Measurement of the Proton Spectrum with CALET on the ISS

Abstract

The CALorimetric Electron Telescope CALET on the International Space Station has been collecting science data since mid October 2015 with excellent performance and no significant interruptions. In addition to its primary goal of exploring the electron spectrum in the poorly known energy region above 1 TeV, CALET is investigating the hadronic component of cosmic rays with direct measurements of the energy spectra, relative abundances and secondary-to-primary ratios of elements from proton to iron and above (up to atomic number Z=40). Equipped with a thick (30 X$_{0}$ and $\sim$1.3 $\lambda_{I}$) calorimeter, comprised of a finely segmented 3 X$_{0}$ pre-shower section with imaging capabilities followed by a 27 X$_{0}$ homogeneous calorimeter, and with two independent sub-systems to identify the charge of the incident particle, CALET has an excellent energy and angular resolution and electron/proton discrimination of order 10$^{-5}$. For the first time CALET covered, with a single instrument, the whole energy interval (from a few tens GeV to the multi-TeV region) previously investigated in separate sub-ranges by magnetic spectrometers and calorimetric instruments. The observation of the proton differential spectrum confirms a deviation from a simple power-law, as previously reported by other experiments. In this paper, the proton spectrum measured by CALET is presented and details of the analysis are given.