Quartz monocrystal neutron filter

A diagram is presented of a monocrystalline quartz neutron filter installed before a triaxial neutron spectrometer at the VVR-Ts reactor to separate a neutron beam from fast neutrons. The filter consists of a set of disks 100 mm in diameter, cut out of monocrystals of quartz in such a way that the hexagonal crystallographic axis is normal to the disk plane. The disks are put in a channel surrounded by a sheath with liquid nitrogen. The cooled assembly is placed in a cylinder, its integrity being provided with an aluminium diaphragm 0.8 mm thick. The filter has a stub pipe system for delivering liquid nitrogen, nitrogen vapour release and for pumping out the vacuum cavity. The results are reported for measurements of the dependence of the neutron flux attenuation in monocrystalline quartz as a function of neutron wavelength at 293 and 85 K in the range 0.3 to 4.9 A. The length of the column of quartz disks being 20 cm, the flux of the neutrons with a wavelength of 0.3 A is attenuated by 74 times, and that with a wavelength of 1.3 A is attenuated only by 1.7 times (at 85 K). Under these conditions the background due to fast neutrons turns out to be small. The obtained results are compared to the measurement results of the factor of neutron flux attenuation by monocrystals of silicon oriented in the direction[3]. It has been found that for long wavelength the attenuation factor for silicon is nearly the same as for quartz, and for a wavelength of about 0.1 A it is twice less than that for quartz