Prompt Decay Radiation from Isotopes

Advanced Features and User Routines
5

Dear Davide

Thank you very much for your detailed reply and please forgive me for my basic questions. I’m still a novice in FLUKA. To clarify my post: My goal is to compare experimental spectra with simulated spectra from an NaI(Tl) scintillation detector with radioactive sources. One of these sources is Am-241 and I know its activity as well as the time since its creation in pure isotopic form. Considering my goal, I have to rely on the direct simulation of the radioactive source. Moreover, I use the detect card, because it’s much easier to post-process compared to the eventbin card. The eventbin card stores a lot of unnecessary repetitive clauses, which increase the file size and decrease the postprocessing speed dramatically (EVENTBIN output simplification? - #3 by Fgeser). Based on your explanations, I have the following follow-up questions:

  1. Semi-analogue mode: I still have problems to understand the way FLUKA treats the decay chain of radioactive isotopes using the semi-analogue mode. During a single event simulation (1 primary), does FLUKA simulate only one decay of an arbitrary radionuclide (parent or descendent) or always the parent nuclide decay together with all its descendants nuclides (where the decays are sampled according to the decay scheme, i.e. decay channel probabilities)? In my understanding, the Monte Carlo sampling described by you is equal to say that the radioactive source is in secular or at least transient equilibrium with all its descendants (daughters, grand-daughters,…), i.e. we assume that our original source consisting only of parent nuclides had “enough” time to reach an activity equilibrium with its descendants (at least, that was also stated in the cited paper). Is that correct? If this is not correct, at which “reference” time (where “time zero” is equal to a pure source containing only the parent radioisotope) is FLUKA calculating the results? With other words: the relative activity of the parent nuclide and its descendants is time dependent and one has to provide or assume a time in order to simulate the correct isotopic composition and correspondingly the correct emission spectrum from a radioactive source. Or does FLUKA apply some integration over a time period and hence provide a “time-averaged” spectrum? It is important to add that, at least in my understanding, one has to distinguish between the time, when a single parent nuclide decays (this follows an exponential function as you correctly stated), and the reference time of the source, i.e. the time between the creation of the pure isotopic source and the performed measurement or simulation, when the source contains also descendent nuclides and emits therefore a different decay radiation compared to time zero.

  2. Normalization: You stated that FLUKA will normalize the result per “decay event”. Based on my understanding, the number of “decay events” is equivalent to the number of primaries, which is equal to the number of parent nuclide decays (in my case Am-241 decays) and not the total number of decays (decay of parents but also decay of descendants). Otherwise, one couldn’t use the activity of the radioactive source to scale the resulting simulated spectrum, could one?

  3. RADDECAY: You stated that one can use the RADDECAY card in active mode to simulate the emission spectrum of a radioactive source after a predefined cooling time. I worked always under the assumption that this is not possible for isotopic sources. In the FLUKA manual, there is the following note under the raddecay card:

“8. When the source is a radioactive isotope (defined by command BEAM with SDUM = ISOTOPE and by command HI-PROPErt), RADDECAY must be used in semi-analogue mode (WHAT(1) > 1). The detector results are then expressed per isotope decay. Note that command DCYSCORE must be issued with WHAT(1) = -1, and must be applying to all relevant estimators and detectors. Without DCYSCORE, no scoring will occur (see Note 8 to command BEAM). “

So, I’m still confused, can I use now the active mode for isotopic sources or not?

  1. Alpha Emission: You stated that no alpha particles are emitted from isotopic sources in FLUKA. So I wonder, is the energy of the alphas deposited directly at the parent source? Is there any simulation of secondary electrons or X-rays created by the alpha particles. Is there a way to activate the alpha emission?

Thank you very much for your help and support
Take care
David