Orsi, S

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29th International Cosmic Ray Conference Pune (2005) 3, 369372Pre-flight Performance Studies of the Anticoincidence Systems of thePAMELA Satellite ExperimentS. Orsi, P. Carlson, P. Hofverberg, J. Lund and M. PearceRoyal Institute of Technology (KTH), Albanova University Centre, S-106 91 Stockholm, SwedenPresenter: S. Orsi (silvio@particle.kth.se), swe-orsi-S-abs1-og15-oralThe PAMELA satellite experiment will be launched on-board a Resurs DK1 earth observation satellite towardsthe end of 2005. During the three year mission, the primary objective of PAMELA is to measure the fluxof antiprotons (80 MeV - 190 GeV) and positrons (50 MeV - 270 GeV) in the cosmic radiation. The wideenergy range and large statistics,  antiprotons and  positrons, will allow sensitive tests of cosmicray propagation models and searches for exotic sources of antiparticles, such as the annihilation of dark matterparticles. The PAMELA experiment contains two anticoincidence systems built from plastic scintillators readout by photomultipliers. One system surrounds the permanent magnet spectrometer and the other surroundsthe volume between the first two time-of-flight layers. The pre-flight performance of both anticounter systemshas been studied using data from ground tests of PAMELA.1. IntroductionPAMELA [1][2] is built around a permanent magnet silicon spectrometer (tracker), surrounded by a plasticscintillator anticoincidence shield. The electromagnetic calorimeter, mounted below the tracker, is used forparticle identification and energy measurements. The time of flight system (TOF) consists of scintillator planes(S1-S3), which provide the main trigger to the experiment. Simulations have shown that the majority (   75%)of triggers in space are expected to be false triggers [3], i.e. where the coincidental energy deposits in the TOFscintillators are generated by secondary particles, produced in the mechanical structure of the experiment. Fig-ure 1 (left) shows a schematic view of the PAMELA experiment and a false trigger: a proton interacts with themagnet and generates a shower of particles which fulfill the trigger condition. The aim of the anticoincidence(AC) systems is to identify these events during offline data analysis. A second level trigger [4] based on infor-mation from the AC detectors and from the calorimeter provides an online identification of false triggers andis capable of reducing the data downloaded to Earth by 60% (figure 1, right). The second level trigger can beenabled via uplink commands from ground.The performance and stability of the AC detectors and of the electronic read-out system has been investigatedand is discussed in section 3.2. The Anticoincidence SystemThe main AC system [5] consists of 4 plastic scintillators (CAS) surrounding the sides of the magnet and onecovering the top (CAT). The second AC system consists of 4 scintillators (CARD) covering the sides of thevolume between the first two time-of-flight layers. Each scintillator (Bicron BC-448M) is coupled via a 7 mmthick optical pad to 2 (8 for CAT) photomultiplier tubes (Hamamatsu R5900 PMT), operated at 800 V and read-out by to 2 independent data acquisition boards, for a total of 24 electronic channels. For each channel binaryhit information is generated indicating whether the deposited energy exceeds 0.5 mip. The hit information isrecorded in a time window of length 1.28  s centred on the trigger time. Within this window, the hit can belocated with an accuracy of 80 ns. Hits coincident with the trigger are registered in the central 2 bits, whilethe first (last) bit refers to particles interacting   600 ns before (after) the trigger. The time window has been370 S. Orsi et al.INCOMINGPARTICLES1S2S3S4Neutron    Det.CalorimeterTrackerCARDCATCAS1.2m10−11001011020102030405060708090100Kinetic Energy (GeV)Rejection Ratio (%)10−1100101102024681012Kinetic Energy (GeV)Inefficiency (%)protonselectronsFigure 1. (left) A proton entering from the side generates an hadronic shower and triggers the experiment (false trigger).(right) The inefciency of the second level trigger (top) is shown for electrons and protons. The rejection ratio (bottom) isthe fraction of false proton triggers correctly identied. The total data reduction is 	 60%. Adapted from [4].chosen to allow an overlap with the calorimeter self trigger [6], which is issued a few hundred of ns after theinteraction of a particle.A miniature low intensity light emitting diode (LED) is glued directly onto each scintillator (2 for CAT). TheLEDs are placed far away from the PMTs, in order to provide a long path for the photons to reach the PMT,which increases the sensitivity to opacity. The LEDs allow the functionality and stability of the AC systems tobe verified in-flight [5]. Once per orbit, on the ascending node, the data acquisition is stopped and a calibrationprocedure is performed. All PAMELA detectors are calibrated, sequentially and independently. Each LEDemits short light pulses ( nm), that reproduce mip-generated pulses inside the scintillator with respectto pulse height, shape and timescale. The monitoring procedure requires less than one second to be performedand provides an integral spectrum with statistical errors of the order of 1% (to be discussed in section 3 andfigure 4).3. Performance of the SystemIn 2004 and 2005, extended acquisitions of cosmic particles at ground level with PAMELA detectors andread-out system in flight configuration have been performed in the laboratory in Rome (Italy) and in Samara(Russia). The AC system performances have been investigated on ground using a ‘QuickLook’ package, aThe PAMELA Anticoincidence System 371shift3_mEntries  110976Mean    6.917RMS    0.3701Bin0 2 4 6 8 10 12 14 16Content110210Shift register CAS 11 2 3 4 5 6 7 8020406080100Hit PMTs CATNumber of hits1 2 3 4 5 6 7 8110210Hit PMTs CASFigure 2. (left) The ‘time window’ of 1.28  s for the 4 CAS detectors during a data