The AMS-02 experiment has been installed on the International Space Station on May 19, 2011. The main goals of the experiment are the search for antimatter and dark matter and an high-precision measurement of the spectra of

the cosmic rays in the GeV to TeV energy range. This article describes the different components of the detector and presents the performances obtained at the last test

beams

Di Falco, S.

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DOI 10.1393/ncc/i2011-11028-3Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Status of AMS-02 experiment on the International Space StationS. Di FalcoINFN, Sezione di Pisa - Largo B. Pontecorvo 3, 56100 Pisa, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 26 Ottobre 2011)Summary. — The AMS-02 experiment has been installed on the InternationalSpace Station on May 19, 2011. The main goals of the experiment are the searchfor antimatter and dark matter and an high-precision measurement of the spectra ofthe cosmic rays in the GeV to TeV energy range. This article describes the differentcomponents of the detector and presents the performances obtained at the last testbeams.PACS 07.87.+v – Spaceborne and space research instruments, apparatus, and com-ponents (satellites, space vehicles, etc.).PACS 29.40.Vj – Calorimeters.1. – IntroductionRecent results from space-borne experiments excited a new interest in astroparticlephysics, making the search for the missing constituents of the universe (antimatter, darkmatter, dark energy,. . . ) one of the most promising fields of discovery for the next years.In particular the Pamela experiment [1] has measured a e+/(e+ + e−) fraction thatshows a large deviation from the predictions of the positron fluxes based on standardmodels of comsic-ray propagation and interaction: the origin of this discrepancy is stillunclear and more precise measurements from AMS are eagerly awaited.AMS is an international collaboration that includes more than 60 institutes from 16different countries for a total of more than 500 physicist. The AMS-02 experiment hasfinally been installed on the International Space Station (ISS) on May 19, 2011 where itwill take cosmic data until the end of the ISS life, currently estimated for not earlier than2020 and, most likely, around 2028. The ISS is orbiting at an altitude of about 386Kmover the earth surface, with a revolution period of 92 minutes and a precession period of71 days. The maximum of the geographical magnitude is 51.6◦. The AMS axis is tiltedby 12 degrees with respect to the axis of the ISS.The numbers of the AMS-02 detector (fig. 1) are quite unusual for a space-bornedetector: a volume of 12m3 for 7 tons of weight, 20000 electronic channels for a totalpower consumption of ∼ 2 kW.c© Societa` Italiana di Fisica 120STATUS OF AMS-02 EXPERIMENT ON THE INTERNATIONAL SPACE STATION 121Fig. 1. – The AMS-02 detector: the Italian flags tag subdetectors with Italian contributions.The different AMS subdetectors are described in the following sections together withtheir main features and performances measured at the test beams.2. – The Transition Radiation Detector (TRD): electron/hadron separationSituated on top of AMS, it’s made out of 20 layers of foam and straw tubes filled witha Xe/CO2 gas mixture [2]. The orientation of the straw tube depends on the layer andallows a 3D tracking. The gas leakage is kept low enough to guarantee the efficiencies ofthe straw tubes for at least 24 years. Looking at the transition radiation it is possible todistinguish light particles from heavy particles. Test beam results show a rejection factorfor protons with respect to positrons better than 102 up to 300GeV, in agreement withMonte Carlo calculations (fig. 2).3. – The Time of Flight (TOF): time, trigger and chargeThe measurement of the time of flight of the particles is performed by 2 stations ofscintillators located above and below the Silicon Tracker. Each station is made of 2 layersof scintillator paddles read at both ends by two or three photomultipliers (PMTs). Theresolution in time is Δt  160 ps, which corresponds to a precision in the speed mea-surement Δβ/β ∼ 4% at β ∼ 1 [3]. The dE/dx measurements allow to distinguish thecharge of the lightest nuclei. TOF is the main ingredient of the different charged triggerstreams: protons (and antiprotons) together with Anticoincidence Counters, electrons(and positrons) together with the Electromagnetic Calorimeter and heavy or slow nuclei.Last but not least, TOF is used to distinguish between upgoing and downgoing particles.122 S. DI FALCOFig. 2. – Left: TRD rejection factor for protons/positrons as function of energy: comparisonbetween test beam results and Monte Carlo simulation. Right: rigidity resolution and maximumdetectable rigidity obtained by the silicon tracker on protons with the current Permanent Magnetlayout (upper curve) and the original Superconducting Magnet layout (lower curve): the shadedarea shows the difference between the two curves.4. – The Permanent Magnet (PM)In the original design of the detector, when AMS was supposed to run only for threeyears, a very powerful Superconducting Magnet had been chosen. The lifetime of thismagnet was limited by the evaporation of the superfluid helium, needed to keep themagnet cold, to not more than 2–3 years.When the time schedule for the ISS changed, extending AMS running time up to morethan 15 years, a permanent magnet became the best choice. The mechanical design ofthe detector was, by purpose, compatible with the old permanent magnet used for theAMS-01 prototype(1): a set of permanent dipoles that gives a dipolar magnetic field of1.5 kG