Detection of low energy antiproton annihilations in a segmented silicon detector

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Detection of low energy antiproton annihilations in a segmented silicon detector
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PUBLISHEDBYIOPPUBLISHINGFORSISSAMEDIALAB
RECEIVED:November18,2013
REVISED:February14,2014
ACCEPTED:April25,2014
PUBLISHED:June25,2014
Detection of low energy antiproton annihilations in a
segmented silicon detector
2
0
AEgIS collaboration 1
S. Aghion,a,b O. Ahle´n,c A.S. Belov,d G. Bonomi,e,f P. Bra¨unig,g J. Bremer,c 4
R.S. Brusa,h G. Burghart,c L. Cabaret,i M. Caccia,j C. Canali,k R. Caravita,l
F. Castelli,l G. Cerchiari,m S. Cialdi,l D. Comparat,i G. Consolati,a,b J.H. Derking,c J
S. Di Domizio,n L. Di Noto,h M. Doser,c A. Dudarev,c R. Ferragut,a,b A. Fontana,f
I
P. Genova,f M. Giammarchi,b A. Gligorova,o,1 S.N. Gninenko,d S. Haider,c
N
J. Harasimowicz,p T. Huse,q E. Jordan,m L.V. Jørgensen,c T. Kaltenbacher,c
S
A. Kellerbauer,m A. Knecht,c D. Krasnicky´,r V. Lagomarsino,r A. Magnani,f,s
T
S. Mariazzi,t V.A. Matveev,d,u F. Moia,a,b G. Nebbia,v P. Ne´de´lec,w N. Pacifico,o

V. Petra´cˇek,x F. Prelz,b M. Prevedelli,y C. Regenfus,k C. Riccardi,s,f O. Røhne,q
A. Rotondi,s,f H. Sandaker,o A. Sosa,p M.A. Subieta Vasquez,e,f M. Sˇpacˇek,x 9
G. Testera,n C.P. Welschp and S. Zavatarellin
aPolitecnicodiMilano, P
PiazzaLeonardodaVinci32,20133Milano,Italy 0
bIstitutoNazionalediFisicaNucleare,Sez. diMilano,
6
ViaCeloria16,20133Milano,Italy
cEuropeanOrganisationforNuclearResearch,PhysicsDepartment, 0
1211Geneva23,Switzerland 2
dInstituteforNuclearResearchoftheRussianAcademyofSciences,
0
Moscow117312,Russia
eUniversityofBrescia,DepartmentofMechanicalandIndustrialEngineering,
ViaBranze38,25133Brescia,Italy
fIstitutoNazionalediFisicaNucleare,Sez. diPavia,
ViaAgostinoBassi6,27100Pavia,Italy
gUniversityofHeidelberg,KirchhoffInstituteforPhysics,
ImNeuenheimerFeld227,69120Heidelberg,Germany
hDepartmentofPhysics,UniversityofTrentoandTIFPA-INFN,
ViaSommarive14,38123Povo,Trento,Italy
iLaboratoireAime´ Cotton,CNRS,Universite´ ParisSud,ENSCachan,
Baˆtiment505,Campusd’Orsay,91405OrsayCedex,France
1Correspondingauthor.
(cid:13)c CERN2014forthebenefitoftheAEGIScollaboration,publishedundertheterms
oftheCreativeCommonsAttribution3.0LicensebyIOPPublishingLtdandSissa doi:10.1088/1748-0221/9/06/P06020
Medialabsrl. Anyfurtherdistributionofthisworkmustmaintainattributiontotheauthor(s)andthe
publishedarticle’stitle,journalcitationandDOI.
jInsubriaUniversity,
Como-Varese,Italy
kUniversityofZurich,PhysicsInstitute,
Winterthurerstrasse190,8057Zurich,Switzerland
lUniversityofMilano,DepartmentofPhysics,
ViaCeloria16,20133Milano,Italy
mMaxPlanckInstituteforNuclearPhysics,
Saupfercheckweg1,69117Heidelberg,Germany
nIstitutoNazionalediFisicaNucleare,Sez. diGenova,
ViaDodecaneso33,16146Genova,Italy
oUniversityofBergen,InstituteofPhysicsandTechnology, 2
Alle´gaten55,5007Bergen,Norway
0
pUniversityofLiverpoolandtheCockroftInstitute,Liverpool,Sci-TechDaresbury,
1
KeckwickLane,Daresbury,Warrington,WA44AD,UnitedKingdom
qUniversityofOslo,DepartmentofPhysics, 4
SemSælandsvei24,0371Oslo,Norway

