GB2374976A

GB2374976A - Neutron detector units

Info

Publication number: GB2374976A
Application number: GB0209211A
Authority: GB
Inventors: Kevin Roy Whitehouse; Oliver Paul Nicols; Alan Peter Simpson
Original assignee: BNFL IP Ltd
Current assignee: BNFL IP Ltd
Priority date: 2001-04-26
Filing date: 2002-04-23
Publication date: 2002-10-30
Anticipated expiration: 2022-04-23
Also published as: JP2002341045A; GB2374976B; GB0209211D0

Abstract

A neutron detector unit comprises a neutron detector 202, layers 204, 208 of a neutron moderating material such as polyethylene, layers 206, 210 of a thermal neutron screening material such as cadmium and a layer 212 of a neutron absorbing material such as a boron loaded polymeric material. The layer 212 shields against lower neutron energies and also serves to thermalise some higher energy neutrons.

Description

IMPROVEMENTS IN AND RELATING TO DETECTORS This invention concerns improvements in and relating to detectors, particularly, but not exclusively, neutron detectors and systems which employ such detectors.

A number of situations call for radioactive materials to be monitored and investigated. Neutron detectors are used in some of these situations to monitor neutron emissions in their vicinity, with the results arising being processed in one or more of a number of potential ways to obtain the desired information.

Where separate readings of neutron emissions for adjacent locations are needed there can be considerable problems from neutrons emitted in one location being detected in another location. Such problems are particularly problematical where a number of neutron detectors are used for inventory monitoring in different locations throughout the whole or part of a process plant.

The present invention has amongst its aims the provision of detectors more capable of discriminating the source location of such neutrons, the provision of detectors which provide more accurate neutron counts for their locations of concern and the provision of systems employing such detectors to provide more accurate material accountancy.

The present invention seeks to address these aims through three principal forms which are discussed in detail below.

According to a first aspect of the first form of the invention we provide a neutron detector unit, the unit comprising a neutron detector, a neutron moderating material around the detector, a thermal neutron screening material around the detector and a neutron absorbing material around the detector.

According to a second aspect of the first form of the invention we provide a neutron detector unit, the unit comprising a neutron detector with at least two different neutron effecting materials provided around the detector, the neutron effecting materials being such that the neutron detector unit has high detection efficiency for high energy neutrons and low detection efficiency for low energy neutrons.

One of the neutron effecting materials may be a thermal neutron screening material. One of the neutron effecting materials may be a neutron absorbing material, for instance a polymeric material loaded with a neutron absorbing material.

One of the neutron effecting materials may be a neutron moderating material, such as a hydrogen containing material.

Preferably at least three different neutron effecting materials are provided, most preferably the three types of material are a thermal neutron screening material, a neutron absorbing material and a neutron moderating material.

The first and/or second aspects of the first form of the invention may include the following features, options or possibilities.

The neutron detector may be a single detector or a plurality of detectors may be provided within the detector unit. The detectors may be provided on a common axis. The detectors may be provided on different, preferably parallel, axis. One or more of the detectors may be of the boron trifluoride type. One or more of the detectors may be of the helium-3 type.

The neutron detector may have the form of a cylinder, for instance a right cylinder. The neutron detector may be 100mm to 3000mm long.

Preferably the neutron moderating material is a material containing one or more elements of atomic weight 14

or less. The neutron moderating material may be a hydrogen containing material. The neutron moderating material may contain hydrogen and/or deuterium and/or carbon and/or beryllium. The hydrogen containing material may be water and/or a water containing material, but is preferably a polymer, for instance polythene.

The neutron moderating material is preferably, at least partially, provided between the neutron detector and the thermal neutron screening material and/or neutron absorbing material.

The thermal neutron screening material may be a metal, such as cadmium or indium, and/or a metalloid, such as boron.

Mixtures of such materials may be used. Preferably the thermal neutron screening layer is provided in elemental form.

The thermal neutron screening material is preferably, at least partially, provided between the neutron moderating material and the neutron absorbing material.

The neutron absorbing material is preferably a polymeric material loaded with a neutron absorbing material.

The polymeric material may be rubber, natural or manmade.

The neutron absorbing material may be cadmium or indium, but is preferably boron.

The neutron absorbing material is preferably provided externally of the neutron detector and thermal screening material or neutron moderating material. More preferably the neutron absorbing material is provided externally of the neutron detector and both the thermal screening material and the neutron moderating material.

The detector unit and particularly at least the thermal neutron screening material may be enclosed by a non-toxic material, such as aluminium or PVC.

Neutron detection efficiency at one energy level or energy band may be compared with efficiency at other levels

or bands by comparing the interaction rate with the neutron detector for those energies or bands. The efficiencies may be normalised relative to the maximum efficiency for the detector unit.

The neutron effecting materials may be such that the detection efficiency for high energy neutrons is at least twice that of low energy neutrons, preferably is at least two and a half times that of low energy neutrons and more preferably is at least three times that of low energy neutrons. These ratios are particularly preferred where high energy neutrons may be thought of as those which have an energy greater than 10keV, for instance between 10 keV and 400 keV, upon reaching the outside of the detector unit and low energy neutrons may be thought of as those which have an energy less than 10 eV, for instance between 0.4 eV and 10 eV, upon reaching the outside of the detector unit. These ratios may be derived from comparison of the normalised detection efficiencies for an energy level or band of energy levels.

