Speaker
Description
Neutron guides transport neutrons through long paths by exploiting total external reflection from highly polished surfaces. Neutron supermirrors transport even more neutrons by diffraction from multilayers of materials with contrasting scattering lengths. These supermirrors typically have thousands of alternating layers of nickel and titanium on the neutron beam-facing surface of the mirror. The MCNP guide extension, first published in 2009[1] and updated in 2020[2], implemented a phenomenological model of neutron reflection from conventional and supermirror guide surfaces. These extensions accurately describe the propagation of neutrons through a neutron guide network but do not address the gamma production from those neutrons that are not guided down the beamline. Simulation of the absorption of neutrons in these layers would require adding a few micron thick and likely homogenized layer to the model and which will overestimate or underestimate neutron capture in the supermirror layers depending on where the reflecting surface is assumed to be. Instead, the guide extension has been updated from the 2020 version to add a virtual material for the supermirror layers and uses the model presented by Kolevatov [3] to produce gammas from capture in the supermirror layers with splitting at the mirror surface. This gamma production is significant for the determination of gamma dose rates surrounding a neutron guide. Additionally, this update of the guide extension has added the ability to simulate a polarizing supermirror by specifying an input beam polarization and spin-up/spin-down reflectivity profiles. The machinery used to implement this functionality in MCNP 6.2/6.3.0 [4] will be discussed along with examples of the gamma production from a neutron supermirror and from a V-cavity polarizer will be discussed.
- Gallmeier, F.X., Wohlmuther, M., Filges, U., Kiselev, D., Muhrer, G., 2009. Implementation of Neutron Mirror Modeling Capability into MCNPX and Its Demonstration in First Applications. Nuclear Technology 168, 768–772. https://doi.org/10.13182/NT09-A9304
- Magán, M., Bergmann, R.M., 2020. Supermirror physics with event biasing in MCNP6. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 955, 163168. https://doi.org/10.1016/j.nima.2019.163168
- Kolevatov, R., Schanzer, C., Böni, P., 2019. Neutron absorption in supermirror coatings: Effects on shielding. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 922, 98–107. https://doi.org/10.1016/j.nima.2018.12.069
- Grammer, K.B., Gallmeier, F.X., 2026. Updating the MCNP neutron guide extension for supermirror absorption, gamma production, and neutron polarization. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 1082, 171067. https://doi.org/10.1016/j.nima.2025.171067