Abstract
The Materials Testing Reactor (MTR) velocity selector consists of a pair of Fermi-type phased choppers and a pair of rotating collimators. The four rotors are driven at identical speeds up to 5225 rpm by four hysteresis synchronous motors. With the appropriate angular phase difference established between the choppers and the collimators, bursts of monoenergetic neutrons are selected from the MTR's Maxwellian flux. Phase instability broadens the initial neutron burst by only 12%, as demonstrated by actual measurements of time distributions. The monochromatic neutrons produced by the selector are used in measuring inelastic scattering cross sections by means of time-of-flight equipment. Intensities per time channel are sufficient to give reasonable statistics in a 24 hour run. Wavelength resolutions between 2% and 8% are realized. The paper describes the layout and characteristics of the velocity selector in detail, and discusses problems associated with backgrounds and time and intensity calibrations. Inelastic scattering data taken on samples of methane gas and on a beryllium single crystal are displayed and analyzed to show the counting rates and background levels achieved with the selector, and to demonstrate the versatility possessed by this type of monochromator.
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