Patent Description:
There are many situations in which it can be important to reliably measure the amount of nuclear fissile material present in a sample (that is to say to perform nuclear assay measurements). For example, national and international nuclear regulatory agencies need to maintain an inventory of the quantity of fissile material present within their domain. To this end, so-called nuclear safeguards instrumentation may be used to detect the diversion of nuclear material from declared facilities, or to detect the clandestine production, diversion or processing of nuclear material. The verification of the physical inventory at processing facilities hinges on the use of non-destructive assay (NDA) instruments which are typically based on either neutron, gamma, or calorimetric detection. Other situations where it is important to reliably measure the amount of nuclear fissile material present in a sample include Arms Control and Treaty Verification measurements and irradiated fuel measurements to quantify how much nuclear fuel remains 'unburnt' when fuel elements are removed from a reactor before their return to a storage or re-processing facility. Another scenario is for characterising nuclear waste to help prevent fissile material being wrongly consigned in nuclear waste.

A known approach to quantify fissile materials, such as uranium and plutonium present in a sample, is based on identifying the rate of 'coincident' detections of neutrons arising from the spontaneous or induced fission in the sample. The general aim of nuclear assay measurements is to estimate the number of fissile nuclei present within a sample of material. This can be done by measuring the rate of neutron detection events seen using a plurality neutron detector elements surrounding a sample which can be attributed to fission events in which multiple neutrons are simultaneously ejected from the sample, typically with an average energy of around <NUM> MeV. Since these neutrons must first thermalise before detection, the neutrons associated with an event in which multiple neutrons are produced are rarely detected in strict coincidence. Consequently a technique has become established based on a shift-register analysis to measure a degree of timing correlation between potentially large numbers of neutron detection events within an instrument [<NUM>]. With this kind of shift-register analysis, a coincidence gate is opened when a trigger neutron pulse from a detector is received at a shift register. The duration (gate-width) for the coincidence gate is selected according to the die-away time of the detector. When another neutron is detected during the coincidence gate (coincidence window), it is counted. Neutrons detected within the coincidence gate may be in "true" coincidence (i.e. associated with a common neutron decay event in the sample), or may be in "false" coincidence, i.e. associated with uncorrelated events. The significant coincidence rate for assay purposes is the "true" coincidence rate (coincidence rate after subtracting the "false" coincident rate) which can provide an estimate of the quantity of fissile material present within the sample.

In this regard there is now a well-established and internationally validated shift-register package available "off the shelf' for processing neutron detection event signals for nuclear assay purposes, for example of the kind described by D. Reilly in "Passive Nondestructive Assay of Nuclear Materials" [<NUM>]. A range of different neutron detection approaches have been developed to detect neutrons and provide corresponding neutron detection event signals for processing in accordance with this recognized processing approach. Two broad types of detection system exist. One type may be referred to as passive interrogation detectors which operate to measure neutrons generated by the spontaneous fission of nuclei within the material sample. The other type may be referred to as active interrogation detectors and these use an external source to stimulate fission events, for example for low self-activity samples (such as uranium bearing materials) or to boost sample fission signature counts above a high gamma-ray background (e.g. in a spent fuel assay implementation).

<FIG> schematically represents a known neutron coincidence well counter based on the use of helium-<NUM> proportional neutron counters, namely Canberra's JCC-<NUM> (www. com/products/waste_safeguard_systems/pdf/JCC-<NUM>-SS-C36906. pdf) [<NUM>]. The upper part of the figure represents the device in horizontal cross-section and the lower part of the figure represents the device in vertical cross-section. The detector is a portable non-destructive assay system used for high mass Pu bearing samples (<NUM>-<NUM>) based on passive neutron counting. The device is considered to set a baseline target system for coincidence counting applications due to its wide use in the Nuclear Safeguards Community and its ability to handle relatively high count rates from kilogram-range mass Pu samples. The device has a ring of eighteen Helium-<NUM>-based neutron detectors (<NUM>-He tubes) around a sample cavity in which a sample to be assayed is received. The JCC-<NUM> measures the effective mass of fissile material in the sample by detecting coincidence neutrons from the spontaneous fission of nuclei as discussed above. The JCC-<NUM> has a cylindrical-shaped sample cavity <NUM> high by <NUM> in diameter. A cadmium sleeve surrounds the sample cavity to prevent the re-entry of thermalized neutrons into the sample, which could induce fission in the sample and adversely affect the results. Outside the cadmium sleeve is a ring of high-density polyethylene and the eighteen <NUM>-He tubes. The <NUM>-He tubes are divided into six groups of three with each group wired together and connected to one amplifier / discriminator circuit board for coincidence analysis.

