n34-1: annihilation of low energy antiprotons in silicon...
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Annihilation of low energy antiprotons
in silicon sensorsA. Gligorova, Member, IEEE, S. Aghion, O. Ahlen, A. S. Belov, G. Bonomi, P. Braunig, J. Bremer,
R. S. Brusa, G. Burghart, L. Cabaret, M. Caccia, C. Canali, R. Caravita, F. Castelli, G. Cerchiari,
S. Cialdi, D. Comparat, G. Consolati, J. H. Derking, C. Da Via, S. Di Domizio, L. Di Noto, M. Doser,
A. Dudarev, R. Ferragut, A. Fontana, P. Genova, M. Giammarchi, S. N. Gninenko, S. Haider, T. Huse,
E. Jordan, L. V. Jørgensen, T. Kaltenbacher, A. Kellerbauer, A. Knecht, D. Krasnicky, V. Lagomarsino,
S. Lehner, A. Magnani, C. Malbrunot, S. Mariazzi, V. A. Matveev, F. Moia, C. Nellist, G. Nebbia,
P. Nedelec, M. Oberthaler, N. Pacifico, V. Petracek, F. Prelz, M. Prevedelli, C. Regenfus, C. Riccardi,
O. Røhne, A. Rotondi, H. Sandaker, M. A. Subieta Vasquez, M. Spacek, G. Testera, E. Widmann,
P. Yzombard, S. Zavatarelli, J. Zmeskal
Manuscript received November 22, 2013.This work was supported by the Research Council of Norway and the
Bergen Research Foundation.A. Gligorova, is with the University of Bergen, Institute of Physics
and Technology, Allegaten 55, 5007 Bergen, Norway (e-mail: an-gela.gligorova@cern.ch).
S. Aghion and F. Moia are with Politecnico di Milano, Piazza Leonardoda Vinci 32, 20133 Milano, Italy and Istituto Nazionale di Fisica Nucleare,Sez. di Milano, Via Celoria 16, 20133 Milano, Italy.
O. Ahlen, J. Bremer, G. Burghart, J. H. Derking, M. Doser, A. Dudarev,S. Haider, L. V. Jørgensen, T. Kaltenbacher and A. Knecht are with theEuropean Organisation for Nuclear Research, Physics Dept., 1211 Geneva 23,Switzerland.
A. S. Belov and S. N. Gninenko are with the Institute for NuclearResearch of the Russian Academy of Sciences, Moscow 117312, Russia.
G. Bonomi and M. A. Subieta Vasquez are with the University ofBrescia, Dept. of Mechanical and Industrial Engineering, Via Branze 38,25133 Brescia, Italy and Istituto Nazionale di Fisica Nucleare, Sez. di Pavia,Via Agostino Bassi 6, 27100 Pavia, Italy.
P. Braunig and M. Oberthaler are with the University of Heidelberg,Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, 69120 Heidelberg,Germany.
R. S. Brusa and L. Di Noto are with the University of Trento, Departmentof Physics and TIPFA, Via Sommarive 14, 38123 Povo, Trento, Italy.
L. Cabaret, D. Comparat and P. Yzombard are with Laboratoire AimeCotton, CNRS, Universite Paris Sud, ENS Cachan, Batiment 505, Campusd’Orsay, 91405 Orsay Cedex, France.
M. Caccia is with the Insubria University, Como-Varese, Italy.C. Canali and C. Regenfus are with the University of Zurich, Physics
Institute, Winterthurerstrasse 190, 8057 Zurich, Switzerland.R. Caravita, F. Castelli and S. Cialdi are with the University of Milano,
Dept. of Physics and Istituto Nazionale di Fisica Nucleare, Sez. di Milano,Via Celoria 16, 20133 Milano, Italy.
G. Consolati and R. Ferragut are with Politecnico di Milano, PiazzaLeonardo da Vinci 32, 20133 Milano, Italy and Istituto Nazionale di FisicaNucleare, Sez. di Milano, Via Celoria 16, 20133 Milano, Italy.
