GB2404780A - Neutron detector - Google Patents

Abstract

The neutron detector consists of a support layer of diamond 12, which typically may be in the range 200-1000 žm thick, onto which is grown a CVD diamond layer 14 doped with <10>B, and possibly other dopants including <11>B and other elements, typically 5-15 žm thick. A further layer 16 of undoped CVD diamond, of a quality suitable for the detection of alpha particles ('detector grade' diamond) and typically 20-50 žm thick, is grown on the doped layer 14. The structure is completed by a contact layer 18 or layers over the undoped CVD diamond layer 16. An optional second layer of doped CVD diamond may be grown onto the detector grade diamond overlayer 16. The diamond of the support layer 12 may be of standard quality and synthesised at high rate. It provides mechanical robustness and protection to the final structure, and improves the selectivity of the device to neutrons by absorbing other particles with a high capture cross-section in diamond.

Description

RADIATION DETECTOR

BACKGROUND OF THE INVENTION This invention relates to a radiation detector and, more particularly, to a neutron detector formed from diamond. The detection of neutrons is more difficult than for some other particles, since they are electrically neutral and have a low collision cross-section with most materials. Neutrons can be indirectly detected by utilising an atomic reaction with 10B, which generates an alpha particle, and then by detecting the charge carriers generated in the detector by the passage of the alpha particle. Neutron detectors which utilise diamond doped with 10B are known in the art. It has been proposed to create a neutron detector which comprises a diamond crystal, a surface layer of which is doped with 10B or other neutron-sensitive material. Such a device is described in US Patent No. 3,805,078 to Koslov. However, the device described in this patent specification is relatively complex, and only a relatively small portion of the diamond crystal is actually sensitive to neutron radiation. It has also been proposed to utilise a diamond sheet implanted with boron ions for detecting neutrons. A device of this kind is described in an article by A E Luchanskii et al in Atomnaya Energiya, Vol. 63, No. 2, pp 132-133, August, 1987.It has also been proposed by R J Keddy, T L Nam and R C Burns (Carbon, Vol. 26, No. 3, pp 345-356, 1988) and by R J Keddy (Advances in Ultrahard Materials Applications Technology, 1988, De Beers Industrial Diamond Division (Proprietary) Limited) to use diamond crystals as neutron detectors. A neutron detector is also disclosed in US 5,216,249 which comprises a thin layer of polycrystalline diamond material, deposited by a chemical vapour deposition (CVD) process, and containing 10B as a dopant uniformly throughout the layer. The diamond layer may be deposited on a supporting substrate, or a free-standing diamond layer may be formed by first depositing a diamond film on a substrate and then separating at least a portion of the diamond layer from the substrate. Typically, part of the substrate is removed, leaving a supporting ring or strip attached to the diamond layer. Electrical contacts may be applied to the diamond layer to form a conduction mode detector, or the detector may be used in a thermoluminescent mode.A limitation of this type of detector operating in conduction mode is that the B acts as a dopant in the diamond and enables a significant leakage current through the diamond even in the absence of a neutron flux. There are also many applications where diamond based neutron detectors are excluded on the basis of limited available area or high cost.

SUMMARY OF THE INVENTION According to one aspect of the invention, a neutron detector comprises: a support layer of polycrystalline diamond material providing a large grain polycrystalline surface for further layers to be grown onto; a first doped, electrically conductive diamond layer grown on the support layer; an overlayer of diamond grown on the first doped layer, the diamond overlayer being of a quality sufficient for the detection of alpha-particles; and optionally, dependent on the configuration of the detector, a second doped diamond layer grown onto the detector grade diamond layer, wherein the first doped layer and/or the second doped layer, when present,

At least one contact layer or counter electrode is preferably applied to an exposed surface of the detector grade diamond or to the second doped diamond layer, if present, and a further contact made to the first doped diamond layer. The support layer is typically grown as a thick diamond layer, typically with a minimum thickness of greater than 200 Microm, preferably greater than