The neutron effecting materials may be such that the detection efficiency for high energy neutrons is at least 200 times that of low energy neutrons, preferably is at least 250 times that of low energy neutrons and more preferably is at least 300 times that of low energy neutrons. These ratios are particularly preferred where high energy neutrons may be thought of as those which have an energy greater than 400keV, for instance between 400 keV and 10 MeV, upon reaching the outside of the detector unit and low energy neutrons may be thought of as those which have an energy less than 0.4 eV upon reaching the outside of the detector unit. These ratios may be derived from comparison of the normalised detection efficiencies for an energy level or band of energy levels.

It is preferred that the detector is provided within one or more layers of neutron moderating material, for instance of polythene, a layer of thermal neutron screening

material, such as cadmium and a layer of neutron absorbing material, such as boron loaded rubber. Preferably at least one polythene layer is provided between the detector and one or both of the cadmium layer and the boron loaded rubber.

Preferably the cadmium layer is provided between one of the polythene layers and the boron loaded rubber layer.

In an embodiment, the detector is provided within a structure comprising a layer of neutron moderating material, a layer of neutron absorbing material, a further layer of neutron moderating material and a layer of thermal neutron screening material.

In one embodiment the neutron detector may have a diameter of less than 60mm. The detector may have a diameter of between 60 mm and 40 mm, for instance 50 mm.

In an alternative embodiment the neutron detector may have a diameter less than 30 mm. The detector may have a diameter of between 30mm and 20mm, for instance 25 mm.

In one embodiment the neutron moderating material may have an inner diameter of less than 60 mm. The inner diameter may have a diameter of between 60 mm and 40mm, for instance 50 mm. The neutron moderating material may have an outer diameter of less than 180mm. The outer diameter may have a diameter of between 180mm and 120mm, for instance 150mm.

In an alternative embodiment neutron moderating material may have an inner diameter of less than 30 mm. The inner diameter may be between 30 mm and 20 mm, for instance

25mm. The neutron moderating material may have an outer diameter of less than 150mm. The outer diameter may be between 150mm and 100mm, for instance 125mm.

Preferably the neutron moderating. material has a thickness of between 40 mm and 60 mm, ideally 50 mm.

The thermal neutron screening material may have a thickness of less than 5mm, for instance between 0.1 and 2

mm, more preferably 0.3 and 0.8 mm, for instance 0.5 mm +/- 10%, and ideally of 0.5 mm.

The neutron absorbing material may have a thickness of between 1 and 20mm, preferably 2mm and 10mm, more preferably 3 and 8 mm, for instance 5 mm +/-10%, ideally 5 mm.

The neutron absorbing material loading may be between 1 and 50 wt%.

In a first particularly preferred embodiment the detector has a diameter of 50mm, the hydrogen containing material has an inner diameter of 50mm and outer diameter of 150mm, the thermal neutron screening material is 0.5mm thick and the polymeric material is 5mm thick, most preferably +/- 10% for all the dimensions.

In a second particularly preferred embodiment the detector has a diameter of 25mm, the hydrogen containing material has an inner diameter of 25mm and outer diameter of 125mm, the thermal neutron screening material is 0.5mm thick and the polymeric material is 5mm thick, most preferably +/- 10% for all dimensions.

The materials are preferably in contact with one or both of the adjacent materials, most preferably both. The contact is preferably continuous.

According to a third aspect of the first form of the invention we provide a method of detecting neutrons, the method comprising exposing one or more neutron detecting units to neutrons, the neutrons passing through a neutron moderating material, a thermal neutron screening layer and a neutron absorbing material to reach the neutron detector and generate a signal indicative of their detection.

The third aspect of the first form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a fourth aspect of the first form of the invention we provide a neutron monitoring system for a plurality of locations, the system comprising a plurality of neutron detector units, at least one neutron detector unit being provided in each location being monitored, signals from the neutron detector units being conveyed to processing means, the processing means determining a characteristic of neutron emitting sources in one or more of the locations, one or more of the detector units being provided according to the first and/or second aspect of the first form of the invention.

The locations may be of the same or different types.

Types of location include process location, process vessels, storage locations, storage vessels and the like. The locations may comprise sequential locations along a process route, for instance a spent fuel reprocessing plant or part thereof.

One or more detector units may be provided in a given location. The detector units at a location may be operated separately from one another and/or may be operated in combination with one another, for instance as an array of detectors. Detector units may be provided at different positions within a location.

The processing means may be a computer and/or computer software. The processing means may analyse the signals from a neutron detector unit individually and/or in combination with one or more other detector unit's signals.

The characteristic may be the mass of sources in the location and/or the distribution of sources in the location and/or the mass of sources in a plurality of locations and/or the distribution of sources in a plurality of locations. The characteristic may be the difference between the amount of sources monitored as entering the location and/or plant and

the amount of sources measured as leaving the location and/or plant.

The fourth aspect of the first form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a fifth aspect of the first form of the invention we provide a method of monitoring neutron emissions from a plurality of locations, the method comprising obtaining signals from at least one neutron detector in each of the locations, processing the signals to determine a characteristic of neutron emitting sources in one or more of the locations, the neutrons being detected by one or more of the detectors according to the method of the third aspect of the first form of the present invention.

The fifth aspect of the first form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a first aspect of the second form of the invention we provide a neutron detector unit, the unit comprising a neutron detector, a first body of neutron moderating material around the detector, a first thermal neutron screening material around the detector, a second body of neutron moderating material around the detector and a second thermal neutron screening material around the detector.

