Cs 137 spectrum - gamma source or radioactive source?

Hi everyone,

I am trying to obtain the spectrum of Cs137 using the number of scintillation photons produced(reaching the PMT).

However, I am not sure whether to model Cs137 as a source or to directly model a gamma source having 662 keV energy?

Also, if someone can highlight the possible differences between the two it would be very helpful.

Thank You