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SSC Pacific scientist awarded patent for nuclear detection device.

A new invention by a Space and Naval Warfare Systems Center (SSC) Pacific scientist, Dr. Wayne McGinnis, may enable U.S. Navy and Department of Homeland Security personnel to more effectively detect radioactive materials, such as plutonium, that are being transported illegally on ships, through ports of entry, or by other means.

Such materials present a critical threat to the United States because they can be used in manufacturing nuclear weapons.

A patent for the invention, titled "Omnidirectional solid-state thermal neutron detector," was awarded on June 29 of this year under U.S. Patent 7745800.

"This device furthers the work that a number of us at SSC Pacific began about a decade ago to find an efficient and convenient means to detect concealed radioactive materials," McGinnis said. "We developed a solid-state thermal neutron detector that was patented in 2008 and provides the basis for the new invention."

The detector consists of a semiconducting or insulating sheet of neutron-reactive material, such as boron nitride, sandwiched between two parallel conducting electrodes formed of a metal, such as titanium or zirconium. The total thickness of the boron nitride detector element is typically less than one millimeter. A small voltage applied between the two electrodes produces a continuous electric field within the neutron-reactive material. Because the boron nitride is highly resistive (not electrically conductive), only an extremely small background current will flow between the two electrodes when no neutrons are present.

Thermal neutrons emitted from a radioactive substance interact with the nuclei of boron atoms within boron nitride. Energetic alpha particles (helium nuclei) are one product of this nuclear reaction. The alpha particles then collide with other boron and nitrogen atoms within the boron nitride, knocking electrons from their atomic orbit. These newly freed charge carriers are swept toward the positive electrode, producing a current pulse (larger than the background current), thus indicating the presence of the neutron-emitting nuclear material.


"The solid-state thermal neutron detector provides improvements in sensitivity, size, weight, power consumption, operator safety, transportability and cost compared to other available detectors such as gas proportional counters and scintillation counters," McGinnis said. "It is more compact and therefore more portable. Operation at low voltage also means that the electronics used to apply the electrode voltage and to measure the detector current can be simpler, more compact, safer to the user, and much less power consuming."

However, like the helium-based counters, this device was unable to determine neutron flux direction, so McGinnis went back to work to find a solution to this remaining deficiency. He found that by arranging multiple planar neutron detector elements orthogonally, i.e., at right angles with respect to each other, omnidirectional detection could be attained. Each element will absorb a fraction of the incident neutrons. By comparing the detection (or current pulse) count rate for each detector element, the direction of the neutron flux can be determined.

"This arrangement can take many forms. One example is a configuration wherein six planar solidstate neutron elements are arranged to form the faces of a cube," McGinnis said. "Each pair of opposing sides of the cube forms a directional neutron detecting apparatus, in this instance one of three orthogonally arranged pairs of elements. For a given detector element pair, the neutron source will be on the cube side with the highest count rate." (See Figure 1.)


The new omnidirectional detector has potential use in locating hidden nuclear radiation sources, monitoring nuclear worker safety, hazardous materials assessment, and nuclear weapons surveying.

With support from the Defense Threat Reduction Agency (DTRA) and the Department of Homeland Security, McGinnis has been working with the University of Michigan to develop prototype individual detector elements for evaluation. Future efforts will focus on optimization of boron nitride as a neutron detection material, and demonstration of its use in a solidstate detector that includes directional capability.
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Author:Dakis, Ann
Date:Oct 1, 2010
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