According to a second aspect of the second form of the invention we provide a neutron detector unit, the unit comprising a neutron detector with at least two different neutron effecting materials provided around the detector, the neutron effecting materials being such that the neutron detector unit has high detection efficiency for high energy

neutrons and low detection efficiency for low energy neutrons.

One of the neutron effecting materials may be a thermal neutron screening material. One of the neutron effecting materials may be neutron moderating material, such as a hydrogen containing material. Preferably at least two different neutron effecting materials are provided, most preferably the two types of material are a thermal neutron screening material and a neutron moderating material, such as a hydrogen containing material.

The first and/or second aspects of the second form of the invention may include the following features, options or possibilities.

Preferably the neutron moderating material is a material containing one or more elements of atomic weight 14 or less. The neutron moderating material may be a hydrogen containing material. The neutron moderating material may contain hydrogen and/or deuterium and/or carbon and/or beryllium. The hydrogen containing material may be water and/or a water containing material, but is preferably a polymer, for instance polythene.

The neutron moderating material is preferably, at least partially, provided between the neutron detector and the first thermal neutron screening material and/or second thermal neutron screening material. Preferably a first body

of neutron moderating material is provided between the detector and the first thermal neutron screening material.

Preferably a second body of neutron moderating material is provided between the first thermal neutron screening material and the second thermal neutron screening material.

The first and/or second thermal neutron screening material may be a metal, such as cadmium or indium, and/or a metalloid, such as boron. Mixtures of such materials may be used. Preferably the thermal neutron screening layer is provided in elemental form.

Neutron detection efficiency at one energy level or energy band may be compared with efficiency at other levels or bands by the comparing the interaction rate with the neutron detector for those energies or bands. The efficiencies may be normalised relative to the maximum efficiency for the detector unit.

The neutron effecting materials may be such that the detection efficiency for high energy neutrons is at least twice that of low energy neutrons, preferably is at least two and a half times that of low energy neutrons and more preferably is at least three times that of low energy neutrons. In a particularly preferred embodiment of the invention the detection efficiency for high energy neutrons may be at least four times, more preferably at least five times and ideally at least six times that of low energy neutrons. These ratios are particularly preferred where high energy neutrons may be thought of as those which have an energy greater than 10keV, for instance between 10 keV and 400 keV, upon reaching the outside of the detector unit and low energy neutrons may be thought of as those which have an energy less than 10 eV, for instance between 0.4 eV and 10 eV, upon reaching the outside of the detector unit. These

ratios may be derived from comparison of the normalised detection efficiencies for an energy level or band of energies.

The neutron effecting materials may be such that the detection efficiency for high energy neutrons is at least 200 times that of low energy neutrons, preferably is at least 250 times that of low energy neutrons and more preferably is at least 300 times that of low energy neutrons. In a particularly preferred embodiment of the invention the relative detection efficiency of high energy neutrons may be greater than 0.5, more preferably greater than 0.6 and ideally greater than 0.65. The relative detection efficiency for low energy neutrons may be less than 0.01, preferably is less than 0.005, more preferably is less than 0.002 and ideally is less than 0.0001. These ratios/values are particularly preferred where high energy neutrons may be thought of as those which have an energy greater than 400keV, for instance between 400 keV and 10 MeV, upon reaching the outside of the detector unit and low energy neutrons may be thought of as those which have an energy less than 0.4 eV upon reaching the outside of the detector unit. These ratios may be derived from comparison of the normalised detection efficiencies for an energy level or band of energies.

It is preferred that the detector is provided within at least two layers of neutron moderating material, such as polythene and at least two layers of thermal neutron screening material, such as cadmium. Preferably at least one of the neutron moderating layers is provided between a thermal neutron screening layer and the detector. Preferably a thermal neutron screening layer is provided outside of the outermost neutron moderating layer.

In one embodiment the neutron moderating material may have an innermost diameter of less than 60 mm. The innermost diameter may have a diameter of between 60 mm and 40mm, for instance 50 mm. The innermost diameter is preferably the inner diameter of the first body of neutron moderating material. The outer diameter of the first body of neutron moderating material may be between 120mm and 80mm, for instance 100mm. The neutron moderating material may have an outermost diameter of less than 184mm. The outermost diameter may have a diameter of between 184 and 120.4mm, for instance 151mm. The outermost diameter is preferably the outer diameter of the second body of neutron moderating material. The inner diameter of the second body of neutron moderating material may be between 124mm and 80.4mm, for instance 101mm.

In an alternative embodiment the neutron moderating material may have an innermost diameter of less than 30 mm.

The innermost diameter may be between 30 mm and 20 mm, for instance 25mm. The innermost diameter is preferably the inner diameter of the first body of neutron moderating material. The outer diameter of the first body of neutron moderating material may be between 90mm and 60mm, for instance 75mm. The neutron moderating material may have an outermost diameter of less than 144mm. The outermost diameter may be between 144mm and 100.4mm, for instance 126mm. The outermost diameter is preferably the outer diameter of the second body of neutron moderating material.

The inner diameter of the second body of neutron moderating material may be less than 94mm, for instance between 94mm and 60. 4mm, for instance 61mm.

Preferably the neutron moderating material bodies have a combined thickness of between 40 mm and 60 mm, ideally 50

mm. The thickness may be made up of two or more separate neutron moderating material layers. Preferably the two or more layers are of equivalent thickness. The neutron moderating material may be provided in two layers, each between 20 and 35mm thick, ideally two layers 25mm thick are provided.