<FIG> schematically represents another previously-proposed <NUM>-He-based device known as an Epithermal Neutron Multiplicity Counter [<NUM>]. The left-hand part of the figure represents the device in vertical cross-section and the right-hand part of the figure represents the device in horizontal cross-section. The device represented in <FIG> is based on broadly the same detection technologies as the device of <FIG>, but comprises <NUM><NUM>-He tubes arranged in four rings around a sample cavity having a diameter of <NUM> and a height of <NUM>. The detector is reported to have an efficiency of <NUM>% and a die-away time of <NUM> microseconds. The higher efficiency allows for neutron multiplicity counting.

Some sample types, such as fuel assemblies, are difficult to assay in a well-counter configuration because of their size. In this regard collar-type (as opposed to well-type) detector configurations may be used to accommodate larger sample types, for example Canberra's JCC-<NUM> (www. com/products/waste_safeguard_systems/pdf/JCC-<NUM>-<NUM>-<NUM>-SS-C38898. pdf) [<NUM>]. This detector may be configured for active or passive interrogation operation. The device is presented for use for quantifying <NUM>-U in fresh PWR (pressurized water reactor), BWR (boiling water reactor) and CANDU (Canadian light water reactor) fuel assemblies, and for measuring plutonium in MOX (mixed oxide) fuel. In the passive mode the device comprises four banks of neutron counters, each comprising several <NUM>-He tubes, around a sample space. In the active mode, one of the banks is replaced by an interrogating neutron source, to measure <NUM>-U.

A drawback with the above-described detectors is their reliance on <NUM>-He. There has over the years been a significant reduction in the availability of <NUM>-He following the reduction in the stock-pile of nuclear weapons. Due to the shortage of <NUM>-He in recent years, there has been recognised a need to replace the neutron detection technology that has been at the heart of these systems since their conception. For many years, the 'gold standard' for the detection of thermal neutrons was set by proportional counters based on the use of <NUM>-He, but the Nuclear Safeguards Community now seeks alternative technologies which do not rely on <NUM>-He.

There are several properties of <NUM>-He which make it well suited for nuclear assay detectors. For example, detectors based on <NUM>-He typically have high thermal-neutron detection efficiency, good neutron-gamma discrimination, a short dead-time, and various practical operational advantages, such as high stability, non-inflammability, non-toxicity, and long operational lifetime. This means that finding a suitable replacement technology able to provide comparable performance represents a significant technical challenge. The issue has been investigated extensively over the past few years, and several <NUM>-He replacement technology options have been proposed, such as those based on boron-lined tubes or plates, boron trifluoride as a proportional gas, boron loaded plastic scintillators, as well as LiF:ZnS(Ag) screens on wavelength-shifting light-guides. These alternate technologies have been widely discussed, for example in documents [<NUM>], [<NUM>], [<NUM>], [<NUM>], [<NUM>], [<NUM>], [<NUM>], [<NUM>] and [<NUM>]. Some <NUM>-He alternate technology detectors have been implemented in safeguards counters with some degree of success, but several are still in the developmental stage.

