The Alpha Magnetic Spectrometer (AMS-02) is a general purpose high-energy particle physics detector. It was installed on the International Space Station (ISS) on 19 May 2011 to conduct a unique long duration mission (20 years) of fundamental physics research in space. The first AMS results reported in this Letter are based on the data collected during the initial 18 months of operations on the ISS, from 19 May 2011 to 10 December 2012. This constitutes 8% of the expected AMS data sample. The positron fraction, that is, the ratio of the positron flux to the combined flux of positrons and electrons, is presented in this Letter in the energy range from 0.5 to 350 GeV. Over the past two decades, there has been strong interest in the cosmic ray positron fraction in both particle physics and astrophysics [1]. The purpose of this Letter is to present the
accurate determination of this fraction as a function of energy and direction (anisotropy).

AMS detector.—The layout of the AMS-02 detector [2] is shown in Fig. 1. It consists of nine planes of precision silicon tracker, a transition radiation detector (TRD), four
planes of time of flight counters (TOF), a permanent magnet, an array of anticoincidence counters (ACC), surrounding the inner tracker, a ring imaging C ˇ erenkov detector (RICH), and an electromagnetic calorimeter (ECAL). The figure also shows a high-energy electron of 1.03 TeV recorded by AMS.

The AMS coordinate system is concentric with the center of the magnet. The x axis is parallel to the main component of the magnetic field, and the z axis points vertically. The (y-z) plane is the bending plane. AMS is mounted on the ISS with a 12 roll to port to avoid the ISS solar panels being in the detector field of view; terms such as ‘‘above,’’ ‘‘below,’’ and ‘‘downward-going’’ refer to the AMS coordinate system... (Read the full article ->>)