Lechanoine-Leluc, C

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29th International Cosmic Ray Conference Pune (2005) 3, 381384AMS - A magnetic spectrometer on the International Space StationC. Lechanoine-Leluc on behalf of the AMS-02 collaborationUniversity of Geneva, D.P.N.C., 24 Quai E.Ansermet, 1211 Geneva, SwitzerlandPresenter: C. Lechanoine-Leluc (catherine.leluc@physics.unige.ch), swi-lechanoine-leluc-C-abs1-og15-oralThe Alpha magnetic Spectrometer (AMS) is a particle physics detector designed to measure charged cosmicrays spectra up to the TV region, with high energy photon detection capability up to few hundred GeV. Withits large acceptance, long duration (at least 3 years) and state of the art particle identification techniques, AMSwill provide the most sensitive search for the existence of anti-matter nuclei and for the origin of dark matter.The detector is being constructed with an eight layers Silicon Tracker inside a large superconducting magnet,providing a   0.8 Tmbending power and an acceptance of   0.5m. A Transition Radiation detector and a 3DElectromagnetic Calorimeter allow for electron, positron and photon identification, while independent velocitymeasurements are performed by a Time of Flight scintillating system and a Ring Image Cerenkov detector.The overall construction is due to be completed by 2006.1. IntroductionThe Alpha Magnetic Spectrometer (AMS) is a high energy particle physics experiment in space to be installedon the International Space Station (ISS) in 2008 for at least three years of operation. It is a large acceptance(   m.sr) superconducting magnetic spectrometer able to investigate the composition of cosmic rays withhigh statistics up to the TV region [1] and provide the most sensitive search for the existence of anti matternuclei and for the origin of dark matter [2]. It will also measure high energy photons up to few hundredGeV [3].A ’scaled-down’ version, AMS-01, was built and successfully flown on the shuttle Discovery (STS-91) for 10days in June 1998 at a mean altitude of 370 km. The heart of the AMS-01 detector was a magnetic spectrom-eter, using a 0.15T permanent dipole magnet in magic ring configuration and a partially equipped six planesilicon tracking detector. The set-up was completed by a two double layer scintillator time-of-flight system, anAerogel threshold Cherenkov counter and veto counters located on the magnet inner wall. The detector thusmeasured the charge  , rigidity 	   , velocity  and specific energy loss ﬀ of particles travers-ing its aperture. The main goal of the 10 days STS-91 flight in June 1998 was docking to and resupplying theMIR station. For AMS, it was a mission to test the equipment under stringent conditions during the launch andin the space environment, and to familiarize the AMS team with operations in space. After MIR undocking,AMS had control of the shuttle orientation for ninety hours. Data were taken with variable attitudes, at altitudesbetween 320 and 390 km above sea level. The space shuttle’s orbit covered latitudes between ﬁ ﬂﬃ "! and alllongitudes. A total of ﬂ#ﬀ$ events were registered. Results have so far been obtained on rates and spectra ofelectrons and positrons protons antiprotons and deuterons helium the search for antihelium nucleias well asheavy anti-ions [4].Due to the environmental conditions in space, AMS has to fulfill various special requirements concerningweight (limited to 7 tons), thermal vacuum, vibration, cosmic radiation, electromagnetic interference, powerconsumption (limited to 3kW) and data communication.382 C. Lechanoine-Leluc2. The AMS-02 detectorFollowing the experience gathered during the AMS-01 mission, a more ambitious detector with extendedrigidity range from a few GV to the TV region was designed for installation on the ISS. This detector, AMS-02, is based on a spectrometer with a superconducting magnet. Its main components are shown in Figure 1.In addition to providing an extended rigidity range for charged particles, it also implements more performantparticle identification, additional redundancy for the measurement of charge and velocity and less trigger biasagainst heavy elements.Figure 1. Full view of the AMS Spectrometer.The superconducting magnet [5] consists of two helmholtz coils and two series of six smaller racetrack coilscircumferentially distributed, cooled by superfluid helium at 1.8 K. This configuration was chosen for threereasons: to increase the overall dipole field, to reduce the stray field and to suppress