Measurement of antiproton annihilation on Cu, Ag and Au with emulsion films

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by  S. Aghion

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e Measurement of antiproton annihilation on Cu, Ag
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1 S. Aghiona,b, C. Amslerc,d, A. Arigac, T. Arigac∗, G. Bonomie,f, P. Bräunigg, R. S.
v: Brusah,i, L. Cabaretj, M. Cacciab,k, R. Caravital,m, F. Castellib,n, G. Cerchiario, D.
Xi Comparatj, G. Consolatia,b, A. Demetriog, L. Di Notol,m, M. Doserp, A. Ereditatoc, C.
r Evansa,b, R. Ferraguta,b, J. Feselp, A. Fontanaf, S. Gerberp, M. Giammarchii, A.
a
Gligorovaq, F. Guatierih,i, S. Haiderp, H. Holmestadr, T. Huser, J. Kawadac, A.
Kellerbauero, M. Kimurac, D. Krasnickýl,m, V. Lagomarsinol,m, P. Lansonneurs, P.
Lebruns, C. Malbrunotd,p, S. Mariazzid, V. Matveevt,u, Z. Mazzottab,n, G. Nebbiav, P.
Nedelecs, M. Oberthalerg, N. Pacificoq, D. Paganoe,f, L. Penasah,i, V. Petracekw, C.
Pistilloc, F. Prelzb, M. Prevedellix, L. Ravellih,i, B. Rienaeckerp, O. M. Røhner, A.
Rotondif,y, M. Sacerdotib,n, H. Sandakerr, R. Santorob,k, P. Scampolic,z, M. Simond, L.
Smestadp,aa, F. Sorrentinol,m, G. Testeram, I. C. Tietjep, S. Vamosid, M. Vladymyrovc,
E. Widmannd, P. Yzombardj, C. Zimmero,p, J. Zmeskald, N. Zurlof,bb
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aPolitecnicoofMilano,PiazzaLeonardodaVinci32,20133Milano,Italy
bINFNMilano,viaCeloria16,20133,Milano,Italy
cLaboratoryforHighEnergyPhysics,AlbertEinsteinCenterforFundamentalPhysics,
UniversityofBern,3012Bern,Switzerland
dStefanMeyerInstituteforSubatomicPhysics,AustrianAcademyofSciences,Boltzmanngasse3,
1090Vienna,Austria
eDepartmentofMechanicalandIndustrialEngineering,UniversityofBrescia,viaBranze38,
25123Brescia,Italy
fINFNPavia,viaBassi6,27100Pavia,Italy
gKirchhoff-InstituteforPhysics,HeidelbergUniversity,ImNeuenheimerFeld227,69120
Heidelberg,Germany
hDepartmentofPhysics,UniversityofTrento,viaSommarive14,38123Povo,Trento,Italy
iTIFPA/INFNTrento,viaSommarive14,38123Povo,Trento,Italy
jLaboratoireAiméCotton,UniversitéParis-Sud,ENSCachan,CNRS,UniversitéParis-Saclay,
91405OrsayCedex,France
kDepartmentofScience,UniversityofInsubria,ViaValleggio11,22100Como,Italy
lDepartmentofPhysics,UniversityofGenova,viaDodecaneso33,16146Genova,Italy
mINFNGenova,viaDodecaneso33,16146Genova,Italy
nDepartmentofPhysics,UniversityofMilano,viaCeloria16,20133Milano,Italy
oMaxPlanckInstituteforNuclearPhysics,Saupfercheckweg1,69117Heidelberg,Germany
pPhysicsDepartment,CERN,1211Geneva23,Switzerland
qInstituteofPhysicsandTechnology,UniversityofBergen,Allégaten55,5007Bergen,Norway
rDepartmentofPhysics,UniversityofOslo,SemSælandsvei24,0371Oslo,Norway
sInstituteofNuclearPhysics,CNRS/IN2p3,UniversityofLyon1,69622Villeurbanne,France
tInstituteforNuclearResearchoftheRussianAcademyofScience,Moscow117312,Russia
uJointInstituteforNuclearResearch,141980Dubna,Russia
vINFNPadova,viaMarzolo8,35131Padova,Italy
wCzechTechnicalUniversity,Prague,Brehova´7,11519Prague1,CzechRepublic
xUniversityofBologna,VialeBertiPichat6/2,40126Bologna,Italy
yDepartmentofPhysics,UniversityofPavia,viaBassi6,27100Pavia,Italy
zDepartmentofPhysics“EttorePancini”,UniversityofNapoliFedericoII,Complesso
UniversitariodiMonteS.Angelo,80126,Napoli,Italy
aaTheResearchCouncilofNorway,P.O.Box564,NO-1327Lysaker,Norway
bbDepartmentofCivilEngineering,UniversityofBrescia,viaBranze43,25123Brescia,Italy
E-mail: [email protected]
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ABSTRACT:Thecharacteristicsoftheprocessoflowenergyantiprotonannihilationonnuclei(e.g.
