A question about the I125 energy spectrum

I was recently doing simulations on the I125 radioactive source and in my experiments I was using GPS to create primary particles and I defined my radioactive particles in the macro file like this.

/gps/source/add 1
/gps/particle ion
/gps/ion 53 125
/gps/pos/centre 0 0 0 
/gps/pos/type Plane
/gps/pos/shape Annulus
/gps/pos/rot1 1 0 0
#/gps/pos/rot2 0 1 0
/gps/pos/radius 0.5 mm
/gps/pos/inner_radius 0 mm
/gps/ang/type iso

But I have seen in some papers that some experimenters use Primary photons when defining I125 originating from 125I with a spectrum.

I would like to know if there is any difference between the two and if I want to use the energy spectrum to define I125 I should use G4 in how to achieve this.

See the arbitrary spectrum definition for the General Particle Source.

Thank you for your reply I will log on to this site to find the answer