NEUTRON DETECTION

Abstract

A neutron detection system is described comprising a neutron scintillator detector having a detection area, wherein the detection area is segmented into a plurality of discrete sub-regions, and a light readout system is provided with a corresponding plurality of discrete channels each to detect a respective output of a respective discrete sub-region.

Claims

1-22. (canceled)

23. A neutron detection system comprising a neutron scintillator detector having a detection area, wherein the detection area is segmented into a plurality of discrete sub-regions, and a light readout system is provided with a corresponding plurality of discrete channels each to detect a respective output of a respective discrete sub-region, wherein the neutron scintillator detector comprises a neutron responsive scintillator comprising a combination of a neutron capture isotope with a scintillating compound, and provided in association with a photodetector to detect light emitted by the scintillating compound; and wherein the light readout system is a segmented light readout system having a plurality of discrete channels each to detect a respective light output of a respective discrete sub-region so that the output of each segmented sub-region can be separately processed.

24. The detection system in accordance with claim 23 wherein the segmented light readout system comprises a solid-state, distributed light guide readout.

25. The detection system in accordance with claim 23 wherein the segmented light readout system comprises a plurality of fluorescent light guides.

26. The detection system in accordance with claim 23 wherein the scintillator detector comprises a neutron responsive scintillator, a plurality of separately addressable photodetectors, and a corresponding plurality of light guides each disposed to couple light from one of a plurality of areas on the scintillator to a corresponding one of the separately addressable photodetectors.

27. The detection system in accordance with claim 26 wherein each light guide is a wavelength shifting light guide.

28. The detection system in accordance with claim 27 wherein each wavelength shifting light guide is a wavelength shifting fibre.

29. The detection system in accordance with claim 28 wherein each fibre is fabricated from one or more materials comprising a high refractive index core with low refractive index cladding.

30. The detection system in accordance with claim 26 wherein each light guide is provided with a coating of a neutron capture material and scintillating material mixture.

31. The detection system in accordance with claim 30 wherein the coating comprises one or more neutron capture materials selected from LiF, BN mixed or chemically combined with one or more inorganic scintillating compounds selected from ZnS(Ag), ZnO, YAlO3:Ce.

32. The detection system in accordance with claim 23 wherein the photodetector comprises a silicon photomultiplier.

33. The detection system in accordance with claim 23 wherein the photodetector is divided into a plurality of separately addressable regions.

34. The detection system in accordance with claim 33 wherein the each separately addressable region of the photodetector is configured for a separate and discrete light read out via suitable control electronics.

35. The detection system in accordance with claim 23 wherein the detection area of the detector is segmented into a plurality of discrete sub-regions in a two-dimensional area array.

36. The detection system in accordance with claim 35 wherein the detector comprises a large area neutron responsive scintillator, a plurality of separately addressable photodetectors, and a corresponding plurality of light guides disposed to couple light from each of a plurality of areas on the scintillator defining a two-dimensional area array to a corresponding one of the separately addressable photodetectors.

37. The detection system in accordance with claim 23 wherein the neutron scintillator detector comprises a composite scintillator distributed in a moderator.

38. The detection system in accordance with claim 23 wherein the detector has a detection area of at least 500 cm2.

39. The detection system in accordance with claim 23 wherein the detector has a detection area of at least 5000 cm2.

40. A method of neutron detection comprising the steps of: providing a neutron scintillator detector having a detection area in the vicinity of an object/source to be tested; segmenting the detection area into a plurality of discrete sub-regions; obtaining a light readout output of each respective discrete sub-region; wherein the step of segmenting the detection area into a plurality of discrete sub-regions is carried out by provision of a neutron responsive scintillator, a plurality of separately addressable photodetectors, and a corresponding plurality of light guides each disposed to couple light from one of a plurality of areas on the scintillator to a corresponding one of the separately addressable photodetectors; and by provision of a segmented light readout system having a plurality of discrete channels each to detect a respective light output of a respective discrete sub-region so that the output of each segmented sub-region can be separately processed.

Description

[0032] Particularly preferred examples of boron nitride composite scintillation detectors are discussed by way of example below with reference to the figures in which:

[0033] FIG. 1 is a micrograph of an example .sup.10BN scintillator material;

[0034] FIG. 2 is an example solid state distributed light guide readout for use in an embodiment of the device of the invention;

[0035] FIG. 3 is a scintillator and fibre system for use in conjunction with the light guide readout of FIG. 2 in an embodiment of the device of the invention.

