Certain aspects of the calibration and resolution of slow neutron spectrometers

Abstract

The method of calibrating and determining the resolution of slow neutron spectrometers by the measurement of Bragg breaks is discussed. The folding of a Gaussian resolution function with a linear function presenting a sharp break is analytically studied. The ratio between the resulting “break width” and the resolution width is determined; in practical cases it is 1.0645. A relation between the mean abscissa, corresponding to the half-height of the resulting curve, and the true position of the break is given; they coincide only for symmetrical breaks. The effect of a Gaussian resolution in the transmission, inverse transmission, and total cross section, is theoretically evaluated for the iron (110) Bragg cut-off, for many resolution widths and several sample thicknesses. It is concluded that the analysis of the measured transmission curve gives the most direct and accurate result. The characteristics of a curved-slit slow neutron chopper and time-of-flight spectrometer built at the IEA are presented. The time resolution for the present conditions of geometry, with a very small detecting area, is 1% for a 3 m flight path and 4.046 Å neutrons. A calibration independent of the chopper speed, desirable when a wide energy range must be covered, has been attained within the precision of 4 μsec. The spectrometer can be used from 0.5 Å upto 12 Å for total cross section measurements.