Hello,
You can use the General Particle Source (GPS) to place Cs137 at rest in your simulation. It then decays and you get the Gammas.
Macro command to place ions with GPS:
/gps/particle ion
#Atomic Number / Atomic Mass / Charge of ion / Excitation energy in keV
/gps/ion 55 137 0 0 #Cs137 - 662 keV gamma from Ba137*
/gps/position 0 0 0 #GPS position of point source
/gps/energy 0. eV #at rest to study decay
/run/beamOn 1000
However, what you have attached is NOT the gamma Spectrum of decaying Cs137 - this would be a peak at 662 keV.
What you have attached is the detector response of Cs137 with the 662 keV doing different interactions. If you want to feed this into your simulation, you should take a look into this (maybe):
Can someone explain how to use a histogram energy spectrum as a source input for GPS - Particles, Track, Event, Run and Biasing - Geant4 Forum (cern.ch)
and use the GPS with an input histogram of your plot.
Regards
Max