# Volumetric source sampling

Good afternoon Experts,

I have a basic question on how does the calculation work for a volumetric source.

If I have a RCC source of 1.4 cm radius and 15 cm L and I give no. of primaries as 1.0E07. Then from where the photons would originate ? As in the case of a point source, the primaries seem to originate from the point itself… how would the photons be travelling from a volume source ? Would it be 1.0E07 from the full volume or no. of photons per unit volume will originate from a unit specific volume?

Another way to ask, if I choose to use a volumetric source in a shielding problem, should I use more no. of primaries to get a good statistics, in comparison to a point source with smaller no. of primaries ?
( thought is: photons concentrated at a point Vs. photons over a defined volume - how it works ?).

Kindly explain.

Thanks and regards,
Raksha Rajput.

Dear Raksha,

the actual starting points of each primaries in a volumetric source are sampled uniformly in the volume.

I would reverse your logic about the required number of primaries. The number of primaries needed doesn’t depend inherently on the size of the source. You should always aim for reasonable statistics with your results, and you will see if you need more primaries or not.

Cheers,
David

Dear David,

Thank you for explaining.
Regards,
Raksha.