Cosmic Ray lifetime in Galaxy
CR cannot travel through however - all high-m particles break up.
Notes:
Estimates show that cosmic ray particles cannot pass through more than 50 kg/m^2 of material however, because even the particles with the very highest masses will have broken up.
We can use this number to calculate the lifetime of cosmic rays in the disk of our galaxy.
The distance travelled is roughly equal to the speed of light multiplied by the time taken. So the amount of material that a particle passes through is the distance times the mean density, ro, and this, we know, is a maximum of 50 kg /m^2.
Since the ISM is dominated by protons and there are about a million protons per cubic metre in the ISM - and the mass of a proton is 1.67e-27kg, we calculate a lifetime of 3 million years for a cosmic ray particle. Note however, that this could be 10-100 times longer in the halo of the galaxy where the density is lower.
So, since the disc of our galaxy is only 10kpc, it would take only 3000 years for the particles to escape (if they’re moving at v~c), however the Galaxy’s magnetic field would trap them forever.