C. Da Via and C. Nellist are with the University of Manchester, OxfordRoad, Manchester M13 9PL, United Kingdom.
S. Di Domizio, D. Krasnicky, G. Testera and S. Zavatarelli are withIstituto Nazionale di Fisica Nucleare, Sez. di Genova, Via Dodecaneso 33,16146 Genova, Italy.
A. Fontana and P. Genova are with Istituto Nazionale di Fisica Nucleare,Sez. di Pavia, Via Agostino Bassi 6, 27100 Pavia, Italy.
M. Giammarchi and F. Prelz are with Istituto Nazionale di Fisica Nucleare,Sez. di Milano, Via Celoria 16, 20133 Milano, Italy.
T. Huse and O. Røhne are with the University of Oslo, Dept. of Physics,Sem Sælands vei 24, 0371 Oslo, Norway.
E. Jordan, A. Kellerbauer and G. Cerchiari are with Max Planck Institutefor Nuclear Physics, Saupfercheckweg 1, 69117 Heidelberg, Germany.
V. Lagomarsino is with the University of Genoa, Dept. of Physics, ViaDodecaneso 33, 16146 Genova, Italy.
A. Magnani, C. Riccardi and A. Rotondi are with the University of Pavia,Dept. of Nuclear and Theoretical Physics, Via Bassi 6, 27100 Pavia, Italy andIstituto Nazionale di Fisica Nucleare, Sez. di Pavia, Via Agostino Bassi 6,27100 Pavia, Italy.
Abstract—The aim of the AEgIS experiment is to measurethe gravitational acceleration for anti-hydrogen in the Earth’sgravitational field, thus testing the Weak Equivalence Principle,which states that all bodies fall with the same accelerationindependent of their mass and composition. AEgIS will makeuse of a gravity module which includes a silicon detector, inorder to measure the deflection of anti-hydrogen from a straightpath due to the Earth’s gravitational field, by detecting theannihilation position on its surface. A position resolution betterthan 10 µm is required to determine the gravitational accelerationwith a precision better than 10%. The work presented here ispart of a study of different silicon sensor technologies to realisea silicon anti-hydrogen detector for the AEgIS experiment atCERN. We here focus on the study of a 3D pixel sensor withFE-I4 readout, originally designed for the ATLAS detector at theLHC, and compare it to a previous monolithic planar detectorstudied, the MIMOTERA. The direct annihilation of low energyanti-protons (∼ 100 keV) takes place in the first layers and weshow that the charged annihilation products (pions and nuclearfragments) can be detected by such a sensor. The present studyaims at understanding the signature of an annihilation eventin a 3D silicon sensor, in order to assess the accuracy thatcan be achieved by such a sensor in the reconstruction of theposition of annihilation, when the same happens directly onthe detector surface. We also present a comparison betweenexperimental data and GEANT4 simulations and previous dataobtained with a silicon imaging detector. These results are beingused to determine the geometrical and process parameters to beadopted by the silicon annihilation detector to be installed inAEgIS.
C. Malbrunot is with European Organisation for Nuclear Research, PhysicsDept., 1211 Geneva 23, Switzerland and Stefan-Meyer-Institut fr subatomarePhysik, Boltzmanngasse 3, 1090 Vienna, Austria.
S. Mariazzi, S. Lehner, E. Widmann and J. Zmeskal are with Stefan-Meyer-Institut fr subatomare Physik, Boltzmanngasse 3, 1090 Vienna, Austria.
V. A. Matveev is with the Institute for Nuclear Research of the RussianAcademy of Sciences, Moscow 117312, Russia and Joint Institute for NuclearResearch, 141980 Dubna, Russia.
G. Nebbia is with Istituto Nazionale di Fisica Nucleare, Sez. di Padova,Via Marzolo 8, 35131 Padova, Italy.
P. Nedelec is with Claude Bernard University Lyon 1, Institut de PhysiqueNucleaire de Lyon, 4 Rue Enrico Fermi, 69622 Villeurbanne, France.
N. Pacifico and H. Sandaker are with the University of Bergen, Instituteof Physics and Technology, Allegaten 55, 5007 Bergen, Norway.