The thermal neutron screening material may have a thickness of less than 5mm, for instance between 0.1 and 2 mm, more preferably 0.5 and 1.5mm, for instance 1. 0mm +/- 10%, and ideally of 1. 0mm. The thermal neutron screening material thickness may be made up of two or more separate thermal neutron screening material layers. Preferably the two or more layers are of equivalent thickness. The thermal neutron screening material may be provided in two layers, each between 0.1 and 1.00mm thick, ideally two layers 0. 5mm, preferably +/-10%, thick are provided.

The neutron absorbing material loading may be between 1 and 50 wt%.

In a first particularly preferred embodiment the detector has a diameter of 50mm, the neutron moderating material is provide in two parts separated by a layer of thermal neutron screening material 0.5mm thick, the inner part of the neutron moderating material having an inner diameter of 50mm and outer diameter of 100mm, the outer part of neutron moderating material having an inner diameter of 101mm and an outer diameter of 151mm, and the outer layer of thermal neutron screening material being 0. 5mm thick, most preferably +/-10% for all the dimensions.

In a further particularly preferred embodiment the detector has a diameter of 25mm, the neutron moderating material is provide in two parts separated by a layer of thermal neutron screening material 0. 5mm thick, the inner part of the neutron moderating material having an inner diameter of 25mm and outer diameter of 75mm, the outer part of neutron moderating material having an inner diameter of

76mm and an outer diameter of 126mm, and the outer layer of thermal neutron screening material being 0. 5mm thick, most preferably +/- 10% for all the dimensions.

According to a third aspect of the second form of the invention we provide a method of detecting neutrons, the method comprising exposing one or more neutron detecting units to neutrons, the neutrons passing through a first thermal neutron screening material, a first neutron moderating material, a second thermal neutron screening material and a second neutron moderating material to reach the neutron detector and generate a signal indicative of their detection.

The third aspect of the second form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a fourth aspect of the second form of the invention we provide a neutron monitoring system for a plurality of locations, the system comprising a plurality of neutron detector units, at least one neutron detector unit being provided in each location being monitored, signals from the neutron detector units being conveyed to processing means, the processing means determining a characteristic of neutron emitting sources in one or more of the locations, one or more of the detector units being provided according to the first and/or second aspects of the second form. of the invention.

The locations may be of the same or different types.

Types of location include process location, process vessels,

storage locations, storage vessels and the like. The locations may comprise sequential locations along a process route, for instance a spent fuel reprocessing plant or part thereof.

The characteristic may be the mass of sources in the location and/or the distribution of sources in the location and/or the mass of sources in a plurality of locations and/or the distribution of sources in a plurality of locations. The characteristic may be the difference between the amount of sources monitored as entering the a location and/or plant and the amount of sources measured as leaving the location and/or plant.

The fourth aspect of the second form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a fifth aspect of the second form of the invention we provide a method of monitoring neutron emissions from a plurality of locations, the method comprising obtaining signals from at least one neutron detector in each of the locations, processing the signals to determine a characteristic of neutron emitting sources in one or more of the locations, the neutrons being detected by one or more of the detectors according to the method of the third aspect of the second form of the present invention.

The fifth aspect of the second form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a first aspect of the third form of the invention we provide a neutron detector unit, the unit comprising a neutron detector, a neutron moderating material around the detector, a thermal neutron screening material around the detector and a neutron absorbing material around the detector.

According to a second aspect of the third form of the invention we provide. a neutron detector unit, the unit comprising a neutron detector with at least two different neutron effecting materials provided around the detector, the neutron effecting materials being such that the neutron detector unit has high detection efficiency for high energy neutrons and low detection efficiency for low energy neutrons.

According to a third aspect of the third form of the invention we provide a neutron detector unit, the unit comprising a neutron detector, a first body of neutron moderating material around the detector, a first thermal

neutron screening material around the detector, a second body of neutron moderating material around the detector and a second thermal neutron screening material around the detector.

The first and/or second and/or third aspects of the third form of the invention may include the following features, options or possibilities.

The neutron moderating material is preferably, at least partially, provided between the neutron detector and the first thermal neutron screening material and/or second thermal neutron screening material. Preferably a first body of neutron moderating material is provided between the detector and the first thermal neutron screening material.

Preferably a second body of neutron moderating material is provided between the first thermal neutron screening material and the second thermal neutron screening material. The

neutron moderating material is preferably, at least partially, provided between the neutron detector and the thermal neutron screening material and/or neutron absorbing material.

The first and/or second thermal neutron screening materials may be a metal, such as cadmium or indium, and/or a metalloid, such as boron. Mixtures of such materials may be used. Preferably the thermal neutron screening layer is provided in elemental form.

The polymeric material may be rubber, natural or manmade.

Neutron detection efficiency at one energy level or energy band may be compared with efficiency at other levels or bands by comparing the interaction rate with the neutron detector for those energies or bands. The efficiencies may be normalised relative to the maximum efficiency for the detector unit.

The neutron effecting materials may be such that the detection efficiency for high energy neutrons is at least

twice that of low energy neutrons, preferably is at least two and a half times that of low energy neutrons and more preferably is at least three times that of low energy neutrons. In a particularly preferred embodiment of the invention the detection efficiency for high energy neutrons may be at least five times, more preferably at least seven times and ideally at least nine times that of low energy neutrons. These ratios are particularly preferred where high energy neutrons may be thought of as those which have an energy greater than 10keV, for instance between 10 keV and 400 keV, upon reaching the outside of the detector unit and low energy neutrons may be thought of as those which have an energy less than 10 eV, for instance between 0.4 eV and 10 eV, upon reaching the outside of the detector unit. These ratios may be derived from comparison of the normalised detection efficiencies for an energy level or band of energies.

