First production of ultracold neutrons with a solid deuterium source at the pulsed reactor TRIGA Mainz⋆.

The production rates of ultracold neutrons (UCN) with a solid deuterium converter have been measured at the pulsed reactor TRIGA Mainz. Exposed to a thermal neutron fluence of $\ensuremath \sim 1\cdot 10^{13}$ n·cm^-2·pulse^-1, the number of detected very cold and ultracold neutrons ranges up to 200 000 at 7mol of solid deuterium (sD₂) in combination with a pre-moderator (mesitylene). About 50% of the measured neutrons can be assigned to UCN with energies E of $\ensuremath V_{\rm F}({\rm sD}_2)\leq E \leq V_{\rm F}{\rm (guide)}$ where V _F( sD ₂) = 105 neV and V _F( guide) = 190 neV are the Fermi potentials of the sD₂ converter and our stainless steel neutron guides, respectively. Thermal cycling of solid deuterium, which was frozen out from the gas phase, considerably improved the UCN yield, in particular at higher amounts of sD₂. [ABSTRACT FROM AUTHOR]