Change component material during decay time

Hi - I have an activation model of a beam collimator and shielding assembly. I have calculated dose rates of the collimator and shielding after 6 months decay time.

But I have then been asked to then calculate the dose rate of the shielding if the collimator is removed at a point during this decay time.

I know I could set Mat(decay)=air for the collimator but in my understanding this would set the material over the whole decay time.

Is it possible to change the material after a specified decay time?

Or is there any other approach I could take to solve this problem?

Many thanks

Dear @HVavanagh,

Welcome to the FLUKA forum.

Not sure I understood correctly so please feel free to correct me.

Let us say that the irradiation finished at a time t_i and you are interested in the dose rate at t_d. But at some point t_c your collimator was removed, with t_i < t_c < t_d. If you set your collimator mat(decay) to air, your collimator material will be considered as air for the entire decay calculation, as you pointed out. But, how would the dose rate resulting from radioactive decays in your shielding be influenced by the time at which you remove your collimator (provided that t_c<t_d) ? I mean, it doesn’t matter if you removed your collimator just after irradiation or later, the dose rate coming from your shielding would be the same at t_d. Does this makes sense to you?

Kind regards,

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Hello Fancisco

Thank you very much for your swift reply.

Yes, you have understood me correctly.

However, I’m not sure I understand your reasoning.
Myself and my engineer have discussed and he would like me to simulate this effect, because looking at the dose rate in the shielding we believe this would be affected by having a hot collimator inside? No?
If the collimator is removed then there is less radiation ‘source’ contributing to the shielding.
If I take a projection of dose rate over the entire shielding arrangement it is clear to see where the collimator is inside. And the belief is that if we remove the collimator at an earlier time then the dose rate of the shielding may decay more quickly due to there being less ‘source’.

Thank you for any further consideration or advice you can offer.

Hi, the dose rate at the shielding depends on the shielding activation on one side and on the presence of the collimator as an additional radiation source (due to its own activation) on the other side. Since you are interested in a time when the collimator is no longer there, the second contribution is gone. As @fogallar said, it does not matter when, since its former presence (during the cooling phase) does not contribute to the shielding activation, which is determined by the prompt irradiation phase (where the collimator presence has a role).

Thank you both very much your time and for the clarification.