In addition to providing sufficient detection efficiency, <NUM>-He replacement technologies should also allow deployment in configurations having sufficiently short neutron die-away times to allow an appropriate precision for coincidence detection. In this regard, a recognised figure of merit (FOM_2) for characterising the performance of a neutron detector is defined both in terms of single neutron detection efficiency ε (%) and detector die-away time τ, which is a measure of the mean time-delay expected between a fission event and the detection of a neutron following moderation. This figure of merit is conventionally defined as <MAT> and it may be used to compare detector performance for different detector technologies and configurations against one another. A relatively high value for FOM_2 indicates a detector with relatively high efficiency and / or relatively low die-away time. Generally speaking, it can be desirable to have neutron detector elements that are compact and, when in a well counter configuration, do not rely on large amounts of surrounding material for moderation since this can extend the detector's neutron die-away time.

An LiF:ZnS(Ag)-based detector used for Pu assay and independent measurements of neutrons and gamma rays is described in document [<NUM>]. This detector uses LiF:ZnS(Ag) material as a thin screen, coupled to wavelength shifting fibres in the form of a ribbon. These layers can be combined to produce slabs which can then be assembled to surround a sample on four sides forming a well counter. One side consists of <NUM> individual detector bundles per side. As example of such a slab is represented in <FIG>.

<FIG> schematically represents another previously-proposed LiF:ZnS(Ag)-based detector as described in document [<NUM>]. The left-hand part of the figure represents the device in vertical cross-section and the right-hand part of the figure represents the device in horizontal cross-section. The detector is presented as a demonstrator unit to determine the feasibility of utilizing LiF:ZnS(Ag) detectors in a well counter configuration. The detector comprises four walls, around <NUM> tall, surrounding a central cavity for source placement. Each wall comprises alternating layers of five LiF:ZnS(Ag) screens between six polyvinyltoluene (PVT) sheets. A single PMT detector is provided at the top and bottom of each wall to receive light from the LiF:ZnS(Ag) screen which is guided by the polyvinyltoluene (PVT) sheets to the PMT.

Various other previously-proposed designs are described in the above-identified documents. However, none of these previously-proposed designs have been able to provide a practical detector having performance comparable to a <NUM>-He based detector for nuclear assay purposes. There is therefore a need for new methods and apparatus for neutron detection which do not rely on <NUM>-He detectors, but which have improved performance over existing designs, for example in terms of helping to achieve performance comparable to <NUM>-He-basd detectors for nuclear assay purposes.

<CIT> discloses a neutron spectrometer includes two pr more conversion screens (<NUM>, <NUM>, <NUM>) comprising a neutron absorbing material (e.g. LiF) and a phosphor material (e.g. ZnS), two wavelength-shifting light-guides (<NUM>, <NUM>) arranged between the conversion screens. A photodetector detects photons from each respective light guide. The conversions screens are formed of powdered material in a binding material on a substrate. The substrate may be reflecting or translucent in order to concentrate light into the light guides.

<NPL> discloses neutron counters constructed using large area layers of a mixture of lithium fluoride and zinc sulphide, the scintillation light being transmitted to photomultipliers by wavelength shifting light guides. An array of these counters has been assembled to form a neutron multiplicity detector for use in a superheavy element search. This array had a detection efficiency for fission neutrons of <NUM> ± <NUM>% and a neutron diffusion time of <NUM> #AS. The measured background rate of the array, when operated underground at a depth of <NUM> mwet was <NUM> ± <NUM>-<NUM>.

The invention is now described by way of example only with reference to the following drawings in which:.

Aspects and features of certain examples and embodiments of the present invention are described herein. It will be appreciated that aspects and features of the apparatus and methods discussed herein which are not specifically described may be implemented in accordance with conventional techniques.

The invention provides a nuclear assay counter comprising one or more neutron detector modules. Each neutron detector module comprises a neutron moderating block, for example HDPE (high density polyethylene), having a plurality of neutron detector elements embedded therein. For example, the moderating block may be provided with slots to receive the neutron detector elements. In principle the neutron detector elements may be cast into the moderating block during manufacture assuming the detector elements remain stable at the relevant temperature. The neutron detector elements may be referred to as neutron detector blades. Each neutron detector blade comprises a neutron conversion layer on either side of a generally planar light-guide, e.g. formed of a PVT sheet. Each conversion layer comprises a mixture of a neutron absorbing material, such as <NUM>-LiF, and a scintillation material such as ZnS(Ag), in accordance with established conversion-screen neutron detection techniques (e.g. as generally described in document [<NUM>]). The light-guides are arranged to receive photons emitted from the scintillation material of their conversion layers as a result of neutron absorption events in one or other of the conversion layers, and to guide the photons to a photodetector optically coupled to one end of the light-guide. Output signals from the photodetector thus provide an indication of a neutron interaction event in one of the conversion layers for the relevant detector blade.