The support layer provides at its growth surface a relatively large grain size polycrystalline diamond surface, onto which epitaxial growth of further large grain size material may take place. The grain size of polycrystalline diamond layers is a concept generally understood by those well versed in the art. Polycrystalline CVD diamond growth is directional and as such it produces columnar grains. The grain size referred to is that determined on a plane normal to the direction of growth.A grain is then a region of material within which the material is generally related in crystallographic orientation, being either the same or related by twinning, and generally surrounded by incoherent boundaries separating it from material whose crystallographic orientation is unrelated. The grain size is the average 'diameter' of such grains in the plane normal to growth, the average diameter being obtained by averaging the maximum chord across the grain in a number of different directions. Prior to further growth the support layer is preferably processed flat. After fabrication of the other diamond layers onto the support layer it is possible to remove the support layer. However, the preferred embodiment of the invention is where the diamond support layer remains attached and thus provides mechanical support to the final assembly. Where the support layer remains attached to the final device, the final thickness of the layer is generally less than 1000 Microm, preferably less than 850 pm, and more preferably less than 600 Microm. The doped diamond layers are preferably doped with one or more of 10B, "B or optionally other dopants such as P, S and the like. Where 10B doping is required within a particular layer, it may be the sole dopant, to enhance the concentration of 10B and improve detection yields, or it may be in conjunction with another dopant such as 11B to maximise electrical conductivity at minimum cost. In some applications it may be possible to use natural abundance B to provide the 10B doped layer. The diamond overlayer is preferably a high purity CVD diamond layer, also referred to as an overlayer of 'detector grade' diamond. At least one contact layer or counter electrode preferably forms an ohmic contact with the detector grade diamond overlayer. Likewise, the contact with the first doped layer is also preferably ohmic. The invention extends to a process for producing a neutron detector including the steps of: growing a support layer of diamond to a thickness in excess of 200 Microm, polishing it into a plate, and exposing a surface thereof having large grain size diamond particles; growing a first doped layer of diamond on the exposed surface of the support layer; masking an edge or surface of the first doped diamond layer; growing an overlayer of high purity detector grade diamond on the exposed surface of the first doped diamond layer; optionally, growing a second doped layer of diamond on the exposed surface of the detector grade diamond overlayer;and providing at least one contact on the exposed surface or a portion of the exposed surface of the detector grade diamond overlayer, or on the exposed surface or a portion of the exposed surface of the second doped layer, where present, and at least one further contact on an exposed surface or a portion of an exposed surface of the first doped diamond layer. In use the contacts are connected to an external circuit designed to detect the charge carriers generated in the diamond under the influence of the alpha particles. Optionally the method of fabrication may also include release of the diamond support layer, preferably using implantation release methods. In this embodiment the method would include creating a damage layer by implantation through the polished large grain surface of the diamond support layer prior to synthesis of the first doped layer, and later release of the diamond support layer along this damage layer. The release may occur before completing contacting of the device, or it may occur afterwards if a protective encapsulation is provided, which may also provide mechanical support.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention provides a means for the fabrication of large area diamond neutron detectors at low cost. Referring to Figure 1 of the accompanying drawings, a neutron detector 10 of the invention comprises a support layer 12 of diamond, which typically may be in the range 2001000 Microm thick, onto which is grown the first doped layer, a CVD diamond layer 14 doped with 10B, and possibly other dopants including "B and other elements, typically 5-15 Microm thick. A further layer 16 of undoped CVD diamond, of a quality suitable for the detection of alpha particles ('detector grade' diamond) and typically 20-50 Microm thick, is grown on the doped layer 14, and the structure completed by a contact layer or layers 18. An optional second layer of doped CVD diamond (not shown) may be grown onto the detector grade diamond overlayer 16. The diamond of the support layer 12 may be of standard quality and synthesised at high rate. It provides mechanical robustness and protection to the final structure, and improves the selectivity of the device to neutrons by absorbing other particles with a high capture cross-section in diamond. In addition it provides at its growth surface 17 a relatively large grain size polycrystalline diamond surface because of its thickness, onto which further growth of large grain size material may take place by epitaxial means. Typically the grain size of polycrystalline diamond layers, at least up to 1 mm thick, is approximately 10% of the thickness of the overall layer 12, and more generally in the range 5%-15% of the thickness of the overall layer 12, being in part dependent on growth conditions.The grain size here is considered to be the typical lateral dimension of a grain on the exposed polished surface 17 of the diamond support layer 12. Thus, by using a

achieved. Preferably the grain size in the detector grade layer 16 is comparable to or greater than the thickness of this particular layer, the first doped layer 14 and then the detector grade layer 16 each showing an epitaxial relationship to the grain structure of the previous layer. Thus preferably the grain size at the growth face of the support layer 12, onto which the subsequent layers are grown, is greater than 10 Microm, and more preferably greater than 20 Microm, and even more preferably greater than 40

greater than 80 Microm. The role of the first doped layer 14 is twofold. Firstly it provides 10B in a suitable concentration to capture a sufficiently high proportion of neutrons. Secondly it acts as an electrode, providing the back electrical contact for the detector grade diamond overlayer 16 used to detect the charge carriers generated by the alpha particles. The first doped layer 14 may be doped with isotopically pure 10B, or it may be doped with natural abundance or some other ratio of 10B/ B. It may also use 10B in conjunction with some dopant other than B to provide the required electrically conductive layer. Where a second doped layer is used,

