# Transmission of 5.5 MeV electrons across aluminum

Dear FLUKA Experts,

I’m asking once again for your help. I hope I’m not bothering you too much. I’d like to estimate the number of transmitted electrons through aluminum, on which a pencil beam of 5.5 MeV energy normally impinges. To do so, I score with USRBDX the transmitted current at the aluminum/vacuum exit face and then I integrate it in energy. The spectrum is normalized in such a way that its integral is equal to 1 for the impinging beam. The problem I face is the following: by setting the aluminum thickness to a value so big that I expect no electron should be transmitted, I get a residual value of about 0.03 for that integral, i.e. corresponding to a transmittance of about 3%. This is at least my interpretation as a FLUKA beginner. In the figure, as an example, the current spectrum I get at the exit of 30 mm of aluminum (almost three times thicker than the practical range) – as said, its integral is about 0.03. I would have expected a zero spectrum for such a thickness. Can you please help me understand what I’m doing wrong? Thank you in advance.

Kind regards,
Enrico

elettroni_Al.inp (1.4 KB)
elettroni_Al.flair (2.3 KB)

Dear @enrico.nic,

I think the problem is that USRBDX scores the particles passing through the whole surface of the target. I don’t know if this is the easiest solution, but I created another vacuum region in order to score the electrons passing through that side of the target. Also, there is a difference between scoring the beamparticles and the electrons - electrons will take into account also the secondary electrons generated due to the interaction of the beam with the target.

You can find the modified input attached.

Best regards,
Mihaela
elettroni_Al_modified.flair (3.1 KB)

3 Likes

Dear both, I shall just add to @Mihaela_Parvu’s marvellous explanation (and considerably faster than mine) that the (much fewer) secondary electrons still leaving the target on the side and downstream, are generated by photons, which are produced in turn by the primary beam and are able to penetrate substantially more. Cheers, Francesco

2 Likes

Dear Mihaela and Francesco,

you and the guys of this forum I’ve so far interacted with are wonderfully kind and always helpful! Your explanations are clear also in this case. Thank you.

Dear Mihaela, being on holidays from today I will be able to check the modified input you kindly uploaded only on next Monday. I will let you know for sure. Thank you again meanwhile.

And yes, dear Francesco, I believe your comment on photons fully explains the presence of that residual spectrum. I will check with my colleagues what they detect experimentally for thick Al slabs to see how to proceed with simulations.

Kind regards,
Enrico

1 Like

For the sake of (redundant) clarity, note that the integral you had previously obtained is largely dominated by backscattered electrons, as Mihaela showed, and is not representative of what actually leaks beyond 30 mm. Nevertheless, a tiny fraction of it still corresponds to electrons exiting downstream, as in Mihaela’s second plot. My comment on photons applies only to that tiny fraction, as well as the lateral one, which has no visible impact on your spectrum above, since the latter rather illustrates the backscattered electron population.

1 Like

OK Francesco, thank you for your further explanation!

Dear Mihaela and Francesco,

finally I had the chance to test Mihaela’s modified .flair file and see what she changed in it. I believe I have better digested your comments with Mihaela’s project opened and understood how to proceed. I thank you both again for your kindness and useful help.

Kind regards,
Enrico

2 Likes