 7,
mVrms,correspondingtoabout30keV.
12). D: α particle stopped in PD1 (see Figs. 6, 8, 9,11). E: α
PanelBofFig.5presentsthequalityofthefitwiththeshape
particlebarelypunchingthroughPD1andstoppedinCsI1(see
given by an ionization component alone (4), applied to parti-
Figs. 6,8,10,11). F:higherenergyαparticlestoppedinCsI1
cles stopped in PD0. Its location in the identification map is
(seeFigs. 6,8,10,11.)
presentedinFig. 13. Thisfitallowedforderivationofboth,the
riseandfalltimesand,therefore,alsoofthemode(positionof
maximum)oftheassociatedcurrentsignal(2):
beenremovedbysubtractingitswelldefinedfractionfromthe
τ τ τ totalamplitudeandthusmakingtheionizationandscintillation
mode= RD FD log RD (6)
τ −τ τ componentsoftheSCTalmostperfectlyorthogonal.
RD FD FD
Panel C of Fig. 5 shows the waveform of a high energy α
particlestoppedintheCSI2crystal. Thecorrespondinghithas 6. Performance
beenmarkedinFigs. 7and12.
PanelsD-E-FofFig. 5showtheevolutionofthepulseshape Figures 6-13 present various identification (ID) maps ob-
of an α particle detected and stopped in the SCT as its energy tained by using the parameters of the reconstructed waveform
increases.Thecorrespondinghitshavebeenmarked,whenpos- components. ThestrongestlinesinFigs. 6-13correspondto p,
sible,inFigs. 6,8-11. d, t, 3He, α, and so on, from bottom to top, respectively (see
The fitting of signals from PD2 (Fig. 5, panel C) produces Fig. 16forpreciselabeling).
small artificial ionization contributions for particles which ac- The reconstructed amplitude maps for the first two and the
tually do not hit the photodiode (see Fig. 7). It amounts, on lasttwophotodiodesoftheKRATTAmodulearepresentedin
average,to1.7±0.5%ofthetotalamplitude. Correspondingly, Figs. 6and7.
theartificialcontributionofascintillationcomponentforparti- TheidentificationmapinFig. 6showsacomplexspectrum
clesstoppedinthePD1photodiode,andthusproducingnolight with each “ID-line” composed of two parts: an ordinary Si-
(see representative hit in panel D), is about 3.8±0.6%. These Sihyperbolicsegmentatlowenergies,forparticlesstoppedin
numbersspecifythequalityofthepulseshapeparametrization PD1,andamorecurvedpartforparticlespunchingthroughthe
and the systematic uncertainty of the decomposition into dif- PD1 photodiode and stopped in the CsI1 crystal. Due to line
ferent components. The artificial scintillation component has crossing and its complex structure, this kind of a map cannot
5
3000 3000
2500 2500
s] s]
nnel2000 +D nnel2000+D
a a
h h
c c
ude [1500 +E ude [1500+E
plit plit
m m
A1000 A1000
D0 F D0 F
P + P +
500 500
0 0
0 2000 4000 6000 8000 10000 12000 0 2000 4000 6000 8000 10000 12000
PD1 Amplitude [channels] PD1 Scintillation [channels]
Figure 6: ∆E-E ID map for the first two photodiodes, PD0 vs Figure8: DecompositionofthemapfromFig. 6. PD0vsscin-
PD1(SCT), for particles stopped in PD1 or in the thin crystal tillationdetectedbyPD1-forparticlespunchingthroughPD1
(CsI1). andstoppedinCsI1.
