Unexpected Energy Deposition in Photon Beam Simulation

[talktoshubhama (Shubham Agarwal)]

Dear Expert,

I have a question regarding energy deposition behavior when I simulate a photon beam. When I use an incident photon energy of 33.3 keV (3.33e-5 GeV), the beam stops at the first absorber, which is aluminum, and there is no energy deposition in any layer of the structure. However, with a slightly higher photon energy of 33.4 keV (3.34e-5 GeV), I observe energy deposition across all layers.

For example: (values are for just reference)

At 33.3 keV: 0 0 0 0 0 0 0 (no energy deposition)
At 33.4 keV: 4 2 1 0.5 0.4 0.2 0.1
I initially expected that if I gradually increased the incident photon energy, it would yield a more progressive deposition pattern, like this:

0 0 0 0
1 0.5 0 0
2 1 0.3 0
2.5 1.2 0.1 0

In other words, I expected that at lower energies, only the initial layers would show energy deposition, gradually extending to other layers as energy increased. I understand that deposition patterns depend on the incident energy, but the sudden transition between no deposition at 33.3 keV and all layer deposition at 33.4 keV seems unusual.

Could you help me understand this? I am attaching the relevant files for reference.

Thank you!
b_canon_all_geometry.inp (11.7 KB)

b_canon_all_geometry.flair (11.1 KB)

[horvathd (Dávid Horváth)]

Dear Shubham,

please have a look at this post:

USRBDX and USRTRACK

Dear Chailin, The default minimum particle kinetic energy in FLUKA using the PRECISIOn default is generally 100 keV, except for photons and neutrons. In case of photons it is 33 keV and 1e-5 eV for neutrons. The thresholds can be lowered using the EMFCUF card to 1 keV for electrons and 100 eV for photons. You can read more about the EM thresholds here: Cheers, David

Cheers,
David

[talktoshubhama (Shubham Agarwal)]

Dear David,

Thanks. It works, although it takes much longer time to run. I added the cards in the following way with MAt : blckhole to @lastmat because geometry uses many materials. Is that ok? just to be sure.

Thanks

Reagrds
Shubham

[ceruttif (Francesco Cerutti)]

@talktoshubhama

It works, although it takes much longer time to run.

Of course: the code needs now to track (much) more secondary particles, down to a significantly lower energy.
The electron threshold often dominates the simulation time.

@talktoshubhama

blckhole to @lastmat because geometry uses many materials. Is that ok?

Yes.

[talktoshubhama (Shubham Agarwal)]

Dear Francesco,

Thanks.