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Original: "Cosmic ray mass composition measurement in the energy range from $$10^{16.5}$$ eV to $$10^{18.5}$$ eV observed with the TALE hybrid detector"
The TALE detector caught particles from space for five years and noticed that around an energy of $$10^{17}$$ eV, their composition changes sharply.
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Cosmic rays are not light but charged particles bombarding Earth from all directions. Most of them are protons (hydrogen nuclei), but heavier nuclei also show up: helium, nitrogen, iron. When such a particle hits the atmosphere, it spawns a whole shower of secondary particles, and the shape of that shower tells you what exactly arrived.
The TALE detector in Utah, USA, collected data on such showers for five years. It consists of fluorescencetelescopes that see the faint glow of the air and ground-based scintillation detectors that catch particles on the ground. Together, they allow precise determination of the depth at which the shower reaches its maximum, and that depth is directly related to the mass of the primary particle.
The heavier the nucleus, the faster it loses energy in the atmosphere, so the shower is "short"—its maximum is closer to the upper layers. A light proton penetrates deeper, and the shower stretches out.
The main result: the plot of the average depth of maximum versus energy has a kink around $10^{17}$ eV. Below this energy, the composition of cosmic rays becomes increasingly heavy — the fraction of iron and similar nuclei grows, while protons decline. Above it, the opposite happens: protons start to dominate again.
This behavior looks like a change of "supplier." Inside our Galaxy, particles are accelerated mainly in supernova remnants, where heavy nuclei prevail. But beyond the Galaxy, in active galactic nuclei or other powerful accelerators, more protons are produced. The kink falls right at the so-called "second knee"—the energy where the influence of galactic sources is thought to end and extragalactic influence begins.
The work doesn't just refine the picture; it gives a quantitative estimate: after the kink, the proton fraction rises with energy, while the iron fraction falls. This is a strong argument that above $10^{17}$ eV, we are mostly seeing particles from beyond the Milky Way.
A cosmic ray with an energy of $10^{18}$ eV carries as much energy as a well-served tennis ball, but all of it is concentrated in a single subatomic particle.