Experiments with a velocity-spectrometer for slow neutrons

Abstract

The properties of slow neutrons of different velocities have been investigated mainly by methods in which their velocities are determined indirectly, for example, by way of the absorption coefficient of boron, which is believed to vary inversely with the velocity of the neutron. Direct measurements of velocity within the thermal range (~ 2-5 x 10 5 cm./sec.) have, however, been made with velocity-selectors employing high speed rotary shutters which allow neutrons to pass for brief periods at regular intervals (Dunning, Pegram, Fink, Mitchell and Segrè 1935). If two such shutters are arranged at a distance l apart, a neutron of velocity v can pass through both shutters in succession only if the second is open at a time l/v later than the first; if the two shutters are driven at identical frequencies with a suitable difference of phase, only those neutrons whose velocities lie within certain ranges will be transmitted.