hadronization and product multiplicities) are not well known, and Monte Carlo simulation pack-
ages that use different models provide different descriptions of the annihilation events. In this
study, we measured the particle multiplicities resulting from antiproton annihilations on nuclei.
The results were compared with predictions obtained using different models in simulation tools
suchas GEANT4andFLUKA.For thisstudy, we exposedthintargets (Cu, AgandAu) toavery
lowenergyantiprotonbeamfromCERN’santiprotondecelerator, exploitingthesecondarybeam-
line available in the AEgIS (AD6) experimental zone. The antiproton annihilation products were
detectedusingemulsionfilmsdevelopedattheLaboratoryofHighEnergyPhysicsinBernwhere
they were analysed at the automatic microscope facility. The fragment multiplicity measured in
thisstudyisingoodagreementwithresultsobtainedwithFLUKAsimulationsforbothminimally
andheavilyionizingparticles.
KEYWORDS: Particledetectors;Emulsiondetectors;Antiprotonannihilations.
∗Correspondingauthor.
Contents
1. Introduction 1
2. Experimentalsetup 2
3. Dataanalysisandresults 2
4. Conclusions 8
1. Introduction
Emulsion films have recently been considered as possible position detectors for low energy anti-
matter studies. These studies include the AEgIS experiment at CERN [1, 2, 3, 4] whose goal is
themeasurementoftheEarth’sgravitationalaccelerationonantihydrogenatoms. Anothercollab-
oration proposed emulsions for their studies on positrons, as described in [5]. In particular, in the
case of the AEgIS experiment, the position detector must have a micrometer level resolution to
allow the required sensitivity of ∼1% to be obtained for the gravitational acceleration measure-
ment. Therefore, the excellent intrinsic spatial resolution of the emulsion detectors [6] has been
exploitedsincetheyarecapableofreconstructingtheantihydrogenimpactpointsfromtheannihi-
lationproducts. Emulsionfilmsweretestedandtheyexhibitedaspatialresolutionof∼1-2µm[3].
Filmswiththisresolutioncombinedwithatimeofflightdetector,couldallowtheexperimentgoal
to be achieved. In the same paper [3], a preliminary study of antiproton-nuclei annihilations was
also reported. That study assessed particle multiplicities resulting from antiproton annihilations
onemulsionfilmsandaluminium. Recently,againusingtheframeworkoftheAEgISexperiment,
analogous measurements were performed by means of a silicon detector acting at the same time
asanannihilationtarget[7]. Apartfromtheobviousapplicationsinnuclearphysics,measurement
ofthedecayproductsoflowenergyantiprotonannihilationindifferentmaterialsprovidesauseful
check of the ability of standard Monte Carlo packages to reproduce fragment multiplicities, type
and energy distributions stemming from antiproton (or antineutron) annihilation at rest on nuclei.
In fact, although measurements of the multiplicities of pions and other charged particles with en-
ergies higher than approximately 50 MeV are available in the literature [8, 9], the production of
highly ionizing nuclear fragments with short range has not been sufficiently studied. A measure-
mentofthemultiplicitiesofchargedproductsinantiproton-aluminiumannihilationswasreported
in[3],althoughthestatisticsperformedinthisstudywere43events,andthetrackingefficiencyof
thedetectorwaslimitedto80%. Inthispaper,wepresenttheresultsofastudyofthemultiplicities
ofchargedannihilationproductsondifferenttargetmaterials,namelycopper,silverandgold,using
emulsiondetectorsattheAntiprotonDecelerator(AD)atCERN.
