Tracking and identifying radiation sources in the age of nuclear proliferation and well- resourced non-state actors is a national priority. Current neutron detection methods favor large detector volumes and long data collection times. Additionally, portable neutron detection methods have persistent problems with low signal-to-noise (small pulse height) and require large applied voltages.

The microscopic fission cross sections for 235U and 238U as a function of neutron energy.

Conventional neutron detection usually employs scintillators, gas proportional tubes, or semiconductors with separate conversion layers that convert neutrons to charged reaction products. The challenge in conversion-layer devices is to construct a layer of adequate thickness for neutron capture that is also thin enough to allow the resulting reaction products to interact with the charge-sensitive areas. Standard conversion-layer devices typically employ 30 to 40 μm of enriched 10B due to the high thermal capture cross section of the isotope 10B and the ability for the energetic Li (0.84 MeV) and α (1.47 MeV) daughter particles to escape. The conversion layer thickness is thus a compromise between the neutron interaction rate and the ability to capture the charge in an electrically active medium. Some novel solid-state technologies provide a thin-film neutron detector consisting of a (or a stack of) semiconductor diode(s), each surrounded by a thin neutron-absorbing material.

As an alternative to a conversion layer, the conversion atoms can also be incorporated into the detection medium (gas, scintillator, or semiconductor). This method has the advantage of a more efficient structure to capture the reaction products, but often results in a reduced electronic signature (or in the case of scintillators, reduced light output). As an example, icosahedral semiconducting boron carbides have recently garnered interest because a high-quality electronic material can be made that incorporates 10B as a component of the semiconductor material.

Another consideration in 10B based neutron detectors is the energy dependence of the capture cross section. For thermal neutrons (⊕0.025 eV), the capture cross section for 10B is about 3840 barns. However, for neutrons of 1.6 MeV (the mean energy of prompt fission neutrons from 235U) the 10B absorption cross section is 0.2 barns. Therefore, to achieve the highest detection efficiency for neutrons from fission, 10B based neutron detectors require neutron moderation, resulting in a loss of efficiency either due to scattering out or capture outside of the detection medium. Similar arguments can be made for other conventional neutron conversion materials.

For 235U, the thermal fission cross section is 585 barns and the fast neutron (1.6 MeV) fission cross section is 2 barns as shown in the accompanying figure. Also, the 238U thermal fission cross section is 16.8 microbarns and its fast neutron (1.6 MeV) fission cross section is 1.1 barns. Therefore, 235U is approximately 2× more likely to fission with fast neutrons than 238U in materials of the same density. In comparison, 10B has a thermal neutron absorption cross section 6.55 greater than the thermal fission cross section for 235U. However, for neutrons at 1.6 MeV, 235U has a 4× larger fission cross section and 238U has a 2× larger fission cross section than the 10B neutron absorption cross section of approximately 0.5 barns. Therefore, for fast neutron detection, the uranium cross section is preferred.

A uranium-based neutron detector has a substantial energy output advantage as compared to conventional neutron detection devices that incorporate non-fissile or fissionable neutron conversion materials. The fission of uranium results in two or more charged fission fragments with 168 MeV, having the potential of producing more than 25 million electronic transitions across the UO2 2.1 eV band gap. The resulting high-energy fission products may, however, degrade the electronic properties of the UO2 either due to formation of point defects or because of subsequent decays and transmutations providing a larger internal background noise signal. A further consideration is that uranium requires special handling procedures, especially if enriched, to take advantage of the higher fission cross section of 235U, as opposed to the more naturally abundant 238U. Therefore, this research will help to understand the tradeoffs with uranium-based detectors made from UO2, UO2/ThO2, and U0.71Th0.29O2 using 238U, before the complications of using and managing 235U are considered.

This work was done by Lieutenant Colonel Christina L. Dugan for the Air Force Institute of Technology. For more information, download the Technical Support Package (free white paper) here under the Sensors category. AFRL-0272

This Brief includes a Technical Support Package (TSP).
Electrical Characterization of Crystalline UO2, THO2 and U0.71TH0.29O2

(reference AFRL-0272) is currently available for download from the TSP library.

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