Zenith angle distribution sampling in CRY

Hi,everyone, I used CRY to generate muon then statistics incoming angle distribution. I found that the number of muon is most at ~35 degree, which is contradict with the theory of I=I0cos(theta)^2. Some articles said it is caused by solid angle effect, but i am still confusing which zenith angle have most muon, 0 or 35 degree?

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Yes, it is a solid angle effect. Or perhaps more accurately, a phase space effect. The solid angle is proportional to theta^2, so a constant distribution in solid angle comes out as a proportional to theta at small angles. It is more customary to bin in cos(theta).