Bindi, V

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29th International Cosmic Ray Conference Pune (2005) 9, 4144Performance of AMS-02 Time of FlightV. Bindi  , D. Casadei  , G. Castellini  , F. Cindolo , A. Contin  ,F. Costa  , F. Giovacchini  , G. Laurenti , G. Levi  , A. Oliva  , F. Palmonari  ,L. Quadrani  and C. Sbarra (a) INFN Bologna, viale Berti Pichat 6/2, Bologna 40127, Italy(b) Physics Department, Bologna University, via Irnerio 46, Bologna 40126, Italy(c) IFAC-CNR Firenze, via Panciatichi 64, Firenze 50127, ItalyPresenter: F. Giovacchini (giovacchini@bo.infn.it), ita-giovacchini-F-abs1-he21-oralAMS-02 Time of Flight (TOF) plastic scintillators prototypes were tested with ion beams at CERN SPS in2002 and 2003. The response of the polyvinyltoluene counters to the energy lost by ions with 	 wasstudied in order to find the best parametrization and the ion separation power of the TOF detector. The valuesfor the parameters of the Birks-Chou formula were found to be consistent among the different counters. Goodcosmic nuclei separation of the TOF system up to 	 can be anticipated. The operation of TOF scintillatorcounters at 400 km from Earth will be characterized by a large hit rate and an estimated absorbed dose of about0.5 krad/year. Gamma radiation tests were carried on all parts of a TOF counter prototype to study the agingeffects on the detector performance. No visible radiation damage was found up to 15 krad.1. IntroductionThe Alpha Magnetic Spectrometer (AMS-02) [1] is a large acceptance particle spectrometer that will operateon the International Space Station (ISS) for at least three years to measure Cosmic Ray (CR) fluxes. The TOFsystem of AMS-02, completely developed and built at the INFN Laboratories of Bologna, consists of fourplanes of 8, 8, 10, 8 plastic scintillator counters each. Scintillators are 1 cm thick and the light is collected bytwo or three PhotoMultiplier Tubes (PMT) per side. In order to minimize the angle between the magnetic fieldand the PMT axis, a bended light guides variety was produced.The TOF system is expected to: provide the fast trigger to the experiment with a negligible inefficiency;distinguish between upward and downward particles at the 	 level; measure the particle  (  =v/c) withﬀ for protons; estimate the value of the particle charge up to ﬂﬁﬂﬃ [2]. Two beam tests werecarried out to study the TOF system performance and a counter ageing test is in progress to search for radiationdamage.2. Beam testsPrototypes of AMS-02 TOF polyvinyltoluene scintillator counters were tested at CERN in 2002 and 2003 withstandard NIM and CAMAC electronics at the SPS ion beam facility. The primary line (20  GeV/ ! Pb in2002 and 158  GeV/ ! In in 2003) was directed against a Be target producing secondary nuclei from protonto the incident ion itself with a small spread in  . Mass over charge selection was made possible by tuning themagnets of the T8 selection line. Runs with  #" =1, 2, 9/4, 7/3 were used to get large statistic of all importantCR species.During the 2003 test [2], 4 TOF counters equipped with different light guides, were set along the ion beamtogether with AMS-02 RICH [3][4] and tracker prototypes: 3 over 4 scintillators were made of Eljen EJ-200,adopted for the AMS-02 TOF, whereas the last one was made of Bicron BC-408, used in AMS-01. The time of42 V. Bindi et alTable 1. TOF counter mean charge resolution vs atomic number.Ion %$ Ion $ Ion %$ Ion $H 0.15 B 0.21 F 0.29 Al 0.34He 0.14 C 0.24 Ne 0.37 Si 0.5Li 0.17 N 0.29 Na 0.34 P 0.4Be 0.23 O 0.31 Mg 0.34 S 0.40.060.080.10.120.140.160.180.20 1 2 3 4 5 6 7 8 9P1 = 0.159(2) nsP2 = 0.079(1) nst2 2=   (P1/Z)   + P2σ(ns)ZC2 −> C3 time of flight resolutionFigure 1. Time of ight resolution vs atomic number.11010 20 2 4 6 8 10 12 14 16Atomic number ZRelative peak amplitude (a.u.)   4.894    /    14Q = P1 Z 2 / (1 + P2 Z 2 + P3 Z 4)P1 = 0.99 ± 0.02P2 = (3.3 ± 0.7) × 10 −3P3 = (-5 ± 2.9) × 10 −6Figure 2. Fit of the measured average amplitude vs theatomic number.flight between two different TOF counters was measured; the resolution obtained in the worst case for counterswith curved and twisted light guides is shown in Fig.1 and it is 180 ps for protons and about 100 ps for lightions.The charge of a particle crossing the scintillator can be measured by looking at its energy loss: for particlesheavier than electrons the mean rate of energy loss is a function (the well known Bethe Bloch equation) of themedium and of the incident particle velocity and charge. Two charge measurements, which are not statisticallyindependent, are carried on with the PMTs placed in both sides of a given counter. Hence, the best side of eachcounter is used to estimate the particle charge. The charge resolution reported in Table 1 for ions up to S isthe average over the four counters. Custom electronics boards [5] are being developed fo AMS-02, which willextend the charge range up to &ﬃ .The scintillator response is not a simple linear function of the ionization energy density, but it is well modelledby the Birks-Chou law [6]: ')(')* ,+-/.-/01	3254768+-/.-/01259:+;-/.