MCUCN

The MCUCN code was developed at PSI to support the projects of the UCN physics group, with focus on the UCN source and the nEDM-Collaboration projects at PSI. MCUCN has already been used extensively in the optimization of the PSI UCN source, beamlines and experiments. MC simulations can also be deployed for planning time-efficient measurements. UCN simulations are also important for the estimation of systematic effects, for checking data analysis software by toy data, or as part of data analysis (providing e.g. collision rates, energy spectra).

In MCUCN the particles are represented by time, position, velocity and spin vector coordinates. These coordinates can be written out at user-defined 'snaphot-times' or detected by a virtual monitor surface, for example at a beamline exit, or after a detector window. As an option, reflection points on the surfaces and trajectory points can be generated aiming to test the interaction of UCN with the surfaces of the given confinement by visualizing with an independent tool e.g. Matlab. A storage volume or UCN guiding geometry can be modeled using a combination of an arbitrary large number of reflecting or transmitting second-order surface sections (planes, cylinders, cones, etc.). Toroid parts, like bent guides, can be easily built recursively via loops from smaller second-order sections without strongly increasing computing time. Triangulated mesh exports from CAD programs in form of STL ASCII files can also be used as a geometry input for MCUCN. Time dependent shutters were also defined.

The UCN interaction with the surfaces is implemented as quantum reflections on an averaged nucleus-neutron potential, including loss effects due to nuclear absorption or up-scattering (R. Golub, D. Richardson, S. K. Lamoreaux, Ultra-Cold Neutrons, Adam Hilger, 1991). The reflections can be specular and perfectly diffuse weighted by an input parameter, or non-specular based on an interference effect due to by microroughness (A. Steyerl: Zeitschrift für Physik A 254, 169 (1972)). Low magnetic fields (which are not deflecting the trajectories) were implemented using the Bloch equations for calculating the spin precession along the ballistic trajectories. A strong magnetic field, like a superconducting polarizer, or a magnetized analyzer foil, can be modeled in approximation as a local optical potential, or with a more time-consuming step-based algorithm. See below figures with example calculations and a list of publications involving results obtained with MCUCN.

The MCUCN code will be made available for users working in collaboration with us.