the torque resulting fromthe interaction the magnetic moment of the magnet with the Earth magnetic field. The magnetic field reaches0.8T close to the center, six times the field strength of the AMS-01 permanent magnet. It is thermally connectedto a 2500 l vessel of liquid helium for about 3 years of operation without refill.Trajectories of charged particles are localised by double sided silicon sensors [6], arranged in eight layers ofabout one squared meter each on five planes of an ultra-light support structure. Close to 2500 sensors providea tracking resolution of   10 % m in the bending direction,   30 % m orthogonal to it. Together with the improvedmagnet ( &('=0.9Tm), particle rigidity is measured with an accuracy better than 2 ) up to 20 GV and themaximal detectable rigidity is around 1 TV. The measurement of specific energy loss, ﬀ*ﬃ     , in thesilicon serves to identify nuclei. The tracker also measures the direction and energy of photons, converted inthe material above the first tracker layer, with excellent directional resolution and good energy resolution.Separation between light and heavy particles is provided by the Transition Radiation Detector (TRD) [7] ontop of the set-up, which measures the radiation emitted by particle upon changes in the index of refraction ofthe traversed medium. The detector features 20 layers of a fleece radiator and proportional straw tubes with aAMS - A magnetic spectrometer... 383Xe/CO  gas mixture for X-ray detection. Since the intensity of emitted transition radiation (TR) is proportionalto the Lorentz factor + , light particles such as positrons have a much higher probability of emitting TR thanheavy particles such as protons. It will separate hadrons from electrons in the range 10 to 300 GeV. It will alsohelp in the tracking of charged particles, identify photon conversion points and measure *ﬃ .As in AMS-01, two crossed double layers of scintillators placed at the magnet end-caps provide time-of-flight(TOF) [8] and thus measure particle velocity. Due to the higher magnetic stray field of the AMS-02 super-conducting magnet, the phototubes have to be aligned more carefully with the local magnetic field. A timingresolution for protons of   180 ps is nevertheless expected and 100 ps for light ions. In addition to measuringthe velocity of particles and their direction, the time-of-flight system provides a redundant measurement of*ﬃ with about 10% resolution for minimum ionising particles. It is also the principal trigger detector forcharged particles.Below the spectrometer, a Ring Imaging Cerenkov detector (RICH) [9] provides further velocity and chargemeasurements. Light emitted in the combined Aerogel ( ,.- ﬂﬃ ﬀ ) and sodium-fluoride ( ,.- ﬂﬀ /ﬀ/*0 ) radiatorson the top is detected as a close to an elliptical pattern of photons by an array of position sensitive photomultipliers on the bottom, occasionally after reflection off a conical mirror. The emission angle measuresvelocity, with an expected resolution of 123254 ﬂ#768 . The number of photo-electrons measures the chargewith a resolution better than 20 ) for low  ions, thus providing an independent measurement of the charge upto iron.At the very bottom, an Electromagnetic Calorimeter (ECAL) [10] made of 640 kg of lead scintillating fibersandwich completes the set-up. The calorimeter reconstructs the shower with three dimensional sampling usingcrossed layers. Its energy resolution is expected to be 1*94 ﬂ7:ﬂ );=< ?> GeV @BA?C  D ) for electromagneticshowers. It distinguishes hadronic from electromagnetic showers by their lateral and longitudinal shape, thussuppressing protons by a factor of ﬂ76E up to several hundred GeV. The calorimeter will also serve as anindependent photon detector, with an angular resolution of about  9ﬂﬃ! .Particle identification on AMS-02 relies on a very precise determination of the magnetic rigidity, energy, ve-locity and electric charge. Velocity of low energy particles (up to  ﬂﬃ GeV) is measured by the TOF detectorwhile for kinetic energy above the radiator thresholds (0.5 GeV/n for sodium fluoride and 2.1 GeV/n for aero-gel) the RICH will provide very accurate measurements; a target resolution of ﬂ ) and  FGﬂ ) for singlecharged particles is expected, respectively for sodium fluoride and aerogel radiators. The absolute value ofthe electric charge is measured by the silicon tracker and TOF detectors through ﬀ*ﬃ samplings and by theRICH through Cherenkov signal integration. Ion identification at least up to iron is expected. The