V. Petracek and M. Spacek are with Czech Technical University in Prague,FNSPE, Brehova 7, 11519 Praha 1, Czech Republic.
M. Prevedelli is with the University of Bologna, Dept. of Physics, ViaIrnerio 46, 40126 Bologna, Italy.
978-1-4799-0534-8/13/$31.00 ©2013 IEEE
Fig. 1. Principle of the AEgIS antihydrogen beam formation [1].
I. INTRODUCTION
A. Overview of the AEgIS experiment
THE main goal of the AEgIS experiment [1] is a first direct
measurement of the Earth’s gravitational acceleration
for antimatter, with the simplest form of electrically neutral
antimatter - antihydrogen. This would test both the Weak
Equivalence Principle and the prediction of General Relativity
that matter and antimatter should behave identically in the
gravitational field of the Earth. The experiment is built at the
Antiproton Decelerator (AD) at CERN, and during its first
phase a measurement of the gravitational acceleration with
1% precision will be attempted.
B. Principle of formation and detection of antihydrogen
Antihydrogen atoms will be formed by a resonant charge
exchange reaction between Rydberg (n∼20-30) positronium
and cold (100 mK) antiprotons (see fig. 1). The antihydrogen
states will be thus be defined by the positronium states.
The positrons will be supplied from a 22Na source, and
guided towards a slab of nanoporous silica which acts as a
positronium converter [2]. The antiprotons will be first trapped
and cooled down in the catching traps in the 5 T magnet and
the resonant charge exchange will take place in the mixing
trap in the 1 T magnet (fig. 2). A beam of antihydrogen atoms
will be formed by using inhomogeneous electric field, e.g
Stark acceleration [3]. Some of the antihydrogen paths will
be selected through the two gratings of a moire deflectometer
[4], as shown in fig. 3. This effect is purely classical. After
about 1 m of free fall, the rest of the antihydrogen atoms will
be detected with a position sensitive detector, consisting of a
thin (50 µm) silicon strip detector followed by an emulsion
detector and a scintillating fiber telescope. The vertical shift
of the fringe pattern formed by the moire deflectometer is
proportional to the value of g experienced by antihydrogen
(fig. 3). If the distance between the gratings and the detector
is 0.5 m, the vertical shift is expected to be ∼ 20 µm.
Em
uls
ion
Readout ASICs
77 (or 4) K vessel
Strip detector (25 um pitch, 50 um, 300 um support ribs)
moire deflectometer
~0.5 m
Low
tem
pe
ratu
re a
ntiH
ydro
gen (
4 (
100m
) K
)
Fig. 3. Scheme of the moire deflectometer and the hybrid position sensitivedetector [6]. Antihydrogen atoms enter from the left side and get reducedin vertical components from the moire deflectometer and annihilated onthe silicon detector. The annihilation products are detected by the emulsiondetector.
II. TEST BEAM WITH ANTIPROTONS
A. First measurements with monolithic planar sensor
Understanding the process of an antiproton annihilation in
silicon is essential for the design and construction of the silicon
detector where the antihydrogen annihilations will take place.
The main motivation of this study was to test the concept
of using a silicon detector as a position sensitive annihilation
detector. This would be the first step towards the development
of an ad-hoc silicon annihilation detector with a resolution
better than 10 µm. This kind of on-sensor annihilations of
antiprotons has been observed only once, in an experiment
which has made use of antiprotons with a momentum of 608
MeV/c and a non-segmented sensor [5]. In our case, during
the two test periods (test−beams), the AD supplied the initial
antiproton beam with a momentum of 100 MeV/c (kinetic
energy of 5.3 MeV). These particles were further slowed down
with several degraders, reaching the energy value of few 100
keV according to simulations [6], just before impinging on
the detector. The first detector that we tested in June 2012
was a monolithic active pixel sensor, the MIMOTERA [7].
The MIMOTERA was installed in a six way cross vacuum
chamber (∼ 10−6 mbar, at room temperature) which was
attached at the end of the main AEgIS apparatus and detecting
a fraction of the antiprotons not trapped by the apparatus. A
photo of this detector mounted on a PCB is shown in fig. 4.
