In the input file, there is a sphere made of Pu-238, and the calculation is aimed at determining the isotopes formed as a result of natural decay. Although the file includes the DCYTIMES command with specific time points at which the isotopic composition should be evaluated, it’s unclear how and where this should be applied in the input text. In the RESNUCLE card, I specify Semi-Analog in the field where the time from DCYTIMES should be entered, because if I actually enter a time value, the output results turn out to be zero. How can I correctly obtain the number of isotopes formed after a given period — for example, after 1 day, 1 year, or 80 years?
If you are just interested to calculate activities in a decay chain for specific decay times there might be easier ways to achieve this than using FLUKA, e.g. Nucleonica .
See this plot:
Still, if you want to do this with FLUKA, it has to be set up as I illustrate in the attached modified input file.
To explain the differences with respect to your input file:
You correctly decided to use a HEAVYION beam (in the BEAM card), which produces Pu-238 ions that are stopped almost immediately and then considered as unstable residual nuclei. FLUKA then treats them in the decay step and calculates build-up and decay.
The PHYSICS cards with DECAY in your input do not refer to radioactive decay of isotopes.
You can virtually select any kind of target material (here I chose arbitrarily Cu), as long as you adapt the Max Z and Max M values in the RESNUC card to effectively score your Pu-238 and daughters. The material selection of your target region has no effect on the source. Defining and selecting Pu-238 as material does not make it a radioactive source.
To calculate isotopic activities at specific decay times, you need to run in activation mode: You require RADDECAY, IRRPROFI, DCYTIMES and DCYSCORE cards, where you have to select the activation mode.
The buildup in the IRRPROFI is simply a 1 second irradiation with an intensity equal to the inverse of the decay constant - such we produce just the number required for 1 Bq of initial activity, which simplifies the output reading.
For each of the decay times, you need to define a RESNUC card with an associated DCYSCORE card.
One note: FLUKA filters the products from the decay chain shown in the output. After 80 years of decay, you only see the remaining activity of Pu-238, and the buildup of U-234, but not yet the following daughters. For longer decay times they will eventually show up.
