A possible way to measure the ancient speed of cosmic ray strikes using “ paleo-detectors ”

An international team of researchers has proposed a way to indirectly measure the speed of cosmic rays hitting the Earth over millions of years. In their article published in the journal Physical Review Letters, suggest using the imprints made by atmospheric neutrinos in the so-called “paleo-detectors”, natural minerals that express traces of damage deriving from nuclear recoils.

At every moment of every day, the Earth is bombarded with cosmic rays, most of them are light nuclei and protons. And as those cosmic rays pass through the atmosphere, some of them collide with atoms, tearing them apart and causing the production of neutrinos, which rain down on the planet. Astrophysicists have noted that one method of studying the history of this bombardment could reveal more about the sources of cosmic rays. In this new effort, the researchers suggest that rocks hidden deep beneath the Earth’s surface could hold just such a record.

Previous research has shown that when neutrinos are produced in the atmosphere, they can pass through the Earth and head into space. But for some, the journey ends when they collide with atoms within the rock deep beneath the Earth’s surface. This creates a path of debris that displaces some of the atoms in the crystal that encompasses the rock. Physicists call them nuclear recoils.

Researchers suggest that it should be possible to extract some of this rock and study nuclear recoils to learn more about the history of cosmic rays hitting the planet. To find out if this might be possible, the team conducted experiments to estimate the likelihood of finding nuclear recoils in various types of rock. The experiments led to the construction of computer simulations. And simulations have shown that the estimated length of some nuclear recoils should be between 2 and 20 or 50-100 micrometers, distances that would be distinguishable from other background sources. The researchers suggest that nuclear recoil found in deep rock samples could provide a record of cosmic ray attacks dating back 1 billion years. Getting those samples, they note, would require some effort – traditional boreholes would expose the samples to the hits that are happening today, so they would have to be shielded in some way.

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