rUniversityofGenoa,DepartmentofPhysics,
J
ViaDodecaneso33,16146Genova,Italy
sUniversityofPavia,DepartmentofNuclearandTheoreticalPhysics, I
ViaBassi6,27100Pavia,Italy
N
tStefanMeyerInstituteforsubatomicPhysics,
S
Boltzmanngasse3,1090Vienna,Austria
uJointInstituteforNuclearResearch, T
141980Dubna,Russia

vIstitutoNazionalediFisicaNucleare,Sez. diPadova,
9
ViaMarzolo8,35131Padova,Italy
wClaudeBernardUniversityLyon1,InstitutdePhysiqueNucle´airedeLyon,
4RueEnricoFermi,69622Villeurbanne,France
P
xCzechTechnicalUniversityinPrague,FNSPE,
0
Brˇehova´ 7,11519Praha1,CzechRepublic
yUniversityofBologna,DepartmentofPhysics, 6
ViaIrnerio46,40126Bologna,Italy
0
E-mail: [email protected]
2
ABSTRACT: The goal of the AEg¯IS experiment at the Antiproton Decelerator (AD) at CERN, 0
is to measure directly the Earth’s gravitational acceleration on antimatter by measuring the free
fall of a pulsed, cold antihydrogen beam. The final position of the falling antihydrogen will be
detectedbyapositionsensitivedetector. Thisdetectorwillconsistofanactivesiliconpart,where
the annihilations take place, followed by an emulsion part. Together, they allow to achieve 1%
precisiononthemeasurementofg¯withabout600reconstructedandtimetaggedannihilations.
We present here the prospects for the development of the AEg¯IS silicon position sentive de-
tector and the results from the first beam tests on a monolithic silicon pixel sensor, along with a
comparisontoMonteCarlosimulations.
KEYWORDS: Solid state detectors; Detector modelling and simulations I (interaction of radiation
withmatter,interactionofphotonswithmatter,interactionofhadronswithmatter,etc)
Contents
1 Introduction 1
2 DevelopmentofthesilicondetectorforAEg¯IS 2
2.1 Annihilationofantiprotonsinsilicon 2
2.2 MonteCarlosimulations 3
2.3 Detectorrequirementsanddesign 4
2
3 Testbeamsetup 6
0
3.1 Antiprotonsourceandtestfacility 6
1
3.2 TheMIMOTERAdetector 8
4
3.3 CalibrationoftheMIMOTERAdetectorandclustering 9

4 Results 10
J
4.1 Dataselection 10
I
4.2 Backgroundsources 10
4.3 Clustercharacteristics 15 N
4.4 Tracksrecognition 16 S
4.5 ComparisonwithMonteCarlosimulations 16
T

5 Summaryandconclusions 19
9

P
1 Introduction 0
6
TheAEg¯ISexperiment[1]atCERN(figure1)aimsatverifyingtheWeakEquivalencePrinciplefor
0
antimatterbymeasuringtheEarth’sgravitationalaccelerationgforantihydrogen. Severalattempts
2
havebeenmadeinthepasttomeasurethegravitationalconstantforantimatter,bothforcharged[2,
3]andneutralantiparticles[4–6]. However,noneoftheseexperimentsarrivedatconclusiveresults. 0
Recently, a study from the ALPHA collaboration [7] sets limits on the ratio of gravitational mass
to the inertial mass of antimatter but is still far from testing the equivalence principle. Another
experiment,GBAR,[8]hasbeenproposedbutnotyetbuilt.
Coldantihydrogen(100mK)inRydbergstateswillbeproducedthroughthechargeexchange
reactionbetweenRydbergpositroniumandcoldantiprotonsstoredinaPenningtrap[9]. Applying
an appropriate electric field will accelerate the formed antihydrogen in a horizontal beam, with a
typicalaxialvelocitydistributionspanningafew100m/s[10].
Some of the trajectories will be selected through a moire´ deflectometer [11], which will con-
sist of two vertical gratings producing a fringe pattern on a downstream annihilation plane (see
figure 2). This plane will be the first layer of the position sensitive detector where the antihydro-
gen will impinge with energies of the order of meV and annihilate. The vertical deflection of the
–1–
2
0
1
Figure1. SchematicviewofthecentralregionoftheAEg¯ISexperiment.
4