12000 3000
10000 2500
annels]8000 nnels]2000 +D
h a
c h
ation [6000 ude [c1500
PD1 Scintill4000 C PD0 Amplit1000
2000 + 500
0 0
0 2000 4000 6000 8000 10000 12000 0 2000 4000 6000 8000 10000 12000
PD2 Scintillation [channels] PD1 Ionization [channels]
Figure7: ∆E-EIDmapofscintillationsignalsforCsI1vsCsI2 Figure9: DecompositionofthemapfromFig. 6. PD0vsion-
detectedbyPD1andPD2. izationsignalfromPD1-forparticlesstoppedinPD1(produc-
ingnolight).
be used for identification, however thanks to the presence of
many characteristic punch-through points and curvatures, it is TheisotopicresolutionvisiblefromFig. 8canbeimproved
verywellsuitedforenergycalibrationpurposes. bysummingupthereconstructedionizationcomponentsfrom
Fig.7showsa∆E-Emapforscintillationsignalsdetectedby PD0 and PD1, and thus, increasing the effective thickness of
asecondvsthirdphotodiode.Apartfromtheverygoodisotopic the first ∆E layer to 1 mm of Si. See also inset in Fig. 14.
resolution,onecanseeasubstantialbackgroundfromthesec- Fig. 11presentsaclassical∆E-EIDmapobtainedfromFig. 6
ondaryreactions/scatteringsinthecrystalsandalsofromparti- thankstothepulseshapedecomposition(seediscussionofFig.
cles crossing the module at an angle and not originating from 17 for more details on this transformation and Fig. 14 for the
thetarget, aswellasback-bendingscorrespondingtoparticles correspondingmassresolution.)
punchingthroughthethickcrystal(seediscussionattheendof Figures10and12showthe“Fast-Slow”IDmapsfortheCsI1
thissection).Thankstothepulseshapedecomposition,theion- and CsI2 crystals, respectively. The latter represents a variant
izationcomponentforparticleshittingthephotodiode(nuclear ofthestandardrepresentation,usingthetotallightvstheratio
counter effect) at the back of the thick crystal has been easily SlowoverFast(seee.g. [36]). TheFast-Slowrepresentationis
removed. very useful in many respects in addition to the standard ∆E-E
Usingtheindividualreconstructedamplitudesfortheioniza- one: Onecanobserveacleardoublealphaline(α-αinFigs. 10
tion and for the scintillation components, the map of Fig. 6 and12)whichishiddeninthe∆E-Erepresentationbehindthe
can be decomposed into the PD0-PD1(Si) and PD0-PD1(CsI) Li isotope lines. The Fast-Slow map shows a clear “γ-line” -
components of the SCT segment (Figs. 9 and 8). This makes a strong line below the proton line due to γ-rays (see inset in
effectivelyKRATTAevenafour-foldtelescope. Fig. 10),whichcanbepreciselyisolatedandremovedfromthe
6
7000 14000
α α
6000 α α 12000
s]
s]5000 nel10000
channel4000 st [chan8000
Fast [3000 w+Fa6000
CSI1: 2000 111024000000 +F SI2: Slo4000 +C
800 C
F 600
1000 E + 240000+E γ line 2000
+ 00 200 400 600 800 1000
0 0
0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 0.2 0.4 0.6 0.8 1 1.2 1.4
CSI1: Slow [channels] CSI2: Slow/Fast [channels]
Figure10:FastvsSlowcomponentsofthescintillationinCsI1. Figure12:TotallightvsSlowoverFastcomponentsofthescin-
Theinsetshowsmoreclearlyaγ−lineandthe p, d, tlines. tillationinCsI2. Note,thatunlikeinFig. 7,thepunchthrough
linesdonotcrossthelowerlyingisotopelines.
6000
35
5000
s]
nel 30
n D
ha4000+
on [c +E MeV] 25
nizati3000 ude [ 20
D1 Io2000 mplit 15
0+P +F D0 A +B
PD P 10
1000
5
00 2000 4000 6000 8000 10000 12000 A
PD1 Scintillation [channels] 0 +
0 20 40 60 80 100 120 140 160 180
PD0 Mode [ns]
Figure11: SummedPD0+PD1ionizationcomponentsvsscin-
tillation in PD1 for particles stopped in CsI1. Note a slightly Figure13:Amplitudevsmodeofthecurrentsignalforparticles
improvedresolutioncomparedtoFig. 8(seeinsetinFig. 14) stoppedinPD0,obtainedfromasingleSichip.
