Characterization of a tagged $γ$-ray beam line at the DA$Φ$NE Beam Test Facility

1.4 MB·English

by  P. W. Cattaneo

Characterization of a tagged γ-ray beam line at the DAΦNE
Beam Test Facility
2 P.W.Cattaneo,g,A.Argana,F.Boffellig,A.Bulgarellie,B.Buonomou,A.W.Chenc,d,
∗
1 F.D’Ammandov,L.Foggetta ,d,T.Froyslandb,d,F.Fuschinoe,M.Gallih,F.Gianottie,
∗∗
0 A.Giulianic,F.Longof,M.Marisaldie,G.Mazzitelliu,A.Pellizzonir,M.Prestl,G.
2
Pucellam,L.Quintieriu,A.Rappoldig,M.Tavania,b,M.Trifoglioe,A.Troisr,P.

n Valentei,E.Vallazzaf,S.Vercellones,A.Zambrac,G.Barbiellinif,P.Caraveoc,V.
a Coccoa,E.Costaa,G.DeParisa,E.DelMontea,G.DiCoccoe,I.Donnarummaa,Y.
J
Evangelistaa,M.Ferocia,A.Ferrarid,q,M.Fiorinic,C.Labantie,I.Lapshova,F.
9
Lazzarottoa,P.Liparii,M.Mastropietroj,S.Mereghettic,E.Morellie,E.Morettif,A.
1
Morsellik,L.Pacciania,F.Perottic,G.Pianoa,b,k,P.Picozzab,k,M.Pilial,G.

] Porrovecchioa,M.Rapisardam,A.Rubinia,S.Sabatinia,b,P.Soffittaa,E.Strianib,k,V.
t
e Vittorinia,b,D.Zanelloi,S.Colafrancescon,P.Giommin,C.Pittorin,P.Santolamazzan,
d F.Verrecchian,L.Salottio
-
s
n aINAF/IASF-Roma,I-00133Roma,Italy
i bDip.diFisica,Univ.TorVergata,I-00133Roma,Italy
s. cINAF/IASF-Milano,I-20133Milano,Italy
c dCIFS-Torino,I-10133Torino,Italy
i eINAF/IASF-Bologna,I-40129Bologna,Italy
s fINFNTrieste,I-34127Trieste,Italy
y
gINFN-Pavia,I-27100Pavia,Italy
h
hENEA-Bologna,I-40129Bologna,Italy
p
iINFN-RomaLaSapienza,I-00185Roma,Italy
[ jCNR-IMIP,Roma,Italy
2 kINFNRomaTorVergata,I-00133Roma,Italy
lDip.diFisica,Univ.Dell’Insubria,I-22100Como,Italy
v
mENEAFrascati,I-00044Frascati(Roma),Italy
7
nASIScienceDataCenter,I-00044Frascati(Roma),Italy
4
oAgenziaSpazialeItaliana,I-00198Roma,Italy
1 pOsservatorioAstronomicodiTrieste,Trieste,Italy
6 qDip.Fisica,Universita´diTorino,Torino,Italy
1. rINAF-OsservatorioAstronomicodiCagliari,localita’PoggiodeiPini,strada54,I-09012Capoterra,Italy
1 sINAF-IASFPalermo,ViaUgoLaMalfa153,I-90146Palermo,Italy
1 tDip.FisicaUniv.diTrieste,I-34127Trieste,Italy
uINFNLab.Naz.diFrascati,I-00044Frascati(Roma),Italy
1
vINAF-IRABologna,ViaGobetti101,I-40129Bologna,Italy
:
v
i
X
r
a
Abstract
∗Correspondingauthor
NowatLAL-CNRS,F-91898Orsay,France
∗∗
Emailaddress:[email protected] (P.W.Cattaneo)
PreprintsubmittedtoElsevier January20,2012
AtthecoreoftheAGILEscientificinstrument,designedtooperateonasatellite,
thereistheGammaRayImagingDetector(GRID)consistingofaSiliconTracker(ST),
aCesiumIodideMini-CalorimeterandanAnti-Coincidencesystemofplasticscintil-
lator bars. The ST needs an on-groundcalibrationwith a γ-ray beam to validate the
simulationusedtocalculatetheenergyresponsefunctionandtheeffectiveareaversus
theenergyandthedirectionoftheγrays.Ataggedγ-raybeamlinewasdesignedatthe
BeamTestFacility(BTF)oftheINFNLaboratoriNazionaliofFrascati(LNF),based
onanelectronbeamgeneratingγraysthroughbremsstrahlungina position-sensitive
target. Theγ-rayenergyisdeducedbydifferencewith the post-bremsstrahlungelec-
tronenergy[1]-[2]. Theelectronenergyismeasuredbyaspectrometerconsistingofa
dipolemagnetandanarrayofpositionsensitivesiliconstripdetectors,thePhotonTag-
gingSystem(PTS).TheuseofthecombinedBTF-PTSsystemastaggedphotonbeam
requiresunderstandingtheefficiencyofγ-raytagging,theprobabilityoffaketagging,
theenergyresolutionandtherelationofthePTShitpositionversustheγ-rayenergy.
