Patent Number: 052895106
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to radiation detectors employing scintillation or wave length shifting optical fiber arrays arranged in a two-dimensional X-Y plane for providing signals indicative of the locations of nuclear reactions occurring in the X-Y plane. More particularly, the present invention is directed to such radiation detectors wherein the optical fiber arrays are each provided by a plurality of side-by-side optical fibers with these optical fiber arrays disposed perpendicular to one another and in the X-Y plane and with a planar layer of nuclear reactive material operatively associated with the optical fiber arrays. Each nuclear particle or ray that contacts and reacts with the layer of nuclear reactive material will simultaneously provide a light pulse in a single optical fiber in each optical fiber array with these light pulses together being used to determine the X-Y coordinates of each nuclear reaction with a resolution of less than one millimeter (mm). Position sensitive radiation detectors are of significant interest in many experimental and production applications. For example, the determination of essentially the precise location of neutron activity in research facilities, including those employing cold and thermal neutron scattering experiments, is often of considerable importance in achieving the goal of the research. Also, in production facilities involving nuclear activity such as in the production of tritium in a nuclear reactor, the dimensional profiles of the reactor target materials are important considerations in the operation of the reactor and the production of the tritium. Further, an accurate determination of fission density distribution in a nuclear reactor is a significant criterion in the design and operation of the reactor. Also, with the storage of fissile material in storage vaults, the imaging of the fissile material stored in each location in the vault would provide a valuable monitoring mechanism. Various types of position sensitive neutron detectors have been recently developed and are described in the literature such as in the article, "Experience With Position-Sensitive Neutron Detectors at the Intense Pulsed Neutron Source", R. K. Crawford et al, Nuclear Instruments and Methods in Physics Research, A299 (1990) pp. 17-24. The position sensitive neutron detectors described in this article include detectors employing .sup.6 Li-glass scintillators in connection with multiple channel photomultiplier arrays and .sup.3 He proportional counters for providing two-dimensional (X-Y) coordinates of neutron activity with spatial resolutions in the range of about 0.7 to 14.0 mm. SUMMARY OF THE INVENTION It is an object of the present invention to provide improved position sensitivity radiation detectors for monitoring nuclear activity with spatial resolutions in two dimensions of less than 1 mm, preferably 0.1 mm or less. Generally, the position sensitive, two-dimensional radiation detectors of the present invention each comprise nuclear reaction sensing means defined by adjacently disposed first and second planar arrays of elongated scintillation or wave length shifting optical fibers oriented perpendicular to one another in X and Y directions. The planar arrays of the optical fibers are each provided by a plurality of elongated optical fibers disposed in a side-by-side relationship. A planar layer of nuclear reactive material is operatively associated with surface regions of the planar arrays of optical fibers corresponding to a substantial length of each of the optical fibers in the first and second planar arrays. A nuclear reaction occurring in the planar layer of nuclear material upon being contacted by a nuclear particle or ray will virtually simultaneously provide a light pulse in a single optical fiber in the array oriented in the X-direction and a light pulse in a single optical fiber oriented in the Y-direction. Circuit means are coupled to each of the optical fibers in each planar array and include coincidence counting means that are adapted to utilize the simultaneous light pulses from the optical fibers for providing a signal indicative of the X-Y coordinates of each nuclear reaction. The planar layer of nuclear reactive material is characterized by being capable of undergoing a nuclear reaction when contacted by a nuclear particle or ray for providing at least two particles which in some nuclear reactions both particles have active energy and are virtually if not simultaneously discharged from the layer of nuclear reactive material. In other nuclear reactions only one of the two particles produced by the nuclear reaction will have enough active energy to produce light. In the detector embodiment used for detecting nuclear reaction where the two energetic particles are discharged from the layer of nuclear reactive material, scintillation optical fibers are used in the optical fiber arrays. The planar layer of nuclear reactive material is positional between the first and second planar arrays of the scintillation optical fibers in a contacting relationship therewith. One of the two energetic particles is received by a single scintillation optical fiber in the first planar array and the other energetic particle is received by a single scintillation optical fiber in the second planar array at the nexus of these two optical fibers for virtually simultaneously, if not simultaneously, effecting light pulse producing scintillation reactions in both of these optical fibers contacted by the energetic particles. In the detector embodiment used for detecting nuclear reactions where only one effective energetic particle is produced in the layer of nuclear reactive material, wave length shifting optical fibers used in the optical fiber arrays and the nuclear reactive material is doped with a phosphor. The planar layer of the nuclear reactive material is placed in contact with the surface regions of only one optical fiber array. The active energy particle will react with the phosphor at the site or essentially at the site of the nuclear reaction to produce a light pulse which passes from the layer of nuclear reactive material into a single fiber in each of the optical fiber arrays for providing light pulses therein which are transmitted to the circuit means used for determining the X-Y coordinates of each nuclear reaction. Another object of the present invention is to provide position sensitive radiation detectors wherein each nuclear reaction of a plurality of sequential nuclear reactions is detected with the X-Y location of each reaction determined with a resolution in the order of less than about 1 mm, preferably about 0.1 mm or less. This relatively high level of spatial resolution provided by the subject detectors is limited only by the diameter or cross-sectional thickness of the scintillation or wave length shifting optical fibers employed in each of the optical fiber arrays. A further object of the present invention is to provide position sensitive radiation detectors for monitoring neutron activity in two dimensions by forming the planar layer of nuclear reactive material from enriched .sup.6 Li, preferably as .sup.6 LiF. A still further object of the present invention is to provide circuit means for utilizing the light pulses from the optical fibers to determine the X-Y location of each nuclear reaction with circuit means including pulse amplifying means having position analyzing means associated therewith adapted to provide signals from the planar arrays of optical fibers to coincidence counting means that are indicative of the X and Y location of the particular nuclear reaction. A still further object of the present invention is to provide position sensitive two-dimensional radiation detectors wherein the nuclear reaction sensing means comprises a plurality of stacked pairs of adjacently disposed first and second planar arrays of elongated scintillation or wave length shifting optical fibers oriented in the X and Y directions. In one embodiment of the detector a planar layer of the selected nuclear reactive material is disposed between the first and second planar arrays of scintillation optical fibers in each pair of the plurality of stacked pairs of scintillation optical fiber arrays. In another embodiment of the detector a planar layer of the selected nuclear material containing a phosphor is disposed between each pair of the plurality of stacked pairs of wave length shifting optical fiber arrays. The optical fibers in the same X or Y location in the plurality of stacked pairs of the optical fiber arrays in both of these embodiments of the detector are coupled to a common location on the pulse amplifying means for providing a light pulse thereto that is indicative of the location of the nuclear reaction in the X or Y direction regardless of the location in the Z direction in the plurality of stacked pairs of optical fiber arrays in which the nuclear reaction occurs. By employing a plurality of optical fibers in each planar array, the signals to the pulse amplifying means provide for the X-Y coordinates of sequential nuclear events in a manner with significantly more spatial resolution than that provided by previous position sensitive detectors such as described in the aforementioned article. In the present invention the instrumentation used for determination of the spatial resolution of the nuclear events is reduced by a factor of n/2 over that required in previously known radiation detectors wherein n.sup.2 is the number of monitoring positions of interest in the two-dimensional X-Y plane. Other and further objects of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.