The Sandia National Laboratories’ Annular Core Research Reactor (ACRR) is a unique pool-type research reactor that can pulse up to 300 MJ in energy. The ACRR maintains a dry, 9-in. (22.9 cm) diameter central cavity that extends through the center of the core region and allows for experiment irradiations at the peak neutron flux of the core. An epithermal/fast neutron flux exists in the cavity that allows the neutron energy spectrum to be modified to meet the requirements of the experimenter. Using a moderating material such as water or polyethylene in an annular geometry in the cavity allows a greater thermal neutron energy spectrum to be attained. Using a thermal neutron-absorbing material such as boron carbide or cadmium in an annular geometry in the cavity allows for a more epithermal-fast neutron energy spectrum. The gamma-ray fluence can be decreased by adding a high-Z material such as lead in an annular geometry. The gamma-ray fluence can be enhanced by adding a radiative capture material such as cadmium or gadolinium to a moderating material. Both neutron energy spectrum modification and gamma-ray attenuation/enhancement can be attained simultaneously. Different types of spectrum-modifying “buckets” are currently available for use by experimenters, and others can be custom designed and fabricated. This paper presents the results from the neutron and prompt gamma-ray characterization work for several of the environments in the ACRR central cavity, including the free field, polyethylene-lead-graphite, lead-boron-44 in., and cadmium-polyethylene bucket environments, and for the ACRR-Fueled Ring External Cavity-II. These environments represent typical neutron and gamma-ray spectrum modifications that can be attained at the ACRR.