Trouble with Primary Weights, USRBDX, and USRYIELD


I am attempting to model an electron beam (~35 MeV) impinging upon a metal bar. I want to see the bremsstrahlung photon spectrum. I am able to generate plots in FLAIR for this quantity using the USRBDX scoring cards, but the reported particle weights in the respective files are greater than 1. To my knowledge, I have not altered any source weighting. This leaves me questioning the results. I have referred to the manual for the USRBDX card, but can not find any clear issues or solutions. I suspect the “area” input of WHAT(6), but I am entering the surface area of the designated region and do not know what else to do.

Additionally, I can not get the USRYIELD card to provide any data to plot. The “Data Processor” function of FLAIR does not return any files for the given location (54BIN).

Any assistance would be greatly appreciated. I have attached the input file.

Thank you,
Brem_Target.inp (2.3 KB)

Dear @RJacob,
thank you for your question. If I understood what you mean, when you refer to the total weight of the primaries run, it is correct that it should be more than 1 (see below a snapshot from a *sum.lis from your simulation). Dividing by the total number of primaries you would get that, since there is no biasing applied, the weight of each primary particle is 1.0 as it should. What you have done with the area normalization is also correct.
I didn’t have problems generating the *lis files for the output unit 54 but maybe you had run a simulation and added the scoring at later stage or you were opening the *inp file instead of the flair project. What you can do (see below) is either click on “New” adding the usryield detector and then “Add” the fortran files corresponding to unit 54 or you could select all the detectors and then click on “Remove” allowing Flair to automatically refresh and detect the scorings.

I hope this was helpful. Please let me know in case you still encounter some issues.
All the best,

If you wonder why the resulting electron current, integrated over the considered surface, is larger than 1 (=1.11 in Davide’s sum.lis), this is due to secondary electrons generated in the metal. As for the spectrum units, as a function of energy, these are GeV^-1 cm^-2, so the value 1 does not represent a meaningful expectation.

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Hi Davide,

I removed the detectors and forced FLAIR to refresh and it found the unit 54 detector immediately. Thank you for your input. My (poorly asked) question about weighting was referring to the particles/primary number of 1.11, but I believe that was answered by another user.


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