What Does Imp:n=0 Mean in MCNP and How Does it Affect Tally Results?

[seedsluis]

hello, I am new to MCNP, could somebody tell me how to use imp:n, what is imp:n=0 means, if neutron importance is 0 in one cell, why the F4 tally is 0 in this cell? how about imp:n=1 or some large number?
Thanks for all.

 

[QuantumPion]

Importance allows you to focus which particles you want to spend computational resources on. You can cease calculating interactions for particles that leave the problem area or give a higher importance to particles that make it through shielding. It is of primary importance in deep shielding applications since only a small percentage of particles will make it through the shielding, but you don't care about calculating the tracks of all the ones that don't make it through. Manipulating the importance let's you focus your computational power on the particles that do make it through without having to vastly increase the number of generations run.

 

[seedsluis]

Importance allows you to focus which particles you want to spend computational resources on. You can cease calculating interactions for particles that leave the problem area or give a higher importance to particles that make it through shielding. It is of primary importance in deep shielding applications since only a small percentage of particles will make it through the shielding, but you don't care about calculating the tracks of all the ones that don't make it through. Manipulating the importance let's you focus your computational power on the particles that do make it through without having to vastly increase the number of generations run.

Thanks, but if I do not care much about interactions, for example, I define a void cell, what is the difference between imp:n=1 and imp:n=0?

 

[QuantumPion]

Thanks, but if I do not care much about interactions, for example, I define a void cell, what is the difference between imp:n=1 and imp:n=0?

Zero neutron importance tells MCNP to forget about any neutrons that enter that cell. If the cell is void and is the problem boundary (i.e. those neutrons have no way of being reflected back or otherwise traveling through to another area with fuel in it), you don't need to track them any more (in the context of criticality analysis). If the cell is void but is not the boundary of the problem (e.g. spacing between fuel assemblies in air) you would not set this cell to imp:n=0 because doing so would isolate the fuel regions from each other non-physically.

 

[alphy]

Hello and welcome to MCNP! Imp:n stands for neutron importance and it is a parameter that is used to determine the contribution of a neutron to the tally results. When imp:n=0, it means that the neutron has no importance in that particular cell and therefore will not contribute to the tally results. This could be due to various reasons such as the neutron being outside of the cell or having a low energy that does not have an impact on the tally results.

On the other hand, when imp:n=1 or a large number, it means that the neutron has a high importance in that cell and will have a significant contribution to the tally results. This could be due to the neutron being in a high-density material or having a high energy that will contribute to the tally results.

It is important to understand the imp:n value and how it affects the tally results in order to accurately interpret the results. You can adjust the imp:n value in the input file to see how it affects the tally results and determine the appropriate value for your simulation.

I hope this helps and feel free to reach out if you have any further questions. Happy simulating!