-/01=<(1)where(is the scintillator luminescence, 476 is proportional to the density of excited or damaged molecules,which act as quencher, 9 is a small correction parameter due to saturation. For particle crossing the samePerformance of AMS-02 Time of Flight 43Figure 3. TOF counter: exploded view.wavelength (nm)400 450 500 550 600 650 700 750 800)-1 (mµ051015202530 3000 Gy10000 Gy15000 Gy15000 Gy (after 66 hours)15000 Gy (after 30 days)wavelength (nm)380 400 420 440 460 480 500 520 540 560 580Photon Emission (%)110210Figure 4. Induced absorption coefcient measured inthe wavelenght range 400-800 nm for different absorbeddoses. Also the recovery after 1.5 Mrad is shown. In theinset the EJ-200 emission spectrum is reported.material and with the same  , the mean energy loss is a function only of their atomic number. The four fitsof the luminous response vs  give compatible results. Fig.2 shows the global fit obtained with the averagedamplitude peaks of the four detectors. The uncertainties associated to each point are of the order of 5%.3. Radiation hardnessThe TOF materials must be radiation hard enough to keep their characteristics stable for 10 years in space: theestimated absorbed dose is about 0.5 krad/year. It is known that the radiation induced darkening occurs in themajority of optical materials. Indeed organic materials are sensitive to radiation because of the generation ofreactive intermediate free radicals, which take place when covalent bounds are excited or ionized by radiation.A set of studies about the radiation damage on each single part making up the TOF counter was performedusing gamma sources.One side of the TOF counter, including epoxy joints and light guides (see Fig.3), was irradiated at the GIF(Gamma Irradiation Facility - CERN) >@?BA Cs gamma source. At 60 cm from the source the dose rate was ofthe order of 11.5 rad/h (in water); 4.5 C 0.4 krad at the center and 3.1 C 0.3 krad at one side were achieved. Nomeasurable changes were found in the attenuation length and in the number of photoelectrons, when comparedto the measurements done before the irradiation and on the non-irradiated side.In collaboration with the ISOF-CNR Institute of Bologna, several radiation damage tests were carried out onevery TOF counter part: silicon optical pad, a plexyglass conical light guide and a small sample of scintillator(42 D 55 D 10 mm ? ). Facilities employed: the Gamma Cell of EGF Co (ISOF-CNR) characterized by an activityof 4.27 D 10 >@? Bq (21 Feb ’05) and a dose rate of 1.51 C 0.22 krad/min (in water); a Perkin-Elmer spectrometer(lambda 45 model) with 2 lamps (deuterium lamp for the UV region and a halogen one for HIJKﬃ nm)and a monochromator with 0.1 nm resolution. First, the optical pad was irradiated at several dose levels upto 15 krad, corresponding to three times the maximum absorbed dose expected in ten years. The opticaltransmission spectra of the pad in the range 190-1100 nm were measured before and after each irradiation bythe spectrometer. The multiplication of the optical pad transmission spectrum with the scintillator emission44 V. Bindi et alspectrum (inset of Fig.4) does not show any appreciable variations of the integrated light output. A similarstudy was performed on the light guide. Even though the integrated transmittance spectrum in the scintillatoremission region is stable within 5  , it is possible to recognize few small absorption structures.The two most important types of radiation damage in plastic scintillators [7] are the production of new stableabsorption centers that decreases the light transmission, and the deterioration of fluor and shifter moleculesthat reduces the light production. Both effects produce a loss of light output to the exit face. The concentrationof colour centres is estimated from optical transmission measurements because it is proportional to the inducedabsorption coefficient L defined as follows:LM	'ONQPSRUTFTWV(2)whereTFandTare the transmission before and after irradiation and'is the sample length in meters. Thescintillator sample was irradiated up to 400 krad observing negligible variation on the transmission spectrum.That dose corresponds to about hundred times the expected dose for the whole AMS mission. Above thisthreshold an absorbing peak at about 410 nm increases as is shown in Fig.4, where the radiation inducedabsorption coefficient L is plotted as a function of the dose. The absorption in the lower wavelength region ofthe spectrum can be related to a deterioration of the wavelength-shifter molecules. In Fig.4 the measurementsdone some hours after the irradiation are also included to appreciate the recovery of the scintillator. A deeperinvestigation of recovery time of our scintillators is in progress.4. ConclusionsTOF scintillator counters show a good charge resolution in particular up to Nitrogen and Oxygen nuclei. A firstpreliminary estimation of the Birks-Chou parameters 476 and 9 for polyvinyltoluene EJ-200 scintillator canbe anticipated: 476XX+@	)Y ﬃZC[Y]\1DM	;%? g MeV ^> cm <and 9_X+@`ZY abC[1Dc	;7A g<MeV <cm %d . TheTOF system has a good radiation hardness and in particular neither optical pad nor light guide suffer radiationup to an absorbed dose of about 15 krad. The scintillator transmission spectrum is unchanged up to 400 krad.5. AcknowledgementsWe wish to thank the many organizations and individuals listed in the acknowledgments of Ref. [1].References[1] C. Lechanoine-Leluc, ”AMS-02 A magnetic spectrometer on the International Space Station”, these pro-ceedings.[2] L.Amati et al., to appear in Nuclear Physics B (Proc. Suppl), (2005).[3] B. Baret et al., Nucl. Instr. & Meth. A 525, 126 (2004).[4] M. Aguilar Benitez, ”The Ring Cherenkov detector (RICH) of the AMS experiment”, these proceedings.[5] V. Bindi et al., ”Time of Flight read out system of AMS-02 experiment”, these proceedings.[6] C. N. Chou., Phys. Rev. 87, 904 (1952).[7] S.Ilie, H. Scho¨nbacher, M. Tavlet, Nuclear Physics B (Proc. Suppl.) 32, 384 (1993).

Performance of AMS-02 Time of Flight

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