A cooled polyethylene moderator on a pulsed neutron source

A 40 mm thick polyethylene moderator cooled by liquid nitrogen was tested on the Harwell 5 kW electron linac. The scientific considerations underlying its design are discussed. Figures are given for its performance in terms of neutron intensity and pulse width as a function of wavelength, and compared with those from the same moderator at ambient temperature. The cooled moderator shifts the Maxwellian peak to longer wavelengths, with a factor of five gain in intensity in the 3 A region. The epithermal slowing down spectrum is extended to longer wavelengths, so that sharp pulses are preserved in the 1 A region. This is useful for diffraction experiments on a pulsed neutron source, and the loss of intensity at 1 A is compensated in terms of figure-of-merit by a 30% decrease in pulse width. The pulse shapes are also compared with those from a similar polythylene moderator homogeneously poisoned with a thin gadolinium foil. The choice between these two methods of pulse improvement depends on the wavelength and resolution requirements of each instrument. (Auth.)