This paper describes this study comparing data taken during the AGILE calibration
occurredin2005withsimulation.
Keywords: Electronandpositronbeam;Photonbeam;Position-sensitivedetectors;
bremsstrahlung
PACS:41.75.Fr,41.85.p,29.40.Gx,41.60.-m
1. Introduction
AGILE(Astro-rivelatoreGammaaImmaginiLEggero)isaSmallScientificMis-
sionoftheItalianSpaceAgency(ASI),dedicatedtohigh-energyastrophysics.Itcom-
binestwoco-alignedimagingdetectorsoperatingrespectivelyintheXandγ-raybands
withlargeField ofViews(FoV).TheSiliconTracker(ST),atthecoreoftheAGILE
satellite,isdesignedtodetectandimageγraysinthe30MeV-50GeVenergyrange
[3]-[4]-[5]-[6].
Theon-groundcalibrationofanastronomicalinstrumentisimportantfortheinterpre-
tationofitsresults. Thegoalistoreproduceinlaboratory,undercontrolledcondition,
theresponseoftheinstrumentinflight. Thistaskrequiresataggedphotonbeamwith
position, directionand energyof each photonknownwith a precision better than the
instrumentresolution.Therealizationofsuchabeamturnsouttobeachallengingen-
deavour. TheBeamTestFacility(BTF)intheDAΦNEcollidercomplexattheINFN
LaboratoriNazionaliofFrascati(LNF)[7],wastheelectedsiteforrealizingthetagged
photonbeamexploitingbremsstrahlunginathintargetandperformingthecalibration.
PreliminaryresultsonthecalibrationoftheAGILESThasbeenpresentedin[8]and
willbesubjectofafuturepaper.
Thispaperpresentstheexperimentalsetup,adetailedMonteCarlostudyofthesystem
andthecomparisonwiththeexperimentalresultscollectedduringthecalibration.
2. Theexperimentalsetup
The experimentalsetup is a complex system consisting of the BTF e beam, the
−
targetto generate bremsstrahlungphotons, the spectrometer magnetand the detector
2
to measurethe energyof post-bremsstrahlungelectrons. The varioussubsystems are
describedinthefollowingwiththeconventionthatyisthecoordinateperpendicularto
theBTFlineplane,xtheonetransversetothebeamatthetargetintheBTFlineplane
andzistheonealongthebeamatthetarget,thatisthedirectionofthebremsstrahlung
photons.
Figure1:TheinitialsectionoftheBTFtransferline.
2.1. Theelectronbeam
Thee susedforgeneratingthephotonbeamaredeliveredbytheBTF.TheBTFis
−
fedbytheDAΦNEcomplexthatprovidese undercarefullycontrolledconditionwith
±
predefinedmultiplicity.
2.1.1. TheBeamTestFacility(BTF)
FortheST calibrationweusedtheBTFin theDAΦNEcollidercomplexatLNF,
whichincludesaLINACathighe+/e currents,anaccumulatorofe+/e andtwostor-
− −
ageringsat510MeV.Thee+/e beamfromtheLINACisdirectedintotheaccumula-
−
tionringtobesubsequentlyextractedandinjectedintheMainRing. Whenthesystem
injectordoesnottransferthebeamstotheaccumulator,thebeamfromLINACcanbe
extractedand transportedin the test beam area througha dedicatedtransfer line: the
BTFline(Fig.1).TheBTFcanprovideacollimatedbeamofe+/e intheenergyrange
−
20-750MeVwithapulserateof50Hz. Thepulsedurationcanvaryfrom1to10ns
andtheaveragenumberofe perbunchN rangesfrom1to1010[7]-[9]-[10].
