Liquid Helium Scintillation Counter as a Neutron Polarimeter

A liquid helium scintillation counter has been constructed and used in analyzing the spin polarization of neutrons in the range of energy near 20 Mev. Details of construction and operation of the device are described. The glass scintillation flask contains 8 moles of helium, and is supplied by a reservoir which provides 7 hours continuous operation under experimental conditions. The pulse-height spread for a source of natural alpha particles is approximately 25%. It is found that the attenuation of the helium scintillation light in the liquid is not appreciable at a distance of 10 cm, while the wavelength is observed to be less than 1600 A. Consequently, use of a fluorescent material as a wave shifter within the helium flask is required. The intensity of the wave shifted helium light compares favorably with that from CsI(Tl). The helium counter is used in coincidence operation with a plastic scintillator neutron detector. Under these circumstances, the width of a gated helium recoil pulse-height distribution is generally less than 30%. The geometry and electronics for a typical neutron polarization experiment are described.