Question about RADDECAY modes

Dear FLUKA experts,

My input file describes a monoenergetic thermal neutron beam which hits a cylindrical target made of B-10 + water and a LaBr3 scintillator, positioned 5 cm away from the target at 90° with respect to the beam direction.
The goal of my simulation is to count inside the scintillator how many 478keV gamma rays from B-10 radiative neutron capture and 487keV gamma rays from the decay of La-140 have occurred during a given amount of time (let’s say a 10 min experiment).
The part regarding the 478keV from the B-10 neutron capture is not a problem since FLUKA uses the pointwise cross section for this element. Indeed, by using a USRTRACK card I can get the fluence in the scintillator and I can recognise the 478 keV gamma.
Instead, the generation of the other gamma occurs as follows: La-139, which is naturally present in the LaBr3 crystal undergoes radiative neutron capture and generates La-140. This isotope is unstable and, with an half-life time of 1.6781 days it undergoes beta- decay (100% of the times) and amongs the various gamma lines there’s my 487 keV gamma. Since it’s a decay product, I’m using the card RADDECAY to take it into account. I’m unsure whether to use “Semi-Analogue” or “Active” as the “Decay” option of the card. From what I understood:
- “Semi-Analogue” picks randomly the decay time (also daughters, radiation but it’s not relevant in my case), but then scores the TOTAL decays, like as if every single La-140 has decayed. This is not what I want sice, being the half life much bigger than the typical duration of the experiment, only a few of the activated La-140 atoms would decay in that time interval.
- with “Active” I can input a given irradiation profile (IRRPROFI) and cooling time (DCYTIMES) and then get the punctual fluence in that cooling time. In the Manual it’s specified that, when using the “Active” mode, the units of the plots are per second, but I don’t think that USRTRACK gets from photons/pri/GeV/cm^2 to photons/pri/GeV/cm^2/s, am I correct?

In the end, I’m unsure which settings to use to get the desired quantity. I’m attaching the input file so that you might take a look.

Thank you in advance,
Giacomo

Test_478.inp (5.2 KB)

Dear Giacomo,
many thanks for your question and welcome to the forum. If I understand correctly, you have the need to estimate both the prompt photons from the neutron capture on B-10 and the “delayed” photons from the decay of La-140.

If this is the case, then using RADDECAY with Active is not what you need since if you score at a give cool-down time you will loose the contribution from radiative capture. Instead I think you should use RADDECAY in semi-analogue mode and associate your desired scoring card (e.g. your USRTRACK) to a DCYSCORE card specifying “Semi-analogue” in the field for the cooling time (WHAT(1) = -1): this will score both prompt and residual particles. You results will still be expressed per primary particle and will be easier to normalize once you know the intensity of your neutron beam.

Your understanding of the semi-analogue mode is also correct: for each single radioactive nucleus a random decay time, random daughters, random radiation are selected and tracked. This does not mean that the information on when the unstable nucleus is made decay (and decay radiation transported) is lost. You can for example use a generic TIME-CUT card to input a time cut-off for photons (in ns!) and discard those that would be emitted after the duration of your experiment.
Note that if you would be interested in energy deposition only, the TCQUENCH card would suffice.

I hope this was helpful.
Best,
Davide

Dear Davide,
thank you for your quick reply. Yes, you understood correctly what I need to estimate and your answer was clear. However, I have additional questions:

  1. I don’t think it’s a problem that in Active mode the prompt gammas are not scored. I added another USRTRACK card without any associated DCYTIMES and from that I get only the prompts. Afterwards I can compare the two, correct?

  2. I found an issue with the TIME-CUT card: my experiment lasts for 10 mins, so 6E11 ns. This number seems to be too big for FLUKA, which gives an error (see attached image “TIME-CUT error”). With smaller numbers the simulation runs properly.

  3. Moreover, I believe that the TIME-CUT approach won’t give me exactly the result I want. If I understood correctly, FLUKA is tracking the particle’s time since when it’s created and, if the elapsed time becomes larger than the cut time I’m setting with TIME-CUT, the particle is not killed. However, in the real experiment, the neutron flux lasts for 10 mins, meaning that a neutron emitted 9m59s after the start of the experiment will have only 1 s to generate valid delayed gammas, so the probability of seeing a 487keV from an activated La140 by this neutron is smaller. Is there a way in which I can implement this?

Bonus question: If I’m scoring photons and I’m also specifying the volume of the target, the USRTRACK output is in ph/(cm^2 pri GeV). I can then multiply the score by the corresponding energy bin width and by the number of primaries expected in the real experiment and I’m left with ph/cm^2. From what I understood, 1/cm^2 is given by track length/volume of the region. How can I retrieve the track length to get the unit to ph? I’m also interested in using the DETECT card instead of USRTRACK, but I can’t find on the manual the unit of its output.