The neutron effecting materials may be such that the detection efficiency for high energy neutrons is at least 200 times that of low energy neutrons, preferably is at least 250 times that of low energy neutrons and more preferably is at least 300 times that of low energy neutrons. In a particularly preferred embodiment of the invention the relative detection efficiency of high energy neutrons may be greater than 0.8, more preferably greater than 0.9 and ideally greater than 0.95. The relative detection efficiency for low energy neutrons may be less than 0.01, preferably is less than 0.005, more preferably is less than 0.002 and ideally is less than 0.0001. These ratios/values are particularly preferred where high energy neutrons may be thought of as those which have an energy greater than 400keV, for instance between 400 keV and 10 MeV, upon reaching the outside of the detector unit and low energy neutrons may be thought of as those which have an energy less than 0.4 eV upon reaching the outside of the detector unit. These ratios

may be derived from comparison of the normalised detection efficiencies for an energy level or band of energies.

It is preferred that the detector is provided within one or more layers of neutron moderating material, for instance of polythene, one or more layers of thermal neutron screening material, such as cadmium and a layer of neutron absorbing material, such as boron loaded rubber. Preferably at least one polythene layer is provided between the detector and one or both of the cadmium layer and the boron loaded rubber. Preferably a cadmium layer is provided between one, most preferably the outermost, of the polythene layers and the boron loaded rubber layer.

In one embodiment the neutron moderating material may have an innermost diameter of less than 60 mm. The innermost diameter may have a diameter of between 60 mm and 40mm, for instance 50 mm. The innermost diameter is preferably the inner diameter of the first body of neutron moderating material. The outer diameter of the first body of neutron

moderating material may be between 120mm and 80mm, for instance 100mm. The neutron moderating material may have an outermost diameter of less than 184 mm. The outermost diameter may have a diameter of between 184 and 120.4 mm, for instance 151mm. The outermost diameter is preferably the outer diameter of the second body of neutron moderating

material. The inner diameter of the second body of neutron moderating material may be between 124mm and 80.4mm, for instance 101mm.

The innermost diameter may be between 30 mm and 20 mm, for instance 25mm. The innermost diameter is preferably the inner diameter of the first body of neutron moderating material, The outer diameter of the first body of neutron moderating material may be between 90mm and 60mm, for instance 75mm. The neutron moderating material may have an outermost diameter of less than 144mm. The outermost diameter may be between 144mm and 100. 4mm, for instance 126mm. The outermost diameter is preferably the outer diameter of the second body of neutron moderating material.

The inner diameter of the second body of neutron moderating material may be between 94mm and 60. 4mm, for instance 61mm.

Preferably the neutron moderating material bodies have a combined thickness of between 40 mm and 60 mm, ideally 50 mm. The thickness may be made up of two or more separate neutron moderating material layers. Preferably the two or more layers are of equivalent thickness. The neutron moderating material may be provided in two layers, each between 20 and 35mm thick, ideally two layers 25mm thick are provided.

The thermal neutron screening material may have a thickness of between 0.2 and 2 mm, more preferably 0.5 and

1. 5mm, for instance 1. Omm +/-10%, and ideally of 1. 0mm. The thermal neutron screening material thickness may be made up of two or more separate thermal neutron screening material layers. Preferably the two or more layers are-of equivalent thickness. The thermal neutron screening material may be provided in two layers, each between 0.2 and 1. 00mm thick, ideally two layers O. 5mm, preferably +/-10%, thick are provided.

The polymeric material may have a thickness of between 1 and 10 mm, more preferably 3 and 8 mm, for instance 5 mm +/-10%, ideally 5 mm.

The neutron absorbing material loading may be between 1 and 50wt%.

In a first particularly preferred embodiment the detector has a diameter of 50mm, the neutron moderating material is provided in two parts separated by a layer of thermal neutron screening material 0.5mm thick, the inner part of the neutron moderating material having an inner diameter of 50mm and outer diameter of 100mm, the outer part of neutron moderating material having an inner diameter of 101mm and an outer diameter of 151mm, the outer layer of thermal neutron absorbing material being 0.5mm thick, the layer of neutron absorbing material being 5mm thick, +/-10% for all the dimensions.

In a particularly preferred embodiment the detector has a diameter of 25mm, the neutron moderating material is provide in two parts separated by a layer of thermal neutron screening material 0.5mm thick, the inner part of the neutron moderating material having an inner diameter of 25mm and outer diameter of 75mm, the outer part of neutron moderating material having an inner diameter of 76mm and an outer

diameter of 126mm, the outer layer of thermal neutron shielding material being 0.5mm thick, and the polymeric material being 5mm thick, most preferably +/-10% for all the dimensions.

According to a fourth aspect of the third form of the invention we provide a method of detecting neutrons, the method comprising exposing one or more neutron detecting units to neutrons, the neutrons passing through a two'-layers

of neutron moderating material, two thermal neutron screening layers and a neutron absorbing material to reach the neutron detector and generate a signal indicative of their detection.

According to a fifth aspect of the third form of the invention we provide a method of detecting neutrons, the method comprising exposing one or more neutron detecting units to neutrons, the neutrons passing through a first thermal neutron screening material, a first hydrogen containing material, a second thermal neutron screening material and a second hydrogen containing material to reach the neutron detector and generate a signal indicative of their detection.

