# What is cosmic ray albedo neutron decay (CRAND)?

I need a small description of what is cosmic ray albedo neutron decay (CRAND), where it comes from and what effects they have?

I've never heard the specific acronym CRAND.

However, cosmic rays which reach Earth's ground level lose energy by, among other processes, spallation on the heavy nuclei in the soil and rock. Some of these spallation neutrons will leak back out of the surface of the ground, having partially thermalized$^1$. These neutrons basically form a very tenuous part of Earth's atmosphere, with a scale height somewhat higher than the nitrogen/oxygen/ozone parts of the atmosphere, because the neturons' temperatures are comparable to the other gas species, but the neutrons are less massive. I think an astronomer might consider these "albedo neutrons from cosmic rays" (your CRAN).

A thermal neutron at the ground, with average speed $v\sim 2\rm\,km/s$, can certainly reach the ionosphere/exosphere within its 1000 second half-life. The fraction that decay above the atmosphere will contribute to the population of electrons and protons trapped there.

My dumb Google search also finds solar proton albedo neutron decay, which sounds like the same effect but triggered by the solar wind rather than by cosmic rays. That would be more important on a planet without an atmosphere or magnetosphere to modulate the solar wind protons.

My slightly less dumb search finds this 2017 Nature paper that says I mostly guessed right, but that my involvement of the ground rather than the upper atmosphere was some terrestrial chauvinism on my part.

$^1$ One of the early papers describing these neutrons has a great comparison between the detectable neutron flux in St. Petersburg in a boat in the harbor (where the spallation neutrons from the seabed are mostly captured in the seawater), over bare ground (poorly thermalized), and over a meter of snow (well-thermalized). All I'm finding today are balloon flights: Flerov et al., JETP 9, 511 (1959); Kopff, Phys. Rev. 59, 949 (1941).

• ::chuckles:: I nominate the use of "very tenuous" here for understatement of the year. – dmckee --- ex-moderator kitten May 6 '18 at 21:48
• @dmckee I'd like to have a number in my head for the cosmic ray neutron number density, to compare to e.g. the metal ion atmosphere of Mercury, but I couldn't do it in fifteen minutes. That seems too homeworky for a followup question, though. – rob May 6 '18 at 22:13
• Unfortunately, the resources that come to my mind immediately are for fairly deep neutrino detectors, so they don't give you the near-surface production rates or the fraction that wander back to emerge above ground. But I think you can get some sense of the scale from the lack of a line-item for this process in background radiation source charts. – dmckee --- ex-moderator kitten May 6 '18 at 22:24