[en] Among the accelerator-based neutron sources presently in operation, the highest-flux source is the Intense Pulsed Neutron Source (IPNS), a user facility at Argonne. Neutrons in this source are produced by the interaction of 500-MeV protons from a synchrotron with either of two ₂₃₈U target systems. In the Radiation Effects Facility (REF), the ₂₃₈U target is surrounded by Pb for neutron generation and reflection. The REF has three separate irradiation thimbles. Two thimbles provide irradiation temperatures between that of liquid He and several hundred degrees centigrade. The third thimble operates at ambient temperature. In the Neutron Scattering Facility, the second ₂₃₈U target is surrounded by Be, and there are two irradiation thimbles that operate at ambient temperature. The energy distribution of the neutron flux has been measured. The spatial distribution of the neutrons within the REF irradiation thimbles has been determined