I am trying to calculate the true coincidence summing factor (TCSF) for a HPGe detector for gamma-ray sources like Ba-133, Eu-152, and Co-60. My approach is as follows:
1. I define the sources as isotopes using the BEAM card. I assign the radionuclides Ba-133, Eu-152, or Co-60 using the HI-PROPErt card. Radioactive decay is simulated in semi-analogue mode using the RADDECAY card, and scoring of decay particles is enabled using the DCYSCORE card. The USRBIN card is used to score the energy deposited per primary decay. The DETECT card is employed to obtain the energy deposition spectra. Gaussian energy broadening is implemented in the DETGEB card using experimentally obtained FWHM as a function of gamma energy. The full-energy peak efficiencies (FEPE) at various energies are determined from these spectra.
2. The process is repeated with individual gamma-ray lines (monoenergetic) corresponding to the emissions of the above radionuclides. Again, the FEPE values are determined at various energies.
3. The FEPE values obtained from step 2 are divided by those obtained at the corresponding energies from step 1 in order to determine the TCSF values.
4. The experimentally obtained FEPE values at various energies are compared with those obtained from step 1 for validation of the Monte Carlo model. These experimental FEPE values can then be corrected by using the TCSF values obtained from step 3.
The methodology you describe would be (almost) correct if the FLUKA ISOTOPE source generated correlated decay products (i.e. realistic combinations of decay products in each history). This is not the case, as the products are sampled from the full distributions. The simulations in your step 1 will therefore not implicitly reproduce summing effects. For a simple case like Co-60 you could use a source routine and load the two main photons together in the stack. For more complex decay schemes like Eu-152 this is not really practical.
Work on generation of correlated decay products is ongoing, but I am not sure when it will be available.
Why “almost”? when defining an ISOTOPE source, the results are expressed per decay of the source. When simulating a monoenergetic beam, the resulting values from the DETECT output are per emitted photon, and should be multiplied by the transition intensity I_\gamma (emissions per decay) to obtain a FEPE.
With the release of FLUKA-4-5.2 I found the following in the release notes “In low-energy-neutron-induced fission, the relaxation of excited fission fragments is now performed by FLUKA’s gamma de-excitation module (which provides a correlated gamma cascade), instead of relying on evaluated nuclear databases (which just provide inclusive photon spectra).”
The recent improvement you mention deals with a different problem and does not apply to radioactive sources, for which work is ongoing to eventually enable correlated decay emissions based on data (branching ratios, intensities etc.) from relevant databases.