<FIG> schematically shows in perspective view one such neutron detector blade <NUM> for use in a nuclear assay counter in accordance with an embodiment of the invention. The detector blade <NUM> has a generally rectangular planar form and comprises a generally layered structure. The largest surfaces of the detector blade (i.e. the uppermost and lowermost surfaces represented in <FIG>) may be referred to as the detector blade's faces, while the surfaces connecting between the detector blade's faces may be referred to as its edges. In particular, the longer pair of edges may be referred to as side-edges and the shorter pair of edges may be referred to as end-edges. Similar terminology may also be used in respect of the individual layers comprising the detector blade.

The neutron detector blade <NUM> comprises a pair of neutron absorbing conversion screens 4a, 4b arranged on either side of a wavelength-shifting light-guide <NUM> in the form of a PVT plank. The light-guide <NUM> is coupled to a photodetector <NUM>, e.g. a silicon photomultiplier detector. Output signals S from the photodetector <NUM> (schematically shown by arrows <NUM>) are passed to a processor for processing as discussed further below. The detector blade <NUM> in this example comprises a single photodetector <NUM> on one end-edge of the light-guide <NUM>. However, in other implementations there may be multiple photodetectors associated with each detector blade, for example comprising a plurality of photodetectors arranged along one end-edge, or otherwise arranged around the edges, of the light-guide. The principles of operation and construction for the individual detector blades <NUM> comprising a detector in accordance with an embodiment of the invention may follow broadly conventional techniques for conversion-screen neutron detectors. Although not represented in <FIG>, the detector blade may be wrapped in a light-tight material to prevent the risk of external light from reaching the photodetector.

The characteristic scale of the detector blade is as schematically shown in the figure (although it will be appreciated that some aspects of the figures are not drawn to scale). Thus in this example, the detector blade <NUM> is in the form of a rectangular sheet with an overall length L of around <NUM>, a width W of around <NUM>, and an overall thickness T of around <NUM> (e.g., comprising a <NUM>-mm thick light-guide and two <NUM>-mm thick conversion screens). The photodetector <NUM> is sized to match the thickness of the light-guide <NUM>, for example the photodetector may comprise a <NUM> square silicon photomultiplier optically coupled to the middle of an end edge of the light-guide <NUM>, as schematically represented in <FIG>.

It will, of course, be appreciated that other sizes of neutron detector blade may be selected according to the desired sensitivity and overall geometry. For example, in accordance with various embodiments of the invention, and according to implementation, the detector blades may have a characteristic length L within a range selected from the group comprising: <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>; and / or a characteristic width W within a range selected from the group comprising: <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>; and / or a characteristic thickness T less than or equal to an amount selected from the group comprising: <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM>; <NUM> and <NUM>.

The conversion screens <NUM> (defining the neutron-sensitive active area) and the light-guide <NUM> have faces with broadly corresponding areal extents. In this example embodiment the conversion screens <NUM> each comprise a layer made up of a mixture of the neutron absorbing material and scintillating material mounted on a substrate. Each substrate in this example embodiment is an aluminium sheet with a reflective face on the side of its respective conversion layer. The reflective face may be provided by polishing the aluminium or by an intermediate coating, e.g. a diffusively reflecting white coating. In other examples the substrate of the conversions layers may be a polyester sheet with a reflective backing on the outside faces of the conversion screens and facing inwards towards the light-guide <NUM>. The reflective backing may be affixed to the substrate or may comprise a separate element. Alternatively, or in addition, the substrate of the conversions layers may be translucent, e.g., comprising Mylar (RTM) or Melinex (RTM).

