Photon Normalisation - Photon count/dose seems far too low

User Support
1

Hello!

I am using photon and neutron tallies, loosely following the code given in the example: https://github.com/fusion-energy/neutronics-workshop
When I output the dose tallies, after normalising by (power/heating [ev/particle]), as suggested for a non fixed-source calc, I get the gamma dose coming out to be much too small (10x smaller than neutron dose with no gamma shielding).
I’m wondering now, if there is a different method for normalising the photon tallies, do you need to do a heating rate for both neutrons and gammas individually?

Thanks,
Harry

2

Hi Harry,

Are you using heating or a dose function?
Photon heating in OpenMC is the sum of photon, electron and positron heating.
Did you score all three?

Perry

3

Hello, thankyou for the reply.

I am currently scoring ‘heating’ over the entire core region, I was presuming that this accounted for both neutron and gamma heating. Are you saying there is a specific heating tally for photons that I should be using?

Currently I am normalising neutron and gamma dose by the same normalisation factor, and this feels wrong.

Thanks again,
Harry

4

As a follow up question, both neutron and gamma ‘current’ scores are given in particles/source particle. Does this ‘source particle’ value include both neutrons and gammas? Or for the neutron tally, is it the number of source neutrons?

5

I’d have to go double check myself to be sure, but if you just have it as heating over the entire region and there are no particle filters, then neutron and all photon heating should be accounted for in the heating score.

The specific tally to separate between neutron and gamma heating would be to use particle filters
https://docs.openmc.org/en/stable/io_formats/tallies.html
If trying to get gamma heating, then make sure to sum photon, electron and positron.
There is also nuance

Both tallies (neutron and gamma dose) should be normalized by the same factor. It would be very unique, niche problems where you don’t.
The normalization constant is the number of source particles per second (n/s, (n+p)/s, etc.) (assuming you are using a fixed source, fusion type calculation)
Typically even if your source was 1000 neutrons/second, this should be your normalization constant for the gamma tally

https://docs.openmc.org/en/stable/usersguide/tallies.html?highlight=scores#scores
https://docs.openmc.org/en/stable/methods/energy_deposition.html#methods-heating

6

Yes, it includes both particle types. All source particles

7

I see. Thanks a lot for the reply, couldn’t seem to find this on the documentation anywhere.

Much appreciated.

8

So it is not a fixed source calc, rather a k-eigenvalue calc (with neutron and photon tracking) so doesn’t have a known neutron/gamma source. Does this still hold in this case?

9

In eigenvalue mode the source particles are neutrons. For an eigenvalue problem you will need a power normalization factor (PNF), still neutrons/second but it is determined by the physics and power level of your problem.

10

So if I also have photons being tracked and tallied, are they still normalised by this same factor?

11

Yes, the photon tally results of an eigenvalue problem should be normalized by the same factor.

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