∆E-Emap. Last,butnotleast,ascanbeseenfromFig. 12,the Table2)from8.3toabout2.5MeVforprotons,wheretheyare
punch through segments do not cross the lower lying isotope stillresolvablefromdeuterons(seetwobottomlinesinFig.13).
linesinawaytheydoincaseofthe∆E-Emaps(Fig. 7). Thus, Thiseffectivelycorrespondstothereductionofthethicknessof
theFast-Slowmapsallowtoisolatethepunchthroughsegments thefirst∆Elayerfrom500toabout65µmofSi.
inamuchmorepreciseway. Unfortunately,thepunchthrough One should stress, that the time scale presented in Fig. 13
segmentseventuallymergewiththeγ-lines,whichmakesitdif- exceedsbyaboutafactorof3thecollectiontimesestimatedon
ficulttodiscriminatebetweenthesetwo. thebasisofthemobilitiesofthecarriersalone,thustheplasma
Figure 13 demonstrates yet another powerful feature of the delay, or other effects, seem to be quite important in slowing
pulseshapeanalysis. Itallowedtoperformanidentificationof downthecollectionprocess.
themajorityoflightparticlesstoppedinthefirstphotodiodeby The mass resolution for particles stopped in the thin crystal
plottingtheamplitudevsmodeofthereconstructedcurrentsig- (CsI1)andinthethickone(CsI2)canbeviewedfromFigs. 14
nal(6).Duetotheconstantvalueofthefieldwithintheintrinsic and15,respectively.
region of the PIN diode the enhancement of the resolution for The background in Fig. 14, resulting mainly from the sec-
stoppedparticlesisnotexpectedtobeaspronouncedasinthe ondaryreactions/scatteringinthecrystal,amountstoabout6%
caseofreversemountPNdetectors[37],nevertheless,therela- forthehitsbelowthe p, d, t peaks. ForenergeticZ = 1parti-
tionbetweentherangeandthecarriercollectiontimeseemsto clestraversingthethickcrystal(12.5cmofCsI)thesecondary
bestillstrongenoughtoenabletheisotopicseparationoflight reaction probability amounts to about 46% (Fig. 15). These
particles. This method allowed to reduce the lower identifica- probabilities agree reasonably well with the simple estimates
tionthreshold,duetothethicknessofthefirstphotodiode,(see obtainedusingthenuclearcollisionlengthof22.30cmforCsI
7
8000
104
6000
103 4000 7000
counts102 200000.8 0.9 1 1.1 nels]6000
10 an5000
h
c
1 de [4000
1 2 3 4 Z 5 6 7 8 plitu 15N
m3000
Figure14:Particleidentificationspectrum(chargedistribution) 0 A 12,13C
D
obtained from the map of Fig. 11, for particles stopped in the P2000 10,11,12,13B
thincrystal. Theinsetshowsthe p, d, t peaks(inlinearscale)
1000 7,9,10Be
orebstpaeincetidvefrlyo.mtheFig.11(solid)andFig.8(dashedhistogram), 00 2010,20,3H4000 60030,4,6H80e00 10000 1260,70,80,91L4i000 16000 18000 20000
PD1 Amplitude [channels]
Figure16: SameasFig. 6butwiththesuperimposedIDlines
104 calculatedusingtheATIMArange-energytables.
103
counts102
parameter Light-Energy relation [39], applicable for particles
10
stoppedinthescintillator:
1
1 2 3 4 Z 5 6 7 8 (cid:32) (cid:34) (cid:35)(cid:33)
E
Light=a E−a AZ2log 1+ (7)
Figure15: SameasFig. 14,butobtainedfromthemapofFig. 1 2 a AZ2
2
7,forparticlesstoppedinthethickcrystal.
withEbeingtheenergy,A,Z-massandatomicnumbersofthe
stoppedparticle,a ,a -thegainandquenchingparameters.
