|Fig. 1: A schematic of a typical gas-filled proportional counter. Source: Wikimedia Commons|
Special nuclear material (SNM) such as plutonium-239, uranium-233, or uranium-235 is used in the creation of nuclear explosives and atomic bombs due to its fissile nature, or capability to self-sustain a chain reaction with neutrons.  As such, the detection and control of SNM is essential to global nuclear threat reduction. One method used to identify SNM is through the sensing of fast and slow neutrons, which have energies above and below 0.5-1.0 keV, respectively.  This approach is attractive for several reasons. First, since the ambient background of neutrons is fairly low (0.005-0.02 neutrons/(s cm2)), any measured neutron levels above the background value is a strong indicator that SNM is present. Moreover, neutrons cannot be easily concealed or shielded from detection since they can penetrate several meters with only 1 MeV in energy.  These particles also have unique fission signatures (i.e. release of gamma-rays and neutrons) that depend on their specific interaction with the detection material.  Together, these characteristics make SNM identification via neutron detection an appealing approach.
Neutrons can be detected using helium-3-filled gas proportional counters (Fig. 1). A typical counter consists of a gas-filled tube with a high voltage applied across the anode and cathode. A neutron passing through the tube will interact with a helium-3 atom to produce tritium (hydrogen-3) and a proton.  The proton ionizes the surrounding gas atoms to create charges, which in turn ionize other gas atoms in an avalanche-like multiplication process.  The resulting charges are collected as measureable electrical pulses with the amplitudes proportional to the neutron energy. The pulses are compiled to form a pulse-height energy spectrum that serves as a "fingerprint" for the identification and quantification of the neutrons and their energies. 
However, one significant concern regarding the use of helium-3 gas proportional detectors is the global shortage of helium-3.  This isotope is rarely found in nature but exists as a decay product of tritium, which is primarily produced in nuclear reactors. [6,7] A recent estimate by GE Reuter Stokes estimates that the United States' demand for helium-3 is approximately 65 kliters per year while the total supply is only 10-20 kliters per year.  As a result, there has been a rising and urgent demand for the development of new strategies for neutron detection.
There are several alternatives for neutron detection that are currently in use or under development. Boron trifluoride (BF3) is one of the most widely-used fill gases for proportional counters.  In this case, each neutron interacts with BF3 to create lithium-7 and an alpha particle, the latter of which serves as the ionizing charge in the gas proportional counter.  However, BF3's toxicity makes it undesirable to handle, especially in applications that utilize larger detectors. 
Other boron-related detectors employ powdered boron-10. Recent work at Los Alamos National Laboratory has demonstrated that boron-10 detectors can achieve neutron detection efficiencies comparable to those of helium-3 detectors.  Boron-10 is also non-toxic, in abundant supply, and offers excellent discrimination between neutrons and gamma-rays.  This material is used as wall-lining in gas-filled proportional counters, as thin films coupled to the surface of semiconductor detectors, and in scintillators (materials that interact with ionizing radiation to produce detectable light). 
Lithium-6 is another material that is being used for neutron detection. It is used in crystal scintillator compounds like Cs2LiYCl6, Cs2LiLaCl6, and LiI, as well as liquid scintillators and glass fibers. [2,10,11] However, one main drawback of lithium-6 is its lower detection sensitivity.  Additional options for neutrons under development include liquid argon detectors, scintillating-dye-loaded polymers, and graphene field effect transistors. [3,13,14]
It is clear that no material for neutron detection can meet all of the demands for high efficiency, high abundance, good discrimination, low cost, low toxicity, and ease of scalability. Many of the future solutions will likely rely on the improvement and modification of existing methods and will be designed to be application-specific.  Although it is uncertain when the next major breakthrough in neutron detection will occur, shortcomings in certain detection methods can be overcome by coupling complementary and multiple techniques for a more comprehensive analysis. 
© Stephanie Lam. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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