Dear Geant4 Experts,
I am currently performing a simple efficiency calibration of a NaI detector using standard radioactive sources: 22Na, 60Co, and 137Cs. The simulation setup uses a modular physics list and includes G4RadioactiveDecayPhysics() to model the radioactive decays. I am employing the General Particle Source (GPS) to simulate the sources.
For both 22Na and 60Co, the simulation performs as expected, and the resulting spectra match the known decay characteristics. However, I encounter unexpected behavior when simulating ¹137Cs. The resulting spectrum deviates significantly from what is expected experimentally.
My suspicion is that this discrepancy may be due to how Geant4 handles isomeric states. ( In contrary to 22Na and 60Co, 137Cs decays via beta- emission to the isomeric state 137mBa, which subsequently decays to the ground state of 137Ba with the emission of a 662 keV gamma ray). I am afraid RadioactiveDecayPhysics() does not model these isomeric states by default.
To account for this, I modified my physics list to explicitly allow for isomer production, but unfortunately, the results remained unchanged. I’ve double-checked my source and decay settings and ensured that all relevant processes are registered, yet the simulation does not reflect the expected gamma emission pattern for 137Cs. I have identified that, what comes up in the spectrum of 137Cs is beta particle depositing its energy in NaI, but no 662 keV gammas. I have done a sanity test by replacing the 137Cs with 662 keV gammas, but then the spectrum comes out as expected. I hope someone might have idea why this might be happening.
I am attaching the relevant physics list and 137Cs macro file to provide further context along with the 137Cs and 60Co that I have simulated.
Thank you very much in advance for your time and help!
Best regards,
Gokul
PhysicsList.cc (3.8 KB)
cesium1.mac.txt (384 Bytes)
_Geant4 Version:_11.1.2
_Operating System:_UBUNTU 22.04LTS