Re: [fluka-discuss]: USRBIN volume

From: <alfredo.ferrari_at_mi.infn.it>
Date: Tue, 15 Jun 2021 17:02:31 +0200

Dear Ryan

indeed FLUKA does exactly what you indicated: it scores the energy
deposited in a volume between r, r+dr and z, z+dz and then divides by the
volume according to the formula you wrote.

Where you are in error is about how a gaussian beam looks in cylindrical
coordinates. A gaussian beam is a beam gaussian in X and Y, which (you can
work out the math yourself) ends up when expressed in phi and r as
constant in phi and proportional to 2r exp(-r^2/2sigma^2) dr, or if you
want,
proportional to exp(-r^2/2sigma^2) dr^2. If you divide such a ditribution
for the radial area between r and r+dr indeed you get a gaussian as it
should be. Indeed multiple Coulomb scattering is gaussian in X and Y, or
in r if you express it as a function of dr^2.



> Dear FLUKA experts,
>
> I am trying to understand how FLUKA determines the volume when calculating
> dose in USRBIN (228). Does FLUKA calculate the energy deposited in a
> volume
> element, then divide by the volume of that element to get the dose?
>
> I ask because for a dose binning into volume elements in
> cylindrical coordinates with symmetry in θ a volume element ends up as
> V=dzπ(r2)^2-dzπ(r1)^2.
>
> For a proton (or other charged particle) beam incident normal to the end
> of
> such a cylinder (in the z direction) one expects a gaussian fluence
> distribution in n r due to coulomb scattering. The energy should therefore
> be normally distributed along the fluence distribution as it passes
> through
> each cylinder element with thickness dz.
>
> The volume of each cylinder element increases with r, however, and
> dividing
> the energy deposited in each element by the volume of that element
> produces
> a dose distribution which is non-gaussian in r.
>
> However, when scoring dose distribution using USRBIN in FLUKA, I
> consistently obtain gaussian distributions in r.
>
> So my question is, am I considering FLUKA's dose calculation wrongly in
> that energy deposited in each volume element are not divided by the volume
> of each element? If not, then what volume is considered for the dose
> calculation? Is it the volume of the entire region to be scored?
>
> Thanks very much for your kindly assistance.
>
> Ryan Brosch
> Arizona State University
>


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Received on Tue Jun 15 2021 - 18:40:22 CEST

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