The Anticoincidence Counter System of AMS-02

arXiv:0906.1068v1 [astro-ph.IM] 5 Jun 2009 PROCEEDINGS OF THE 31st ICRC, Ł ´OD´Z 2009 1 The Anticoincidence Counter System of AMS-02 Ph. von Doetinchem∗, Th. Kirn∗, K. L¨ubelsmeyer∗and St. Schael∗ ∗I. Physics Institute B, RWTH Aachen University, Sommerfeldstr. 14, 52074 Aachen, Germany Abstract. The AMS-02 experiment will be installed on the International Space Station at an altitude of about 400 km in 2010 to measure for three years cosmic rays. The total acceptance including the electromagnetic calorimeter is 0.095m2sr. This work focuses on the anticoincidence counter system (ACC). The ACC is a single layer composed of 16 interlocking scintillator panels that surround the tracker inside the inner bore of the superconducting magnet. The ACC needs to detect particles that enter or exit the tracker through the sides with an efficiency of better than 99.99 %. This allows to reject particles that have not passed through all the subdetectors and may confuse the charge and momentum measurements which is important for an improvement of the antinuclei-measurements. In 2007/2008 all subdetectors were integrated into the AMS-02 experiment and atmospheric muons were collected. These data were used to determine the ACC detection efficiency. Keywords: AMS-02, Veto, Inefficiency I. THE AMS-02 DETECTOR The AMS-02 experiment will be installed on the In- ternational Space Station at an altitude of about 400 km in 2010 for about three years to measure cosmic rays without the influence of the Earth’s atmosphere [1]. The detector consists of several subdetectors for the determination of the particle properties, namely a tran- sition radiation detector (TRD), a time of flight system (TOF), a cylindrical silicon microstrip tracker with eight layers surrounded by an anticoincidence counter system (ACC) in a superconducting magnet with a field of 0.8 T strength, a ring image ˇCerenkov detector (RICH) and an electromagnetic calorimeter (ECAL) (fig. 1). II. THE ANTICOINCIDENCE COUNTER The AMS-02 anticoincidence counter [2], [3] sur- rounds the silicon tracker and can be used as a veto for the trigger decision made by the TOF (fig. 2, upper). This is important for rejecting events with particles entering the detector from the side or with particles from secondary interactions inside the detector which could distort the charge measurement. To improve existing upper limits on antihelium an inefficiency of the ACC smaller than 10−4 is needed according to MC simula- tions. The inefficiency is the ratio of missed to the total number of particle tracks crossing the ACC. Another important task of the ACC is to reduce the trigger rate during periods of very large flux, e.g. in the Fig. 1. The AMS-02 detector. South Atlantic Anomaly. For that purpose, it is important to use a detector with a fast response. The ACC cylinder has a diameter of 1.1 m and a height of 0.83 m and is made out of 16 scintillation panels (Bicron BC-414) with a thickness of 8 mm. The ultraviolet scintillation light through ionization losses of charged particles is absorbed by wavelength shifting fibers (WLS, Kuraray Y-11(200)M) which are embedded into the panels. The WLS fibers are coupled to clear fiber cables (Toray PJU-FB1000) for the final light transport to photomultiplier tubes (Hamamatsu R5946). A set of two panels is read out by the same two photomultipliers, one on top and one on the bottom, via clear fiber cables (Y-shape) in order to have redundancy and to save weight (fig. 2, lower). The slot between two panels is realized with tongue and groove and is crucial for the determination of the inefficiency because of less scintillator material and a smaller active (WLS) to passive (scintillator) material ratio. The qualification of the individual components is discussed in references [2], [3]. III. ACC INEFFICIENCY DETERMINATION WITH ATMOSPHERIC MUONS AMS-02 was pre-integrated with all subsystems but the magnet in 2007 and 2008 at CERN, Geneva. The data used here are taken after this installation. 2 PH. VON DOETINCHEM et al. THE ACC OF AMS-02 Fig. 2. Upper) The anticoincidence counter system after integration (left) and the principle of component arrangement (right). Lower) ACC working principle. In most runs a trigger is given by any two out of the four TOF planes. The trigger condition for events used in this analysis used tracks with hits in both upper TOF layers. TRD and tracker tracks are extrapolated to the ACC in order to find the intersection on the ACC cylinder and to determine the ACC detection efficiency. A. Event Reconstruction Fig. 3 shows a typical event used in the following analysis. In addition, the AMS-02 coordinate system is shown. The origin is located at the center of the tracker. The axis of symmetry of the ACC cylinder is the same as the z axis of the AMS-02 coordinate system. A track fit was developed to find tracks which point to the ACC. The requirement of a reasonable agreement between TRD and tracker track acts as an effective momentum filter such that onl

…(Full text truncated)…

This content is AI-processed based on ArXiv data.