Thank you in advance,
Giacomo

TIME-CUT error

Dear Giacomo,

  1. If you run with RADDECAY active and provided that you entered your correct beam parameters (intensity and irradiation time) in IRRPROFI, you still have to be careful with the units: the photon fluence scored with an associated cool-down time will be expressed in GeV/cm^2/s (so no normalization needed in principle) while the one scored without an associated cool-down time (i.e. prompt only) will be in GeV/cm^2/primary so you need to multiply it by the beam intensity in primary/s (i.e. the same that you place in IRRPROFI).
  2. That is because you exceed the 10 character length for the WHAT(1) of the card. In Flair you can enter directly an expression and it will be formatted in the correct way, e.g. enter directly =10*60*1e-9 or the number with the exponential.
  3. On the contrary, particles having a “life counter” greater than what you set by TIME-CUT are killed. TIME-CUT is meaningful only if you decide to run with RADDECAY in semi-analogue.
  4. Yes, if you give the volume, the USRTRACK output is a differential fluence. You can find the integrated values if you inspect the *sum.lis files. I am not sure I get the question: if you want to obtain the total number of photons entering the scoring volume you should score a current, not a fluence (see USRBDX)
  5. For DETECT (which scores deposited energy) you don’t need to specify a unit, it is always unit 17. Mind that DETECT always scores both prompt and residual radiation. See the page manual in particular note 1 and 3.

Best,
Davide

Dear Davide,

  1. Ok I understand your answer, thanks.

  2. Ok, if I type =10*60*1E9 it works, but it doesn’t if I type 6E11 as I did in the first place. Thanks again.

  3. Sorry it’s a typo, I wanted to write “the particle is killed”. Nevertheless, do you think that my question has an answer?

  4. If I understood correctly, the difference between fluence and current is that the latter isn’t corrected by the cosine of the angle between the surface and the direction of the particle. So indeed that would mean that scoring a current with USRBDX is more meaningful if I want to score the number of the particles entering the volume, correct?

  5. Sorry, I wanted to say which is the unit of measurement of the scoring of the DETECT card. On the manual it’s specified that “scores energy deposition on an event by event basis”, but I can’t find the unit in the two output files *_17.dtc and *_17_tab.lis.

Thank you in advance,
Giacomo

Dear Giacomo,

  1. Unless you are using a free format input, the numerical values of the WHATs should be entered in Flair with =<number> if their length exceeds the 10 characters so that Flair properly formats them. This may not be necessary in the Geometry section since I am pretty sure that you are using a free format geometry (default in Flair).

  2. Apologies for the misunderstanding. With FLUKA you can’t have directly a time dependent source, but your irradiation conditions are reproducible. What happens is that you run a certain number of histories (with neutrons as your primary particles) and for each primary particle there is a certain probability that certain radionuclides may be produced. If you run using RADDECAY in active, the time evolution on the pure radionuclide yield is then calculated analytically (via the familiar Bateman equations) using the user defined time dependent irradiation profile (duration and intensity of your primary source) and all daughter nuclei and all associated radiation are considered at fixed times.

  3. Yes, you understood correctly. The integral (which you can also find directly in the *sum.lis files) of the differential current will give you the average current (particles entering per unit surface area) while the integral of the differential fluence will give you the average fluence across the surface. You can also refer to these slides for a more detailed explanation

  4. With DETECT you should obtain a pulse height spectrum (i.e. counts per primary)

Hope is more clear. Best,
Davide

Dear Davide,

  1. Understood, thanks.

  2. Ok, that’s exactly what I’m currently doing. The only thing that’s still unclear to me is the meaning of “all daughter nuclei and all associated radiation are considered at fixed times”.
    I’ll try to explain my understanding with an example. Let’s say that the activated nuclei A have only one decay branch (say betha-, decay constant lambda with the emission of a single gamma photon) to the nuclei B, with B stable. So, in RADDECAY Active mode, FLUKA would simulate how many A nuclei N_A are activated during the irradiation time. Then, the number of resulting gammas will be calculated as N_A*(1-e^(-lambda*t)), where t is the irradiation duration in IRRPROFI. Am I correct?

  3. and 5. Understood, thank you for the explanation.

Thank you for your time,
Giacomo

Dear Giacomo,

yes, it should be basically like that. In RADDECAY with active mode, decay particles are transported once and then, at scoring time, a factor that depends on the decay time associated to the detector is applied (this is why it is said at fixed time). This factor accounts for the buildup and decay of the parent nucleus at the specified decay time and is essentially related to the Bateman coefficients.

Best,
Davide

1 Like

Dear Davide,
thank you a lot for your answers!

Best,
Giacomo