From introduction: The principal work described below is the development and tests of a slow neutron flux generator based on a generator developed at the University of California (UCRL 8359, W. Patterson, Roger Wallace, "A Method of Calibrating Slow Neutron Detectors"). It is shown that a tolerance flux density of slow neutrons results within a cubical cavity 15 inches on an edge with 4-inch-thick walls of water or paraffin wax when a plutonium-beryllium source emitting about 7 million neutrons per second is placed within the cavity according to a standard procedure. The principle on which the generator is based is that the fast neutrons from the source are slowed to near thermal velocities by scattering from the cavity walls. (Although the principal interest is in the slow flux, a primary fast flux is present at any point depending on the inverse square of the distance from the source.) The variation of slow neutron flux density over the walls of the cavity is unimportant for the calibration of thermal neutron detectors which follow a 1/v or 1/E response law (where v and E are the neutron velocity and energy), so that the detector is sensitive mainly to the slow neutron flux. As stated, the method provides a slow neutron flux density (instead of the neutron density only) so that a calibration for detectors placed within the cavity can be given either in terms of flux density of slow neutrons or, by simple conversion, in terms of dose-rate.