The mixture of neutron absorbing material and scintillator material comprises powdered forms of each which are mixed in a resin binder and spread onto the substrate, e.g. in a layer perhaps around <NUM> to <NUM> thick, and left to set. In other example embodiments the neutron absorbing material and scintillating material mixture may be deposited directly on the light guide <NUM> (that is to say, the light-guide may in effect provide a substrate for the conversion screens <NUM>). However, this approach may be less appropriate in some situations, for example for relatively long detector blades, because of the potential for a reduction in light-guiding efficiency in the light-guide due to the absence of total internal reflection. In this example, the neutron absorbing material comprises <NUM>-Li enriched LiF and the scintillator material comprises ZnS(Ag). In other examples the neutron absorbing material may be based on / include other neutron-absorbing elements, e.g. <NUM>-B based material, such as a <NUM>-B<NUM>O<NUM> mixture. Equally, in other examples the scintillator material may be based on / include other scintillator material, e.g. using pure Csl or yttrium aluminium perovskite (YAP) in powdered / granular form.

In this example the light-guide <NUM> comprises a sheet of wavelength-shifting plastic scintillator material, e.g. based on polyvinyltoluene (PVT) such as the EJ-<NUM> materials available from Eljen Technology, Texas, USA.

The light-guide <NUM> is placed in loose contact with the conversion screens 4a, 4b so that optical photons from the scintillator material in the conversion screens are readily coupled into the light-guide <NUM>. The conversion screens in this example are in loose contact and not bonded contact with the light-guide <NUM>. In other examples the conversion screens may be optically bonded to the wavelength shifting light-guide <NUM>, but again this approach may be less appropriate in some situations, for example for relatively long detector blades, because of the potential for a reduction in the occurrence of total internal reflection within the light-guide.

The role of the conversion screens <NUM> is to convert incidents neutrons into light. Thus a neutron from a sample under assay which is incident on the detector blade <NUM> may be absorbed by the associated neutron absorbing material in one or other of its conversion screens 4a, 4b by interacting with one of the <NUM>-Li nuclei. This reaction (<NUM>-Li3 + 1n0 → 3H1 + 4α2 + <NUM> MeV) results in reaction fragments that readily excite the intermixed scintillator ZnS(Ag), causing it to radiate photons. These photons are emitted in all directions, and since the conversion layers <NUM> are relatively thin, for most interaction sites the light-guide <NUM> presents a solid angle of around 2π such that close to half the scintillator photons from the neutron interaction that escape the conversion layer enter the light-guide directly. Furthermore, the remaining half of scintillator photons (i.e. those initially travelling away from the light-guide) may be reflected back into the light-guide <NUM> following reflection from the substrate.

Thus a relatively large fraction of the neutron-induced scintillator photons may be expected to enter the light-guide <NUM>. In general, the initial directions of the photons entering the light-guide <NUM> will be such that the photons will exit the light-guide the opposing side (e.g., because they enter at too steep an angle). The light-guide comprises a wavelength-shifting material, such as a PVT based material as noted above, and so the scintillator photons from the ZnS(Ag) intermixed with the neutron-absorbing LiF in the conversion screens <NUM> may be absorbed in the light-guide <NUM> and corresponding longer-wavelength photons re-emitted. Significantly, the wave-length shifted photons will be emitted over a broad range of directions such that a good fraction of them are guided to the photodetector <NUM> for detection. A fraction of the photons will be guided along the light-guide <NUM>, e.g. by total internal reflection at the surfaces of the light-guide, to interact with the photo-detector <NUM>, and a corresponding output signal <NUM> generated in the usual way therefrom. These output signals may be processed to determine when neutrons are deemed to be detected in accordance with conventional techniques. For example, output signals may be compared with a threshold signal, and if an output signal is greater than the threshold (and / or satisfies one or more other criteria for example based on pulse shape), it may be assumed the signal corresponds with a neutron interaction event in the corresponding detector blade. Additional processing may be applied in accordance with conventional techniques for identifying from the output signals when neutron detection events are deemed to have occurred, for example processing may be applied to discriminate neutron detection events from gamma-ray detection events in accordance with known techniques (such as described in document [<NUM>]).