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2. Experimentalsetup
Emulsion detectors were used to study the antiproton annihilation products generated in different
materials. ThiswasconductedusingasecondarybeamlinelocatedintheAEgISzoneattheAD.
Before reaching the targets, the 5.3 MeV antiprotons from the AD (3×107 pbar/shot every 100
s) were moderated in successive steps by means of different titanium and aluminium foils with
variable thicknesses. Finally, the beam was collimated in a vacuum test chamber after crossing
a vacuum separation window of titanium with a thickness of 12 µm. The emulsion detector was
situatedatthedownstreamendofthevacuumchamber(∼1minlength)whereitcouldbereached
byadefocusedbeamoflowenergyantiprotons(∼100keV).Thisdistancefromthedegradinglayer
wasnecessarytoreducethebackgroundfromannihilationstakingplaceatthemoderator. Asketch
of the experimental setup is shown in Fig. 1. The emulsion detector was operated under ordinary
vacuum conditions (10−5−10−6 mbar). The antiproton intensity measured by the detector was
approximately150/cm2/shot.
Figure1. Schematicsetupoftheexperiment. Anenlargedviewofthetargetregionisshownontheright
side.
Forthisstudytheemulsiondetectors(forareviewontheemulsiontechnologysee [6])were
producedattheLaboratoryforHighEnergyPhysics(LHEP)oftheUniversityofBernbypouring
the emulsion gel, provided by Nagoya University (Japan), on a glass plate. Glycerin was added
to the emulsion gel so that it could operate in vacuum [3]. This emulsion features a very low
backgroundwithanumberofthermallyinducedgrainsofapproximately1-2grains/1000µm3 [5].
Foilsofcopper,silverandgold,eachhavingathicknessof10µm,wereplacedastargetsatthe
endofthevacuumchamber,infrontoftheemulsiondetectors(onesetupwithcopperandsilverand
theotheronewithgold). Withthesesetupsandgiventheantiprotonenergies,alltheannihilations
are expected to take place within a few µm of the surface. Fig. 2 shows the targets (2×2 cm2)
fixedtotheemulsionfilm(leftandmiddle)andanantiprotonannihilationtakingplacedirectlyon
theemulsionsurface(right). Duringdatataking,wecollectedapproximately1500antiprotonsper
cm2 inabout10ADshots.
3. Dataanalysisandresults
Data recorded by the emulsion detectors were automatically scanned by an optical microscope
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Figure2. Leftandmiddle: Targetarrangementinthetwosetups,fixedtotheemulsionfilms. Targetsother
thanCu,AgandAuwerenotincludedinourstudy. Right: Antiprotonannihilationsonthebareemulsion
surface.
and then analysed by exploiting a recently developed fast tracking algorithm [10]. The measured
tracking efficiency of our detector was approximately 99% for minimally ionizing particles for a
wideangularrangeasreportedin[10].
m) 40000 100 ks
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Cu Ag
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X (m m)
Figure3. Profileofdetectedtracks(XYpositionsoftracksattheemulsionlayer)inoneofthesetups. The
toprightpartwasnotincludedinourstudy.
Fig.3showstheprofileofdetectedtracks(XYpositionsoftracksattheemulsionlayer)inone
of the setups. Among the tracks reconstructed by the above algorithm, we only considered those
tracksthatwerelongerthan30µmandinthedirectionoftheantiprotonbeamtoavoidconsidering
tracks that were due to the background. An angular cut of 0.4<tanθ<2.0 (22◦<θ<63◦), with
θ representing the track angle with respect to the beam direction, was applied. To reconstruct a
vertex,twothree-dimensionaltrackswererequired. Theefficiencyofvertexfindingwasestimated
by applying the criteria given above to the output of the FLUKA simulation and was found to be
22%forcopper,24%forsilver,and18%forgold.
