I have been trying to score the ambient dose rate in regions (cubics) from a volumetric radioactive source (Be7) distributed over a watercourse. I have tried to maximize the size of the detectors (detect1 to detect7), increase the primaries number, and activate/deactivate AUXSCORE, but nothing worked, scoring zeros all the time. (see attached)

I saw a first error in the definition of your detec regions. I let you see the differences but in a nutshell, you added -void in your detector regions definition instead of -det... in your VOID region.
Please have a look to the proposed modification and meanwhile I will investigate to solve your problem.

Mainly your problem was that the definition of your beam card was wrong. I modified it and now it’s corresponding to where is the water.
I tried with 1e8 primaries and I obtained a non-zero results.

Please let me know if it’s working now on your side.
In conclusion:

Be careful at the definition of your regions, you can use flair to check that your region definition is meaningful

Thank you very much for your help! I truly appreciate it.

However, I’m still a bit confused about your correction regarding the beam position card for volumetric sources, particularly in the context of FLUKA’s CART-VOL geometry.
In most FLUKA courses, it’s explained that the center of the volumetric source geometry is defined by the position specified in the beam position card. The lengths (x, y, and z) then represent the extent of the source in each direction, starting from this central point and reaching outwards.

Based on this understanding, I assumed that each length value (X, Y, and Z) represented half the total distance the source extends in that particular direction, measured from the center.
However, in your correction, you seemed to be referring to the entire side length. This doesn’t quite match the description I’ve encountered. Could you please clarify if I’m misunderstanding something?

As explained in the most recent course (slide 21), the parameter is the actual length.

Looking at your definition, and considering that your source corresponds to the region made of water, I deduced dX = 4000 cm, dY = 150 cm and dZ = 500 cm.

*Define the beam position
BEAMPOS 0 4000 0 150 0 500CART-VOL

However, you still have to give the center position of your source. Here again, looking at your geometry, I deduced it was (x,y,z) = (0,-75-250). It corresponds to: