Defining an AmBe neutron spectrum

I perform an activation study to estimate the residual dose rate from the activation drum. I use 241Am-Be with the energy sampled of the respective spectrum (I set the distribution to be isotropic in the BEAM card). Again, I have neutron emission rate (n/s) obtained using the relation from literature.

Different energy ranges in the spectrum have different intensities, and I know I have to define the irradiation profile to start the simulation.

I am confused. What neutron intensity do I consider to define irradiation profile in this case?
Thank you

Dear Vincent,

Thanks a lot for your question and let me see if I understood it correctly.
The irradiation profile is set by the IRRPROFI card, in which the subsequent time durations (in seconds) and the beam intensities (in particles per second) are defined. If you know what the irradiation intensity will be you can insert a IRRPROFI card and will get the results for the specified intensities. The energy spectrum and the particle type are set in the BEAM card, so they do not have to be specified in the irradiation profile. And please do not forget to switch on the radioactive decays with the RADDECAY card.
Please tell me if it answers your question.

Best wishes,

Thank you for responding.
My point is, I get the neutron emission rate that I think is “average emission rate” in Be(\alpha, n) reaction when using the activity-to-emission rate conversion factor. My understanding is that I do not have to use emission rate as the beam intensity to feed to IRRPROF cards.
Let us also look at the Am-Be energy spectrum below. Different energy ranges have different intensities. I am confused about what intensities are appropriate to use for my simulation without underestimating the dose rate because I am also considering the neutron intensities at different energies? My understanding is that irradiation intensities, in this case, will come from intensities in the spectrum.
Not unless Fluka can allow for specifying different intensities at different energies.

I am sorry about that picture. I meant to upload this one here

Dear Vincent,

thank you for the clarification of your question! Indeed, in FLUKA one can specify different intensities at different energies with help of source user routines - are you familiar with those? There is an intro e.g. under the following link:

This helps you to set up the source with the neutron spectrum corresponding to the distribution in the figure you have attached. Then, if you are interested in the resulting activation, RADDECAY card should be used as described above. In particular, the irradiation profile should give the (energy integrated) neutron emission rate, knowing that your primary particle is one neutron of energy randomly sampled from the spectrum you have defined.

I hope this helps, please keep us updated.

Best wishes,

Dear @Motlatsi_Vincent,

in addition to Alex’ answer, I would like to point you to this topic:


I am not familiar with the routines so I first have to visit their content. I will keep you updated.

Thank you @horvathd, I am using that already

I went through subroutines and I got familiar with source.f for my case.

I am interested in the residual dose rate after irradiation in the activation drum. I have successfully used the same input file before, but for a different purpose.

The problem arises when I have to set the irradiation intensity that corresponds to the intensity of neutrons as seen from the figure above. Which why I asked if, for setting irradiation intensity, FLUKA allows for specifying intensity at different energies as it appears in the spectrum.

Any suggestions? Here is my input file and spectrum (2.0 KB) AmBe90.txt (2.2 KB)

As neutron irradiation profile, you do not need to input specific intensities for different neutron energies (which is not possible in the IRRPROFIle card), rather the neutron emission rate (neutrons per second) integrated over the energy spectrum, as @agerbers indicated above. In parallel, you need a source routine to sample at each primary event the neutron energy according to your spectrum, meaning that you will select more frequently (or with higher statistical weight) certain energy values, as the spectrum prescribes. For this purpose, refer to @horvathd 's post above. Cheers