<FIG> represents in schematic perspective view a neutron detector module <NUM> for use in a detector according to an embodiment of the invention. The nuclear assay counter comprises a moderating block <NUM> in which a plurality of detector blades <NUM> are embedded. In this example neutron detector module <NUM> there are eight neutron detector blades arranged within the neutron detector module <NUM> in a separated parallel stack arrangement. The moderating block <NUM> in this example implementation comprises high density polyethylene (HDPE) formed into a suitable shape, for example by casting or machining. Other moderating materials, for example other moderating plastics, could be used in other examples. The detector blades <NUM> may be embedded during casting (so long as they can withstand the casting temperature) or by virtue of being inserted into slots in the body of the moderating block <NUM>. Although not represented in <FIG>, the moderating block <NUM> comprising the detector module <NUM> may be wrapped in a light-tight material to help prevent external light from reaching the photodetectors of the respective neutron detector blades. The moderating block may also be provided with a cladding, example comprising a cadmium sheet, to help retain thermalized neutrons within the detector module.

The characteristic scale of the detector module <NUM> / moderating block <NUM> is as schematically shown in the figure (although it will be appreciated that some aspects of the figures are not drawn to scale). Thus in this example, the detector module <NUM> in the form of a rectangular block with an overall length X of around <NUM>, a width Y of around <NUM>, and an thickness less X of around <NUM>. However, it will be appreciated that other sizes of moderating block may be selected according to the implementation at hand and having regard to a desired amount of moderation and overall geometry. For example, in accordance with various embodiments of the invention, and according to implementation, the detector module <NUM> / moderating block <NUM> may have a characteristic length X within a range selected from the group comprising: <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>; and / or a characteristic width Y within a range selected from the group comprising: <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>; and / or a characteristic thickness Z within a range selected from the group comprising: <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>.

In line with the terminology used for the detector blades, the largest surfaces of the detector module may be referred to as the detector module's faces, while the surfaces connecting between the detector module's faces may be referred to as its edges. In particular, the longer pair of edges may be referred to as side-edges and the shorter pair of edges may be referred to as end-edges. The blades <NUM> in this example are arranged in the moderating block <NUM> with their faces parallel to each other and to the side edges of the moderating block <NUM>, with the detector blades separated from one another by around <NUM> to <NUM>. The faces of the outermost blades may be separated from the side-edges of the moderating block <NUM> by a similar distance, or perhaps more, for example somewhere in the range <NUM> to <NUM>. The end-edges of the respective detector blades <NUM> are in this example separated from the perspective end-edges of the moderating block <NUM> by around <NUM> and the side edges of the respective detector blades <NUM> are in this example separated from the respective faces of the moderating block <NUM> by around <NUM>, but in other cases other separations may be used, for example somewhere in the range <NUM> to <NUM>. It will be appreciated that in other examples there may be more or less moderating material around the group of detector blades according to the amount of moderation desired.