Its total size is 2x2 cm2 and the thickness of the sensitive
volume is 14 µm, glued on a silicon mechanical support with
a total thickness of ∼ 600 µm. The MIMOTERA is back-
illuminated through an entrance window of ∼ 100 nm (SiO2
passivation layer). The maximum readout rate is configurable
up to 20 MHz. The readout system of the detector is divided
into four sub-arrays of 28 x 112 pixels, which are readout in
parallel. Each of the pixels consists of two independent readout
matrices, each of them built by 2 x 81 interconnected diodes,
5 x 5 µm2 each. This double-readout architecture allows the
MIMOTERA to operate without dead-time. The detector was
mounted ∼ 4 cm off axis (the same position adopted for
Fig. 2. Schematic view of the AEgIS apparatus. It consists of two magnets, 5 T and 1 T, which enclose the antiproton trap and the mixing trap respectively.The positrons are guided towards the main apparatus by their own transfer line. The gravity module is attached at the end of the 1 T magnet.
the 3D sensor, described later on) in order to have a lower
luminosity detecting only the antiprotons slightly deviated by
a fringe field from the 1 T magnet used for the antihydrogen
production. The simulated kinetic energy distribution of the
antiprotons implies that most of the antiprotons annihilate
within the first few microns of the detector. A schematic
view of the cross−section of this detector is given in fig.
5 showing an antiproton annihilation and the annihilation
products penetrating the active sensor volume.
Fig. 4. The MIMOTERA detector mounted on its PCB. The dimensions ofthe sensor are 2x2 cm2.
SiO2 layer (100 nm)
EPI silicon active
bulk (14 um)
A B A B A BA B readout matrices
Carrier substrate (600 um)
...
Antiprotons
protons
nucl.
fragments
pions
Fig. 5. Schematic view of the MIMOTERA detector, showing an annihilationevent occurring in the first layers of the detector and annihilation prongstravelling inside the sensitive volume.
B. Second measurements with 3D pixel sensor
While the small thickness and the relatively low granularity
of the MIMOTERA detector (153 µm pixel pitch) was im-
portant to determine the typical energy range of the clusters
and thus the needed dynamic range of the final detector, we
also wanted to study in more detail the tracks to understand
better the possible achievable resolution of the annihilation
point. For this reason we chose to test a thicker pixelated
detector because of a better tracking quality, and a 3D pixel
sensor (CNM 55)[8] was installed during the second test-beam
(fig. 6). The installation was done in the same way as for the
Fig. 6. The CNM 55 sensor bump bonded the FE-I4 R/O chip mountedon single chip card. The whole system is mounted on a flange before beinginstalled in the six way cross vacuum chamber.
MIMOTERA detector, providing as similar conditions with the
first test beam as possible.
This 3D pixel sensor and the readout ASIC [9] were origi-
nally designed for the Insertable B Layer [10] currently being
installed at the ATLAS [11] experiment at CERN, optimised
for looking at high energy particles from events occurring
every 25 ns. In spite of this the technology offered interesting
features relevant for our application described below.
The sensor consists of 80 (columns) x 336 (rows) = 26880
cells. The pixel size is 250x50 µm2 and the electrodes have a
diameter of 10 µm. The thickness of the active volume is 230
µm and its passivation layer is ∼ 3 µm thick and composed of
three layers of material: 1.5 µm Al, 0.8 µm doped polysilicon
and 1.150 µm thermal oxide, one on top of the other. A
schematic view of the architecture of the pixels is given in
fig. 7. The internal box drawn with a dotted line represents
a typical cell that consists of two readout n-columns around
six ohmic p-columns. The 3D design of the electrodes results
in a higher average electric field between the electrodes and a
shorter collection path, which is the reason for a larger signal
to noise ratio than the one for the planar pixel technology.
The USBpix hardware, used for the readout, is based on a
multipurpose IO-board (S3MultiIO) with a USB2.0 interface
to a PC and an adapter card which connects the S3MultiIO
to the Single Chip Adapter Card (SCC), where the FE and
the sensor are mounted. The S3MultiIO system contains a
programmable (Xilinx XC3S1000 FG320 4C) FPGA, which
provides and handles all signals going to the FE.
III. RESULTS
A. Monolithic planar sensor
The results of the MIMOTERA detector are fully reported
in [6]. What follows is a summary of the features of antiproton
Fig. 7. Schematic view of the 3D electrodes in CNM 55 pixel sensor. Thevolume within the dashed lines represents one pixel consisting of two readout(red) and six ohmic (blue) electrodes.
