In this paper , we present the analysis and results of a direct measurement of the cosmic-ray proton spectrum with the CALET instrument onboard the International Space Station , including the detailed assessment of systematic uncertainties .
The observation period used in this analysis is from October 13 , 2015 to August 31 , 2018 ( 1054 days ) .
We have achieved the very wide energy range necessary to carry out measurements of the spectrum from 50 GeV to 10 TeV covering , for the first time in space , with a single instrument the whole energy interval previously investigated in most cases in separate subranges by magnetic spectrometers ( BESS-TeV , PAMELA , and AMS-02 ) and calorimetric instruments ( ATIC , CREAM , and NUCLEON ) .
The observed spectrum is consistent with AMS-02 but extends to nearly an order of magnitude higher energy , showing a very smooth transition of the power-law spectral index from -2.81 \pm 0.03 ( 50–500 GeV ) neglecting solar modulation effects ( or -2.87 \pm 0.06 including solar modulation effects in the lower energy region ) to -2.56 \pm 0.04 ( 1–10 TeV ) , thereby confirming the existence of spectral hardening and providing evidence of a deviation from a single power law by more than 3 \sigma .