Well-defined, intense beams of fast neutrons have been produced by using shields of paraffin and lead in conjunction with the 8-Mev deuteron-beryllium source of a cyclotron. Essentially, the beam is delimited by an outward tapering channel through a wall of paraffin (or water) over 50 cm thick. Accompanying gamma-radiation was greatly reduced by lining the channel with 3 cm of lead and by walling the outside of the hydrogenous shield with over 2.5 cm of lead. Gamma-rays from the source are suppressed by a 3-cm thick lead filter in the channel. Ionization measurements and photographs showed that the fast neutron effects are mainly localized to a collimated beam which radiates out along the projection of the aperture through the lead channel. To distinguish between neutron and gamma-radiations the responses of small ionization chambers with different walls were studied. The ionization produced by the fairly uniform background of gamma-radiation is only a few percent of that caused by fast neutrons in the beam. With an ordinary deuteron current of 60 μa the ionization produced in air at the beam outlet by fast neutrons alone is 1.14 e.s.u./cc per min. To produce a neutron beam of this intensity would require a Rn-Be source of over ${10}^5$ curies. The beam was found suitable for many physical and biological experiments.

• Received 25 July 1939

DOI:https://doi.org/10.1103/PhysRev.56.714