pattern is proportional to the gravitational constant to be measured. Over a flight path of ∼1 m,
J
the deflection is expected in the order of ∼20 µm for a 1 g vertical acceleration [1]. A vertical
I
resolution better than 10 µm is required to meet the goal of 1% precision on the g¯ measurement
N
with600reconstructedandtimetaggedannihilations[12].
S
According to the current design, the position sensitive detector will be a hybrid detector
consisting of an active silicon part, where the annihilation takes place, followed by an emulsion T
part[12,13]. Thesilicondetectorwillprovideonlinemeasurementanddiagnosticsoftheantipro-

tonannihilationsaswellasthenecessarytimeofflightinformation.
9
The aim of the present study is to perform the first measurement and direct detection of slow

antiproton(∼few100keV)annihilationsinsilicon. Thisisthefirststeptowardsunderstandingthe
P
signatureofantihydrogenannihilations,whichisoneofthemostfundamentalaspectsofdesigning
a silicon position sensitive detector for AEg¯IS. To our knowledge, only in one other experiment 0
were annihilations in a silicon sensor directly detected and simulated [14]. However, much faster 6
antiprotonswereusedinthatstudy(608MeV/c)thaninthestudypresentedhere.
0
2
2 DevelopmentofthesilicondetectorforAEg¯IS
0
InAEg¯IS,thesilicondetectorwillactastheannihilationsurface. Kineticenergyoftheantihydro-
genatomwillbeinsufficienttogenerateadetectablesignal,sotheantihydrogenwillbeindirectly
detected through the detection of the annihilation products. We will now present available exper-
imental data on the annihilation process of antihydrogen (antiprotons) in matter and the available
MonteCarlotoolsforitssimulation. ThisconstitutesthebasisforthedesignoftheAEg¯ISsilicon
detector,whichwillbepresentedin2.3.
2.1 Annihilationofantiprotonsinsilicon
The annihilation process of antihydrogen in matter is similar to the one of an antiproton as the
positron annihilates immediately when meeting an atomic electron. Previous experiments at
–2–
Readout ASICs
77 K vessel
K)
m
0
0
1
n (
e
g
dro mE
y antihy osuiln
g
er
n
e
w ~0.5 m
o
L
Moire Strip detector (25 um pitch, 50 um, 300 um support 2
deflectometer ribs, 20x20 cm^2 area covered)
0
Figure 2. The moire´ deflectometer producing a pattern on the position sensitive detector, where several
1
particlepathsintersectatthedetectorplane.
4

LEAR [15] have studied annihilations of antiprotons in elements with different Z. In this pro-
J
cess,theantiprotonlosesenergyasittraversesmatterandannihilateswithaprotonatrestcreating
I
charged (1.53±0.03 per annihilation per charge sign) and neutral pions (1.96±0.23 per anni-
N
hilation). For elements with atomic numbers >1 the average ratio is shifted towards producing
morenegativelychargedpions,duetothepossibleannihilationoftheantiprotonwithnuclearneu- S
trons. Thepionsproducedintheannihilationmayfurtherinteractwithothernucleonsresultingin T
nuclear fragments and isolated neutrons and protons. For silicon, the stopping power of the low-

est incoming antiproton energy so far measured (0.188MeV) shows it to be 32% lower than for
9
protons[16].

