PAMELA Measurements of Cosmic-Ray Proton and Helium Spectra

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PAMELA comprises a number of high-performance detectors, capable of identifying particles through the determination of charge (Z), rigidity (R = pc/|Z|e, where p is the momentum of a particle of charge Ze, c is the speed of light, and e is the electron charge), and velocity (β = v/c, where v is the velocity) over a wide energy range. The device is built around a permanent magnet with a six-plane double-sided silicon microstrip tracker, providing absolute charge information and track-deflection (η = ±1/R, with the sign depending on the sign of the charge derived from the curvature direction) information. A scintillator system, composed of three double layers of scintillators (S1, S2, S3 in fig. S2) provides the trigger, a time-of-flight measurement, and an additional estimation of absolute charge. A silicon-tungsten tracking calorimeter, a bottom scintillator (S4), and a neutron detector are used to perform lepton-hadron discrimination. An anticoincidence system is used off-line to reject spurious event triggers generated by particles interacting in the apparatus. Respect to balloon-borne experiments, PAMELA has the advantage of a substantially longer period of uninterrupted observing time. Furthermore, taking data in space is not affected by environmental systematics such as those due to correction for secondary particles produced in the residual atmosphere that affects balloon-borne experiments. A more detailed description of PAMELA and the analysis methodology can be found in (32, 33) and in the SOM.

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The changing spectral characteristics of the proton spectrum between 30 and 230 GV may be partly due to heliospheric effects. Although solar-modulation effects are considered negligible above 30 GV in the spherical force-field approximation (15), more detailed models, which use the full Parker equation to describe the propagation of cosmic rays in a two- or three-dimensional heliosphere (34, 35), may be needed to fully understand the impact of this effect.

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