A possible way to measure the ancient rate of cosmic ray strikes using ‘paleo-detectors’

An international team of researchers has proposed a way to indirectly measure the rate of cosmic rays hitting Earth over millions of years. In their article published in the journal Physical examination letters, they suggest using fingerprints made by atmospheric neutrinos in so-called “paleo-detectors” – naturally occurring minerals that express traces of damage resulting from nuclear recoils.

Every moment of every day, the Earth is bombarded by cosmic rays – most of them are light nuclei and protons. And when these cosmic rays pass through the atmosphere, some of them collide with atoms, destroying them and causing the production of neutrinos, which rain down on the planet. Astrophysicists noted that a method to study 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 such a record.

Previous research has shown that when neutrinos are produced in the atmosphere, they can travel all the way through Earth and head into space. But for some, the journey ends when they collide with atoms inside rock deep below the Earth’s surface. This creates a debris path that moves some of the atoms in the crystal that includes the rock. Physicists call them nuclear rollbacks.

The 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 was possible, the team conducted experiments to estimate the probability of finding nuclear recoils in various types of rocks. The experiments led to the construction of computer simulations. And simulations have shown that the estimated length of some of the nuclear recoils should be in the range of 2 to 20 or 50 to 100 microns in length – ranges that would stand out from other background sources. Researchers suggest that nuclear recoil found in deep rock samples could provide a record of cosmic ray impacts dating back 1 billion years. Obtaining those samples, they note, would require some effort – traditional drilling would expose the samples to the impacts that are occurring today, so they would have to be protected in some way.

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