Output signals <NUM> from the photodetectors of the respective detector blades <NUM> are output to a processing unit <NUM>. The processing unit is configured to determine from the output signals <NUM> whether a neutron detection event has occurred, for example using broadly conventional techniques, and to output a signal indicating when a neutron detection event is deemed to have occurred. In this example the processing unit <NUM> is represented as receiving separate output signals from the respective detector blades <NUM> comprising the detector module <NUM>, and may process these separately, and so potentially provide information indicating which detector blade was involved in the neutron detection event (although typically this information will not be considered especially relevant for assay purposes). In another example the output signals <NUM> from some, or all, of the detector blades comprising a detector module may be combined before processing so the processing unit <NUM> is only able to determine whether at least one of the detector blades associated with the output signals which have been combined was involved in a neutron detection event. Combining output signals from multiple detector blades for processing together reduces the amount of processing required, but with a loss in positioning resolution. However, for assay purposes the positioning resolution of a detector is often not of primary significance. In this example a single processing unit is provided for the detector module <NUM>. However, it will be appreciated that in some examples the processing of signals from one (or more than one) detector blades may be performed using separate processing units. Furthermore, in a detector comprising multiple detector modules, there may, for example, be one processing unit per detector module, or a single processing unit for handling signals received from a plurality of detector modules. In this regard, it will be appreciated the manner in which the output signals are collected for processing is not significant. The functionality of the processing unit <NUM> can be provided in various different ways, for example using a single suitably programmed general purpose computer, or suitably configured application-specific integrated circuit(s) / circuitry. In some examples in which each detector blade is associated with its own processing unit for providing output pulses in response to neutron detection events, the respective processing functionality may be provided at the detector blade itself (e.g. using circuitry adjacent the blade's photodetector) such that the output signals from the detector blades comprise TTL pulses indicating a neutron detection event. These pulses may be analysed separately or may be combined before analysis (e.g. using a logical "OR" gate) for subsequent coincidence analysis in accordance with conventional coincidence detection techniques for nuclear assay purposes (i.e. using established shift register techniques).

<FIG> schematically represents in perspective view a nuclear assay counter <NUM> comprising four detector modules <NUM> of the kind represented in <FIG>. The four detector modules <NUM> are arranged around a sample volume / space <NUM> for receiving a sample to be assayed. The neutron detector modules <NUM> are arranged so the detector blades <NUM> embedded within the respective detector modules do not squarely face the sample space but are arranged generally side on to the sample space <NUM> (i.e. generally with their respective side edges facing the sample space). Put another way, the detector blades are arranged such that their largest-area surfaces (at least for the majority of detector blades) are more closely aligned with a radius from the centre of the sample space <NUM> than a direction which is orthogonal to a radius from the centre of the sample space <NUM>. The inventors have identified that providing the neutron detector blades in this type of side-on configuration relative to the sample to be measured can in some scenarios provide enhanced performance. Nonetheless, in other examples, the detector blades may be arranged square on to the sample space (i.e. generally parallel, as opposed to perpendicular, with respect to the largest surfaces of the respective detector modules). Although not represented in <FIG>, output signals from the respective detector blades may be routed for processing in one or more processing units of the kind represented in <FIG> to provide neutron detection event signals (e.g. TTL pulses) in response to neutron detection events. These neutron detection event signals may be processed to establish a coincidence rate in accordance with conventional techniques for nuclear assay detectors. That is to say, in some respects the detector modules <NUM> in accordance with embodiments of the invention may be seen as "drop-in" replacements for, for example, banks of <NUM>-He tubes in known nuclear assay detectors. In this regard, the processing unit(s) <NUM> coupled to the respective detector blades may be configured to provide neutron detection event signals (trigger pulses) corresponding to those provided from a conventional <NUM>-He based detector. Accordingly, the subsequent processing of the neutron detection event signals to determine coincidence rates may be performed using existing processing apparatus and techniques.

Thus, a detector of the kind represented in <FIG> may provide a detector for nuclear assay purposes having a broadly corresponding geometry to existing <NUM>-He detectors, for example as represented in <FIG>. The inventors have identified through modelling that such detectors can also have comparable performance capabilities. Furthermore, by providing groups of detector blades in detector modules as described here in it becomes a simple task to provide different detector configurations by simply using different numbers of detector modules in different configurations. For example, <FIG> schematically represent other example arrangements of detector modules <NUM> of the kind recommended in <FIG> to provide other detector assemblies around respective sample spaces <NUM> according to different embodiments of the invention. <FIG> represent the respective detectors in horizontal cross-section (i.e. in a plane parallel to the end-faces of the respective detector modules). The detector of <FIG> comprises a single "ring" of detector modules, and differs from the arrangement of <FIG> by in-effect providing detector blades in the corner regions of the detector assembly (i.e. in the empty spaces adjacent the site-edges of the respective detector modules <NUM> in the arrangement of <FIG>). The detector of <FIG> has some similarities to the detector of <FIG>, but comprises a second ring of detector modules arranged around the first ring of detector modules.

