Study the decay of daughter nuclides up to a certain point in time

Dear FLUKA community,

I’m interested in simulating a Th-232 source which is not in secular equilibrium with its daughters.

I would like to explore, for example, how the spectrum of emitted photons look like after a decay of 5 years, after 10 years, and so on, until equilibrium is reached, starting from “pure Th-232”.

I read in other posts (as e.g. this one) that the card TIME-CUT could be used for this purpose.

If I understand correctly from the manual entry on the TIME-CUT card, “If the source has been defined as a radioactive isotope (command BEAM with SDUM = ISOTOPE), transport of each isotope decay secondary starts with an age equal to the time of decay.”, meaning that I should set the cut-off time as the time of decay of Th-232 + 5 years, + 10 years and so on. Is this correct?

Given that the cut-off time in the TIME-CUT card is expressed in nanoseconds, it is very difficult to set the half-life time of Th-232 + 5 years in the available 10 digits of WHAT(1).

Is there something I’m misunderstanding? Maybe a better way to tackle this problem? Is the only solution to define the source.f with the correct gamma spectrum?

Thank you all very much in advance for any help you provide!

Best regards,
Federico

Hi Federico,

The decay of rad. isotopes with the BEAM card is only allowed in ‘semi-analogue’ mode, but not in ‘activation’ mode (with IRRPROFIle and DCYTIMES).

To quote the manual BEAM card Note 8:

As the emitted secondary radiation from decay products are inclusive (“semi-analogue”) decay secondaries are sampled over the whole decay time from zero to infinity, and all scoring will refer to the time integral of isotope activity (dose, fluence, current, yield or residual nuclei per decay , not the corresponding rates at particular decay times as it happens in the activation study mode).

The ‘time of decay’ is a random time sampled between zero and infinity, not the half-life or lifetime. You would not be able to eliminate any short-lived daughter by setting a TIME-CUT. In the referenced post this works somehow because of a short-lived mother that should be isolated from very long-lived daughter isotopes. (thanks to @ceruttif for pointing out)

So in short, there is no way at the moment to sample a spectrum for a specific moment in time, starting from an isotope defined in the BEAM card.

Indeed, you would need to compose your spectrum with other tools and sample from this spectrum.

Cheers,

Markus

Hi Markus

Thank you very much for your answer.

I thought this might be the case.

Do you think a solution like the one proposed in this post of using a usrmed.f subroutine is also not applicable in this case?

Best regards,
Federico

Hi Federico,

indeed I believe there is a way to isolate a single isotope using the usrmed.f routine. It seems that the variable IAZTRK kindly provides the origin of the particle and could be used to filter.

      INCLUDE 'trackr.inc'
      IF (IAZTRK .NE. 90232) THEN
         WEE = ZERZER
      END IF

Cheers,

Markus