MIMOTERA WIDTH [pixel #]20 40 60 80 100
MIM
OT
ER
A H
EIG
HT
[p
ixe
l #]
20
40
60
80
100
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
Pix
el E
nerg
y [k
eV
]
no Al9 μm
6 μm
3 μm
Fig. 8. Sample of a triggered frame from the monolithic planar pixel detectorafter applying the noise cut. The dashed lines mark the areas that were coveredwith 3, 6 and 9 µm Al foil for studying the number of annihilations in eachof them [6]
tot
clu
s)/
Npix
(nclu
sN
-410
-310
-210
-110
1 FTFPChips
Data
Cluster Size [pixels]
2 4 6 8 10 12 14 16 18 20
DA
TA
/NS
IMN 0
2
4
Fig. 9. Cluster size (in number of pixels) distribution for clusters observedin the monolithic planar pixel detector and comparison with two GEANT4models. FTFP matches the data points better than CHIPS.
annihilations in a planar pixel detector. In these first tests we
successfully detected on-detector annihilations for the very
first time at such low antiproton energies. Some areas of the
detector were covered with 3, 6 and 9 µm Al foil for studying
the number of annihilations in each of them. The analogue
readout of the MIMOTERA detector required an off-line noise
cut. This cut was fixed to 150 keV, the value being extracted
as 5 RMS (30.3 keV) from the noise distribution of single
pixels. A sample frame (after the noise cut) is shown in fig.
tot
clu
s(E
)/1
Me
V/N
clu
s N
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
FTFPChips
Data
[MeV]totE
0 5 10 15 20 25
D2σ
+S2
σ)/
D-p
S(p
-15
0
15
Fig. 10. Cluster charge distribution for clusters observed in the monolithicplanar pixel detector and comparison with two GEANT4 models. Again, FTFPis closer to the data points, especially for energies > 5 MeV.
8.
For the cluster analysis, clusters were defined as conglom-
erates of pixels neighbouring in the horizontal, vertical and
diagonal direction, without using a seed-driven algorithm.
This would require assumptions on the geometrical profile
of the energy deposited, which is not possible due to the
heterogeneous nature of the clusters. In spite of the small
thickness, the relatively high statistics allowed observation
of a number of tracks from long-range annihilation products
travelling inside the thin detector plane. We found prongs
(most likely being protons) with lengths up to 2.9. mm.
The small ratio depth/pixel width resulted in small clusters
of mostly one pixel (∼ 70%), about 20 % of two pixels,
and the remaining part included three or more pixels. Part
of the one pixel cluster was due to secondary particles from
annihilations that happen elsewhere in apparatus. This noise
was simulated separately and also studied in the data [6]. The
cluster size (in number of pixels) is shown in fig. 9. The cluster
sizes ranged between 1 and 20 pixels. The total deposited
energy for each cluster was also measured and the results are
shown in fig. 10. This energy was measured to be up to 40
MeV. Charge saturation of pixels was observed in very few (<
10) annihilation events, due to the generation of slow charged
fragments characterized by a high dE/dx.
The cluster parameters (size and energy) from data analysis
were compared with two GEANT4 models: CHIPS (QGSP
BERT) [12] and FTFP (FTFP BERT TRV) [13], both suitable
for anti-proton at-rest-annihilation with nuclei, though none
of the models was to-date validated for annihilations at rest in
silicon. Pertinent validation was performed at rest for CHIPS
with Uranium and Carbon data, while the newer FTFP still
lacks validation for antiproton energies below 120 MeV [14].
The cluster energy distribution in fig. 10 shows an excess
of clusters with energy < 1 MeV in the data compared with
both simulations, which can be ascribed to a lower energy
deposition from the noise present in the data. The FTFP model
shows a better agreement, especially for energies > 5 MeV.
The cluster size distribution (fig. 9) shows that there is a
small systematic overestimation of the cluster size by CHIPS,
but the overall agreement is good with the two simulation
models adopted, with FTFP being the closer one to the data
points.