Antimatter annihilation has been detected with silicon sensors previously [17], through the
detectionofpionsemittedintheannihilationprocess. ThesepionsareMinimumIonizingParticles P
(MIPs)depositing∼0.3keV/µm[18]inmatter,anegligiblefractioncomparedwiththeiraverage 0
momentumof∼350MeV/c[19].
6
However,inourpresentapplication,forthefirsttimetheantiprotonannihilateswithanucleon
0
in the bulk of the detector itself. When the annihilation takes place on-sensor, the largest fraction
of deposited energy is due to the heavy fragments. These fragments are Highly Ionizing Particles 2
(orHIPs). Energydepositsandrangesinsiliconfordifferentannihilationproductssimulatedusing 0
theSRIM[20]packageareshowninfigure3and4. HIPs(slowprotonsandheavierions)deposit
locally(withinafewortensofµmfromtheinteractionpoint)alloftheirkineticenergy. Itbecomes
thus evident that being able to discriminate between the signal produced by HIPs or MIPs in the
detectorcanhelpincreasingsignificantlytheresolutionontheannihilationposition.
2.2 MonteCarlosimulations
In the present work we compare data with Monte Carlo simulations, using GEANT4, release
4.9.5.p01, interfaced with VMC (Virtual Monte Carlo) software, release v2-13c [21]. Two par-
ticularGEANT4modelswerestudied,CHIPS(QGSPBERT)andFTFP(FTFPBERTTRV).
The CHIPS (CHiral Invariant Phase Space) model [22] is a 3D quark-level event genera-
tor for the fragmentation of excited hadronic systems into individual hadrons, whereas the FTFP
–3–
m] m]
M. dE/dx [keV/µ103 1233HHHHe 468LHHHieee pping Range [µ110034
E. 102 Sto102 12HH
3H
10 3He
10 4He
6He
1 8He
Li
10 20 40 60 80 100 10-10 20 40 60 80 100
Kinetic Energy [MeV] Kinetic Energy [MeV] 2
Figure 3. Energy deposition in silicon for different Figure4. Stoppingrangeinsiliconfordifferentnu- 0
nuclearfragmentsthatcanbegeneratedinanannihi- clearfragmentsthatcanbegeneratedinanannihila-
1
lationevent,calculatedwiththeSRIMpackage[20]. tionevent,calculatedwiththeSRIMpackage.
4

model[23]reliesonastringmodeltodescribetheinteractionsbetweenquarks.
J
The CHIPS and FTFP models differ in the production rate and in the composition of the
annihilation products. CHIPS produces heavy nuclear fragments in only 20% of the events while I
FTFPgeneratesheavyfragmentsinallofthem. Inaddition,CHIPSproducesmorethanthreetimes N
asmanyprotons,neutronsandalphaparticlesineachcollision,asseeninfigure5,whichprovides
S
themultiplicitiesforthedifferentproductsforannihilationsatrest.
T
Bothmodelscansimulateannihilationofantiprotonswithnuclei,thoughcomparisonofsim-

