Gamma Compensated, Self Powered Neutron Detector

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission and pertains generally to self-powered neutron detectors and more particularly to such detectors that have internally designed gamma compensation.

A variety of self-powered detectors presently exist for the measurement of reactor thermal neutrons. The large thermal neutron cross sections result in easily measured signal currents with minor contributions from gamma induced currents. As attention focused on an application of self powered techniques to the measurement of fast neutrons, reduced neutron cross sections resulted in greater relative contributions of the interfering gamma induced currents. Accordingly, in order to provide a representative fast neutron detector output, a method or technique is required which will effectively eliminate or significantly reduce the gamma current contributions.

Presently, in analogous state of the art devices, compensation for gamma induced currents is achieved by introducing a second detection device responsive only to the gamma effects. The neutron caused and gamma caused currents are then combined in a summing network to provide a measurement of the desired parameter (neutron representative current). This compensation technique necessitates the use of a two conductor shielded cable and generally requires a complex and difficult to fabricate structure.

One example of prior art devices can be found in the patent to Hilborn U.S. Pat. No. 3,375,370, which accomplishes gamma compensation by using two identical detectors, one of which is sensitive only to gammas. Currents generated by the two detectors are summed at the input to a differential current meter so that only the neutron induced current is indicated. A second example can be found in the patent to Linden U.S. Pat. No. 3,067,329, which provides gamma compensation in a manner similar to the Hilborn method except that the two detectors are constructed concentrically instead of separately. The Linden method requires two separate cables or a twin conductor cable coupled to current summing equipment to balance out the gamma induced current.

Preferably, a structural arrangement is desired for fast neutron, self-powered detectors, which enables direct, internal cancellation of gamma induced currents and eliminates the need for two coaxial cables or for twin conductor cables at the detector output. Desirably, measurements should be made with one single ended output current lead.

Briefly, this invention provides an improved, self-powered, neutron detector having an integral structure which inherently provides gamma compensation. Gamma interaction with the detector materials is nullified at the collector through direct cancellation of the currents induced by Compton scattered electrons.

In its preferred form this is accomplished by providing a detector having two electrically isolated concentric electrodes with a centrally positioned emitter formed from a material which emits beta particles in a field of neutron bombardment. The central emitter and outer electrode are maintained at a common potential while the intermediate concentric electrode, which serves as a collector, is electrically insulated from the other detector components. The beta responsive current is read across the common and collector potentials and is available for transmission by a single coaxial connecting cable. Collector absorbed electrons resulting from gamma particle bombardment of the outer electrode are directly cancelled by the generation of an equal and opposite current induced by Compton scattered electrons that result from gamma interaction with the collector materials.

For a better understanding of the invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating the basic operation of the detector of this invention;

FIG. 2 is an equivalent circuitry diagram illustratively showing the gamma and beta induced currents generated by the detector of FIG. 1; and,

FIG. 3 is a sectional view of the detector of FIG. 1 illustrating the basic construction thereof.

The source of gamma induced current in a self-powered detector is complex. However, the primary cause is due to Compton electrons scattered towards the emitter. The movement of these charges between the inner and outer conductors of a normal coaxial assembly constitutes the gamma induced current, which combines with the neutron representative beta current to provide a non-representative signal at the detector output. Some form of compensation is required as previously set forth to normalize the total detector output current to the current induced by gamma interaction with the detector materials.

In accordance with this invention a structural variation is employed which supplies a second cylindrical concentric electrode in a triaxial design which makes it possible to generate two equal but opposing gamma induced currents at the detector output as illustrated in FIG. 1.

Referring to FIG. 1 of the drawings it will be appreciated that a central rod emitter 10 is provided, constructed of a material that is responsive to neutron bombardment to provide a representative beta current output. A concentric cylindrical electrode 12 is positioned coaxially around the center electrode 10 and serves as a collector to absorb the beta current generated at the emitter. A second current I.sub..gamma..sub.2 is concurrently generated at the collector, induced from the gamma interaction with the collector materials, which provides an electron flow between the collector and the emitter. To cancel the effect of this second current a second cylindrical electrode 14 is positioned concentrically around the collector. The second electrode 14 is designed to generate a gamma, induced current output I .sub.1 substantially equal to I.sub..gamma..sub.2. Note, that the current generated by beta (.beta.) particles emitted at the center emitter (I.beta.) constitutes the desired measurement parameter. With the central emitter 10 and outer conductor 14 maintained at a common potential, direct cancellation of the gamma induced currents will occur at the collector interface. The equivalent electrical analog is illustrated in the circuit of FIG. 2 which shows that direct current cancellation enables the use of a simple coaxial cable to connect the sensor to the readout instrumentation.

FIG. 3 is a cross sectional view showing the actual construction of the preferred form of the detector of this invention. The sensor basically is formed from two cylindrical electrodes 12 and 14 constructed from a material such as stainless steel, a central rod emitter 10 constructed of a material such as beryllium (.sup.9 Be), and cylindrical and disc insulators. The beryllium rod emitter provides the .sup.9 Be (n,.alpha.).sup.6 He reaction with neutrons exceeding about 0.67/MeV in energy. The generated .sup.6 He decays by beta emission with an 0.81 second half life. These betas emitted from the rod move toward the collector, thus causing a current to flow into the external circuit. The rod 10 is electrically coupled to the outer stainless steel sheath 14 through an electrically conducting base plate 16 which establishes a common potential between the central emitter and outer electrode. The second cylindrical electrode 12 is supported between and insulated from the outer electrode 14 and the central emitter 10. Electrical communication is established at the end plate 18 of the collector to the central conductor 20 of the coaxial connecting cable 22 for transmission to the readout electronics shown schematically as a meter 24. The ground connection of the coaxial cable is coupled to the end cover 26 of the outer electrode to establish the common potential across the outer electrode and central emitter. Insulation is maintained between the several components of the detector unit by the plurality of insulating washers 28, 30, 32, 34 and 36 formed from a material such as MgO or Al.sub.2 O.sub.3. The specific design illustrated provides a compact durable structure that is relatively easy to fabricate. Compton electrons generated in the outer sheath 14 move towards the collector and compensate for the Compton electrons generated at the collector which move away from the collector towards the beryllium rod. The sheath, collector, and insulator thicknesses are chosen for proper current generation rates and for complete absorption of Compton electrons and beta particles.

Thus, the structure of this invention provides direct cancellation of gamma induced currents and allows the use of a simple coaxial cable to connect the detector to the measurement equipment. This is extremely desirable in a reactor application wherein the detectors are located in the reactor environment and the measurement equipment is located external to the reactor containment in a separate control room.