# How many anti-particles hit the ground?

I am curious to know the amount of flux of anti-particles that arrive to the ground in the cosmic rays. The reason is that I thought it should be very improbable that an anti-particle traveling through the atmosphere to the ground will survive being annihilated during its journey. Nevertheless, the positron was discovered on the ground, without waiting much for it. Therefore I want to evaluate the probabilities. I have two questions regarding this issue:

1- How much is the flux of anti-particles hitting the atmosphere from space.

2- Can someone show me how to calculate the probability for a positron or an anti-proton to be annihilated before hitting the ground ?

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By the time you reach inhabited elevations most of the cosmic flux is in the form of (anti)muons, and they will not annihilate with anything they find along they way, rather you have to wait for them to decay to get either electrons (positrons). Plus some neutrinos, of course, but you don't detect those. So the problem can be roughly simplified into figuring the proportion of antimuons, which I have a vague memory may be on order of a third. – dmckee Aug 26 '12 at 19:11
I assume most anti-matter reaching the ground is not cosmic rays themselves but was actually formed in the atmosphere by cosmic ray collisions with the atmosphere. – Brandon Enright May 29 '13 at 5:16

2) The problem of interactions with air affects all charged particles we might want to detect on the ground. Electrons and positrons are discussed in the 1937 article "The Passage of Fast Electrons and the Theory of Cosmic Showers" by Bhabha and Heitler (can't seem to find a working link any more). Here, though, they mention a mechanism that does allow high-energy particles to make it through the atmosphere. When a single particle interacts, it might have so much kinetic energy that its byproducts are still energetic enough to cause further showers (and that energy need not be equally distributed amongst its byproducts). As they say, "The chance that an electron of $10^{12}$ e-volts should penetrate to sea-level and retain an energy $> 10^8$ e-volts is only about $10^{-5}$. We shall try to show in this paper that these difficulties are only apparent..."