Why dose in air medium is changed with detector size

Dear fluka experts,

For a point source at 1 m, dose scored in detectors of radius 5 cm and 0.5 cm shows dependence on the volume. I have scored energy using USRBIN with region type, so I have normalised both the outputs by detector mass. I expected after normalization, both these values should be same. For example, to get the value of specific gamma ray constant, detector is placed at 30 cm from the point source. The output should not depend on the detector size, I guess.

Can you please explain the reason for the present input where output result (dose) is not constant? May be I am lacking some serious conceptual clarity. It will be helpful if you kindly guide.

for radius 5 cm, USRBIN energy = 2.1484E-08 GeV/primary, dose = 4.95E-03 Gy/primary
for radius 0.5 cm, USRBIN energy = 2.1615E-09 GeV/primary, dose = 4.98E-01 Gy/primary

65keV.inp (1.7 KB)


To get a constant dose, the detector signal (i.e. ENERGY deposition you score) must scale with the detector volume, which is only true if the radiation impacts the detector homogeneously. If you input a square beam spot of 10 cm x 10 cm (to do so, you should much more conveniently send the beam along the z-axis, in order to directly profit from the BEAM card capabilities in terms of transverse size settings), the 5 cm radius detector will get an ENERGY 1,000 times larger than the 0.5 cm radius detector, resulting in the same dose. In your case instead, the radiation travels on the x-axis, so the detector signal just scales with the detector size along that incidence axis, i.e. by the reported factor 10.

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Thank you @ceruttif , so to reproduce the gamma ray constant values, I should use a broad square beam source, right ? The same confusion I also got when in some texts, the kerma values at 1m are given for x ray sources from xray machine (point beam source), with no information on the detector used. If I change the detector size, then kerma values are changing. Do you have any suggestion regarding this or any standard protocol on detector size when it comes to kerma calculation or gamma ray constant estimation ?


You should rather use a point-like isotropic source.
However, this - available through the BEAM card - will spend CPU time over the whole 4 pi solid angle, so one can consider to implement a source user routine to limit the emission over the relevant solid angle portion.

Thank you @ceruttif