Optimization of a Cylindrical Para-Hydrogen Moderator for Cold Neutrons

Using Monte Carlo simulations we investigated the properties of a small cylindrical para-hydrogen moderator delivering cold neutrons. It may be situated within a regular beam tube of the FRM II. We present a separate optimization of both length and radius of this cylindrical moderator and a theoretical description of the simulation results. It is found that there is an optimum length, whereas the radius should be chosen in dependence of the experimental needs. Furthermore the spectral output of such a cold neutron source is investigated and a theoretical interpretation of the results is suggested. These results are compared to an ideal model of the existing cold source at the FRM II facility. It is found that a para-hydrogen moderator is capable of delivering a higher neutron flux than the conventional cold source, which moderates neutrons by subsequent elastic scattering events. This is achieved by an inelastic collision exciting the rotational transition in para-hydrogen.