The unit of electron fluence scored by USRBIN

Hi Fluka experts,

I am writing to talk about the unit of electron fluence scored by USRBIN, it seems that the unit should be particles/cm^3/primary instead of particles/cm^2/primary specified by the manual. Following are my considerations.

The electron beam is coming from the left, passing through the titanium window, the air and then into the water in the container. The water volume is a cylinder with 2.43 cm radius and 4 cm length.
Then I scored the electron in water region and the corresponding R-Phi-Z coordinate system with only 1 bin there, shown in the following two figures.

The simulation results are
3.6752E+00 and 4.9529E-02
respectively, the former is equal to the latter multiplying by the volume of water (~75ml).
If the former value is the total electrons per primary, then the unit of the latter one should be electrons per volume and per primary.
And there is a topic " The unit of energy deposition and fluence" mentioned that Fluka calculate the volume of the bin, I think that is similar to this situation.
If I set 2 bins along z direction, hold 1 bin along r direction. Then the results are
6.4229E-02 3.4828E-02
The total electrons per primary in the region can be calculated by
6.4229E-2 * 74.2/2+3.4828E-2 * 74.2/2 = 3.67,
which is around the result scored by the region.
Could you help check if this is a correct understanding? I am using this to score the 2D and 1D projection of electron fluence in the water region. Thanks.


Dear Xi,

it is important to remember, that the USRBIN fluence scoring is a track length estimator, meaning FLUKA will calculate the average distance [cm] travelled by a (pseudo)particle type in a bin / region per primary. Which is when divided by the volume of the bin / region, will give the expected [1/cm^2/primary]

However, there is a big difference between the coordinate system and region type scoring with USRBIN.

  1. For the coordinate system type scorings, FLUKA can calculate the actual volume of a bin, thus it can report the correct fluence value.

  2. However with the region type scoring, FLUKA always assumes that the volume is 1 cm^3. This means to get fluence, these results has to be normalized with the region volume by hand.

In your calculation you apply the volume multiplication to the fluence result, so you are getting the actual track length values [cm] for each bin. Because track length is an extensive (additive) physical property, when you add those together you get the same value as with the region type scoring.

I hope this clarifies it.


1 Like

Hi David,

That makes sense to me now. Thank you very much.