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Thepositionofthereconstructedannihilationverticescanbeusedtocalculatethemagnitude
oftheactualgapbetweentheemulsionfilmandmetalfoils,whichinprinciplewereputincontact
with the detector. The surface topography obtained from the reconstructed vertices is shown in
Fig. 4 for copper, silver and gold targets. In our analysis, we only considered regions less than
100µmfromtheemulsionfilmsurfacebecausethevertexfindingefficiencywasuniformwithina
fewpercent. Theanalysedfiducialareawas1.68cm2 forthecoppertarget,1.96cm2 forsilverand
0.80cm2 forgold.
Figure4. Surfacetopographyforcopper, silverandgoldtargetsobtainedfromthereconstructedvertices.
The vertical scale refers to the distance from the emulsion film. The precision of target foil position is
approximately14µmintheverticaldirection.
Themainsourceofthebackgroundsignalinthereconstructedverticeswasduetoaccidental
coincidences of tracks coming from annihilations taking place upstream in the apparatus, which
were not completely excluded by the angular cut due to broad angular distribution. The number
of background tracks was much larger than the number of signal tracks, while the signal fraction
became significant if vertex reconstruction at the position of the target foil was required. The
rateofbackgroundverticeswasestimatedbyrandomizingthetrackpositionsintheanalysedarea
(keeping the slope information constant), reconstructing the vertices and counting the number of
verticesthatmimickedannihilationsinthetarget.
The estimated background, which depends on the number of prongs in the event, is shown
ontheleftsideofFig.5, whiletherightsideshowsthemultiplicitydistributionsaftersubtraction
of the background, compared with the Monte Carlo predictions based on the CHIPS [11] and
FTFP (FTFP_BERT_TRV) [12] models in the GEANT4 (4.9.5.p02) and FLUKA (2011.2c) [13]
frameworks. A total of 617 signal annihilation vertices were reconstructed for copper, 882 for
silverand219forgold.
We could also discriminate between heavily ionizing particles (HIPs) such as protons and
nuclearfragmentsandminimallyionizingparticles(MIPs),namelypions. Continuousdensetracks
corresponded to HIPs, while faint tracks are produced by MIPs, since the aligned grains of these
lasttracksareseparated. Thelocalenergydeposition(dE/dx)ofeachtrackcanthenbeassessedin
termsofsignaldensity(S.D.)alongthereconstructedtracksusing,
S.D.= ∑ S /L.
xyz
x,y,z∈C
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ntries600 DBaactak g(arollu)nd a.u. 0.81 DMaCt a(CHIPS)
E
MC (FTFP)
400 0.6 MC (FLUKA)
0.4
200
0.2
0 0
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Total multiplicity Total multiplicity
Copper
ntries800 a.u. 0.81
E
600 0.6
400 0.4
200 0.2
0 0
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Total multiplicity Total multiplicity
Silver
ntries200 a.u. 0.81
E
150
0.6
100 0.4
50 0.2
0 0
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Total multiplicity Total multiplicity
Gold
Figure5. Left: Backgroundcontributionstothetotalmultiplicitiesforthedifferenttargets. Right: Multi-
plicitydistributionsaftersubtractionofthebackground. ThehistogramsshowtheMonteCarlopredictions.
D. 20000 10
S. 18000 9
16000 8
14000 7
12000 6
10000 5
8000 4
6000 3
4000 2
2000 1
0 0
0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
cosq
Figure6. Signaldensity(S.D.)distributionvstrackanglewithrespecttothebeamdirection. Tracksbelow
theblacklinearedefinedasbeingminimallyionizing.
Here, x, y and z are the coordinates of voxels in the 3D image data. C is a group of voxels in a
cylinder along the track, and S is an 8-bit grey-scale signal of the voxel. L is the length of a
xyz
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u. 0.8 Minimum ionizing u. 0.8 Heavily ionizing
a. a.
Data Data
0.6 0.6
MC (CHIPS) MC (CHIPS)
MC (FTFP) MC (FTFP)
0.4 0.4
MC (FLUKA) MC (FLUKA)
0.2 0.2
0 0
0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
Multiplicity Multiplicity
Copper
u. 0.8 u. 0.8
a. a.
