Dose absorbed by Al2O3 sample due to Co60 decay

Good day, fluka experts,

I am having difficulty understanding the results of my simulation. As far as I can tell the simulation has been set up correctly, the simulation involves a Co-60 source which decays, and an Al2O3 sample (1cm diameter & 1mm thickness), I am trying to measure the absorbed dose of the Al2O3. The Co-60 has an activity of 70TBq.

The output of my simulations shows that the absorbed dose lies mainly towards the surface and almost trails off to 0 at 0.1 mm into the sample, as far as my theoretical understanding goes the absorbed dose should gradually trail off the deeper the photons go into the sample.

Any insights on this would be greatly appreciated,

Regards,
Cameron
xMeV_Gammas_Al203_dose.inp (1.6 KB)

Please note the following normalisation factor is applied post processing:

1.6e-7 * 70e12 * 60 * 60 * 24 * 14 * 1e-3

1.6e-7 is the normalization factor

70e12 is the activity of the Co60

60 * 60 * 24 * 14 (14 days of irradiation)

1e-3 conversion to kGy

the simulations should have a maximum of 50 kGy, however the result is much higher.

Dear Cameron,

what you observe is due to the electrons produced by the decay.

Cheers,
David

Hi David,

Thank you very much for your reply. Does the dose scoring not take all particles produced by the decay into account? I’d assume the gammas would travel deeper into the material.

Many thanks,

Cameron

Dear Cameron,

of course the photons are also taken into account, but while all electrons are stopped in the target, most of the photons don’t interact with it, so you need more primaries to see their contribution.

And this leads to the optimization of your geometry: You are using an isotropic source 50 cm away from the target, thus the majority of your source particles won’t reach the target.
Maybe it would be benefitial to use an extended parallel photon beam with the energies equalling the peaks of the decay to have a more efficient simulation.

Cheers,
David

Hi David,

This simulation is an effort to reproduce an actual irradiation that occurred where the sample was 50cm away and thus most of the particles produced by the source wouldn’t reach the sample anyways.

In this case would the photon beam be applicable?

Regards
Cameron

Dear Cameron,

it would be an approximation, and you will have to normalize correctly.
However, using an isotropic source is also fine, if you keep in mind that you need much more primaries to get a meaningful result.

Cheers,
David

Hi David,

When you say I will have to normalise correctly, does the normalisation I explained in the beginning of the post not suffice?

Thank your timeous responses I really appreciate the support.

Regards,
Cameron

Dear Cameron,

it would be a different element of the normalization.

But first, let’s take a step back.
Instead of the extended beam, let’s use a pencil beam with a flat angular distribution and imagine a sphere centered around the position of the source.
We can calculate the area covered by the source on the sphere, and get a ratio with the whole area of the sphere.
So if we have this source in a FLUKA simulation, we will need to multiply all our results with the ratio mentioned above, because only this portion of the primaries go toward the target.
Of course, this only works if the beam still covers the whole region of interest, and the omitted directions don’t interact with it via backscattering, etc.

Cheers.
David

Dear David,

I’m not sure I quite follow. I’m not aiming to only model the photons of this interaction but the whole interaction (electrons etc). Would using a photon beam not skew the results.

I am fine with the simulation if the outcome I am getting is reasonable (mainly due to electrons, most photons passing through). Would there still need to be an additional normalisation as you have mentioned?

Many thanks,
Cameron

Dear Cameron,

if you want to model an experiment, then you need to model the source as accurately as necessary: the Co-60 is in a container, and probably you need to have AIR instead of VACUUM.
This means, the electrons won’t reach your target anyway.

Cheers,
David

Hi David,

Thank you for the reply I will change this!

I think from here the simulation should run as expected.

Many thanks
Cameron