Characterizing the Experiment for Calibration with Uranium (Excalibur) Neutron Source for Use in Warhead Verification

Neutron sources can play a variety of roles in warhead verification. For transmission radiography, 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 neutron generator specified and measured to be capable of producing 14 MeV neutrons at rates of up to 8.2$\times$10$^8$ neutrons/s. The generator is enclosed in a carbon-steel 32$^{\prime\prime}$ diameter, 23.62$^{\prime\prime}$ high carbon-steel cylinder that moderates 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$^{\prime\prime}$$\times$48$^{\prime\prime}$ box that is 30$^{\prime\prime}$ 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. Its collimating capability is demonstrated by measuring the neutron flux profile. 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. Neutron angular and spectral measurements using both a nested neutron spectrometer and a commercial liquid scintillator coupled with a $^3$He detector show the expected softer neutron spectrum in moderated mode in good agreement with MCNP6 calculations. The gamma-ray spectrum from Excalibur is also in good agreement with MCNP modeling. Based on these findings, the future application of Excalibur in its two configurations is discussed.