The fourth and/or fifth aspects of the third form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a sixth aspect of the third form of the invention we provide a neutron monitoring system for a plurality of locations, the system comprising a plurality of neutron detector units, at least one neutron detector unit being provided in each location being monitored, signals from the neutron detector units being conveyed to processing means, the processing means determining a characteristic of neutron emitting sources in one or more of the locations, one or more of the detector units being provided according to the first and/or second and/or third aspects of the third form of the invention.

The locations may be of the same or different types.

Types of location include process location, process vessels, storage locations, storage vessels and the like. The locations may comprise sequential locations along a process

route, for instance a spent fuel reprocessing plant or part thereof.

The characteristic may be the mass of sources in the location and/or the distribution of sources in the location and/or the mass of sources in a plurality of locations and/or the distribution of sources in a plurality of locations. The characteristic may be the difference between the amount of sources monitored as entering the location and/or plant and the amount of sources measured as leaving the location and/or plant.

The sixth aspect of the third form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

According to a seventh aspect of the third form of the invention we provide a method of monitoring neutron emissions from a plurality of locations, the method comprising obtaining signals from at least one neutron detector in each of the locations, processing the signals to determine a characteristic of neutron emitting sources in one or more of the locations, the neutrons being detected by one or more of the detectors according to the method of the fourth and/or fifth aspects of the third form of the present invention.

The seventh aspect of the third form of the invention may include any of the options, possibilities or features set out elsewhere in this document.

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which :- Figure 1 illustrates a transverse cross-section through an embodiment of a detector according to the first form present invention; Figure 2 illustrates a longitudinal cross-section through the embodiment of Figure 1; Figure 3 illustrates the relative efficiency response against incident neutron energy for the embodiment of Figure 1; Figure 4 illustrates a cross-section through a second embodiment of a detector according to the first form of the present invention; and Figure 5 illustrates the relative efficiency response against incident neutron energy for the embodiment of Figure 4; Figure 6 illustrates a transverse cross-section through an embodiment of a detector according to the second form of the present invention; and Figure 7 illustrates the relative efficiency response against incident neutron energy for the embodiment of Figure 6; Figure 8 illustrates a transverse cross-section through an embodiment of a detector according to the third form of the present invention; and Figure 9 illustrates the relative efficiency response against incident neutron energy for the embodiment of Figure 8.

A great variety of situations call for the monitoring of radioactive materials. Neutrons arising from the materials/sources are often considered due to their penetrating nature which overcomes attenuation by the sources surroundings prior to detection. The requirements of such detectors vary considerably from situation to situation.

The applicant has found particular applicability for neutron detectors in providing inventory monitoring for plutonium in process plants. Simultaneous measurement of neutron levels at a variety of locations throughout the plant in question enable mass and distribution of the process material to be investigated. The neutron levels are measured by taking the count rates at individual detectors, or groups of detectors, and processing them. Total neutron count rates are used in this process. Mathematical deconvolution of the detector responses is used to determine the neutron emissions from areas of the process plant, from vessel or other areas as specified in the design of the investigating system. The system is advantageous in being non-intrusive, nondestructive and capable of operating in real-time.

An issue, particularly with systems such as those described above, which monitor areas in conjunction with monitoring of various adjacent areas, is the ability to distinguish emissions from the area under consideration rather than arising from other areas.

Detectors generally used in neutron monitoring in such cases have been found in practice to have their highest efficiencies at low energy levels (for instance 0. 5 to 10eV).

Such a detector is typified by a 50mm diameter detector within 25mm of polythene and a 0. 5mm thick outer skin of cadmium (referred to below in comparisons as Prior Art 1).

The applicant's have determined that the problems associated with cross over of neutrons from one area into another area where they are detected, giving a false

contribution to the reading in that area, can be reduced considerably according to the present invention.

Careful design of the detectors used can result in the exclusion of low energy neutrons from detection and the efficient detection of high energy neutrons. The overall effect of this is to avoid the counting of lower energy neutrons and count only high energy neutrons. This is achieved in the present invention whilst still providing detectors which are sufficiently small to be readily deployed, even in severely confined circumstances, and yet which are simple in structure and hence relatively cost effective to produce.

As low energy neutrons will correspond to scattered neutrons the effect is to discount neutrons which have passed from adjacent areas through scattering. Scattering occurs on collision of a neutron with wall, vessels and the like with a consequential reduction in energy down from the emission energy. The detectors do successfully detect high energy neutrons, which if of sufficiently high energy (such as 1 to 2 MeV), can only have arisen from that area. Neutrons in adjacent areas will inevitably be scattered before reaching that detector.

The result of this system is that more accurate neutron count rates for an area are obtained and as a consequence more accurate accounting for material in that area can be made. This accuracy is achieved even though the actual number of neutrons reaching the detector is significantly reduced by the techniques of the present invention when compared with prior art detectors.

Illustrations of the first form of the invention The embodiment of the invention illustrated in Figures 1 and 2, Example 1, is an elongate cylindrical neutron detecting unit.

In cross-section, Figure 1, the neutron detecting unit is formed by a neutron detector 2 which is centrally provided and which extends along the axis of the detector unit. The detector is a BF3 detector. Concentrically provided with the detector 2 is a 50mm thick layer of polythene 4, a hydrogen containing material. Concentrically provided with the detector 2 and polythene 4 is a shielding layer of cadmium 6 which is 0.5 mm thick. Concentrically provided with the detector 2, polythene 4 and cadmium 6 is a polymeric layer 8 which is 5 mm thick and loaded with boron.

An aluminium skin (not shown), 2 mm thick, is provided over the outside of the polymeric material 8 and at the ends of the detector unit.

