The Evolution of Neutron Detectors
- By Duncan Williams -
The most important particle in nuclear reactor physics is the neutron. Without it, there can be no sustained nuclear fission and no electricity generated from nuclear power. Given its importance in today’s reactor physics, it is hard to imagine that no one had even heard of a neutron until 1932. Prior to the discovery of neutrons, atoms were thought to contain only negatively charged electrons and positively charged protons. The fact that neutrons have no electrical charge hid them from discovery until a British scientist named James Chadwick conducted experiments in 1932 proving their existence.
Neutrons are necessary to sustain a fission reaction in nuclear reactors. The fission reaction begins when the nuclear fuel (e.g., uranium-235 or plutonium-239) absorbs a neutron. As a result of neutron absorption, the nuclear fuel undergoes fission releasing a number of particles that includes more neutrons. These neutrons go on to be absorbed by other nuclear fuel atoms, thus sustaining a fission chain reaction.
During operation, a nuclear reactor is controlled according to its power level. The power level of a nuclear reactor is usually expressed as a percentage from 0%-100%. The 100% power level is determined by nuclear designers and indicates the maximum amount of neutrons that should be produced during the fission process in order to prolong the life of the nuclear fuel. Therefore, unlike a car engine, it is possible for nuclear reactors to exceed the 100% power level since it is simply a design parameter. But continuously operating a nuclear reactor over 100% power would cause the nuclear fuel to burn out earlier than intended, resulting in costly refueling procedures. Neutron detectors are used in nuclear reactors to monitor the amount of neutrons produced during operation, known as the neutron flux, so that the 100% design parameter is not exceeded. However, the science of detecting the neutrons produced in an operating nuclear reactor has proven to be no easy task and is continuously striving to improve the accuracy of measuring a neutron flux.
Neutrons produced during the fission process in nuclear reactors are categorized according to its energy level. For example, lower-energy neutrons are called thermal neutrons, while higher-energy neutrons are called fast neutrons. Since many commercial nuclear power plants rely on thermal neutrons for fission, and for controlling reactor power, many of the patents relating to neutron detection are focused on measuring thermal neutrons.
U.S. Patent No. 3,390,270, issued on June 25, 1968, and assigned to the U.S. Atomic Energy Commission, describes a neutron detector which does not need an outside power source to operate.
As can be seen in the diagram of the detector from the patent, neutrons produced during fission will first be absorbed by a hollow emitter electrode (12) located in the center of the detector. The emitter electrode is a cylinder made from nickel and coated on its outer surface with cadmium-113, gadolinium-, samarium-149, or mercury-199. These coatings readily absorb thermal neutrons, and emit gamma rays as a result. The gamma rays, in turn, travel into the dielectric insulator (16), which is made from alumina, beryllium oxide, or magnesium oxide.
The gamma rays undergo a process known as Compton scattering in the dielectric insulator (16), resulting in the emission of electrons that are collected at the collector electrode (14). The collector electrode (14) is made from a material that is not likely to absorb neutrons, thereby allowing neutrons to pass through the collector electrode (14) and be absorbed by the emitter electrode (12). The patent indicates that the collector electrode (14) can be made from stainless steel, titanium, nickel, aluminum, or magnesium. The accumulation of electrons at the collector electrode (14) causes an electrical current to form which is transferred to a coaxial cable (22) and inner conductor (24) at one end of the detector (right-side of the diagram). The patent indicates that even though the sensor can operate for long periods of time without need for replacement, the lifetime of the detector can be increased by replacing the hollow cylindrical emitter electrode (12) with a solid cylinder.
Accurate neutron detection is complicated by particles that interact with the cables leading from the detector to the signal-processing circuitry. Often, particles emitted from the reactor collide with the cabling causing interference, or noise, which can distort the electrical signal resulting in inaccurate signals. An electrical circuit described in U.S. Patent No. 4,002,916, issued on January 11, 1977, and assigned to the United States government, uses an averaging circuit from two detectors in order to cancel out the noise that may be caused by particles interacting with the cabling on any one of the detectors. If the signal received by one of the detectors reaches a preset trigger threshold, the electrical signal is blocked and the signal-processing circuit only processes the signal from the detector which is not experiencing noise.
U.S. Patent No. 4,103,165, issued on July 25, 1978, and assigned to the U.S. Department of Energy, describes an improved neutron detector which has increased accuracy over previous detectors (e.g., U.S. Pat. No. 3,390,270 discussed above).
The neutron detector includes an elongated rod-like centralized emitter (12) made from beryllium. The detector also includes an electrically conductive collector sheath (14) made from inconel or stainless steel. A vacuum space (16) separates the emitter (12) and the collector sheath (14), while ceramic spacers (18) hold the emitter (12) securely in place. The patent explains that the neutrons are absorbed by the beryllium emitter (12) resulting in the emission of electrons. The electrons easily travel through the vacuum space (16) to the collector sheath (14), and are not impeded by a dielectric as in U.S. Pat. No. 3,390,270. Since none of the electrons are trapped in a dielectric layer, the detector has an overall improved accuracy for detecting neutrons.
Another type of neutron detector, as described in U.S. Pat. No. 3,956,654, issued on May 11, 1976, and assigned to Westinghouse Electric Corporation, utilizes a coating of boron that produces electrons as a result of absorbing neutrons.
