SiO2(Quartz), MgO, PbF2, and Bi as Low-Pass Neutron Velocity Filters

We have measured the transmission of monochromatic slow neutrons over the wavelength range 1–5 Å for monocrystalline and polycrystalline samples of SiO2(quartz), MgO, PbF2, and Bi at 83 K and 300 K. These materials were selected as apparently suitable for use as neutron ``low-pass'' velocity filters; SiO2 and Bi have already been used in this manner at several laboratories. Single crystals of MgO cooled to 83 K are remarkably transparent to thermal neutrons; more than 70% of the incident 2 Å neutrons are transmitted by a 15 cm sample whose transmission for 1–2 MeV neutrons is about 0.01. SiO2 is nearly as transparent, in agreement with several earlier measurements by other workers. A comparison of our measurements on Bi with other reported work shows that neutron transmission is low and quite variable from sample to sample for neutron wavelengths in the range 1–2 Å but fairly high for all samples at wavelengths >4 Å (∼50% for a 15 cm sample). For the latter wavelength range, we find slowly cooled Bi castings, in which large grains have developed, to be about as transparent as (nominally) monocrystalline samples. A single crystal of PbF2 exhibited higher transmission in the range 1–3.5 Å than did any Bi sample. The theoretical basis for the neutron filtering action of crystalline materials is briefly discussed.