ulations to data for low-energy antiprotons in silicon is missing. CHIPS simulations have been
previously compared with uranium and carbon data, while the newer FTFP still lacks comparison 9
todataforantiprotonenergiesbelow120MeV[24].
Table 1 shows a comparison of experimental values obtained for 12C and 40Ca, the two el-
P
ements closest to silicon, with LEAR [25], and the simulated values for the same elements and
0
silicon. However, the values presented are for higher energies (> 6MeV) than in this study. The
6
table shows that for the kinetic energy range of 6-18MeV, FTFP describes the data obtained for
protonsbetterthanCHIPS.Ontheotherhand,CHIPSdescribesbettertheexperimentalvaluesfor 0
ionspecieswithhigheratomicnumbersandforhigherenergies. 2
0
2.3 Detectorrequirementsanddesign
As already shown in figure 2, the AEg¯IS silicon position sensitive detector will act as a separa-
tionmembranebetweentheultra-highvacuumoftheantihydrogenformationandtransportregion
and the secondary vacuum where the emulsion planes will be positioned. The resulting design
includes an array of co-planar single-sided silicon strip sensors, built with a strip pitch of 25 µm
and mounted on a silicon mechanical support wafer, hosting the readout electronics. This system
will provide the one-dimensional vertical (y) deflection information, though an approach based
on resistive strips, able to provide the x coordinate as well, as demonstrated in [26], is currently
understudy.
A further requirement of the silicon detector will be a thickness, in the active regions, of
50µm. Thiswillallowtominimizethescatteringofannihilationproducts,detectedfurtherdown-
–4–
Table1. Measuredandsimulatedproductionyields(for100annihilations)forthemostimportantnuclear
fragmentsproducedinannihilationofantiprotonswithhighAnuclei. ExperimentaldataisfromLEAR[25]
for12Cand40Ca,thetwoelementsclosesttosilicon. Energyreferstothekineticenergyoftheannihilation
products. These measured values are compared with the simulated values for calcium, carbon and silicon
using the two GEANT4 models, CHIPS and FTFP. FTFP describes the data obtained with protons better
thanCHIPS,whileCHIPSseemstobeabetterdescriptionforionspecieswithhigheratomicnumbersand
higherenergies.
28PSi ±.08 ±.03 ±.01 ±.004 0 2
FTF .586 .122 .15 .016 0
28SSi ±.10 ±.04 ±.02 ±.005 ±.01 1
CHIP .1700 .159 .30 .023 .18 4

0Ca .08 .03 .01 .004 J
4FTFP .±602 .±120 .±10 .±013 0 I
N
CHIPS40Ca .±.172010 .±.14904 .±.2702 .±.022005 .±.1901 ST

AR Ca ±.41 ±.11 ±.04 ±.017 ±.016 9
LE40 .742 .181 .57 .222 .218
12PC ±.08 ±.03 ±.01 ±.003 0 P0
FTF .560 .121 .13 .011 6
12SC ±.10 ±.04 ±.02 ±.001 ±.01 0
CHIP .1680 .159 .28 .019 .18 20
12C .20 .08 .04 .017 .012
R ± ± ± ± ±
LEA .233 .93 .45 .172 .114
Energy (MeV) 6-18 8-24 11-29 36-70 36-70
e
p d t H α
3
–5–
city 8 Chips
ultipli 7 FTFP
M
6
5
4
3
2
1
0 p+ p- p0 K h, h'l p n d Atnnih3Hilaetioan prhoedauvcygt isons 2
0
Figure5. Multiplicityofdifferentannihilationproducts(perannihilation)aspredictedbythetwomodels
1
CHIPSandFTFP,overthewholekineticenergyspectrum.
4

streambytheemulsiondetector,allowingforaprecisevertexreconstruction. Toachievethegoal, J
thick support ribs will guarantee the mechanical stability of the system, with size and position of
I
theribsbeingoptimizedtoallowforthemaximumefficiencyofthedetectorinareaswhereahigher
N
beamluminosityisexpected.
S
Finally,inordertoavoidtheblackbodyradiationcomingfromthedetectorincreasingthean-
tiproton plasma temperature (which would increase the thermal velocity of the antihydrogen), the T
wholedetectorsystemwillbekeptatcryogenictemperatures(77Korlower). Thiswillrequirethe
electronicstobedesignedforsuchconditions. ThefeasibilityofoperationofstandardCMOSread-
9
outASICsincryogenictemperatureshasalreadybeenprovenin[27]. TheASICdesignforAEg¯IS,