− e
TheBTFcanbeoperatedintwoways:
LINACmode:operatingwhenDAΦNEisoff,withatunableenergybetween50
•
MeVand750MeVandanefficiencyaround0.9
3
DAΦNEmode: operatingwhenDAΦNEison,withafixedenergyat510MeV
•
andanefficiencyaround0.6
Theextractedelectronsare transportedto the BTFhall, where the finalsection is lo-
cated (Fig. 2); the experimentalequipment under test is positioned at the exit of the
spectrometermagnetDHSTB02. All orpartof theequipmentcan bemountedunder
vacuumcontinuingthebeamlineor,alternatively,thebeamlinecanbeterminatedwith
athinwindowandtheequipmentmountedinair.
In spite of some disadvantagefromthe pointof view of background,this last option
wasadoptedwitha0.5mmBewindowbecauseofthedifficultiesinimplementingan
extendedvacuumlineincorporatingalltherequiredequipment.
Figure2:ThefinalsectionoftheBTFtransferlineincludingthelastspectrometermagnetDHSTB02.
2.1.2. e multiplicityperbunch
−
The calibrationof the ST should be ideallyperformedin a single-photonregime,
avoidingsimultaneousmulti-photonproductionto reproducetheastrophysicalcondi-
tions. Multi-photonevents should ideally identified and rejected otherwise they will
biasthecountingstatistics.
On the other hand, bremsstrahlungis a continuousprocessand multi-photongenera-
tion(withphotonenergyE aboveagiventhreshold)ispossiblealsowhenasinglee
γ −
crossesthe target. Thefractionof multi-photoneventsis approximatelyproportional
tothesingle-photonemissionprobability. Thatimpliestheneedofacompromisebe-
tweenthephotonbeamintensityandthesingle-photonbeampurity.
Considering that the targetthickness was constrained by the availability of the hard-
ware, by the needto guaranteefulldetectionefficiencyofthe beamelectronsand by
4
theneedtomeasureelectronstwicebothinthexandydirectionsforstudyingthebeam
sizeanddivergence,theonlyfreeparameteristhee multiplicityperbunch.
−
Another constraint is the limited time available for the calibration campaign and the
requestofcalibratingmanydifferentSTgeometricalconfigurations.Thatputsalower
limittotherequiredphotonfluxandthereforeonthee multiplicityperbunch.
−
In DAΦNE mode with 5 e /bunch the fraction of multi-photon events having E >
− γ
20MeVcanbeestimatedtobe 10%bytheformulaeintheAppendix. Thisuncer-
≈
taintyisgreaterthantheaccuracyrequirementontheeffectiveareameasurements.On
theotherhands,theDAΦNEmodewith1e /bunchisconsistentwiththeaccuracyre-
−
quirementsbutthetimenecessarytocollectenoughstatisticsisincompatiblewiththe
timeavailableforthecalibration.TheSTcross-sectionforphotonswithE <20MeV
γ
is notnegligibleand thusthe fractionof interactingsecondaryphotonswill be larger
than the numbers calculated in the Appendix. Taking into account the above con-
siderations,thebestconfigurationforSTperformanceandcalibrationwouldbewith1
e /bunch,butthefluxrequirementforcedtoselecttheconfigurationwith 3e /bunch.
− −
≈
non
interacting Bremsstrahlung photon
electrons
bending magnet
Si target
impact of reduced
momentum electron
tagging detector
BTF e− beam
Figure3:Aschematicviewoftheγ-rayline:thetarget,thespectrometermagnetandthePTS.
2.2. Thebremsstrahlungtarget
PhotonsintheenergyrangerelevantfortheSTareproducedbybremsstrahlungof
electronsinatarget;subsequentlyamagnetbendsawaytheelectronswhiletheγrays
cantraveltowardstheAGILEinstrument(seeFig.3).
