Pushing the Limits of Direct Spectra and Composition Measurements
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ELEMENTARY PARTICLES AND FIELDS Experiment
Pushing the Limits of Direct Spectra and Composition Measurements S. Coutu* Institute for Gravitation and the Cosmos, Departments of Physics and of Astronomy & Astrophysics, Penn State University, University Park, PA 16802, USA Received June 14, 2019; revised June 14, 2019; accepted June 14, 2019
Abstract—The recent years saw the implementation and deployment of a new generation of instruments flown in space or on stratospheric balloons. They are targeted at the study of a variety of energetic cosmic particles, including protons and nuclei, electrons, antimatter particles (positrons and antiprotons), secondary nuclei (including isotopes), ultraheavy nuclei, all complementing gamma-ray studies. Thus a new wealth of data is providing fresh insights on high-energy phenomena in the Galaxy. The instruments are large and deployed for long exposures, providing for an energy reach that permits direct crosscomparisons with ground-based measurements. We briefly review the state of the field, focusing on present and near future efforts. DOI: 10.1134/S1063778819660141
ENERGY SPECTRA Direct measurements of cosmic-ray (CR) nuclei with balloon and satellite experiments are possible at present up to the 100-TeV range, limited by the fast falling CR power-law spectra, coupled with the finite instrumental apertures and exposure times. These particles are almost certainly of Galactic origin, and the idea that they gain their energy in diffusive shock acceleration processes associated with supernova remnants remains valid (for a classic treatment, see, e.g., [1]). For the past 20 years or so, balloon and satellite experiments have pushed these direct measurements ever closer to the cosmic-ray spectral knee with ever shrinking statistical uncertainties, thanks to multi-week exposures in Antarctic balloon flights (e.g., CREAM [2]), or multi-year exposures on free-flying satellites (e.g., PAMELA [3]) or the International Space Station (ISS) (e.g., AMS-02 [4]). A compilation of such measurements is shown in Fig. 1. This figure presents CR differential energy fluxes without any rescaling, as a function of total kinetic energy per nucleus. Shown are all-particle flux values (top points), and direct measurements for a number of the more abundant CR species (p, He, C, O, Ne, Mg, Si, Fe). This figure is confusing because of so many overlapping measurements, but it does illustrate a number of points. First, the power-law spectra are readily apparent beyond about 1011 eV, past any solar modulation effects. Next, the very large range of energies and flux values is obvious (more *
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than 6 orders of magnitude in energy, up to 15 orders of magnitude in flux values), illustrating the extreme experimental challenge at the higher end of the energy range shown. Also apparent is the fact that very high statistics measurements now exist at the lower end of the energy range shown, particularly for the longexposure space-borne experiments PAMELA and AMS-02, while at the higher end the experimental points ha
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