acquisition in the laboratory withcosmic particles. Hits simultaneous with the trigger are in the centre, while the rst (last) bin corresponds to hits 	 600 nsbefore (after) the trigger. The two distributions correspond to different read-out boards and are shifted due to differencesin the timing of the trigger signals sent independently to each board. (centre) The number of PMTs hit in CAS (histogram)per trigger for a given run with cosmic rays. The CARD detectors show a similar behaviour. (right) The number of PMTshit in CAT per trigger for the same run.ROOT based tool which will be used during flight to monitor the PAMELA apparatus almost1 in real time.Quicklook (QL) has been successfully used for the study of cosmic ray data taken on ground with PAMELA inflight acquiring data in flight configuration. The Quicklook software provides detailed performances analysisspecific to each of the PAMELA subdetectors in the form of histograms or plots.Apart from monitoring error flags that indicate system anomalies (parity errors in data transmission, regulatorlatch-ups, high temperature warnings, ...), the QL software checks the stability of the AC systems monitoringthe time variation of specific parameters. Figure 2 (left) illustrates the stability of the shift register: the temporaldistribution of hits for a specific run with cosmic particles is shown. The majority of hits occupy 2 bins atthe centre of the time window, i.e. in time with the trigger, as expected (section 2). The histogram in thecentre (right) shows the number of PMTs hit in the CAS (CAT) detector per cosmic ray trigger. The read-out redundancy is clearly illustrated. Each electronic channel records the detector singles rate between twoconsecutive triggers. This allows to monitor the ‘singles rate’ in each channel. Figure 3 (left) shows the‘singles rate’ for a typical run (   2 h) with cosmic particles measured in Rome. The two distributions refer tothe 2 PMTs facing the same scintillator. Since the sensitive area and the thresholds are the same, the histogramsalmost overlap and show a peak around 60 Hz. The plots in the centre and right show the stability of the ‘singlesrate’ for the same detector over a period of   5 months, where the  -axis displays the days elapsed since the1  January 2005. On the  -axis are shown the mean of the peak (centre) and its RMS (right). At the end ofMarch (day 90) PAMELA was transported from Rome to Samara, where the different laboratory set-up (tallerbuilding, i.e. larger amount of concrete placed above PAMELA) caused a significant singles rate decrease inall scintillators.During data acquisition in the laboratory, the AC detectors have been monitored several times each day withthe LED-based monitoring system. Figure 4 (left) shows a typical result from the monitoring procedure, i.e. thenumber of LED-generated pulses above threshold, where the threshold varies from 0 to 1 V (0 to several mips).The QL software shows the stability of the plateau at low thresholds (centre) and of the falling edge (right).The characteristics of the AC detectors have not changed since the first measurements were taken with the ACsystem integrated on PAMELA in flight configuration. The LED data allows to determine that the singles ratedecrease is not due to a degradation of the system.As soon as data are downloaded to Earth (few times a day), the QuickLook software checks PAMELA stability and performances.372 S. Orsi et al.counter11_mEntries  693Mean    60.63RMS     5.54320 40 60 80 100 120 140020406080100120140160Counter register CAS 320 40 60 80 100 120 14001234567891020 40 60 80 100 120 1400102030405060708090100Figure 3. The ‘singles counters’ monitor the hit rate between consecutive triggers in a CAS detector. The lled and emptysquares refer to the 2 PMTs facing the scintillator. The hit rate distribution for one run is shown as function of the frequency(Hz) (left). The position (RMS) of the peak shown in the left is shown in the centre (right). In the central plot, the rst gap(day 	 20) is due to the vibration test. The rise (day 45) is due to the increased high voltage to the ight value. The secondgap is the transport to Samara. The RMS jumps are due to commissioning tests.C A R D2 (MA IN), P MT  059E ntries  256Mean  0.1238R MS  0.07985DA C  [V ]0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Events01000200030004000500060007000C A R D2 (MA IN), P MT 05960 80 100 120 1406000610062006300640065006600670068006900700060 80 100 120 1400.10.110.120.130.140.150.160.170.180.190.2Figure 4. The LED-based monitoring system provides a mean of monitoring the system independent of cosmic ray particleshitting the detectors (section 3). The number of LED pulses above the discriminator threshold is shown as function of thethreshold (left). The time stability of the height of the plateau shown in the left gure is shown in the centre (  -axis indays). The edge (dened as the voltage corresponding to 90% of the plateau) is shown on the right.4. ConclusionsThe performance of the PAMELA anticounter systems has been monitored using cosmic ray and LED mon-itoring runs during the commissioning of the experiment. No significant changes in the performance of thesystems was observed.References[1] PAMELA homepage: http://wizard.roma2.infn.it/pamela/[2] M. Boezio for the PAMELA collaboration, these proceedings[3] J. Lund, A Study of the PAMELA Anticoincidence System, licentiate thesis, Stockholm 2002[4] S. Orsi et al., A Second Level Trigger for the PAMELA Satellite Experiment, Astroparticle Physics,submitted. July 2005[5] S. Orsi et al., The Anticoincidence Shield of the PAMELA Space Experiment, Proc. 35th COSPARScientific Assembly, Paris, France, 2004, Advances in Space Research, to appear. July 2005[6] M. Boezio et al., Nucl. Instr. and Methods in Physics Res. A487 (2002) 407-422

Pre-flight Performance Studies of the Anticoincidence Systems of the PAMELA Satellite Experiment

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