"Fast Chopper" Time-of-Flight Measurement of Neutron Resonances

A high-resolution neutron velocity selector, or "fast chopper," has been constructed and is in routine operation at the Brookhaven reactor. It consists of a high-speed rotor that produces bursts of neutrons of about 1-μsec duration, together with a neutron detector at the end of a 20-meter flight path and circuits with which the time of flight is measured. The construction and operating characteristics of the equipment are described briefly. The methods that are used for analysis of sample transmission curves in the energy regions of good and of poor resolution are presented, together with curves used to correct for the effects of Doppler broadening and sample thickness. In the good resolution region the measured total cross section in the vicinity of a resonance can be interpreted to give the total width $\Gamma$ and $g{\Gamma }_n$. In the poor resolution region only the quantity ${\sigma }_0{\Gamma }^2$, which is proportional to $g{\Gamma }_n\Gamma$, is obtained. Measured parameters for levels in the 2 to 200 ev energy region are given for silver, iodine, thorium, gold, manganese, and cobalt. The results are compared with theoretical expectations.