Thebremsstrahlungtargetconsistsoftwopairsofsiliconsinglesidedmicro-stripde-
tectorsofarea8.75 8.75cm2and410µmthick,eachincluding768stripswith114µm
×
pitch. Onlyeveryotherstripisread,sothateachtargetdetectorhas384readoutchan-
nelswith228µmreadoutpitch. Eachpairmeasuresseparatelythe xandycoordinates
transverseto the beam. A spatialresolutionσ 114/√12µm 33µmis expected;
≤ ≈
5
the cluster size is oftenlimited to one strip and thereforethe resolutionis limited by
thestrippitch.
Thetargethastworoles: tomeasurethecoordinateandthedirectionoftheelectrons
and to cause the emission of bremsstrahlung photons. The target detectors are posi-
tionedalongthebeamdirectionbetweenthelastfocusingmagnet(QATB04inFig.2)
andthespectrometermagnet(DHSTB02inFig.2). The xmeasuringonesarethefirst
andthethird,positionedrespectivelyat5.45cmand7.20cmdownstreamtheBewin-
dow,whiletheymeasuringonesarethesecondandthefourth,positionedrespectively
6.45cmand8.20cmdownstreamtheBewindow.
Attheelectronenergymostusedduringcalibration, E = 463MeV, thecontribution
e
tobeamdivergenceduetoCoulombMultipleScatteringineachtargetdetectoriseval-
uatedundertheGaussianapproximationfrom[11]as 2.0mrad.
≈
2.3. ThePhotonTaggingSystem(PTS)
The spectrometer magnet, visible in Fig.2-Fig.3-Fig.4, generates a dipolar field
alongtheydirectionoveranangularrangeof45 . Inbetweenthetwomagnetpoles,
◦
there is a pipe made of stainless steel with rectangular section. It is composed of a
straight section (’photon pipe’) along which the bremsstrahlung γ rays travel to the
ST and a curved section (’electron pipe’) defining the trajectory for e s bent by the
−
magneticfield.
Thepipeis hollowwith an airfilled innersection withsize 5.50 3.50cm2, its wall
×
thicknessis0.35cm. Themagneticfieldinthevolumebetweenthepolesthatincludes
the’electronpipe’isassumedconstantwithstrengthB=0.9TwhenE =463.0MeV,
e
correspondingtoacurvatureradiusR=172.0cm.
Theequipmentforthedetectionofthee sthatlostenergyinthetargetwasdeveloped
−
andinstalledinsidethespectrometermagnetbyourteam: thePhotonTaggingSystem
(PTS).Itconsistsof12micro-stripsilicondetectorspositionedontheinternalwallsof
the spectrometer magnet(see Fig.2, Fig.3 and Fig.4) groupedinto two modulesof 6
detectorseach,locatedintotwohollowrectangularaluminumboxesfewmmthick. In
each module, the detectorsare located along a straight segmentand thereforefollow
onlyapproximatelythecurvedsectionofthe’electronpipe’.Theareaofeachdetector
is11.86 2.00cm2withthickness410µmandissubdividedin1187stripswith100µm
×
pitch.Onlyeverythirdstripisreadresultingin384readoutstripsperdetectorand4608
intotal.
Between each pair of consecutive detectors inside a module, there is a gap 6mm
≈
wide that is effectively a dead area. A larger gap 2.0cm wide is present between
≈
the two modules that contributes to the dead area as well. Electronic noise gives a
small contribution compared to the signals from e amounting to 2keV that is of
−
≈
littlerelevanceforthemeasurements. Dependingontheenergylossinthetarget,the
electronsimpingeondifferentstrips. Thecorrelationintimebetweenthesignalsofthe
e inthetargetandinthePTStagsthephoton;thepositiononthePTSmeasuresthe
−
photonenergy.
ThetriggerforreadingoutthetargetandPTSdataisgivenbythedelayedLINACpre-
trigger; itis readoutindependentlyfromthe ST data. Thispointhasgreatrelevance
forthefollowinganalysis.
6
Figure4: GeometryofthespectrometermagnetMdrawninGEANT3includingthePTSdetectorsSdis-
playedwithane (enteringfromtheright)irradiatingaγrayinthetargetTandhittingthePTSatP.Photons
−
arerepresentedbydashed(blue)linesandelectrons bysolid(red)lines. Inthiseventtheγrayenergyis
75MeV.
