I’m asking once again for your help and I hope I’m not bothering you too much.

I’d like to simulate the interaction of photons with lead. Then I tried to estimate the transmission of photons, not included the reflection of any particle.

Dear @Zong,
can you please specify more accurate what physical value do you want to obtain, like input to output fluence/current ratio over surfaces perpendicular to the beam?
General recommendations are to make scoring of photons crossing boundary 1) the region before the target and target and (incoming photons) and 2) between the region after the target using USRBDX card.
After you can calculate ratios you need.

Please let me know if you need more detailed comments.

I’d like to estimate the transmittance of 1mm lead (or other materials), then know how many photons have passed through the lead target.
So I tried to subtract the deposited energy from the incident energy to get the transmitted energy at first，but I found that some photons would reflect on target surface and didn’t pass through the target. This made my estimated transmittance too large.

Can you please teach me how to solve this problem?

Dear @Zong,
the answer of Mihaela is absolutely correct and comprehensive. It remains for me just to provide you some guidance how to solve your problem in your Fluka code.

You do not need to score energy, but either photon fluence or number (current) of photons leaving the target.
Number of primary photons that penetrate into the target you can take as 1 or according to the normalization of the number of primaries.

To know exact number of photons per primary you may score the number of photons that crossing the border surface between the target and region after it. You should create a “virtual” region after the target to cross “transmitting” photons but not “reflected”.

To find the ratio between photons passed through the target to the number of source particles you have to integrate obtained spectrum (pay attention to the energy binning, it may be calculated in particles PER energy range, but not particles IN the bin depending on how do you plot it). You can also find an exact number of particles per primaries, searching in the output *sum.lis file. It should be similar to this:

Take into account, that scored photons will include not only primary particles but also contribution of secondary photons, if it is somehow important for your physical model.

Do not forget to adjust scored energy range to your beam energy value if you what to plot the spectrum.

It can be necessary to adjust particle transport threshold in your model for correct scoring of energies of secondary electrons with energies <10keV, it may be relevant for the beam energy below 100 keV.

Please, find example flair file attached. photon.flair (2.4 KB)

Dear Mihaela,
If I integrate the data in the 1D image, will I get the total number of photons in the target?
And Could you please teach me how to plot the second and third pictures that you uploaded?
I hope it doesn’t take you too much time.

The total number of photons in the target is a meaningless concept, as recently pointed out. To estimate the transmitted photon fraction you should score photon current on the downstream boundary, as @illia.zymak detailed above.

Tot. resp. (Part/cmq/pr)

is the integral of the energy spectrum on the boundary divided by the input boundary area in cm^2.

As Mihaela has explained, Part/cmq/pr mean number of scored particles per cm2 per number of primary beam particles.

You can try to do this if according to parameters of your model, you expect that significant number of secondaries may have energy below 10keV. Or just change it and check the difference. I believe that for 100keV and 1 mm target, it may have effect to resulting accuracy.

Scoring fluence or current is more about your task than accuracy. Fluence will give absolute value of the vector quantity according to the flux definition and is typically used for transport phenomena, while current will score just number of photons crossing the surface without disregarding their velocity vector.