0.6 0.6
0.4 0.4
0.2 0.2
0 0
0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
Multiplicity Multiplicity
Silver
u. 0.8 u. 0.8
a. a.
0.6 0.6
0.4 0.4
0.2 0.2
0 0
0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
Multiplicity Multiplicity
Gold
Figure 7. Reconstructed multiplicity distributions for annihilations in the copper, silver and gold foils for
MIPs (left) and HIPs (right). The histograms show the Monte Carlo predictions by CHIPS, FTFP and
FLUKA.TheerrorbarsonthehistogramsaccountforuncertaintiesinthedE/dxclassification.
AveragemultiplicityforMIPs AveragemultiplicityforHIPs
Data CHIPS FTFP FLUKA Data CHIPS FTFP FLUKA
Copper 1.07±0.05 0.98+0.07 1.59+0.09 0.83+0.08 1.59±0.06 1.46+0.09 0.60+0.14 1.68+0.11
−0.09 −0.14 −0.11 −0.07 −0.10 −0.08
Silver 1.02±0.04 1.04+0.08 1.64+0.09 0.73+0.07 1.75±0.05 1.33+0.09 0.51+0.14 1.87+0.09
−0.09 −0.13 −0.09 −0.08 −0.09 −0.07
Gold 0.87±0.08 1.21+0.10 1.75+0.09 0.81+0.07 1.67±0.09 1.04+0.11 0.39+0.13 1.60+0.11
−0.11 −0.13 −0.11 −0.09 −0.09 −0.06
Table1. MeasuredaveragemultiplicityandMonteCarlopredictionsbyCHIPS,FTFPandFLUKA.Statis-
ticalerrorsarereportedforeachsetofdata. TheerrorsinthepredictionsaccountforuncertaintiesindE/dx
classification.
reconstructedtrackinthe3Dimagedata. TheS.D.isproportionaltothedE/dxoftheparticleand
doesnotdependontheangle. However,thereisasaturationeffectforhigherdE/dxregions. Fig.6
showsthattheS.D.distributionoftracksrevealedapeakat3000forMIPs. AsthesimulateddE/dx
distribution of MIPs peaked at 1.2 MeV·g·cm−2, we define particles with dE/dx smaller than 2.4
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MeV·g·cm−2, corresponding to an S.D. below 6000 µm−1, as being MIPs. The complementary
onesaredefinedasHIPs.
Fig.7showsthetrackmultiplicitydistributionsforMIPsandHIPs. Theerrorsonthedataare
statistical. The histograms represent the Monte Carlo predictions by CHIPS, FTFP and FLUKA.
TheerrorbarsontheMonteCarlopredictionsaccountforuncertaintiesinthedE/dxclassification.
These uncertainties were estimated using simulations, which smeared the threshold for assigning
tracks to either class of ionizing particles by 20% and checked for effects on the multiplicity dis-
tributions for MIPs and HIPs. The statistical errors for the simulations were 0.01-0.02, which are
significantlysmallerthantheerrorsreportedabove. Theaveragemultiplicitiesthatweremeasured
are summarized in Table 1. Both CHIPS and FLUKA were in good agreement for copper, par-
ticularly in the case of MIPs. Neither CHIPS nor FTFP accurately describe particle multiplicity
for annihilations on silver and gold nuclei, while FLUKA seems closer to the data than the other
models.
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Atomic number
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Figure8. ParticlemultiplicityfromantiprotonannihilationsasafunctionofatomicnumberforMIPs(top)
andHIPs(bottom).
The mean values of particle multiplicity measured for the three target materials are shown
in Fig. 8 as a function of atomic number along with the simulation outcome. Results obtained
for MIPs with the FTFP model do not agree with our experimental data for any material, while
thoseobtainedwithbothCHIPSandFLUKAareincloseagreementasfarascopperisconcerned,
although only FLUKA reproduces the higher atomic number behaviour. Good agreement with
CHIPS was also found for annihilation on bare emulsions and for aluminium [3]. Multiplicity
related to HIPs is well described by the FLUKA simulation, while the CHIPS and FTFP models
clearlyunderestimatethenumberofparticlesproducedbyantiprotonannihilation.
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