GB2480676A - Radiation Detector - Google Patents

Abstract

A multiband neutron detector uses several multi-layer detectors, each consisting of gold electrodes, CVD diamond collectors and boron or lithium activation (neutron capture) layers. The detectors are separated by HDPE moderator regions. The layers are formed concentrically as part of a spherical shell. Each layer can detect neutrons of different energies.

Description

RADIATION DETECTOR

DESCRIPTION

The present invention is a radiation detector. It is a multiband spectrometry instrument utilising a plurality of multi-layer detectors separated by regions of moderator in order to collect radiation over a range of energies. It is particularly suited to detection of neutrons.

CURRENT STATE OF THE ART

To obtain radiation energy information it is well known to use assemblies of point detectors and different types and/or thicknesses of attenuating materials.

The use of Bonner spheres is a well known technique to determine the energy spectrum of a neutron beam. The methodology, which was first described in 1960 by Bonner et al, places point detectors inside spheres of moderator of different sizes typically up to 50 cm in diameter. The resulting range of signals can be processed into an energy spectrum. Nuclear Instruments & Methods; Volume 9, Oct 1, 1960.

Patent US69281 30 describes a dodecahedral neutron spectrometer. Each of the twelve faces has a section of polyethylene where some incident neutrons liberate protons.

There is then a proton-absorbing material shielding a point diode detector. Importantly each face has a different thickness of absorber, so that different energy ranges of protons reach the detector.

Patent US6362485 teaches the use a substantially spherical or cylindrical moderator body with a series of point detectors at different depths, and a central detector in a central cavity.

Diamond is known as a material for radiation detection. It has a high band-gap, breakdown field strength, radiation hardness and carrier mobility. A summary of diamond detectors is available in chapter 9 of "CVD Diamond for Electronic Devices and Sensors", Wiley, ISBN 978-0-470-06532-7 The present invention improves over the prior art by using a plurality of large area detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic cross-section of an embodiment of the invention Figure 2 is a schematic cross-section of an embodiment of a multi-layer detector The drawings are intended to make clear the structure and operation of the present invention but are not to scale and do not show the precise geometry of embodiments (which in any case vary).

ESSENTIAL FEATURES OF THE PRESENT INVENTION

The present invention is a multiband spectrometry instrument utilising a plurality of multi-layer detectors (103) designed to collect neutrons with a range of energies. The range detected extends from thermal neutrons (here defined as those with energy less than 0.025 eV) to fast neutrons (here defined as those with energy greater than I MeV).

Each multi-layer detector (103) within the spectrometer detects neutrons in two distinct energy bands. Fast neutrons in the range above 14.8 MeV (hereafter "high energy neutrons") are detected directly, and an indirect activation technique is used so that less energetic neutrons are also detected.

A preferred embodiment of the invention is based on a sphere of high density polyethylene (HDPE) with an outer radius of 15cm, giving it a mass of about 14 kg, so that it is easily portable. Within this mass of HDPE are multiple concentric spherical shells (101), and on all or most of the outer surface of each shell (101), a large area multi-layer detector (103) is constructed. Cylindrical geometries may also be used.

A design with three multi-layer detectors (103) is preferable in terms of sensitivity versus complexity of construction, and such an embodiment will now be described in detail: Figure 1 shows a cross-section of a spherical embodiment: Before reaching the outermost multi-layer detector (103-1), incoming particles lose only a small proportion of their energy in the outer layer of moderator (101). The detector (103-1) detects both thermal neutrons (indirectly) and high energy neutrons (directly). Neutrons with energies outside these ranges pass straight through this first detector (103-1).

To reach the intermediate multi-layer detector (103-2) neutrons that have passed the outer detector (103-1) traverse a further layer of moderating material (101) and lose energy. Consequently neutrons that were not detected by the outer detector (103-1) are effectively shifted down the energy spectrum and are detectable by the intermediate detector (103-2). Again both thermal neutrons (indirectly) and high energy neutrons (directly) are detected by this detector (103-2).

Similarly, to reach the inner multi-layer detector (103-3) neutrons that have passed both the outer (103-1) and intermediate (103-2) detectors, traverse another layer of moderator material (101). Here they lose further energy and are effectively shifted further down the energy spectrum. Again both thermal neutrons (indirectly) and high energy neutrons (directly) are detected by this detector (103-3).

The responses of the multi-layer detectors (103) and the dimensions of the moderator material (101) are chosen so that incident neutrons over a wide energy range are detected in one or other of the detectors (103).

We turn now to the detailed construction of each multi-layer detector (103).

Each multi-layer detector (103) comprises a plurality of layers of material. Referring to Figure 2 (NB layers are not to scale): onto the moderator region (101) is first deposited an activation layer (203), then an electrode layer (205), a collector layer (207) and a further electrode layer (209). Optionally and preferably there is also a further activation layer (211). Each detector (103) is constructed on the surface of one moderator region (101) and following device assembly lies closely adjacent to another (101).

Preferably each multi-layer detector (103) occupies almost the whole of the surface of a spherical shell (with one or more small gaps in coverage to allow insulated wires to pass to inner detectors (103)). This provides isotropic detection. Optionally each shell may be divided into multiple isolated detectors, so that directional information may be recorded.

The preferred moderator (101) is high density polyethylene (HDPE), chosen because of its suitability for moderating neutrons and to facilitate construction. Alternative moderating materials may be employed dependent on the energy/particle of interest.

