Measurement of the energy spectrum of cosmic-ray helium with CALET on the International Space Station

The CALorimetric Electron Telescope (CALET) is a space instrument designed to carry out precision measurements of high energy cosmic-rays. It was installed onboard the International Space Station in August 2015 and since mid-October 2015 it is collecting data with excellent performance and no significant interruptions. The instrument consists of two layers of segmented plastic scintillators to identify the charge of individual elements from proton to iron, followed by a thick (30 X₀ and ∼1.3 λ_I) calorimeter. It comprises a finely segmented imaging calorimeter (3 X₀), providing accurate particle tracking and complementary charge measurement, and a total absorption (27 X₀) homogeneous calorimeter. In addition to its primary science goal of identifying nearby sources of high-energy electrons and possible signatures of dark matter in the electron spectrum, CALET is carrying out measurements of the energy spectra, relative abundances and secondary-to-primary ratios of individual elements from proton to iron and above, in order to shed light on the mechanism of acceleration and propagation of cosmic rays in the Galaxy. Preliminary measurements of the energy spectrum of cosmic-ray helium, based on the first five years of collected data, will be presented and details of the analysis are given. The observations performed by CALET in the energy interval from a few GeV/n to the multi-TeV region show that the helium differential spectrum does not follow a simple power-law.