That is, the distribution is normalized as it should.
The higher the π0 energy the larger is the spread in the γ -ray energy spectrum. For mono-energetic source of π0 , we will have a rectangular distribution of γ -ray energy as in Fig. 6.14. But if the γ -rays are observed from π0 , of varying energy, as in cosmic ray events the rectangular distributions may be superimposed so that the resultant distributions may look like the solid curve, shown in Fig. 6.15.
Note that if the π0 * were to decay at rest (γ = 1) then the rectangle would have reduced to a spike at E = 67.5 MeV, half of rest energy of π0.