≈
3. TheMonteCarlosimulation
A proper characterization of the BTF requires a careful comparison of the data
withadetailedMonteCarlosimulation.ThesimulationisrealizedusingtheGEANT3
package[12]. Thesimulationincorporatesadescriptionoftheelectronbeamdelivered
bytheacceleratorcomplexwithbeamparametersdeterminedpartlyfromdesignval-
uesandpartlyfrommeasurements. Thenumberofe perbunchN canbefixedtoan
− e
integervalueorcanfollowaPoissondistributionaveragedatanyrealvalueN .
e
Thematerialdistributionofthebremsstrahlungtargetandofthespectrometerissimu-
latedindetail. Thetargetandthepipecanbesimulatedinairorinvacuuminvarious
configurations. The defaultconfigurationis the one actually used duringdata taking
withthetargetandthepipeinair.
TheinteractionsofelectronsandphotonsaredrivenbytheGEANT3routineswiththe
possibilitiesofswitchingonandofftherelevantphysicsprocesseslikeCoulombMul-
tiple Scattering, bremsstrahlung,Comptonscattering, pair creationforallor onlyfor
some of the materials. This option turnedout to be veryuseful in understandingthe
behaviouroftheBTF/PTSsystem. Theenergycutsforthee andphotonsarekeptat
±
theminimumallowedbytheprogram(100keV).Agaugeofthequalityofthesecuts
istheaverageenergylossofaminimumionizingparticlescrossinga 400µmthick
≈
siliconlayercomparabletoatargetdetectororaST layerthickness: itis 100keV.
≈
ThislevelofprecisionisrequiredtosimulatespurioushitsinthetargetandintheST
thatcanaffectthemeasurement.
The digitization simulation in the silicon micro-strip detectors is based on a simpli-
7
fied model: the charge released in the volume below each strip is collected by the
strip without accountingfor diffusion and charge trapping. Exploitingthe capacitive
coupling [13], the charges collected on all strips are fed into the readout strips with
appropriate coefficients as described in [3]. The noise is simulated simply adding a
Gaussiandistributedchargeoneachstriparoundacluster;thewidthisdeterminedby
thedataandamountto 2keV.
≈
Thee beamisgeneratedinsidethelaststraightsectionoftheaccelerator 5cmup-
−
≈
streamthetargetwithabeamspotofellipticalshapewitha2DGaussiandistribution,
with angular divergences perpendicular to the beam generated according 2 separate
Gaussian distributions and with a Gaussian distributed momentum spread. The mo-
mentumspreadisprovidedbytheDAΦNEstaff,whiletheotherbeamparametersare
directlymeasured.
3.1. Thesimulatedphotonbeam
ThephotonbeamdirectedtotheSTissimulatedthroughtheinteractionofthee
−
beam with the target. The photongenerationis due to the bremsstrahlungeffect and
followsEq.2intheAppendix. Thisformulashowsanapproximate1/E dependency,
γ
thatisapowerspectrumwithindex 1. Moreprecisely,afitofEq.2intheinterval
≈ −
0.05 1.00withapowerspectrumreturnsanindex 1.2.
− ≈−
ThesimulatedenergyspectrumofthemostenergeticγraysreachingtheST(butnot
necessarilyinteractingwithit)isshowninFig.5 withapowerspectrumfitreturning
anindex 1.2asexpectedbytheanalyticalformula.Thenumberofe sperbunchis
−
≈−
setto N = 1withoutPoissonfluctuation. ThesameresultisobtainedwhenN = 3.5
e e
with Poisson fluctuation. This result is notobviousbecause of the energydependent
interactionsalongthepaththatcouldchangethespectrum,thatturnouttobesmall.
Anotherimportantelementthatcharacterizesthebeamisthefractionofmulti-photon
events.Becauseinanastrophysicalenvironmenttherearenosuchevents,theymustbe
consideredbackgroundandmustbeminimized. Ifaneventhastwoormorephotons
with energy above the detection threshold in the ST (about few tens of MeV), they
can interact simultaneously in the ST. The reconstruction software, designed for the
astrophysicalenvironment,isnotfittoidentifysucheventsandwillmostlikelyfailor
returnincorrectenergyanddirection. Thepercentagefractionofmulti-photonevents
aboveathresholdisshowninFig.6. EvenforeventswithN = 3.5,thefractionisnot
e
larger than a few % even for E as small as 10 MeV. Thereforetheir contributionto
γ
errorsinthemeasurementoftheSTperformancesisatmostofcomparablesize.
