The EXCALIBUR Neutron Source – Characterization and Current and Planned Operational Modes

Neutron sources play an important role in a variety of verification schemes. For radiographic applications a source of directed high energy neutrons is required, while for applications to detect fissile isotopes, sub-MeV neutrons are preferred. The EXCALIBUR (EXperiment for CALIBration with URanium) neutron source has been built and used in a variety of verification-related experiments. EXCALIBUR is based on a commercial deuterium-tritium generator capable of producing 14 MeV neutrons at rates of up to 3×108 s-1. The generator is enclosed in a 32” diameter carbon-steel cylinder which lowers the mean neutron energy to under 500 keV. This, in turn, is encased in 5%-borated polyethylene such that the entire assembly is a 48” x 48” box that is 30” tall. For radiographic applications, a narrow, tapered channel in the steel and polyethylene allows 14 MeV neutrons to stream directly from the generator to a test object. In the moderated mode of operation, the generator is fully enclosed in the steel, but a large section of the polyethylene is removed, providing a flux of sub-MeV neutrons from a wide range of angles. Importantly, the system can be switched between the two modes in under two hours, therefore providing flexibility in experimental operations. Neutron spectrum measurements using both a nested neutron spectrometer and a commercial liquid scintillator detector are presented characterizing the neutron flux. The commercial detector is cross-calibrated against a diagnostic fast neutron detector located near the generator in order to monitor EXCALIBUR performance. EXCALIBUR is located in a shielded enclosure at the Princeton Plasma Physics Laboratory, comprised of 32”-thick shield walls, resulting in no significant radiation dose to personnel. Justification Well-characterized versatile laboratory neutron sources are necessary for developing active warhead verification schemes and to help develop the requirements for such systems to be used in the field. Significance The EXCALIBUR neutron source is shown to be a flexible and controllable neutron source that provides either a directed beam of 14 MeV neutron or a diffuse beam of sub-keV neutrons, and can be further modified for schemes with special target/detector geometries. This work supported by Defense Nuclear Nonproliferation R&D.