1 2
[38],whichyieldstheprobabilitiesof11%and43%for2.5and Havingthis(inverse)calibrationitwaspossibletosuperpose
12.5cmofCsI,respectively. Thebackgroundmeasuredunder the ATIMA lines on the decomposed ID map for the Single
the p, d, t lines includes also some contribution from the sec- ChipTelescopesegmentoftheKRATTAmodule. Theresultis
ondary reactions of neutrons and heavier charged particles, as presentedinFig. 17andisworthacomment.
well as from the accidental coincidences. The γ and punch-
through hits have been removed from the background, in both
cases.Thehighsecondaryinteractionprobabilityobviouslyde-
terioratestheidentificationqualityanddefinesthelimitsforap- 3500
plyingthetelescopemethodtointermediateenergies.
s]3000
el
n
n
a2500
h
7. Discussionandremarks c
on [2000
Sincetheparametrizationofthepulseshapesisonlyapprox- zati
ni1500
imate and has no deep physical background, one can ask how 1 Io
precise is the decomposition into individual components and PD1000
howphysicaltheyare. Inordertoaddressthesequestions,the
∆E-E map of the SCT (PD1+CsI1) has been compared to the 500
predictionsoftheATIMArange-energytables. Suchacompar- 0
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
ison requires the knowledge of the energy calibration of both, PD1 Scintillation [channels]
thesiliconphotodiodesandoftheCsI(Tl)lightoutput.Thecal-
ibration has been performed using the ID map of Fig. 6 (see Figure 17: Decomposed SCT ∆E-E identification map (ob-
Fig. 16)whichisrelativelyinsensitivetothedetailsofthede- tained with a single readout channel) with the superimposed
composition and is perfectly suited for the energy calibration ID lines calculated using the ATIMA tables. The sequence of
due to its richness in characteristic punch-through points and linesisthesameasinFig. 16.
curvatures.
The calibration routine allowed to adjust the thicknesses of The observed very good agreement between the recon-
the dead and active layers, the energy calibration parameters, structed amplitudes and the range-energy table predictions is
aswellasthelight-energyconversionparameters. Forthelat- quitenon-trivialtakingintoaccountthesimplicityofthepulse
ter,asimpleintegratedBirks’formula(seee.g. [34])hasbeen shape parametrization. In order to obtain this kind of agree-
used,withdE/dx∝ AZ2/E,yieldingacommonlyappliedtwo- ment,whichisstillacompromise,itwasnotenoughtosetall
8
theparametersfreeandsearchfortheminimumofthe χ2 dis- ofthe∆EcomponentforZ = 1particlesresultsmostprobably
tribution. Such an automatic procedure guarantees the best fit fromthesimplicityoftheLight-Energyconversionformula(7)
tothewaveforms,butnotnecessarilyguaranteesthefollowup and (or) from the light output non-uniformity of the crystals.
ofthetrendsdefinedbytherange-energytables. Ingeneral,an The overall agreement is, nevertheless, quite satisfactory and
automatic procedure can lead, depending also on the starting the calculated lines can be used not only to derive the energy
values, to some local minima, produce discontinuities or lead calibrationparameters,butalsoforidentification(seeFig. 15),
in directions diverging from the ATIMA lines. The presented aftersmallmanualadjustments.
agreement has been obtained after searching for the optimal The energy calibration routine based on the ATIMA range-
values and fixing some of the parameters (see Table 3). The energytablesoperatedonallthreemaps(Figs. 16-18)simulta-
agreementwiththeATIMAlinescouldbemadeevenbetter,for neously,allowingforaconsistentdeterminationofthecalibra-
instancebyfreeingtheτRS parameter, howeverattheexpense tionparametersforallphotodiodesandbothcrystals. Specifi-
of loosing completely the mass resolution in Fast-Slow repre- cally, for the CsI(Tl) crystals with 1500 ppm of Tl concentra-
sentation(contrarytothecasepresentedinFig. 10). Thus,the tion,thequenchingparametera wasfoundtobeequalto0.32,
2
presentedagreementisaresultofaniterativeprocedureoffix- avaluecompatiblewiththeonefrom[39]andabout20%larger
ing some of the parameters and also of the energy calibration thantheaverageonequotedfortheINDRAcrystals[40]. The
parameters,butoncethecrucialparametersareconstrainedthe calibration of the SCT allowed also to estimate the efficiency
fittingproceedsautomatically. of the scintillation. The combination of a relatively high Tl
The whole procedure could probably be better automatized doping, high reflectance of the wrapping and large active area
byusingmorerealisticpulseshapeparametrizations,especially of the photodiode, resulted in a relatively high efficiency for
for individual electron and hole components of the ionization energy-light conversion. It was found that only about 6 times
signal, taking into account the plasma delay effects, interac- moreenergywasneededtoproduceanelectron-holepairinthe
tionsbetween carriers, diffusioneffects, etc[33]. However, to photodiodethroughascintillationprocessintheCsI(Tl)crystal
our knowledge, such pulse shape parametrizations are still to thaninthedirectionizationprocessinthephotodiode.Thisfact
bedeveloped. Definitely,amorerealisticpreamplifierresponse isworthnoting,sinceforthe“goodscintillators”quotedin[34]
functionwouldimprovetheresolutionandwouldallowfordis- thisfactoramountsto15-20.
