No Dose Scored Outside a 40 keV Electron Beam Dump: Questions on Physics Settings, USRBIN Normalization, and Lead Shielding

Scoring
1

Versions

Please provide the used software versions.

FLUKA4-5.0
Flair:3.4-1

Description

Dear FLUKA users and experts,

I am currently simulating a 40 keV, 30 mA electron beam incident on a beam dump. I would like to install a lead enclosure around the beam dump and determine the required lead thickness so that personnel can safely remain at a distance of 2 m during long-term operation.

The absorber inside the beam dump is made of copper, while the outer casing and beam pipe are made of SS304 stainless steel. As a first step, I would like to simulate the unshielded baseline case. Therefore, I have temporarily assigned AIR to the region that will later contain the lead shielding.

However, in the current results, there appear to be almost no scored particles or dose outside the stainless-steel casing or in the surrounding air. The relative statistical uncertainties are also very large.

I would therefore appreciate your advice on the following questions:

  1. Do the material definitions or physics settings in my input file need to be corrected?

  2. Are the current electromagnetic transport settings appropriate for 40 keV electrons and the bremsstrahlung photons they produce? In particular, which transport and production thresholds should be used to obtain a reliable unshielded baseline result?

  3. When scoring DOSE-EQ with a USRBIN detector, how should the results be normalized correctly for a continuous 40 keV, 30 mA electron beam? I would like to obtain the dose-equivalent rate in units such as μSv/h.

  4. Which additional scoring cards or detectors should be added to evaluate the dose rate at 2 m from the beam dump and determine whether it meets the criterion for long-term personnel occupancy? Would USRBIN together with point, surface, or boundary-crossing scorings be recommended?

I have attached my FLUKA input file for reference. Any suggestions regarding the geometry, material assignments, low-energy electromagnetic transport settings, variance reduction, scoring configuration, and normalization would be greatly appreciated.

Thank you very much in advance for your help.

Input files

Please upload all relevant files. (FLUKA input file, Flair project file, user routines, and data files)

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EleBeamDump.inp (6.9 KB)

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3

Dear @chengjiang,

Thank you for your question.

  1. Do the material definitions or physics settings in my input file need to be corrected?

Both the material definitions and physics settings are correct.

However, I noticed that you also have a MULSOPT card to switch on single scattering that is currently commented out. Given the low energy of your primary electron beam, this card should be kept active.

  1. Are the current electromagnetic transport settings appropriate for 40 keV electrons and the bremsstrahlung photons they produce? In particular, which transport and production thresholds should be used to obtain a reliable unshielded baseline result?

Your current production and transport thresholds set via the EMFCUT cards are adequate for your study.

  1. When scoring DOSE-EQ with a USRBIN detector, how should the results be normalized correctly for a continuous 40 keV, 30 mA electron beam? I would like to obtain the dose-equivalent rate in units such as μSv/h.

When scoring DOSE-EQ, FLUKA expresses the results in pSv per unit primary weight. To convert this to a dose rate in μSv/h, you first need to convert the beam current into a primary particle rate (electrons/s) as shown below:

893×145 8.04 KB

You then convert from dose-eq. (pSv/primary) into dose rate (μSv/h) as:

1377×199 16.1 KB

  1. Which additional scoring cards or detectors should be added to evaluate the dose rate at 2 m from the beam dump and determine whether it meets the criterion for long-term personnel occupancy? Would USRBIN together with point, surface, or boundary-crossing scorings be recommended?

USRBIN detectors scoring DOSE-EQ are sufficient, provided your meshes cover the regions of interest.

The results you are currently seeing are expected. As per NIST ESTAR, the range of 40 keV electrons in Cu is ~5 μm, meaning the primary beam is completely stopped inside the beam dump absorber. Furthermore, the generated secondary photons are strongly attenuated by the stainless steel casing, causing the prompt dose outside the beam dump to drop drastically.

To record these highly attenuated escaping photons, you have to use biasing techniques. Specifically, you should use the BIASING card to apply region importance biasing at boundary crossing by assigning progressively higher importances to your outer regions. For more information you can consult the dedicated lecture on biasing techniques from the 2026 Beginners’ course.

You may also want to use the EMF-BIAS card (with SDUM = LAMBBREM) to apply mean free path biasing to the electron bremsstrahlung, artificially increasing the initial yield of secondary photons. More details can be found on the dedicated page of the FLUKA manual.

I hope this helps!

Best regards,

Alexandra

4

Thank you very much for your help. I truly appreciate it. I wish you all the best and a happy and wonderful life ahead!

Sincerely.