Direct Cosmic Rays Detection: Protons, Nuclei, Electrons and Antimatter

Abstract

The relative percentage of protons and heavier nuclei in the cosmic radiation is usually referred as the chemical composition of CRs. A detailed knowledge of the chemical composition up to the highest energies is of crucial importance for the understanding of astrophysical sources of CRs and their propagation in the Galaxy. The chemical composition of CRs can be accurately measured through experiments carried at a negligible residual atmospheric depth or outside the atmosphere. When arriving at the top of the atmosphere, primary CRs start to interact with nuclei of air molecules, producing a cascade of secondary particles. Primary nuclei undergo fragmentation processes and the information about their mass cannot easily derived from the indirect measurements that are the subject of the next chapter. In this chapter, we deal about the techniques, Sects. 3.1, 3.2, and the experimental results of direct measurements performed with balloons (Sect. 3.3) and space missions (Sect. 3.4). They measured accurately the flux and the chemical composition of CRs up to $\sim$ $100$ TeV (Sects. 3.6, 3.7), allowing the formulation of models about their galactic origin and propagation. One of the features predicted by the standard model of CR acceleration is that the CR spectra are well described by power laws, with similar spectral indices for protons and heavier nuclei, up to energies of $\sim$ $10^{15}$ eV. The CR sources are thought to be concentrated near the galactic disk, with a radial distribution similar to that of supernova remnants. The propagation of CRs in the Galaxy is usually studied with a diffusion differential equation. The theoretical models of CR acceleration and propagation in the interstellar medium presented in the following chapters are based on the data described here. Measurements from early space-borne experiments refer mostly to energies lower than 1 GeV. They provided relevant information concerning the energy part of CRs affected by the dependence of the Sun activity. Important information on the energy spectra of protons, helium, and heavier nuclei arise from the PAMELA satellite, launched in 2006. Even more important are the physical outputs of the AMS-02 experiment, launched in 2011 with the Space Shuttle and taking data on the International Space Station (ISS). AMS-02 (Sect. 3.5) represents the most sophisticated particle detector ever sent into space, incorporating all the characteristics of the very large detectors used at large particle accelerators. AMS-02 is providing fundamental and detailed information concerning the chemical composition of the cosmic radiation and the presence of primary antiparticles. An important feature of the new experiments, including PAMELA and AMS-02, is the presence of magnetic spectrometers which enable the search for antiparticles and antimatter in space. Experimental evidence indicates that our Galaxy is made of matter. Antiparticles can be created as secondary particles by CRs interactions with the interstellar medium in our Galaxy. Whether or not there is significant amount of primary antimatter is one of the fundamental questions of the origin and nature of the Universe. For instance, the observation of only one antihelium nucleus would provide evidence for the existence of antimatter in space. At present, searches for $\overline{p}$ and heavier antinuclei (Sect. 3.8) give no indication of primary sources of antimatter in our Galaxy. On the contrary, the measurements of electrons and positrons, Sect. 3.9, show unexpected features. In particular, an excess of positrons with respect to the expectation from secondary production reported with large statistical significance from PAMELA and AMS-02, has opened theoretical scenarios about the possible origin from dark matter annihilations. This scenario will be discussed in Chap. 13.