# Cosmic-Ray Composition

## Galactic Cosmic Ray Composition

In the last chapter we saw the CRs spectrum, and we discussed briefly about the composition saying that 80% are protons and 15% He, and the rest are heavier nuclei. However there is mor than that. The following is a plot from the Particle Data Group that shows the relative abundance of CRs chemical composition normalized to Carbon. Of course the composition depends on the energy of the spectrum, so the following plot is at energy of $$10.6 {\rm; GeV/nucleon}$$

![Cosmic Ray Composition Source: Particle Data Group](https://github.com/zemrude/PHYS-F-467/raw/12f2d10ab5e7b0128f70a9bcf8407ece300332c6/images/composition.png)

There are many things to remark from these plot. So, let's go one by one. First of all, as can be seen most of cosmic rays are Hydrogen (protons) and Helium and the rest are heavy nuclei. Together with the relative abundance of CRs, in the plot we can see the Solar System abundance. This is very similar to the abundance of CRs indicating an **stellar origin of cosmic rays**. This is an important piece of information, as at the beginning, CRs were hypothesized to be the decay products of heavy, exotic particles. Their composition however, seems to indicate that CRs are made up of the same stuff of stars, planets, etc. Ie, most likely CRs, are just particle accelerated by some unknown process, and they do not have a exotic origin.  If you are looking at the plot, you might wondering about the obvious differences though. Yes, there are some elements that we see in CRs that are not found in the Solar System. As we will discuss in the next chapter  [Galactic Cosmic Rays](https://astroparticle.gitbook.io/docs/cosmic_rays/cosmic-rays-through-the-galaxy), these are the spallation products when primary heavy CRs fragment in their propagation through the Galaxy. In particular Li, Be, and B are secondary nuclei produced in the spallation of C and O, while Mn, V, and Sc are originated by the fragmentation of Fe.  We called these **secondary cosmic rays**, and their abundance and spectra can tell us a lot about the propagation of CRs in the Galaxy, in particular the study of the primary-to-secondary ratio is a powerful tool to infer the propagation and diffusion processes.  If you keep looking at the figure, you might also wonder about the see-saw shape of the abundance. This is just due to the fact that nuclei with odd $$Z$$and/or $$A$$have weaker nuclear bounds and are less frequent products of thermonuclear reactions.

## Cosmic Ray Electrons

Most of the CRs are protons and heavy nuclei but the there is still a component of electrons. There is a good reason for that, electrons, as we will see in the chapter [Radiation Mechanism](https://astroparticle.gitbook.io/docs/gamma-ray-astronomy/radiation-mechanism), loose energy way more efficiently than protons and heavy nuclei (by a factor of $$\propto m^{-4}$$). So electrons, $$e^{\pm}$$ , can only travel short distances compare to protons. Also the spectrum of electrons is expected to steepen by one power of E at around \~5 GeV because of these radioactive energy losses in the galaxy. The following is a plot of the electron and positron spectrum, together with the proton spectrum. However to show them in the same graph the proton spectrum has been divided by 100. The reason to show electrons and positrons together is that many experiments cannot distinguish among them. Ground-base Cherenkov experiments like  for example, the measure the $$e^-$$and $$e^+$$spectrum by selecting air-shower events compatible with an electromagnetic shower. Only space-borne instruments like PAMELA and AMS can separate $$e^-$$ from $$e^+$$.

![Spectrum of electrons and positrons measured by several experiments. Source: Particle Data Group](https://github.com/zemrude/PHYS-F-467/raw/12f2d10ab5e7b0128f70a9bcf8407ece300332c6/images/electrons.png)

As can be seen, above $$\sim 5 {\rm; GeV}$$the $$e^-$$ and $$e^+$$ spectrum follows a power law with spectral index of $$\gamma\_{e} \sim 3$$ (the y-axis is multiplied by $$E^{3}$$and so the spectrum is flat). The experimental disagreement at low energies is due to the Solar Modulation of the CRs spectrum. In other words, at low energies, CRs are affected by the magnetic fields that fill the heliosphere and therefore fluxes depend on the period when the measurements were taken. The flattening of the spectrum at 5 GeV is associated to the critical energy when electrons start to loose energy. Another important point is the apparent excess of electrons at 600 GeV measured by ATIC. This excess has been object to multiple interpretations, however it has not been confirmed by other experiments.  Another remarkable  feature is the existence of a cut-off at $$\sim 1 {\rm; TeV}$$. This cut-off is one of  the most prominent features in the CRs data and its origin is not well known. One possibility could be that 1 TeV is the critical energy when electrons and positrons start to loose energy, but this is typically assumed to be at energies of GeV when the spectrum becomes flat. In addition although the  $$e^-$$ and $$e^+$$ spectrum seems to be a single power-law between 5 GeV and 1 TeV  precise measurements of AMS has shown a rich spectral shape, as can be seen in the plot below.

{% hint style="info" %}
:mortar\_board: **Question:** Assuming the electron flux is only 1% of the protons. Is it the Earth positive charged-up?
{% endhint %}

## Antimatter Content in Cosmic Rays

Anti-matter is rare in CRs as it is in the Universe today. All we observe are by-product of particle interactions such as CRs interacting with the interstellar gas. Very few experiments are capable to discriminate between matter and anti-matter, specially those CRs surface detectors they can only see the electromagnetic shower generated in the atmosphere. Only space-borne detectors like PAMELA and AMS-02 can clearly separate $$e^-$$from $$e^+$$for example. The following plot is the latest data from AMS-02 on showing the electron and positron spectrum. As can be seen $$e^+$$are significantly lower than $$e^-$$. 

![](https://978429123-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LL-TjWvtGhAa4RFZygl%2F-M-9DFukhYEaWossdxLv%2F-M-9eHuWx5Hq_IWioqVK%2Felectron-positrons.svg?alt=media\&token=6f41b92d-b517-4537-8aae-6efc3444c5e5)

* As antimatter is rare in the Universe today, all antimatter we observe are by-product of particle interactions such as Cosmic Rays interacting with the interstellar gas.
* The PAMELA and AMS-02 satellite experiments measured the positron to electron ratio to increase above 10 GeV instead of the expected decrease at higher energy.

Source: Particle Data Group

![](https://github.com/zemrude/PHYS-F-467/raw/12f2d10ab5e7b0128f70a9bcf8407ece300332c6/images/positron.png)

This excess might hint to to contributions from individual nearby sources (supernova remnants or pulsars) emerging above a background suppressed at high energy by synchrotron losses