I just found some answers through Google which linked to this interesting forum. I will take a look.
I found this post concerning the cosmic ray flux wrote by Chelle :
I would like to know, if the flux is 1000 particles/m^2-s-sr-GeV and the "upper half of the sky" is 6.2 sr, so why the flux of particle won't be 6200 particles/m^2-s-GeV per "upper half of the sky" ?A good overview graph showing the cosmic ray flux as a function of energy can be found here: http://en.wikipedia.org/wiki/Cosmic_ray
Look at the first plot on the right. It shows a graph of flux, measured in number of particles per square meter per second per steradian per GeV (10^9 eV), on the vertical axis, versus particle energy, in electron volts, on the horizontal axis. You will see that the largest flux is at low energy (about 10^9 eV), where the flux is about 1000 particles/m^2-s-sr-GeV. That is, in one square meter, looking over the whole upper half of the sky (2 pi = 6.2 sr), in a bandwidth of 1 GeV, one sees 1000 particles every second.
In the fantastic book "The Particle Odyssey A Journey to the Heart of Matter" they says :
I'm trying to find this flux (20 particles per cm²/s) with the cosmic ray spectrum, but it's not convincing. Anyone can help me ?Roughly 20 ‘primary’ cosmic rays per square centimetre arrive each second at the top of the atmosphere.
Thanks a lot,