actually that’s a rather interesting question.
I was thinking, maybe, the actual particles that get shot out from the suprenova, are helium nucleus that get decomposed halfway, or even arrive at our atmosphere and then they decompose into hidrogens.

Also, take into account that a mean lifetime of 15 minuter means that it is probable that every 15 minutes that particle may OR may not cease to exist(or simply recompose somewhere else).

I hope I helped. Sorry for the misspellings, i’m typing from an iphone lol.

I was initially thinking along the lines of Civil, but after some quick calculations, it just doesn’t work. The upper end of cosmic ray velocities is around 0.99c, which leads to a gamma factor of 7.1. The 15 minute lifetime would appear as around 1 hrs 45 min to an outside observer. Also, the distance the neutron would be traveling is still millions of lightyears, which would take millions of years in the neutrons inertial reference frame.

I know that when cosmic rays enter our atmosphere they collide with atoms and molecules, ejecting free neutrons (among other things).
drkztan is also correct in pointing out that the mean lifetime is not the exact lifetime of all neutrons.

Interesting question. I’ll do some more research