A slow neutron velocity selector has been developed for use with the Columbia University cyclotron. By the method of arc modulation, neutron production is usually confined to intervals of (10-200) microseconds out of a (1000-10,000) microsecond cycle. Neutrons slowed down in paraffin are detected by B${\mathrm{F}}_3$ proportional counters. Excellent collimation is obtained by using an extensive ${\mathrm{B}}_4$C and Cd collimating system. Experiments were conducted principally at a 5.4-meter source detector distance. A special selector system counts all the neutrons detected and also selectively counts those detected in an adjustable timed interval after the cyclotron burst. The slow neutron energy distributions from paraffin "source" slabs were shown to be of a modified Maxwellian form with an asymmetrical high energy "tail." Data for the resonance absorption by Cd were well matched by a one-level Breit-Wigner formula having $E_0=(0.180\mathrm{}\pm 0.008)$ ev, $\Gamma =(0.112\mathrm{}\pm 0.006)$ ev, ${\sigma }_0=(7,800\mathrm{}\pm 800)\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/atom. The results of measurements with several boron filters over the range of 0.01 ev to over 100 ev were well matched by the $\frac{1}{v}$ relation, ${\sigma }_B=(118\mathrm{}\pm 4)E^{-\frac{1}{2}}\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/atom.

• Received 23 August 1945

DOI:https://doi.org/10.1103/PhysRev.70.136