Dear FLUKA experts and users,
In my current modeling of a cyclotron bunker I need to estimate dose rate distribution outside the shielding walls of the bunker. Particularly, I am interested in the dose rate behind the wall shown on the image below (the right-side wall).
It’s made out of concrete with density 2.35 g/cm^3 and has thickness of 214.5 cm. The primary source is neutrons emitted from a point and slightly biassed towards the direction of the wall (almost perpendicularly to it, predominantly into the forward hemisphere since the parabolic-like angular \theta distribution has a subtle skewness) with an external-sampled spectra-based energy and angle distributions. Trying to propagate neutron radiation field outside that wall I applied region importance biassing technique in several option (I did several attempts to achieve an error less than 10% outside the wall):
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[16 slices with the importance base of 2]For the first time I sliced the wall into 16 regions applying importance coefficients such 2^{i_{reg}}, where i_{reg} = 1..16. The region’s after the last slice (outside the wall) –ROI4importance was set to 2^{16} as well as for the last slice. Giving all other regions (including the inner airAIRINNEregion) their default importance values of 1 = 2^{0} I got a sequence of importance values: air_inside \rightarrow 2^{0}, wall_slice1 \rightarrow 2^{1}, …, wall_slice16 \rightarrow 2^{16}, region_of_interest \rightarrow 2^{16};
It does not give the desired results since the neutron fluence drops down before particles reach the last slice and the ROI after it. The dose errors suddenly increases in the last couple of bins within the wall region. -
[36 slices with importance base of ~1.46]In the second attempt I sliced the wall into 36 regions setting the importances from 1.465^{-6} for the 1st slice to 1.465^{29} for the last slice and the ROI next. The result is similar to the 1st case. -
[36 slices with importance base of ~1.64]. Than I decided to change the importance ratio (\frac{I_{i+1}}{I_{i}}) to 1.6495. Now I have 1.6495^{-12} for the 1st slice and 1.6495^{23} for the last slice and ROI next. It also does not work – an error occurs since approx. 20 minutes of running. The.errfiles didn’t show any particle generation progress. -
[16 slices with importance base of 3]The default importance value set to all regions except the sliced wall’s regions is 3^{-7}. The slices’ importances start with 3^{-6} up to 3^{9} for the last slice (outermost) and 3^{10} for theROI4. This version does not work also – running process takes “eternity”, doesn’t finish and gives no result (the .err fileratio3_errors.errattached).
The BLKBODY region importance defined to be 0 in all these cases described above.
As it’s mentioned in the first two cases the extreme increasing in the estimated variable (dose) uncertainties. On the screenshot below you could find the errors value in the two adjacent bins. I tried to increase the bins dimentions and make this binning grid more coarse to accumulate more particles in a bin, but it didn’t help as well.
Trying to investigate the possible reason of that I scored fluence distribution and it looks like the neutrons merely don’t reach the region of interest which suggest the biassing scheme to be unsuitable to that kind of simulation or set in a wrong way (?)
I also concerned about the split parameter described in the info boxed on the screenshots is 0, is it okay?
Generally, I ran 100 runs (each run contains 5-10 cycles), 500,000 neutrons per one cycle. And I also tried to use automatic importance biassing selecting the ROI4 region as the region of interest, but it gave uncertainties about 90% for 10 runs, 5 cycles for each run and 500,000 primary particles per a cycle.
Therefore, I would ask about any suggestions and/or recommendations on a proper variance reduction technique which could help to achieve errors less than 10\% What would be the optimal approach to implement that technique/techniques for this model? Probably the chosen RIB approach is not effective enough in this case or I’ve been using it in a wrong way? I would be extremely grateful for any hints and recommendations.
Cheers,
Ihor
sim.zip (143.7 KB)
FLUKA: 4-5.2
Operating system: Ubuntu 24.04 LTS