Arelatedbutsomehowdifferentissueisthenumberoflowenergyphotons,sayE <
γ
10MeV,accompanyingaphotonintheSTenergyrange(E >30MeV).Thisphotons
γ
are not enough energetic to convertin a e+e pair detectable in the ST, but they can
−
interactincoincidencewithaphotonofhigherenergygeneratingspurioushitsthatcan
influencetheproperreconstructionoftheevent.
4. Resultsanddiscussion
TheanalysisofthetargetandthePTSdataallowsthevalidationoftheMonteCarlo
simulationrequiredtocorrelatetheγ-rayenergyandthePTShitposition.
8
Figure5:Spectrumoftheγ-raybeamreachingtheSTfitwithapowerlawα/Eβ.
4.1. Thebremsstrahlungtargetdata
Theanalysisofthetargetdatastartsfromlookingforstripsaboveathreshold,then
neighbouringstripsaregroupedintoclusters. Theclustercoordinateisobtainedcalcu-
latingitscentroidbyweightingeachstripcoordinatewiththechargecollected.
Ideally,eachclustershouldsignalahitofonee in atargetdetectorandthepassage
−
ofeache shouldbesignaledbyonehitineachtargetdetector.Inpracticethereisthe
−
possibilityofinefficiencyinthedetectionofe hits,noisehits,multipleclustersdueto
−
asinglee andsingleclustersonthesametargetdetectorassociatedtomultiplee s.
− −
Theefficiencyofthehitsearchingalgorithmismeasuredbyselectingtheeventswith
onehitonthreetargetdetectorsandzeroonthefourth.Thisnumberistobecompared
with the events with one hit on each target detector. With the available statistic, no
eventsatisfiesthisrequirement,sothattheefficiencyisbasically100%.
Thefractionofnoiseinducedhitscanbeestimatedbyrequiringonehitononetarget
detectorandzerohitontheothers.Alsointhiscase,noeventsatisfiesthisrequirement
andthereforethefractionofnoisyhitsisbasically0%.
Particularlyinterestingsamplesarethe0-clusterevents,wherenoclusterisdetectedin
thetarget,andthe1-clusterones,whereoneclusterpertargetdetectorisdetected. For
the considerationsdetailed above, the first sample can be safely associated to events
with 0-e events; even these events are triggered because the target signal is not re-
−
quiredbythetriggerlogic. The1-clustersamplemostlyoverlapswith1-e eventsand
−
isusedinthefollowingforcharacterizingthebeam. Themaingoalofthesemeasure-
9
Figure6: Probabilities(in%)ofmulti-photoneventsversusEγ thresholdforNe = 1(fixed,reddots)and
Ne=3.5(Poissondistributed,greensquares).
mentsisinferringthebeampropertiestobeusedintheMonteCarlogeneration.
4.1.1. Beamsizes
Thebeamsizesaremeasuredonlywith1-clustersample. Thebeamprofilesusing
thefirst x andy measuredcoordinatesinthe targetareshownin thetoprowofFig.7
with the results of Gaussian fits superimposed. Using the second x and y measured
coordinatesinthetargetgivescompatiblebeamsizes.
Thebeamsizesareσ 1.5mmandσ 0.5mmwithasignificantnonGaussiantail
x y
≈ ≈
iny. Thisnumbersarerepresentativebutsubjecttosignificantvariationsfordifferent
runs,duetochangesinthebeamsetting.
4.1.2. Beamdivergences
Angulardivergencesaremeasuredusingthe1-clustersample. Theestimatoristhe
difference between the cluster coordinates(x(y),i (1,2)) on the target divided by
i i
∈
theirdistanced
x(y)
s =(x x )/d
x 2 1 x
−
s =(y y )/d (1)
y 2 1 y
−
InFig.7(bottomrow)thedistributionsofs ands areshown.TheGaussianfitreturns
x y
σ(s ) 5.7mradandσ(s ) 4.2mrad.
x y
≈ ≈
10