TITLE:
Numerical Investigation of a UCN Source Based on Solid Deuterium by Combining a Simulation Code with an Analytical Approach
AUTHORS:
Habib Mohammadi, Mohammad Mehdi Firoozabadi, Rouhollah Gheisari
KEYWORDS:
Two-Layer UCN Source Based on D2O/sD2, Scattering Kernel, Monte Carlo Method, Implicit Method, Super-Thermal Principle
JOURNAL NAME:
Journal of Modern Physics,
Vol.5 No.17,
November
14,
2014
ABSTRACT: At thermal ultra-cold neutron (UCN) sources (neutrons in thermal
equilibrium with the moderator) only a very small fraction of neutrons have velocities
~6 m/s. Therefore, the UCN production rate cannot be substantially increased by
simply lowering the temperature of the moderator. The new approach is to use
the super-thermal principle, i.e.,
neutrons not in thermal equilibrium with the converter. We want to investigate
scattering kernels for a super-thermal UCN source based on a two-layer
arrangement of D2O and solid D2. The solid D2 (sD2) at temperature 8 K is kept in close contact with D2O
moderator at room temperature. Using the MCNP code, the fast neutron flux on
the spallation target, the thermal flux in the D2O near the sD2,
and the cold flux in the sD2 are simulated. For a given cold flux,
neutron transport equations are calculated. In order to obtain precise neutron
scattering kernels, and consequently UCN flux and density, 330 neutron energy
groups have been taken. The coupled energy dependent transport equations have
been solved by combining MCNPX code with an analytical approach and using
implicit method in MATLAB. We have obtained an optimal dimension for the UCN
source. A suitable space step has been taken for the numerical stability.