Highest Energy Cosmics Rays are Iron Nuclei!
The composition of the highest energy cosmic rays, those with energies on the order of tens of EeV (= 106 TeV), appears to include a significant number of iron nuclei – a result that I find truly astonishing. I learned about this from a nice post at Diax’s Rake.
The results come from the Pierre Auger Collaboration, and were reported recently in a paper (arXiv:1002.0699):
Measurement of the Depth of Maximum of Extensive Air Showers above 1018 eV
Their detector has two components: a surface detector which gives timing information to measure the direction of the shower, and the florescence detector, which observes the longitudinal development. The longitudinal shower shape is key to the analysis, since the maximum of the shower energy increases logarithmically with energy (as is well known to particle physics experimenters) and decreases logarithmically with the number of nucleons, A, in the incident nucleus. The authors quote:
Xmax = α( lnE – lnA ) + β
with α and β known to be largely energy-independent at these energies. Furthermore, the fluctuations in Xmax decrease with A. Given enough showers in a certain energy interval, the distribution of Xmax relates to the composition of the incident flux.
The authors apply what sound to me like stringent fiducial and quality cuts, and arrive at a sample of 3754 events. This is not a small sample – they certainly will have a good error on the mean and rms of Xmax for several bins of incident energy. Here are their measurements:
The data clearly drift from proton-like composition to iron-like composition passing from 1 EeV to about 40 EeV. (1 EeV = 109 GeV.) Although there is some spread in the predictions of various simulations, the slope of the data is like none of them, especially for RMS(Xmax). The transition occurs across a bit more than a decade in energy, and it is unlikely that the physics of air showers changes in such a strong way over only one decade. Nominally, cosmic rays with energies of 1 EeV are mostly protons, while cosmic rays with energies 50 times larger are mostly “iron” nuclei – in some average sense. All this supports the hypothesis of a transition from galactic to extra-galactic origins for cosmic rays in the energy regime. But how do such iron nuclei get produced and accelerated to such high energies, and why are they so much more numerous than protons at that energy?
Entry filed under: Astronomy.