Fast neutron total cross sections.

A transistorized, neutron time-of-flight spectrometer has been designed and built for measurement of the energy of neutrons generated with the aid of the McGill cyclotron. Circuitry of the type used in fast coincidence measurements has been used, with avalanche transistors as fast discriminators and limiters. The intrinsic electronic resolution is considerably better than one nanosecond. The spectrometer was used to examine the radio frequency structure of proton pulses generated by the cyclotron. This was found to vary greatly with operating conditions and to depend greatly on the stability of the radio frequency oscillator. The short-term width at half height was 4 nanoseconds when the accelerating voltage applied to the dees was 7 kV, and the mean value of this quantity for long runs was 5-7 nanoseconds. With a flight path of 25 feet, the energies of neutrons or protons between 30 and 80 MeV can thus be measured. The total cross sections for neutrons of calcium, potassium and copper were measured in the range 30 to 100 MeV and found to confirm existing measurements except in the case of calcium. The deep minimum found at about 36 MeV by Moody was not confirmed, though energy resolution was sufficient to detect it