neutron importance in MCNP

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 lets you focus your computational power on the particles that do make it through without having to vastly increase the number of generations run.

seedsluis

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

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.