As can be seen in the diagram, a positively charged electrode wire (14) is placed inside a hermetically sealed cylindrical neutron detector (12) containing either carbon dioxide or boron trifluoride gas. The inner surface of the neutron detector is coated with a boron-10 layer (18). To prevent the boron-10 layer (18) from reacting with the gas in the detector, a thin aluminum oxide film (22) is placed on top of the boron-10 layer (18). Neutrons produced in a nuclear reactor are absorbed by the boron-10 layer (18), resulting in the emission of electrons. These particles collide with the carbon dioxide or boron trifluoride gas, which then produces electrons. The negatively charged electrons are magnetically drawn towards the positively charged electrode wire (14) resulting in an electric current, which is then processed to determine the number of neutrons.
More recently, neutron detectors consisting of semiconductors have come to the forefront of research and development. Although semiconductor neutron detectors were originally developed decades ago, the technology was abandoned due to its inability to withstand bombardment of charged particles inherent in a nuclear reactor environment. One of the first semiconductor neutron detectors is described in U.S. Pat. No. 3,227,876, issued on January 4, 1966, and assigned to Hoffman Electronics.
As can be seen in the diagram, the detector includes a N-type silicon wafer (10) encased in an P-type silicon wafer (11). Generally, P-type silicon is doped with atoms that allow the wafer to carry positive ions, while N-type silicon is doped with atoms that allow the wafer to carry negative ions. Placing these two types of silicon wafers together forms what is known as a P-N junction, which creates an electric field that tends to pull in charged particles. Neutrons (23) produced in a nuclear reactor are absorbed by a boron rich layer (12) deposited on the outer surface of the P-type silicon (11). As a result of absorbing the neutrons, the boron rich layer (12) emits charged lithium and helium atoms, which are pulled towards the P-N junction causing an electric pulse to form at the electrodes (13, 14). The electric pulse is then fed to electrical circuitry which determines if the signal resulted from a neutron, as opposed to some other particle, being absorbed in the boron rich layer (12).
However, an undesired result of this process is that the charged lithium and helium atoms tend to embed themselves in the silicon wafers themselves. This causes the wafers to slowly deform over time, resulting in degraded performance. The semiconductor neutron detector described in U.S. Pat. No. 5,940,460, issued on August 17, 1999, and assigned to the Department of Energy, overcomes this problem partly by using materials resulting in a semiconductor thin enough to allow these particles to pass through it entirely.
The patent indicates that the entire semiconductor is only 10 - 15 cm thick. As can be seen in the diagram, a P-N junction is formed by placing a N-type semiconductor (14) next to a P-type semiconductor (16). The P-N junction is sandwiched between a thin conductive contact (18) and a conductive substrate (12), which are both connected to electrical contacts. A neutron converter layer (22), made from lithium or boron, is spaced apart from the P-N junction by way of an insulating material (20) made from silicon dioxide to provide for resistance to higher temperatures. When a neutron is absorbed in the converter layer (22), the resulting charged helium or lithium particles travel in the direction indicated by lines N and/or O. In contrast to previous semiconductor neutron detectors, the charged particles do not come to rest at the P-N junction thereby avoiding long-term damage to the detector. The patent explains that the charged particle simply needs to pass through the P-N junction, as opposed to coming to rest there, in order to form an electrical pulse.
Research and development is continuously increasing our ability to monitor and measure neutrons in an operating nuclear reactor. The recent emphasis on semiconductors for neutron detection has sparked new research and development into technology that has been idle for decades. With the development of new materials that were not available when the technology first originated, semiconductor neutron detectors will substantially increase the accuracy of measuring neutrons and will eventually lead to cheaper and more accurate devices.
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About Duncan WilliamsDuncan Williams graduated from the University of Florida in 1994 with a B.S. in Physics, and a minor in mathematics. Upon graduation, he was commissioned in the U.S. Navy where he completed training in the Navy’s Nuclear Propulsion program. He then served onboard an 
aircraft carrier, the USS Theodore Roosevelt, as a reactor control division officer. Onboard, he was responsible for the operation and maintenance of the electrical and mechanical components that make up the reactor control systems. This includes the control rod drive mechanisms, the reactor safety and emergency systems, the reactor coolant pump systems, and the ion exchangers. He also developed and implemented ship-wide reactor safety drills in order to educate sailors in reactor safety.
Duncan then transferred to the U.S. Naval Academy, where he served as a senior instructor teaching Thermodynamics to senior cadets. While serving as an instructor at the Naval Academy, Duncan attended night law school at the George Washington University Law School. After receiving his J.D. in 2004, he resigned his commission and began working as an intellectual property associate with Kenyon & Kenyon LLP. While at Kenyon & Kenyon, he drafted numerous patents relating to medical devices, electronic devices, telecommunications, as well as other technologies. He also has experience in all stages of patent litigation, and has represented numerous Fortune 500 companies in protecting their intellectual property rights. Duncan is currently an intellectual property associate at Blank Rome LLP.
If you have questions, comments, or know of a patent that you think Duncan should review E-mail Duncan Williams>> duncan@nuclearstreet.com