[0036] The objective of the invention is to realize a large area, wide energy, neutron detector with at least an order of magnitude improvement in cost and performance over existing technology such as .sup.3He, .sup.6Li based systems. Target specification for the detector is 50% efficiency (.sup.252Cf neutrons), 1 m.sup.2 sensitive area, 10.sup.−7 gamma rejection, with a target cost (in quantity) of $10 k/ m.sup.2. These performance and cost points will potentially help realize a new generation of fixed and transportable passive detection systems.

[0037] A critical challenge that the invention seeks to meet is to achieve high sensitivity, low noise, stable sensor operation at room temperatures (>70° F.) for a large sensitive area device, and to scale this to achieve the target specification. In this regard a composite scintillation detector comprising non-enriched boron nitride platelets evenly distributed in a moderator is given by way of example below.

[0038] Neutron scintillator detectors typically comprise a neutron capture isotope such as .sup.6Li or .sup.10B, either enriched or in natural abundance. Compounds containing these isotopes, such as BO, BN, LiF are either mixed or chemically combined with an inorganic scintillating compound such as ZnS:Ag, ZnO, LiI:Eu, or complex organic compounds, whereby the high energy reaction products from neutron interactions with the capture compound produce scintillation in the scintillator.

[0039] In the example embodiment of the invention, a suitable neutron responsive scintillator makes use of non-isotopically enriched boron nitride.

[0040] Non-isotopically enriched boron is readily available as a fine (sub-micron) powder, as a low cost cosmetics ingredient. Its hexagonal platelet structure offers excellent properties as a neutron capture agent in composite scintillating panels (see FIG. 1); whereby this self lubricating fine powder enables effective coupling of the charged neutron reaction products to the scintillating matrix. The resulting thin, high sensitivity material can be evenly distributed in a moderator, for very high efficiency across wide energy range, and low gamma sensitivity; suitable for portable/transportable and scalable to very large detectors.

[0041] Wavelength shifting light guides allow the scintillator to be distributed over a large sensitive area. Segmented solid-state light readout will provide robust gamma rejection (in high gamma fields where pulse pile up can occur in a bulk detector). Integrated signal processing will complete a self contained robust and scalable detection system. An example light guide is shown in FIG. 2. In this figure a housing 1 contains a two dimensional array of guide tubes 2 through each of which a fluorescent fibre 3 couples to a separately addressed photodetector component of a silicon photomultiplier photodetector 4.

[0042] Additionally segmented and bi-directional readout of individual light guides provides great potential for directionality and energy discrimination in the device. A possible device configuration is shown in FIG. 3.

[0043] In FIG. 3 a large area neutron responsive scintillator 11 is coupled to the photodetector via a two dimensional array of fluorescent fibres. Each fibre couples a discrete area of the scintillator to a separately addressed photodetector element to obtain a discrete photoresponse attributable to that area of the scintillator only. Segmentation of the scintillator, readout devices and signal processing of the scintillator effectively reduces the gamma rate impinging on any one part of the detector. This significantly reduces pulse pile up and enables sufficient time for gamma rejection through pulse shape discrimination, thus enhancing gamma rejection rates.

[0044] In the example embodiment of the invention, the fibres are fabricated from a suitable fluorescent material and for example fluorescent polymeric material. The fluorescent fibres are for example polymeric fibres comprising a high refractive index core with low refractive index cladding and are for example fabricated from polystyrene and/or polymethylmethacrylate. The fibres are additionally provided with a coating of a coating of a neutron capture material and scintillating material mixture. The coating for example comprises one or more neutron capture materials selected from boron nitride, lithium fluoride, mixed or chemically combined with one or more scintillating compounds selected from zinc sulphide, zinc oxide.

[0045] Light generated in the scintillator is directly coupled to the fluorescent fibre, inducing luminescence in the fibre. The plural clad fibres transmit the generated light from each respective area of the scintillator by total internal reflection to the respective photodetector elements for separate processing to achieve sensitivity over a wide area scintillator, while maintaining very high gamma rejection.

[0046] Significant design considerations in developing a device for particular applications include: [0047] To meet the specification requirement a new level of sensitivity and gamma rejection is required, demanding improvements in detector efficiency and signal to noise. [0048] Improved coating formulas and application techniques are required, to enhance sensitivity, maintain good light output and enhance stability/robustness. [0049] Efficiently coupling light from the scintillating layer to the photo-detector is key to the successful implementation of the technique. This requires highly efficient light guides, matching the characteristics of the scintillator and photo sensor. [0050] Solid state photo-detectors close to the performance of photomultipliers are at the technical limit of commercially available sensors. Signal processing and stabilization techniques are required. [0051] The new detector platform must be optimized for performance, against size and weight. This requires extensive modeling of neutron interactions and a clear focus on requirement specifications.