entangling between physical and instrumental parameters. A
Obviously, a drawback of the telescope method at high en-
morerealisticparametrizationandanalysiswouldbeworththe
ergies,isthehighlevelofthesecondaryreactions. Inorderto
effort of implementing it to e.g. better understand the relation
handlethisproblem,amoresophisticatedmethods,neuralnet-
between the range and the charge collection times which en-
worksanddiscriminantanalysisarebeingtested,butthisgoes
abletheidentificationoftheparticlesstoppedinthesiliconde-
beyondthescopeofthisarticle.
tectors. Nevertheless, any more sophisticated analysis would
definitelyslowdownevenmoretheanalysis.
12000 8. Summary
10000
s] Anew,lowthreshold,broadenergyrange,versatilearrayof
nel triple telescopes, KRATTA, has been constructed. The mod-
n8000
a
h ules, equippedwithdigitalelectronicschains, allowedforiso-
c
n [ topicidentificationoflightchargedreactionproducts.
Scintillatio46000000 thePucolsmepslheaxpeSCanTalpyuslisseasllhoawpeeds ifnotroreinadliisvtiidcudaelcioomnipzoastiiotinonanodf
1 scintillationcomponents andeventually profitfrom the, other-
D
P wise harmful, nuclear counter effect. The isotopic resolution
2000
obtainedusingasinglereadoutchannelwasfoundtocompete
very well with those obtained using the standard two channel
0
0 2000 4000 6000 8000 10000 12000 readout. The applied pulse shape analysis permitted also the
PD2 Scintillation [channels]
identificationofparticlesstoppedinthefirstphotodiodeandthe
Figure 18: ∆E-E identification map for scintillation signals reductionoftheidentificationthreshold,duetothethicknessof
from CsI1 vs CsI2, with the superimposed ID lines calculated the first photodiode, by a factor of three. Thanks to the pulse
usingtheATIMAtables.Thesequenceoflinesisthesameasin shapeanalysis,itwasalsopossibletoobtaintheballisticdeficit
Fig. 16. Theγ-lineandpunch-throughhitshavebeenremoved freeamplitudes,whichallowedforeasyenergycalibrationand
(cf. Fig. 7). identification based on the predictions of the range-energy ta-
bles.
Finally,Fig. 18showsthecalculatedATIMAlinessuperim- The array has met the expectations, fulfilled the design re-
posed on the ∆E-E identification map for scintillation signals quirements and performed very well during the ASY-EOS ex-
from thin vs thick CsI(Tl) crystals. The slight overestimation perimentatGSI.
9
9. Acknowledgments [39] D.Hornetal.,Nucl.Instr.andMeth.A320(1992)273.
[40] M.Paˆrlogetal.,Nucl.Instr.andMeth.A482(2002)693.
Work made possible through funding by Polish Min-
istry of Science and Higher Education under grant No.
DPN/N108/GSI/2009.
We (S.K.) acknowledge the support by the Foundation for
Polish Science - MPD program, co-financed by the European
UnionwithintheEuropeanRegionalDevelopmentFund.
Stimulating discussions, expertise and help of Giacomo
PoggiaswellasofMarianPaˆrlog,RemiBougaultandHector
AlvarezPolduringthedesignandtestphaseoftheprototypes
aregratefullyacknowledged.
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10