Hello Balaram,

you can use an input file with historgam data. You can find an example with Co60 in the Geant's example folder:

... \examples\extended\radioactivedecay\rdecay01\

or a user defined energy historgam using General Particle Source, as here:

http://hurel.hanyang.ac.kr/Geant4/Geant4_GPS/reat.space.qinetiq.com/gps/examples/examples.html

Hi Balaram,

There is a free G4 compatible library of radioactive sources "RadSrc"

http://nuclear.llnl.gov/simulations/radsrc_v1.6_doc/radsrc.pdf

Source code is  available to download at Livermore lab site.

This one is really helpful if you have to deal with ageing of radioactive sources.

Mikhail Demichev could you recommend how to load Pu-Be spectrum to particleGun and which format ?