An ultracold neutron (UCN) detector with Ti/6LiF multi-layer converter and 58Ni reflector

High efficient detectors for ultracold neutrons (UCN) must be developed for the new high flux neutron source Forschungsreaktor München II (FRM II). On silicon PIN diodes 76 μg/cm2 58Ni was deposited as a UCN reflector. On this 100 double layers of natTi (4.7 μg/cm2) and 6LiF (1.8 μg/cm2) were deposited to function as a UCN converter. On top of this, 33 double layers of natTi (3.4 μg/cm2) and 6LiF (0.92 μg/cm2) were condensed in addition to provide sensitivity to very low-energy UCN. Finally, 6.0 μg/cm2 natV was deposited to protect the multi-layers. Vanadium has nearly zero optical potential for UCN and thus should not hinder their transmission. no expensive isotopes were involved, a source to substrate distance of 24 cm could be chosen, leading to excellent uniformity. The setup designed for deposition under ultrahigh vacuum conditions and the evaporation procedures are described.