Date: Mon, 5 Oct 2009 10:06:24 +0200
Dear FLUKA experts,
I have a question regarding electrons cross sections and energy deposition in the range 1-10 keV. As some of you already know, since long time I have been doing simulation of Tissue Equivalent Proportional Counters response to complex radiation fields. I have now the possibility to perform the comparison with precise measurements done with a miniaturized TEPC at the secondary standard centre of ENEA-Montecuccolino (by Paolo Colautti). I attach the microdosimetric spectra for 60Co and 137Cs spectra in 1 and 2 micrometer tissue size.
The blue points are measurements
All the other lines are FLUKA simulations with default precision:
Red lines: electron threshold everywhere down at 1 keV
Violet lines (label optim): 1 keV in the propane gas and 5 keV in the surrounding A150
Blue line: 1 kev in propane gas and 2 keV in the surrounding A150 (only for Co 1mu).
In general, there is a satisfactory agreement between simulations and measurements. Looking in more detail, however, the simulations consistently have a shoulder around 5 keV/mu not seen by the measurements. This shoulder is due to electrons of 1-5 keV coming or backscattered from the A150 which are then stopped in the gas.
If I change the threshold in A150 from 1 keV to 5 kev this clearly change the shape of the shoulder. Now the question is: why there is this discrepancy between simulations and measurements?
A possibility is that in reality the A150 composition contains impurities not taken into account in the simulations. Another possibility is that the electron cross sections and/or the energy deposition in the 1-10 keV region used in FLUKA are not accurate enough. I have been told that this problem is addressed in an old paper (Projected ranges and effective stopping powers of electrons with energy between 20 eV and 10 keV H Iskef et al 1983 Phys. Med. Biol. 28 535). I do not have the paper so I don't know exactly what it is written inside but I fear I couldn't anyway relate this with what it is used in FLUKA. And for this I kindly ask your help.
There is also a difference in the valley between 1-5 keV/mu where FLUKA is consistently below the measurements independently from the transport threshold.
I would be grateful for a specific answer on the cross sections from the experts or any other alternative suggestion.
A more general answer on the electron energy loss calculations in case of single or multiple scattering and the accuracy of the calculations for these low energy electrons would be also welcome.
Best regards,
Sofia
SOFIA ROLLET
Scientist
Health & Environment Department
Nano Systems
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