When viewed from the side in cross-section, Figure 2, the central position of the detector 2 within the polythene 4, cadmium 6, polymeric material 8, aluminium skin 10 and aluminium retaining plates 12 can be seen. The side view also shows the access tube 14 into which standardisation sources can be introduced for calibration purposes.

The boron loaded polymeric material 8 shields against lower neutron energies and also serves to thermalise some higher energy neutrons entering the detector.

The cadmium 6 acts as a screen against thermal neutrons in the surrounding environment and neutrons thermalised by the polymeric layer 8.

The polythene 4 acts to moderate neutrons which penetrate the outer layers to an energy level where they can readily be detected, in the case of the highest incident energy neutrons, and also serves to exclude from detection a portion of those neutrons which have sufficient energy to penetrate the outer layers, but for which detection is undesirable.

The careful configuration of these layers individually and in combination gives greatly improved detection efficiency in the important part of the spectrum. As'shown

by the results plotted in Figure 3 the relative detection efficiency is very much higher for neutrons having an energy of greater than 100 keV on reaching the detector unit than for neutrons having an energy of less than 100 eV (epithermal neutrons) and those having an energy of less than 0. 05eV (thermal neutrons). Thus improved detection of those neutrons which have originated and directly reach the neutron detector (the neutrons of interest) is obtained, with reduced detection of those neutrons which have bounced around the environment, potentially from other locations, (the neutrons which would give spurious information if detected).

The detector unit illustrated in Figure 4, Example 2, features a 25 mm diameter detector 20 provided within equivalent thicknesses of polythene 22, cadmium 24 and boron loaded rubber 26 to the first embodiment. The reduced overall size and weight of the detector is useful where such factors are important, but provides slightly reduced detection performance when compared with the first embodiment. The performance though is still good, Figure 5, and represents a significant improvement over prior art detector units.

The improvement these detectors represent can be illustrated by a figure of merit, calculated as the ratio of efficiency of fast neutron detection (lOkeV to 400keV) to epithermal neutron detection (0. 4eV to lOeV).

Detector Relative Relative Fast/Slow Design efficiency efficiency efficiency (fast) (slow) ratio Prior Art 1 0.63 1.00 0.63 Example 1 1.00 0.32 3.13 Example 2 1.00 0.34 2.94

Comparisons are made between efficiencies normalised to the maximum detection efficiency. The absolute detection efficiency of one detector unit type relative to another is a less important criteria than the maximum detection efficiency, particularly in inventory monitoring applications.

Illustrations of the second form of the invention The embodiment of the invention illustrated in Figure 6, Example A, is an elongate cylindrical neutron detecting unit.

In cross-section, Figure 6, the neutron detecting unit is formed by a neutron detector 102 which is centrally provided and which extends along the axis of the detector unit. The detector is a BF3 detector. Concentrically provided with the detector 102 is a 25mm thick layer of polythene 104, a hydrogen containing material.

Concentrically provided with the detector 102 and polythene 104 is a shielding layer of cadmium 106 which is 0.5 mm thick. Concentrically provided with the detector 102, polythene 104 and cadmium 106 is a 25mm thick layer of polythene 108. Concentrically provides about these layers is a 0.5mm thick layer 110 of cadmium, as a neutron screening material.

An aluminium skin (not shown), 2 mm thick, is provided over the outside of the cadmium 110 and at the ends of the detector unit.

The cadmium layers 106 and 110 acts as a screen against thermal neutrons in the surrounding environment and neutrons thermalised by the polythene layer 108.

The polythene 104 and 108 acts to moderate neutrons, which penetrate the outer layers, to an energy level where they can readily be detected, in the case of the highest incident energy neutrons, and also serves to exclude from detection a portion of those neutrons which have sufficient

energy to penetrate the outer layers, but for which detection is undesirable.

The careful configuration of these layers individually and in combination gives greatly improved detection efficiency in the important part of the spectrum. As shown by the results plotted in Figure 7 the relative detection efficiency is very much higher for neutrons having an energy of greater than 100 keV on reaching the detector unit than for neutrons having an energy of less than 100 eV (epithermal neutrons) and those having an energy of less than 0. 05eV (thermal neutrons). Thus improved detection of those neutrons which have originated and directly reach the neutron detector (the neutrons of interest) is obtained, with reduced detection of those neutrons which have bounced around the environment, potentially from other locations, (the neutrons which would give spurious information if detected).

The improvement of this detector over the prior art described above can be illustrated by a figure of merit, calculated as the ratio of efficiency of fast neutron detection (10keV to 400keV) to epithermal neutron detection (0.4eV to lOeV).

Detector Relative Relative Fast/Slow Design efficiency efficiency Efficiency (fast) (slow) Ratio Prior Art 1 0.63 1.00 0.63 Example A 1.00 0.17 5.88

Illustrations of the third form of the invention

The embodiment of the invention illustrated in Figure 8, Example X, is an elongate cylindrical neutron detecting unit.

In cross-section, Figure 8, the neutron detecting unit is formed by a neutron detector 202 which is centrally provided and which extends along the axis of the detector unit. The detector is a BF3 detector. Concentrically provided with the detector 202 is a 25mm thick layer of polythene 204, a hydrogen containing material. Concentrically provided with the detector 202 and polythene 204 is a shielding layer of cadmium 206 which is 0.5 mm thick. Concentrically provided with the detector 202, polythene 204 and cadmium 206 is a 25mm thick layer of polythene 208. Concentrically provides about these layers is a 0. 5mm thick layer 210 of cadmium, as a neutron screening material and concentrically provided about that layer 210 of polymeric material 212 which is 5 mm thick and loaded with boron.