well contained inside the volume of the magnet. After 12 years the intensity ofthe field of this magnet has been measured to be the same as before, within the 1%accurancy of the measurement.5. – The Silicon Tracker (ST): momentum, charge and antimatterThe AMS tracker is made of 192 ladders containing the 100μm pitch silicon stripsthat measure the two coordinates orthogonal to the detector axis with a resolution of∼ 10μm. The total active area of 6.45m2 is disposed on 3 double-sided planes located inthe center of the detector (inner tracker) and 3 single-sided planes, respectively locatedon top of TRD, on top of the PM and between the Ring Imaging Cherenkov and theElectromagnetic Calorimeter (fig. 1); this layer layout allowed to mantain the excellentMaximum Detectable Rigidity (MDR) of the original Superconducting Magnet (fig. 2):2.1TV for protons and 3.7TV for helium nuclei [4]. Momenta lower than ∼ 150GVcan be measured using the inner tracker only: in this configuration the acceptance is(1) AMS-01 has successfully flown on the Space Shuttle Discovery for 10 days in June 1998.STATUS OF AMS-02 EXPERIMENT ON THE INTERNATIONAL SPACE STATION 123Fig. 3. – Left: limit on anti-helium abundance with respect to helium after 10 years of datataking with the PM; the steps are due to the different acceptance corresponding to the silicontracker layers used in the momentum fit. Right: charge separation with RICH observing thenumber of photons produced by a 158GeV/n ion test beam.∼ 0.5 sr·m2 and the limit on the abundance of anti-helium relative to helium can bepushed below 10−9 in 10 years (fig. 3).The amplitude of the charge deposited in the silicon sensors allows to measure thecharge of the nuclei up to the nickel.6. – The Anticoincindence Counters (ACC): trigger vetoThe Anticoincidence Counters, made of 16 curved scintillators surrounding the silicontracker, are used in the proton and ion trigger to reject particles entering from the sidesof the detector [5].7. – The Ring Imaging Cerenkov (RICH): speed and chargeThis detector consists of a radiator, a reflecting mirror and an array of 10880 pho-tomultiplier anodes [6]. It is able to measure the speed of the particles through theirCerenkov angle with a resolution Δβ/β ∼ 0.1%. In order to minimize the material infront of the Electromagnetic Calorimeter the radiator is made by 2 different materialswith different refraction index and Cherenkov threshold: a silica aerogel at the center(n = 1.05, βmin = 0.953) and sodium fluoride (n = 1.33, βmin = 0.75) on the sides. Bycounting the number of the emitted photons it is possible to determine the charge of thenuclei up to nickel (fig. 3) in a redundant way with respect to the ST.8. – Electromagnetic Calorimeter (ECAL): energy, trigger and dark matterThe last AMS subdetector is a sampling calorimeter made of lead, scintillating fibersand optical glue. It has an active area of 648 × 648mm2, a thickness of 16.7 cm, cor-responding to 17 radiation lengths, and a granularity of 9 × 9mm2. Each 10 layer offibers (a “superlayer”) the direction of the fibers is rotated by 90 degrees allowing a 3Dreconstruction of the electromagnetic showers. Fibers are read out only at one end by 324photomultipliers, each divided in 4 anodes. Anodes are read out with a double amplifi-cation allowing to span from the MIP energy range (8MeV) to the TeV particles energy124 S. DI FALCOFig. 4. – Left: energy resolution of the electromagnetic calorimeter. Right: residual 400 protonbackground in a 180GeV e+ after TRD, ECAL imaging and E/P matching selection.range (up to ∼ 60GeV in one cell). The PMT signal is also used to build an electromag-netic particle trigger that improves the charged particle trigger efficiency on electrons andpositrons and gives a ∼ 100% trigger efficiency on photons above 2GeV [7]. The energyresolution is shown in fig. 4. The energy linearity has been measured to be better than1% up to 250GeV while the angular resolution is better than 1◦ above 50GeV [8]. Theexcellent 3D imaging of the electromagnetic shower, together with the TRD identifica-tion and the energy/momentum (E/P ) matching brings to an electron/hadron rejectionfactor > 106: fig. 4 shows the residual background on 180GeV positrons due to 400GeVprotons normalized with the proper flux as obtained in the August 2010 test beam. Thelow proton contamination allows a precise determinataion of the positron fraction thatcould reveal the first signal of dark matter annihilation in e+e− (see for example [9]).Other indirect channels involving antiproton or deuterium will also be accessible to AMS.9. – Conclusion and outlookSince last May 19 AMS-02 is taking data on the ISS: the detector is performing well,in agreement with the results of the last test beams. The first results should be publishedquite soon.REFERENCES[1] Picozza P. et al., Nucl. Instrum. Methods Phys. Res. A, 623 (2010) 672.[2] Hauler F. et al., IEEE Trans. Nucl. Sci., 51 (2004) 1365.[3] Amati L. et al., IEEE Nucl. Sci. Symp. Conf. Rec. 2004, p. 366.[4] Haino S., Nucl. Instrum. Methods Phys. Res. A, 630 (2011) 78.[5] Bruch T. and Wallraff W., Nucl. Instrum. Methods A, 572 (2007) 505.[6] Arruda L., Barao F., Goncalves P. and Pereia R., Nucl. Phys. Proc. Suppl., 172(2007) 32.[7] Cadoux F. et al., IEEE Trans. Nucl. Sci., 55 (2008) 817.[8] Di Falco S., Adv. Space Res., 45 (2010) 112.[9] Kounine A., ISVHECRI Proceedings, Batavia (2010).

Status of AMS-02 experiment on the International Space Station

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Status of AMS-02 experiment on the International Space Station

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