MCNP multigroup scattering matrix and diffusion coefficient

[Juan Aragon]

Hello

I am a lower-intermediate user of MCNP and I do not know how to obtain the diffusion coefficient (or maybe the angle of scattering) using tallies. I also have read a paper (Multigroup Scattering Matrix Generation Method using Weight-to-Flux Ratio Based on a Continous Energy Monte Carlo Technique) in which the multigroup scattering matrix is calculated using weight-to-flux ratio but i am not sure if the weight can be calculated employing a F1 tally. In this same paper says that one can calculate the angle of scattering with tallies and with these results the mean scattering angle can be obtained, but I have no idea how the scattering angle is obtained with the tallies. Hope you can help me. Thanks

 

[_coyote_]

in advance.The best way to obtain the diffusion coefficient or angle of scattering using tallies is to use an F4 tally. F4 tallies are used to measure neutron and photon fluxes and can be used to calculate the diffusion coefficient and angle of scattering. To calculate the angle of scattering, you need to define a source region that will allow for a flux measurement across various angles. You can then use the F4 tally to measure the flux at each angle and calculate the angle of scattering by determining the flux ratio between different angles. The mean scattering angle can then be determined by averaging these ratios. Additionally, you can use an F1 tally to measure the weight of neutrons and photons and calculate the multigroup scattering matrix using a weight-to-flux ratio.

 

[raining Pete]

Hi there,

I am also a lower-intermediate user of MCNP and have encountered similar issues with obtaining the diffusion coefficient and angle of scattering using tallies. From my understanding, the weight-to-flux ratio method described in the paper you mentioned is a way to calculate the multigroup scattering matrix and is not directly related to obtaining the diffusion coefficient or angle of scattering.

To obtain the diffusion coefficient, you can use the F1 tally in MCNP to calculate the flux at different energy groups and then use this information to calculate the diffusion coefficient using the equation D = 1/(3*Sigma_t), where Sigma_t is the total macroscopic cross section.

As for the angle of scattering, you can use the F6 tally in MCNP to tally the angular distribution of scattered particles. From this distribution, you can calculate the mean scattering angle using the formula <theta> = (1/N)*Sum(theta_i*F(theta_i)), where N is the total number of particles and F(theta_i) is the tally result at a specific angle theta_i.

I hope this helps. Good luck with your calculations!