underdevelopment,willrelyonanimprovedintegrationandcommunicationprotocol(enablingthe
P
readoutof∼3000strips)andawiderdynamicrange,tocopewiththehighenergydepositedinthe
sensorfromtheannihilationevents. 0
Given the complex nature of the annihilation process, Monte Carlo simulations will be re- 6
quired to validate reconstruction algorithms to be implemented in the final system. Part of the
0
aim of the present work is the validation of the available simulation physics model, in the partic-
2
ular case of direct annihilation in a silicon sensor, with data available for the first time for low
0
antiprotonenergies.
3 Testbeamsetup
3.1 Antiprotonsourceandtestfacility
TheAEg¯ISexperimentissituatedattheAntiprotonDecelerator(AD)whichdelivers∼3×107low
energy (5.3MeV) and bunched (∼120 ns) antiprotons every ∼100 s. During tests in May 2012
thefirstsectionoftheAEg¯ISexperimentwasinplace,comprisinga5Tsuperconductingsolenoid
magnetenclosingaPenningtrapinanultrahighvacuum(UHV)of10−11 mbar.
While passing through the AEg¯IS apparatus, the antiprotons lose energy first through two
aluminum degraders, one fixed (18±2.7 µm) and one mobile (0.8±0.2, 2±0.5, 3±0.75, 4±1 and
–6–
to DAQ
Al degrader (150 um ) Ti foil (2 um)
Si beam counter (55 um)
Al foil (0-5 um)
5T Solenoid Magnet Mimotera
Al foil (18 um)
Mimotera
UHV (~10-9 mbar) 43 cm 2 cm
T r ap region 4 cm 2 cm

Antiprotons Fringe Field
4 cm
(5.3 MeV) 28.4 61.1 0.5 cm 107 cm Outer Vacuum
(~10-6 mbar)
Outer Vacuum (~10-6 mbar)
102 cm
Main apparatus (1.7 m) Six-cross chamber (27 cm)
2
0
Figure 6. Top view (left) and axial view (right) of the test set-up. The center of the silicon detector (MI-
MOTERA)isinstalled40mmoffaxisand430mmfromthemainapparatustoavoidsaturationduetothe 1
highbeamintensity.
4

5±1.25µm),thenasiliconbeamcounter(55±5.5µm)[29]andanotherfixedaluminumdegrader J
(150±15 µm)asshowninfigure6. Afterthis,lessthan1%oftheincomingantiprotonsfromthe
I
ADaretrappedinflightbythePenningtrap,whiletherestcontinuedownstream.
N
Before entering a six-cross vacuum chamber, where the detector was mounted (figure 6) the
S
antiproton beam traversed a 2 µm thick titanium foil used to separate the UHV region from the
secondary vacuum (∼ 10−7 mbar). In the six-way cross the antiprotons were deviated by the T
solenoid fringe field before hitting the silicon detector, which was mounted perpendicular to the
beamand40mmoffaxis(figure6,and7). 9
To overcome the unavoidable small inaccuracies in the stopping power calculation through
thedegraders’totalthickness,thesimulation(seesection2.2)wasindependentlytunedagainstthe
P
antiprotontrappingefficiencyduringthetestsoftheantiprotoncapturetrap. Thesimulatedtrapping
0
efficiency with 229 µm of degrading material was equivalent to the real efficiency obtained with
6
225µmofdegraders. Nevertheless,theeffectofboth225and229µmsiliconequivalentdegrading
materialthicknessesweresimulatedandcomparedwithdatapresentedhereforcompleteness. 0
Figure8showsthekineticenergydistributionoftheantiprotonsjustbeforereachingtheMI- 2
MOTERA detector as simulated with GEANT4. The average kinetic energy according to simula-
0
tionswas∼250keVfor225 µmmaterialand∼100keVfor229 µm. Thisenergyishigherthan
theenergyoftheantihydrogeninthefinalsystem(meV),butmuchlowerthananyenergytestedto
date. Thesamesimulation showsthat∼60%of theantiprotonscomingfrom theADreachedthe
six-waycrosschamber. Thecorrespondingdistributionofannihilationdepthsisshowninfigure9.
FromtheGEANT4simulations(seesection2.2)wecouldalsoestimatethespatialdistribution
of the antiproton beam. The resulting incident angle of antiprotons on the MIMOTERA was of
4.5±1.1◦ withrespecttothenormaltothedetectorplane.
In order to study the absorption effect on antiprotons and to verify them against the simula-
tions,wecovered2/3ofthedetectorsurfacewiththreeverythinaluminumfoils(3,6and9 µm).
The foils were suspended parallel to the detector surface at a distance ∼5 mm by means of three
thincopperwireswithagaugeof300µm,alsorunningonthepartnotcoveredbythefoils.
–7–