new detec-tor thus features sophisticated particle identification devices that will allow to discriminate the small amountof antiprotons and other antinuclei against the large background of electrons, as well as positrons against theoverwhelming background of protons, up to high energies. It will also have two ways of measuring high energyphotons, by conversion in the tracker and in the electromagnetic calorimeter, which will allow systematic longterm observations of the gamma ray sky.3. ConclusionsAMS-02 is a complete and sophisticated detector which should be completed in 2006. Due to its large ge-ometrical acceptance, large momentum range and dynamics, its particle identification capability and its longduration measurements, it will collect close to three orders of magnitude more statistics than in AMS-01 undermuch better instrumental conditions. It will contribute to the observation of cosmics rays and photons in spacein an energy domain and time scale complementary to other projects. A rich and diversified physics programwill result from its at least three years of data taking on the ISS.384 C. Lechanoine-Leluc4. AcknowledgementsThe construction of AMS-02 is an undertaking of many individuals and organizations. The support of NASAand the U.S. Dept. of Energy has been vital in the inception, development and fabrication of the experiment.The interest and support of NASA, the Federal Agency for Atomic Energy, Russia, the Ministry of Scienceand Technology, China, and the European Space Agency is gratefully acknowledged. The dedication of Dr.Robin Staffin, Dr. A. Byon-Wagner and Dr. P.K.Williams of U.S. DOE, the support of the space agencies fromGermany (DLR), Italy (ASI), France (CNES), Spain (CDTI) and China and the support of CSIST, Taiwan,have made the construction possible.The support of GSI-Darmstadt, particularly of Dr. Reinhard Simon made it possible for us to test electronicscomponents for radiation effects. The support of ESA, including Martin Zell, Jean Jamar and Wolfgang Sup-per, will enable the overall thermal vacuum test of ESTEC.The support of INFN, Italy, IN2P3, Region Rhoˆne-Alpes and Haute Savoie, France, CIEMAT and CICYT,Spain, LIP, Portugal, CHEP, Korea, the Chinese Academy of Sciences, the National Natural Science Founda-tion and the Ministry of Science and technology of China, Academia Sinica, Taiwan, the U.S. NSF, M.I.T.,ETH-Zu˝rich, the University of Geneva, National Central University, National Space Program Office, NationalChaio Tung University and National Cheng Kung University, Taiwan, Moscow State University, SoutheastUniversity, Nanjing, Shanghai Jiao Tong University, Sun Yat-sen University, Guangzhou, Shandong Univer-sity, Jinan, RWTH-Aachen, the University of Turku and the University of Technology of Helsinki, is gratefullyacknowledged.We are grateful for the strong support and interest shown from the private sector, including Dr. E.Ettlinger,Linde, Dr. R.Herzog, ILK, Dresden, Mr. M.Molina, CGS, Milan, Mr. F.Petroni, CAEN, Viareggio, CRISA(Astrium), Madrid, Ing. A.Pontetti, G H A Engineering, Italy, Dr. E.A.Werner and Dr. J.Krieger, ISATEC,Aachen, and Dr. H.Bieri, Bieri Engineering, Switzerland.References[1] M. Sapinski, “Cosmic-ray Astrophysics with AMS-02 “, these proceedings.[2] J. Pochon, “Astroparticle Physics with AMS-02”, these proceedings.[3] M. Sapinski, “Detection and measurement of gamma rays with the AMS-02“, these proceedings.[4] J. Alcaraz et al. [AMS Collaboration], Phys. Lett. B 484, 10 (2000).J. Alcaraz et al. [AMS Collaboration], Phys. Lett. B 490, 27 (2000).J. Alcaraz et al. [AMS Collaboration], Phys. Lett. B 472, 215 (2000).J. Alcaraz et al. [AMS Collaboration], Phys. Lett. B 494, 193 (2000).J. Alcaraz et al. [AMS Collaboration], Phys. Lett. B 461,387 (1999).M. Aguilar et al. [AMS Collaboration], Phys. Rep. 366, 331 (2002).[5] R. MacMahon “The Alpha Magnetic Spectrometer Superconducting Magnet”, these proceedings.[6] P. Zuccon, “The AMS Tracker Performance”, these proceedings.[7] J. Olzem et al., “Construction of the AMS-02 Transition Radiation Detector for the International SpaceStation”, these proceedings.[8] V. Bindi et al., “Performance of the AMS-02 Time of Flight”, these proceedings.[9] M. Aguilar Benitez et al., “The Ring Image Cherenkov detector (RICH) of the AMS experiment”, theseproceedings.[10] J. Pochon et al., “ ECAL front-end and electronics”, these proceedings.

AMS - A magnetic spectrometer on the International Space Station

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