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.
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