GB2038531A - Self-powered radiation detectors - Google Patents

Abstract

Flexible self-powered radiation detectors wherein the elongated central conductive emitter is formed of a plurality of small diameter stranded wires of the desired emitter material to improve the flexibility of the detector. Insulating material is provided about the stranded emitter, and a thin conductive sheath is provided about the insulating material.

Description

SPECIFICATION Self-powered radiation detectors This invention relates to self-powered radiation detectors. An exemplary self-powered detector is seen in U.S. 3,872,311. Such devices are termed self-powered in that a signal is generated by action of incident neutraons passing through an outer conductive collector sheath, through insulating means, to a central conductive emitter. No applied potential is required for such devices, unlike ionization chamber detectors or the like. A signal is generated between the emitter and collector electrodes and sensed externally as a function of the differing neutron interactions between the emitter and collector electrodes.

Self-powered detectors have been particularly suggested for in-core neutron flux level monitoring. This means the detector must be insertable into a nuclear reactor core, where it typically remains in place, or may be movable to sense the neutron flux at various positions in the core. The detector must be connected via instrumentation cable to the monitoring station. The self-powered detector must be insertable into the reactor vessel within which the core is disposed through instrumentating tubing or thimbles which are sealed through the reactor vessel. The detector has to be inserted or moved through several feet of such tubing which may have several bends in the tubing because of where it is inserted in the reactor vessel, and because of the sealed containment aspect of the vessel.

The typical self-powered detector may have an outside diameter of from about 0.080 to 0.1 50 inch. The central emitter is typically about 0.010 to 0.080 inch in diameter. The emitter wire is typically one of the metals platinum, rhodium, vanadium, cobalt, cerium, osmium or tantalum. These metals are primarily selected for their neutron interaction properties, but most of the metals which have been used have high tensile strengh and hardness. An insulation means is provided about the center emitter, and is typically compacted aluminum oxide or magnesium oxide which is about 0.010 to 0.040 inch thick. An outer conductive tubular collector sheath is disposed about the insulation means, and is typically Inconel steel with a thickness of about 0.025 inch. Inconel is a trade-marked nickel-steel of the International Nickel Co.

The relative diameter of the center emitter and the relative strength and hardness of the emitter material make it difficult to flex the detector during insertion aand movement of the detector in the instrumentation tubing.

Any flexing that takes place during insertion may result in breakage or fracture of the detector portion which could mean electrical discontinuity and failure of the device.

The radiation sensitivity of the detector is a function of the materials used and the relative dimensions of the respective emitter and collector. For a given emitter metal the sensitivity of the detector increases with increasing emitter diameter. Thus, for most applications the emitter diameter must be kept large enough to provide the requisite sensitivity, but for a solid rod-like emitter this will limit the flexibility of the assembly and make the emitter subject to breaking when the assembly is flexed during insertion into the reactor. It is therefore desirable to maintain a large emitter diameter without making the detector assembly so rigid that insertion through the bent path lead-in tubing is difficult or causes breakage and electrical discontinuity in the emitter cable.

Accordingly, the present invention resides in a self-powered radiation detector of the type comprising a neutron absorptive elongated central conductive emitter, insulating means about the emitter, and a thin conductive collector coaxially spaced about the emitter and the insulating means, wherein the emitter is formed of a plurality of stranded wires of the desired conductive emitter material whereby the detector is made relatively flexible.

The individual wires of the stranded emitter are preferably less than 0.005 inch in diameter. The emitter wires are stranded or helically wrapped about a core wire or in a coreless configuration.

In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a side elevational view in section of a self-powered radiation detector of a first embodiment; Figure 2 is a section view taken along lines ll-ll of Fig. 1; and Figure 3 is a sectional view taken through a self-powered radiation detector of a second embodiment.

Referring to Figs. 1 and 2, self-powered radiation detector 10 comprises an elongated central conductive emitter 12, insulating material 14 about the emitter 12, and a thin generally tubulalr conductive collector 1 6 about the insulating means 1 4.

The emitter 1 2 is formed of a plurality of stranded, spirally wound, small diameter wires 1 8 of the selected emitter metal. The individual wires 1 8 which form the emitter preferably have a diameter of less than about 0.005 inch when the wires 1 8 are of the same size, with the overall stranded emitter being from 0.020 to 0.080 inch in diameter.

The emitter metal is preferably platinum, rhodium, vanadium, cobalt, cerium, osmium or tantalum.

The insulating material 14 is typically com pacted finely divided aluminum oxide or magnesium oxide which is from 0.010 to 0.040 inch thick. The thin collector sheath 1 6 is typically Inconel steel and is about 0.025 inch thick. The overall outside diameter of the detector 10 is from 0.080 inch to 0.1 50 inch.

The use of the relatively small diameter wires 11 8 and stranding them to form the emitter permits the detector to be relatively flexible. Since the collector sheath is relatively thin and tubular it has some inherent flexibility, and the stranded wire emitter is not as rigid as a larger diameter solid rod-like emitter.

In another embodiment of the invention seen in cross-section in Fig. 3, the emitter 22 of the detector 20 comprises a center core wire 24 with a plurality of smaller diameter wires 26 spirally wound or stranded about the center wire. The center wire and plurality of smaller wires about the center wire are formed of the same emitter metal and are in intimate electrical contact. By way of example, the center wire would have a diameter of 0.020 inch, and the outer wires have a diameter of 0.005, so that the overall emitter diameter is 0.030 inch. A variety of other plural wire configurations can be used in forming the emitter with the desired flexibility and sensitivity. The insulating means 28 is disposed about the emitter 22 and insulates it from the conductive outer collector electrode 30.

Claims (5)

1. A self-powered radiation detector of the type comprising a neuutron absorptive elongated central conductive emitter, insulating means about the emitter, and a thin conductive collector coaxially spaced about the emitter and the insulating means, wherein the emitter is formed of a plurality of stranded wires of the desired conductive emitter material whereby the detector is made relatively flexible.

2. A radiation detector according to claim 1, wherein the stranded emitter wires are of equal diameter and each has a diameter of less than 0.005 inch per wire.

3. A radiation detector according to claim 1 or 2, wherein the emitter wire material is composed of platinum, rhodium, vanadium, cobalt, cerium, osmium or tantalum.

4. A radiation detector according to claim 1, 2 or 3, wherein the emitter comprises a center core wire with a plurality of wires stranded about the center core wire.

5. Self-powered radiation detectors substantially as described herein with particular reference to Figs. 1 and 2 or Fig. 3 of the accompanying drawings.