The modularity of detectors provided in accordance with some embodiments of the invention therefore provide a large degree of flexibility in how different sized detectors can be readily configured. This can be more readily achieved when the width of a detector module is an integer multiple of the thickness of the detector module, for example a factor of two, as in the examples represented the <FIG>. It will be appreciated there are many other configurations which could be provided. For example, the detector modules <NUM> comprising a detector in accordance with an embodiment of the invention may be arranged in a ring with their respective side-edges at angles to one another, for example with the detector modules aligned with the size of a pentagon or hexagon, or higher-ordered polygon. Furthermore, detector modules of the kind represented in <FIG> can be arranged above and below a sample space <NUM> to provide near complete coverage around a sample space.

It will further be appreciated that the detector modules need not comprise a rectangular blocks. For example in a detector module <NUM> according to another embodiment of the invention as schematically represented in perspective view in <FIG>, a moderating block <NUM> comprises a cylindrical annulus around a sample space <NUM>. The individual detector blades <NUM> are embedded within the cylindrical annulus at an angle to a radius from the centre of the sample space <NUM>, as schematically represented in the figure. In other examples the detector blades may be embedded radially (i.e. with their largest surfaces aligned parallel to radial lines from the centre of the sample space <NUM>). However, the inventors have found the modelled performance of a configuration of the type represented <FIG> can be improved by having the detector blades arranged generally radially, but inclined with respect to radii, for example by <NUM>° or less. It will be appreciated the detector <NUM> represented in <FIG> may comprise a single detector module (i.e. the annular cylinder of moderating block may be provided as a single element), or may comprise multiple detector modules (e.g. with each module comprising a part of an annular cylinder). The inventors have found from modelling that detector of the kind represented in <FIG> may be expected to display a performance associated with a figure of merit which is perhaps <NUM>% greater than a correspondingly-sized <NUM>-He based detector.

Thus to summarise some aspects of some embodiments of the invention, <NUM>-LiF:ZnS(Ag)-based neutron detection techniques are provided for use in nuclear assay counters for which an <NUM>-He-alternative drop-in technology is highly desirable. The use of relatively compact <NUM>-LiF:ZnS-based detector blades and relatively compact silicon photomultipliers helps to provide detector configurations in which neutron detector blades can be readily embedded within a moderating material to help ensure relatively rapid incident neutron thermalisation and their subsequent detection. Detection of thermal neutrons using a close-packed detector geometry is often desired for the active or passive non-destructive assay of nuclear material. Monte Carlo simulations indicate that a neutron coincidence counter utilizing detector configurations such as those described above in accordance with embodiments of the invention compare favourably with traditional <NUM>-He-based counters of comparable geometry on terms of having a comparable neutron detection efficiency and a shorter die-away time. A shorter die-away time permits the use of shorter windows for coincidence analysis which can help reduce false-coincidences (i.e. unrelated events occurring by chance within a coincidence detection window) and so help improve measurement accuracy over a given measurement period.

Claim 1:
A nuclear assay counter comprising one or more neutron detector modules (<NUM>), each neutron detector module comprising a neutron moderating block (<NUM>) having a plurality of neutron detector blades (<NUM>) embedded therein, wherein each neutron detector blade comprises: a conversion layer (4a, 4b) comprising a mixture of a neutron absorbing material and a scintillation material; a wavelength-shifting light-guide (<NUM>) arranged to receive photons emitted from the scintillation material; and a photodetector (<NUM>) optically coupled to the light-guide and arranged to detect photons generated as a result of neutron absorption events in the conversion layer, wherein neighbouring neutron detector blades in the one or more neutron detector modules are separated by an amount within a range of <NUM> to <NUM>, wherein the one or more neutron detector modules are arranged so that the neutron detector blades at least partially surround a sample volume for receiving a sample from which neutrons are to be detected.