End_of_Descent
You are right with the nature of cosmic radiation. The one importand thing witch isnt mentioned yet is what makes a radiation dangerous.
The danger potential in any form of radiation (aplha, beta [and gamma] the exotic) is the potential to ionize the medium through which they travel.
This ionizising potential is interresstingly, low at very high velocity/energy of the charged particles. It only becomes high at lower velocity/energys. Because energy/velocity is lost through by the ionisation of the medium through which they travel the deacceleration of the charged particles is much faster at lower velocity. The distance travled by charged particles where a dangerous high ionization is caused is quit short. It becomes shorter with increased mass of the traveling particle. Thats also the reason why alpha particles arnt that dangerous (high mass).
As far as I know the Muons that are being created by the cosmic radiation have a very high energy and very short lives ( halflife= 2,2 · 10-6 s) , so that nearly all of them decay into electrons(beta minus rays) and positrons (beta plus rays) before they can to any ionization damage.
The Pions deacy into muons and gamma radiation and have a even shorter half live than the muons.
So I believe most of the dangerous radiation at that altitude will be gamma rays caused by the primary shower, exited atoms, secondary showers and decaying neutral pions. The alpha and beta radiation caused at that altitude should not realy pose any threat.
I hope this forum isnt being now abused by me in becoming scientific forum.
Also have to say a disclamer, im doing my masters in materialphysic not in nuclear physic.