B. 3D sensor
The tests of the 3D sensor provided material for further
analysis of the antiproton annihilation in silicon, in particular
of tracks of the annihilation prongs. Given the same position-
ing of the two detectors in the vacuum chamber, the energy
distributions for the incoming antiprotons were similar. This
distribution is given in fig. 11. A sample acquisition frame is
shown in fig. 12. When compared to the typical frame of the
thin monolithic detector tested, more and longer tracks can
be observed, as a result of the thicker active volume. Tracks
are more likely due to the higher geometrical acceptance for
annihilation products travelling at angles different from the
direction of the beam.
Kinetic energy [MeV]0 0.1 0.2 0.3 0.4 0.5 0.6
tot
p)/
26
.7 k
eV
/Nkin
(Ep
N
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Fig. 11. Kinetic energy distribution for the incoming antiprotons impingingon the 3D sensor.
Cluster charge distribution is shown in fig. 13, the size
distribution in fig. 14. Large clusters are observed, even in
excess of 80 pixels, due to long tracks. Cluster energy is lower
than what was observed with the MIMOTERA detector. The
reason for this discrepancy is to be found in two parameters:
the thicker passivation region is more likely to stop heavy frag-
ments, that would produce high energy deposits. In addition
to this, saturation of the single channel amplifiers was often
observed: the measured charge is thus expected to be lower
than the charge effectively deposited in the bulk in some cases.
When travelling through the detector, annihilation prongs
produce tracks from a few mm to 1.5 cm long. The annihilation
point can be reconstructed by fitting the tracks and calculate
the errors on their interception point. The position resolution
we were able to achieve applying this procedure with the CNM
55 sensor is 56.5 µm on X (pixel size of 250 µm) and 24.3 µm
on Y (pixel size of 50 µm). This resolution could very likely
be improved by using weighted fitting algorithms accounting
the charge of the pixels composing the tracks. However a
sense-full application of such an algorithm would require no
saturated pixels composing the tracks. The results shown in
this paper are the first we obtained using this procedure and
Fig. 12. Frame with antiproton annihilations observed in 3D sensor. Longtracks with different energy deposits, according to dE/dx: pions blue tracks(0.1-0.3 keV/µm); fast protons, green to turquoise tracks (> 0.5 keV/µm).
Fig. 13. Cluster charge distribution for clusters observed in 3D pixel sensor.
our further work will be focused on a better tracking algorithm
for antiproton annihilations, improved clustering etc.
C. Conclusions
Two types of silicon pixel detectors with different electrodes
geometry were used for studying the antiproton annihilation in
silicon. The data taken with the monolithic planar sensor, due
to the small thickness and low granularity, were more suitable
for analyses on the deposited energy. On the other hand, the
3D pixel sensor provided data that allowed a more precise
resolution on the annihilation point and study of the various
tracks produced by the annihilation prongs. The main results
so far, as presented in [6] and in this paper for the 3D sensor,
are:
• First on−sensor detection of antiproton annihilations at
rest in segmented silicon detectors.
• Total energy deposition up to 10 MeV per antiproton
annihilation, value highly dependant on the saturation of
pixels (40 MeV in the case of MIMTOERA).
• Identification of tracks from annihilation prongs up to 1.5
cm long (2.9 mm in the case of MIMOTERA).
Fig. 14. Cluster size distribution for clusters observed in 3D pixel sensor.
Fig. 15. Sample hitmap of the 3D sensor with two fitted proton tracks comingfrom an antiproton annihilation. Such long tracks are used to reconstruct theannihilation point.
• Comparison and agreement to some extent with GEANT4
(CHIPS and FTFP) models for the size and the charge of
the clusters produced from the antiproton annihilation.
• Identification of the annhilation prongs, such as protons,
pions, apha particles, heavy ions etc.
• Position resolution of ∼ 20-30 µm on the annihilation
point.
ACKNOWLEDGMENT
We thank the Bergen Research Foundation and the Research
Council of Norway for their support to this project. We also
thank Ole Dorholt for the technical support and the ATLAS 3D
community asfor the putting their detector at our disposal and
their help with the testing. Finally we thank Andrea Micelli
for his support throughout the test-beam campaign with the
3D CNM-55 sensor.
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