Deviation of energy deposition calculated by FLUKA , stopping power and MCNP

Scoring
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Dear experts, I simulate energy deposition of electron beam shooting a thin Ti window. The energy deposition is used for the temperature calculation of Ti to confirm that the material is safe. The energy of the electron is 0.5MeV，and the thickness of the Ti is 0.1mm and the radius of 8cm. I use three different methods to make it, including the FLUKA with score/usrbin, the stopping power and MCNP with *F8 card. But the results of the FLUKA is almost twice of the result of the stopping power and MCNP with *F8 card.

The model is quite simple but I the deviation of the results confused me a lot. Here are the details of the calculation.

  1. FLUKA: The energy deposition in the Ti window from score card is 0.12875MeV/primary, and is 0.1277MeV/primary from usrbin card (the original results of 6.3520E-05GeV multiply by the volume of the Ti window, which is 3.1415988*0.01cm3). I also calculate the contribution of electron with the AUXSCORE card and the result is 0.1277MeV/primary, which shows that contribution from the electron is dominated and the contribution from photon could be negligible.

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  1. The stopping power is from the NIST ESTAR database（https://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html）， as follows.
    The stopping power for Ti is 1.470MeV/cm2/g for 0.5MeV. since the density is 4.54g/cm3, then the stopping power is 6.67MeV/cm and the total energy loss in the Ti window is 0.0667MeV.
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    image552×538 160 KB

  2. MCNP: The model is exported from FLUKA to MCNP. And the information of sdef is added.
    The calculation is used by the *F8 card and the energy deposition is 0.0586MeV.
    image

Could any expert help to explain the difference of the results, especially that the results of the FLUKA is almost twice of the result of the stopping power and MCNP with *F8 card.

Thank you.
mcnp.inp (896 Bytes)
mcnp.out (15.7 KB)
DC gun.flair (1.5 KB)
DC gun.inp (1.2 KB)

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Hallo, the reason is multiple scattering, which is taken into account by FLUKA and makes its calculation more accurate. At such an energy, the electron does not go straight through the 0.1 mm window, but is deviated by the Coulomb field of titanium nuclei, which makes its path inside the material longer, implying a larger energy loss.
You can see this by suitably increasing the number of radial bins of your USRBIN scoring, such as to get a sub-mm resolution and display that the energy is not deposited only in the central bins on the beam axis, but also at larger radii.
For academic purposes, you can also unrealistically suppress multiple scattering (by means of the MULSOPT card, switching off MCS of electrons in titanium) and you will get back the NIST value.

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Thank you for your kindly help. It helps me to understand the physic process. I add MULSOPT card to replace the multiple scattering with single scattering, but the results seems no change. could you help to have a loot at the project file? thank you very much.
DC gun.flair (1.5 KB)
DC gun.inp (1.3 KB)

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I told you to use the MULSOPT card to switch off MCS, not to replace it with single scattering (which is a more accurate scattering treatment). In the input file, it should look like:

MULSOPT                              3.0  TITANIUM

But please note that this is just to convince yourself that this way you get back the NIST value.
For your real calculation, you should not switch off MCS, since - as you saw - it plays a relevant role making your simplified expectation wrong.

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I got it. You are right and the real calcuation should including the process of multiple scattering. Thank you very much for you help. Have a nice weekend.

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