An aluminium skin (not shown), 2 mm thick, is provided over the outside of the polymeric material 212 and at the ends of the detector unit.

The boron loaded polymeric material 212 shields against lower neutron energies and also serves to thermalise some higher energy neutrons entering the detector.

The cadmium layers 206 and 210 acts as a screen against thermal neutrons in the surrounding environment and neutrons thermalised by the polymeric layer 212 and polythene layer 208.

The polythene 204 and 208 acts to moderate neutrons, which penetrate the outer layers, to an energy level where they can readily be detected, in the case of the highest incident energy neutrons, and also serves to exclude from detection a portion of those neutrons which have-sufficient energy to penetrate the outer layers, but for which detection is undesirable.

The careful configuration of these layers individually and in combination gives greatly improved detection efficiency

in the important part of the spectrum. As shown by the results plotted in Figure 9 the relative detection efficiency is very much higher for neutrons having an energy of greater than 100 keV on reaching the detector unit than for neutrons having an energy of less than 100 eV (epithermal neutrons) and those having an energy of less than 0.05eV (thermal neutrons).

Thus improved detection of those neutrons which have originated and directly reach the neutron detector (the neutrons of interest) is obtained, with reduced detection of those neutrons which have bounced around the environment, potentially from other locations, (the neutrons which would give spurious information if detected).

The improvement of this detector over the prior art described above can be illustrated by a figure of merit, calculated as the ratio of efficiency of fast neutron detection (lOkeV to 400keV) to epithermal neutron detection (0. 4eV to lOeV).

Detector Relative Relative Fast/Slow Design efficiency efficiency efficiency (fast) (slow) ratio Prior Art 1 0.63 1.00 0.63 Example X 0.94 0.09 10.44

Claims

CLAIMS 1. A neutron detector unit, the unit comprising a neutron detector, a neutron moderating material around the detector, a thermal neutron screening material around the detector and a neutron absorbing material around the detector.
2. A neutron detector unit according to claim 1 in which the neutron moderating material is provided between the neutron detector and the thermal neutron screening material.
3. A neutron detector unit according to claim 1 or claim 2 in which the thermal neutron screening material is provided between the neutron moderating material and the neutron absorbing material.
4. A neutron detector according to any preceding claim in which the detector has a diameter of 50mm, the neutron moderating material is polythene and has a thickness of 50mm, the thermal neutron screening material is cadmium and has a thickness of 0.5mm and the neutron absorbing material is boron loaded in a polymeric material, the polymeric material being 5mm thick, all values being +/-10%.
5. A neutron detector according to any of claims 1 to 3 in which the detector has a diameter of 25mm, the neutron moderating material is polythene and has a thickness of 50mm, the thermal neutron screening material is cadmium and has a thickness of 0.5mm and the neutron absorbing material is boron loaded in a polymeric material, the polymeric material being 5mm thick, all values being +/-10%.
6. A neutron detector unit, the unit comprising a neutron detector, a first body of neutron moderating material, a first body of thermal neutron screening material around the

<Desc/Clms Page number 35>

detector, a second body of neutron moderating material around the detector and a second thermal neutron screening material around the detector.
7. A unit according to claim 6 in which the first body of neutron moderating material is provided between the detector and the first body of thermal neutron screening material and the second body of neutron moderating material is provided between the first and second bodies of thermal neutron screening material.
8. A unit according to claim 6 or claim 7 in which the detector has a diameter of 50mm, the first body of neutron moderating material is of polythene and is 50mm thick, the first body of thermal neutron screening material is of cadmium and is 0. 5mm thick, the second body of neutron moderating material is of polythene and is 50mm thick and the second body of thermal neutron screening material is of cadmium and is 0. 5mm thick, all values +/-10%.
9. A unit according to claim 6 or claim 7 in which the detector has a diameter of 25mm, the first body of neutron moderating material is of polythene and is 50mm thick, the first body of thermal neutron screening material is of cadmium and is 0. 5mm thick, the second body of neutron moderating material is of polythene and is 50mm thick and the second body of thermal neutron screening material is of cadmium and is 0. 5mm thick, all values +/-10%.
10. A neutron detector unit, the unit comprising a detector, a first body of neutron moderating material around the detector, a first body of thermal neutron screening material around the detector, a second body of neutron moderating material around the detector, a second body of thermal neutron

<Desc/Clms Page number 36>

screening material around the detector and a body of neutron absorbing material around the detector.
11. A unit according to claim 10 in which the neutron moderating material bodies are of polythene, the thermal neutron screening bodies are of cadmium and the neutron absorbing body is boron loaded in a polymeric material.
12. A unit according to claim 10 or claim 11 in which the detector has a diameter of 50mm, the first body of neutron moderating material is 25mm thick, the first body of thermal neutron screening material is 0. 5mm thick, the second body of neutron moderating material is 25mm thick, the second body of thermal neutron screening material is 0.5mm thick and the neutron absorbing material is 5mm thick, all value +/-10%.
13. A unit according to claim 10 or claim 11 in which the detector has a diameter of 25mm, the first body of neutron moderating material is 25mm thick, the first body of thermal neutron screening material is 0.5mm thick, the second body of neutron moderating material is 25mm thick, the second body of thermal neutron screening material is 0. 5mm thick and the neutron absorbing material is 5mm thick, all value +/-10%.