# [fluka-discuss]: Energy, Dose scoring questions

From: Georgios Dedes <G.Dedes_at_physik.uni-muenchen.de>
Date: Fri, 14 Mar 2014 13:20:09 +0100

Dear FLUKAers,

I've been doing some beginner's mental exercise on USRBIN ENERGY and
DOSE scoring. I came up to some conclusions on how it works, but I would
like to have some feedback from the experts.

First of all, I am using FLUKA 2011.2b.5 and flair 1.2-4.

The setup is extremely simple. An Al (d=2.6989g/cm^3) cube of size
2x2x2cm^3, spanning from -1.0 , 1.0 in all dimensions.

I use a linear source (Deltax=0.5cm) of protons of 1MeV, such us that
the line source is almost equally splitted between two voxels. The
protons stop in the voxel that are created and the energy is completely
contained in those 2 voxels. I run 1000 primaries in 1 cycle, in order
to keep things simple.

Now the scoring:

I score either in X-Y-Z USRBIN or the whole target region. For the X-Y-Z
case, the target cube is divided in x,y,z as 3,2,2. So 12 voxels
dividing a volume of 8cm^3, resulting in a volume per voxel Vi=8/12 cm^3.

I have:

1. X-Y-Z ENERGY USRBIN
2. Region ENERGY USRBIN
3. X-Y-Z DOSE USRBIN
4. Region DOSE USRBIN

I write both in bin and ASCII, in different logical volumes so as to
make it idiot proof.

Now my results:

- In the ASCII file of the X-Y-Z ENERGY USRBIN, I get:
7.9027E-03 7.0856E-03 0.0000E+00 0.0000E+00 0.0000E+00
0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00
0.0000E+00 0.0000E+00

As all the energy is almost equally splitted in two voxels, the
Energy/Primary/Vi = 5GeV/ 1000primaries / (8/12)cm^3 = 7.5E-3 (the
disagreement with the above comes because I assume exactly equal energy
deposit)
So the results is as expected, which means that the ENERGY in the ASCII
is energy density [GeV/primary/cm^3], normalized per primary and per
voxel volume

- In the ASCII file of the region ENERGY USRBIN, I get:
1.0000E-02

which mean that the energy deposit per primary is assigned to the whole
target volume without any volume normalization (it is not the energy
density anymore) [GeV/primary]. Otherwise it would be 1E-2 / 8cm^3. So
for the region NO VOLUME normalization is applied. Correct?

- In the ASCII file of the X-Y-Z DOSE USRBIN, I get:
2.9280E-03 2.6253E-03 0.0000E+00 0.0000E+00 0.0000E+00
0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00
0.0000E+00 0.0000E+00

which is the dose per primary, approximately: 5GeV / 1000primaries /
[(8/12)cm^3 * 2.6989g/cm^3] = 2.7789 (the disagreement with the above
comes because I assume exactly equal energy deposit)
So the dose is calculated per voxel in [GeV/g] and a total dose over the
whole volume should be the average of all voxel doses.

- In the ASCII file of the region DOSE USRBIN, I get:
3.7022E-03

which is obviously not the real dose per primary in the whole Al target
cube. If this was the case the dose would be either the average of the
doses in the X-Y-Z DOSE USRBIN ASCII file, or the total energy deposit
divided by the total mass of the region:
Edep / Nprimaries / (8cm^3 * 2.6989g/cm^3) = 10GeV / 1000primaries /
(8cm^3 * 2.6989g/cm^3) = 4.632E-4 Gy/g

So it seems that the dose it assigns to the whole region is the total
Energy deposit per primary in the region assuming a unit volume (1cm^3).
Right? Note that changing in the REGION card the volume to 8cm^3, won't
change the result. So in general the region scoring is not aware of the
volume, which is OK for the energy, but one should be aware of that when
scoring dose. Are my assumptions so far correct?

Finally, when I take a look into the 2D and 1D plots from flair, using
the bin files produced:

- The 2D xy ENERGY plot has as expected only two pixels filled, with
numbers close to 0.00375. This corresponds to the energy deposit in the
voxel that it actually happened, but normalized using the whole
projected volume on the single x-y pixel. That means 2 voxels depth in
z: 5GeV / 1000primaries / (2*8/12cm^3) = 0.00375 [GeV/primary/cm^3]

- The 1D x ENERGY plot gives again as expected only two bins filled,
with an average value of 0.00375. This corresponds to a normalization to
4 voxels, as 4 y-z combinations will be projected to the same x bin:
5GeV / 1000primaries / (4*8/12cm^3) = 0.001875 [GeV/primary/cm^3]

Similarly for the Dose...

George

--
Dr. Georgios DEDES
Ludwig-Maximilians-Universität München (LMU)
Medical Physics Chair (LS Parodi)
Am Coulombwall 1
85748 Garching
Tel:+49 (0) 89 289-14022
Fax:+49 (0) 89 289-14072

Received on Fri Mar 14 2014 - 14:27:23 CET

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