Moderator regions (101) are preferably half or quarter spherical shells for ease of construction and assembly, although other geometries may be used.

Because neutrons hold no electronic charge, they cannot be directly detected electrically, and require a second-order effect. Some traditional detectors use a hydrogen-rich material where collisions generate free protons which are detected.

Preferably embodiments of the present invention use 10Boron as the reaction species in the activation layer (203), although 6Lithiurn is an alternative. These materials may be provided elementally (preferred for boron) or as compounds (such as lithium fluoride).

Some incident neutrons react with boron in the reaction 10B(n,a)7Li which results in one of two possible reactions. The first reaction (94%) leaves 7Li in its first excited state, which very quickly drops back (-1 013 seconds) to its ground state after the emission of a 480 keV gamma ray. The second reaction (6%) leaves 7Li in a ground state.

High energy neutrons captured directly within each collector layer (207), undergo the 12C(n,a)10Be reaction.

A thicker activation layer (203) gives more collisions, but less chance of detecting the products, because in order for detection, the charged species have to reach the collector layer (207). Embodiments of the present invention preferably employ an activation layer (203) of between one and five hundred microns in thickness.

On top of the activation layer (203) is a first electrode layer (205), preferably gold and alternatively aluminium.

Next is the collector layer (207) and then a second electrode layer (209), preferably gold and alternatively aluminium. The collector layer (207) is preferably thin film diamond. It operates via the generation of an electron-hole pair both directly by a high energy neutron and also when one of the reaction products from the activation layer (203) passes into the collector layer (207) and undergoes a collision. The two electrodes (205 and 209) are maintained at a potential difference, so that the generated electron drifts towards the anode and is collected in an external circuit (and the hole vice versa), creating a signal. Optionally there is a second activation layer (211) mirroring the first activation layer (203) and to which the same comments above apply.

The diamond collector layer (207) distinguishes different types of radiation and detects high energy neutrons directly. Having an activation layer of 10B for the thermal energy neutrons provides the ability to obtain data from this energy region also, reducing the overall amount of moderator material (101) required in the device.

A further benefit due to the selection of 10B and diamond is that both alpha particle and gamma ray are used for detecting thermal neutrons.

In detail, the layers of the detector (103) are constructed as follows: o The electrodes (205 and 209) are made of metallic films, and are deposited by physical vapour deposition, a process well known and understood.

o The boron activation layer(s) (203 and optionally 211) is/are made by sputter coating. This process is also well known and understood and involves ionised gas ions colliding with a (usually heated) cathode, from which the ions physically expel atoms or groups of atoms. These then accumulate on the workpiece.

o The collector layer (207) is preferably created by Chemical Vapour Transport (CVT). Carbon is introduced into a hydrogen rich chamber in the form of a graphite rod, and heated to boiling point via an electrical current. It then vaporises and accumulates as a polycrystalline diamond film on the workpiece.

There are technical issues with the use of plastic materials in reduced pressure deposition environments, and mitigating steps are taken to avoid high defect counts in each multi-layer detector (103). In particular, out-gassing is a problem when the moderator (101) is first placed under vacuum, specifically non-polar gases like H2, N2, 02, and 002. Typically this process subsides after about 30 minutes without the need for substrate heating. Other molecules such as H20 are present bonded to the material (101), and these are removed by placing the substrate (101) under vacuum at standard room temperature for about 24 hours. Some hydrocarbon molecules also outgas.

Typically they are minirnised by baking the substrate (101) at 368K (95°C) for 16 hours.

Optionally the device has a small central cavity containing a single point detector.

Insulated wires or conducting rods (not shown in the figures) are electrically connected to the electrodes (205 and 209) and led out to the exterior of the device where they are connected to the data processing system.

The inventors have developed mathematical models for the signals expected from a plurality of multi-layer detectors (103) with the geometry of the present invention. In survey situations incident flux is usually either parallel or isotropic. A data processing system uses the results of this modelling, and thereby processes the detector signals into an energy distribution, characterising the radiation field spectrum. This spectrum is of use to nuclear professionals.

As is well known to those skilled in the art, preferably each device is calibrated against one or more reference radiation sources (and where appropriate its isotropic performance checked) before deployment.

While the present invention has been described in terms of several embodiments, those skilled in the art will recognize that the present invention is not limited to the embodiments described, but can be practised with modification and alteration within the spirit and scope of the appended claims. The Description is thus to be regarded as illustrative instead of limiting.

Claims (1)

1. CLAIMS1 A device for detection of radiation comprising a plurality of multi-layer detectors separated by regions of a moderator material; and a signal processing system 2 A device as in Claim 1 where the multi-layer detectors and moderator regions are substantially concentric 3 A device as in Claim 2 where the multi-layer detectors and moderator regions are substantially spherical 4 A device as in Claim 3 where each multi-layer detector has substantially the form of a spherical shall or part of a spherical shell A device as in any previous Claim where the moderator material is composed substantially of polyethylene 6 A device as in any previous Claim where the radiation detected is neutrons 7 A device as in any previous Claim where each multi-layer detector consists of at least one activation layer, a collector layer and at least two electrode layers 8 A device as in Claim 7 where each activation layer is composed substantially of boron and/or boron compounds 9 A device as in Claims 7 to 8 where the collector layer is composed substantially of carbon A device as in Claims 7 to 9 where each electrode layer is composed substantially of gold 11 A device as in any previous Claim which is portable 12 A device as in any previous Claim where the signal processing system converts the detector data into a radiation energy spectrum