CHERCAM: The Cherenkov imager of the CREAM experiment

Abstract

A Cherenkov imager, CHERCAM (CHERenkov CAMera), has been designed and built for the CREAM (Cosmic-Ray Energetic and Mass) balloon-borne experiment. The instrument will perform charge measurements of nuclear cosmic-ray over a range extending from proton to iron. It will achieve individual charge separation of the elements over this range [M. Buénerd, et al., in: 28th ICRC, Tsukuba, Japan, OG 1.5, 2003, p. 2157. [2]] (0.25 charge unit rms), allowing measurements of the energy spectra of individual elements by the CREAM instrument in the energy range from 1010 to . CHERCAM is a proximity focused imager, based on a dedicated mechanical structure, equipped with an n=1.05 silica aerogel radiator plane, separated by a 12 cm ring expansion gap from a photon detector plane consisting of a 1600 photomultiplier array, backed with dedicated front-end readout electronics. A prototype of the detector has been recently tested with 100 and 300 GeV/cZ=1 particle beams at CERN. The contribution reports on both the beam test results of the prototype, and of the counter performance in ground operation.

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CHERCAM: aCherenkovimager for the
CREAM experiment
M. Mangin-Brineta, A.Barraua, R. Bazer-BachibO. Bourriona,
J.Bouviera, B.Boyera, M. Bu´ enerda, L. Deromea, L. Erauda,
R. Foglioa, L. Gallin-Martela, O. Ganelc, C. H. Kimc, L. Lutzc,
A. Malininec, A. Menchaca-Rochad, J. N. P´ eri´ eb,
Y. Sallaz-Damaza, J.-P. Scordilisa, E. S. Seoc, P. Walpolec,
S.Y. Zinnc
aLaboratoire de Physique Subatomique et de Cosmologie, Grenoble, France
bCentre d’Etude Spatiale des Rayonnements, UFR PCA - CNRS - UPR 8002,
Toulouse, France
cUniversity of Maryland, College Park, MD USA
dUniversitad Nacional Aut´ onoma de Mexico, Mexico
Abstract
The CREAM experiment (Cosmic Ray Energetics and Mass) is dedicated to the
measurement of the energy spectrum of nuclear elements in cosmic rays, over the
range 1012to 1015eV. The individual elements separation, which is a key feature of
CREAM, requires instruments with strong identification capabilities. A proximity
focused type of Cherenkov imager, CHERCAM (CHERenkov CAMera), providing
both a good signature of downgoing Z=1 particles and good single element separation
through the whole range of nuclear charges [1], is under development. After a brief
introduction, the main features and the construction status of the CHERCAM are
being summarized.
Key words: Cosmic rays, Cherenkov detector, charge identification
PACS: 29.40.Ka, 96.40.-z, 96.40.De, 96.50.Pw
1 Introduction
The CREAM experiment addresses
open questions on the nature and
the origin of charged particles cosmic
rays. It has begun to measure high
energy cosmic rays at the frontier
of the statistical limits accessible to
space-based experiments. Measure-
ments of the cosmic ray spectrum of
nuclear elements from proton to iron
in the energy range ≈ 1012to 1015
eV, will provide invaluable data on
cosmic ray spectral characteristics
and/or abundance changes which
could be related to the acceleration
limit in the shock front of super-
Preprint submitted to Elsevier Science25 May 2006

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novae. They will also provide the first
measurements of secondary to pri-
mary ratios, like B/C, in this energy
domain. The individual nuclear ele-
ments separation will allow to probe
the current models of acceleration
mechanisms, and will provide hints
for the interpretation of the ”knee”
in the inclusive spectrum, and for
the physics of cosmic ray galactic
transport.
CREAM is planned to be the first ex-
periment to be flown on a new Ultra-
Long Duration Balloon (ULDB),
currently under development by the
NASA.
The experiment includes a set of sub-
detectors able to measure the parti-
cle energy and charge [2]. The energy
measurement is performed by means
of a hadronic calorimeter with nearly
constant resolution over the three
orders of magnitude covered. The
calorimeter can be combined with a
TRD in some flight configurations.
The particle charge measurement
will cover the range 1 < Z <≈ 30
with a charge resolution better than
0.3 charge unit expected. This charge
resolution will be achieved by means
of a combination of scintillation ho-
doscope, silicon pad counters, and
a Cherenkov imager CHERCAM,
optimized for charge measurements
and providing a constant resolution
through the range of nuclear charge.
2CHERCAM architecture
CHERCAM is a proximity focusing
imager derived from the solution de-
veloped for AMS experiment, whose
principle is illustrated in Figure 1.
Fig. 1. Scheme of the Cherenkov imager
principle.
The Cherenkov radiator consists in a
1 cm-thick silica aerogel plane, made
of 11x11 cm2tiles with a refractive
index around 1.07.
The Cherenkov radiator plane is sep-
arated by a 10 cm ring expansion
gap, from a photon detector plane
consisting of a 1600 photomultiplier
tubes array(Photonis
backed withdedicated front-endelec-
tronics, power supply, and read-out
electronics. The latter employs the
same 16-channel ASIC circuit than
the one used for the AMS Cherenkov
imager [3].
XP3112),
The mechanical structure of the de-
tector is illustrated in Figure 2. The
counter is divided into two frames for
mechanical reasons. The aerogel will
be fixed onto the top lid while the
lower frame will contains the photo-
multipliers array.
3 CHERCAM simulation
The development of a GEANT4 sim-
ulation of CHERCAM is in progress,
to investigate the detailed features
of the detector, validate the tech-
nical choices and study the limit of
the detector performance. The simu-
lation includes the photomultipliers
modeling and contains as physical
2

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Fig. 2. Exploded view of the CHER-
CAM mechanical structure. The detec-
tor plane is arranged in 5 × 5 blocks of
64 PMT. Each block is divided into 4
modules of 4 × 4 PMT, each being read
out by a single 16-channel FE electron-
ics circuit.
processes, the Cherenkov effect ,
the Rayleigh scattering and reflec-
tion. The photocathode quantum
efficiency is also taken into account
according to the data given by the
PMT manufacturer.
Fig. 3. Example of a simulated event
(top) in the Cherenkov counter (1 TeV
Helium) and photon impacts detected
(bottom)
Reconstruction codes are being de-
veloped, with a special interest in the
control of the geometrical efficiency
determination. The first results of
the simulation show that a combina-
tion of incident particle position re-
construction and ring overlap study
can provide a charge reconstruction
better than ∆Z = 0.3 over the full
range of charges concerned (see fig-
ure 4).
Fig. 4. Charge reconstruction for 11000
simulated events uniformly distributed
in charge.
4Conclusion
The development of the CHERCAM
subdetector for the CREAM exper-
iment is ongoing. The construction
is in progress and two third of the
counter are expected to be ready for
abeamtest atCERN SPSin October
2006. CHERCAM will be integrated
in the CREAM detector at the begin-
ning of next year and participate to
the 2007 flight campaign.
Part of this program has been con-
ducted under theINTAS contract 03-
52-5579.
References
[1] M. Bu´ enerd et al., 28th ICRC,
Tsukuba, OG 1.5, p 2157 (2003)
[2] E. S. Seo et al., Adv. in Space
Research, Vol. 33, Issue 10, 1777
(2004)
[3] L. Gallin-Martel
A504,2783 (2003)
etal.,NIM
3