dopant. Further it may have a non-uniform distribution of the various dopant elements or isotopes throughout this layer. A particular advantageous structure is one where the 10B is concentrated in a sublayer 20 typically 5-15 Microm thick adjacent to the high purity overlayer 16 even though the total thickness of the doped layer 14 is greater than this.The typical thickness range of 5-15 Microm for the layer 14 adjacent to the detector grade diamond containing 10B is based on the behaviour of the 1.47 MeV alpha particles generated by the nuclear reaction of a neutron with 10B. The upper bound of this range is set by the need for such alpha particles to reach the detector grade diamond layer 16 in order to be detected, based

from the boundary 21 than about 15 Microm thus depletes the neutron flux but does not provide detectable alphas. For neutrons moving in the direction towards the detector grade diamond overlayer 16, such depletion of the neutron flux by 10B atoms outside the 5-15 Microm sublayer 20 adjacent to the detector grade overlayer 16 would impact on the device sensitivity.The lower limit of the range is set by the need to ensure sufficient 10B atoms are present in the layer 14 in order to provide sufficient sensitivity. As such this lower limit is also dependent on the concentration of 10B in the diamond layer 14. The overlayer 16 of high purity CVD diamond provides a high degree of electrical isolation between the doped electrode 14 and the counter electrode 18 in the absence of any ionising particles, but provides a high collection efficiency (or high charge collection distance) for electron hole pairs created by the passage of charged particles, and in particular those created by alpha particles emitted by the 10B doped layer 14 under the influence of the incident neutrons. In order to grow polycrystalline diamond of detector grade it is generally advantageous to grow large grain size material, which is possible here because of the initial thick support layer 12, and the epitaxy which continues through the doped layer 14 into the detector grade overlayer 16. A suitable counter electrode 18 can be any known in the art, for example a further doped diamond electrode, using for example B, a metallised electrode, a conductive region generated by ion implantation, or a combination of these. A preferred embodiment is that this electrode 18 forms an ohmic contact with the high purity diamond overlayer 16. Likewise, a contact electrode 24 can be made to the boron doped layer 14 using any method known in the art, typically a metallised electrode. In use the detector 10 can be used in one of two orientations. In a first such orientation the face 22 of the thick standard quality diamond layer 12 is exposed to the radiation. The neutrons would not be attenuated significantly in this layer because of the absence of 10B. The neutron capture layer 14, or rather the sublayer 20 within it, which contains 10B needs to be within 5-15 Microm of the intrinsic high purity overlayer 16, and more preferably within 5-10 Microm of the intrinsic high purity layer 16, since the stopping distance of alpha particles in diamond is about 10-15 Microm. A doped region free of 10B may be used to increase the overall thickness of the doped layer 14 to improve its performance as an electrical contact layer. In a second orientation, the face 26 opposite to the thick standard diamond layer 12 is exposed to the radiation. In this configuration there are a number of advantages. Firstly, the distance traversed by the alpha particles within the 10B doped diamond is decreased, since the neutrons enter from the side 26 in contact with the detector grade diamond overlayer 16. Secondly, alpha particles generated by the nuclear reaction preferentially scatter backwards towards the source of the neutron, thus this is utilised to again increase the alpha flux reaching the detector grade diamond overlayer 16.Thirdly the restriction on limiting the 10B to a layer typically 5-15 Microm adjacent to the detector grade diamond is lifted neutrons absorbed further away than this from the detector grade overlayer 16 may not contribute to the measured signal but the neutron absorbed does not deplete the incoming neutron beam. One potential disadvantage is that the surface 17 of the device exposed to the neutron source is a contact surface, but the contact surface or the whole device can preferably be encapsulated by some means, or else the contact layer 18 can itself be a layer of doped diamond free of a significant concentration of 10B, with only any point of metallization/wire contact to this boron doped contact layer 18 preferably being encapsulated or in some other way protected or made robust. For simplicity the device 10 can be considered to operate in one of four basic configurations: the neutron radiation under detection can enter primarily from the major face comprising the support layer, or from the opposing major face, and the 10B which converts the neutron flux into a detectable alpha flux can be in the doped layer preceding the entry into the detector grade diamond layer of neutron flux, or it can be in the doped diamond layer following the passage of the neutron beam through the detector grade diamond layer. The latter configuration is preferred since the alpha particle generated by the interaction between a neutron and a 10B atom is generally backscattered. The four possible configurations of the invention will now be considered in more detail. In the first configuration, the 10B doped diamond layer lies in the first doped layer 14. If the neutrons impinge from the major face 22 of the support layer, passing through the first doped layer 14 preceding the detector grade diamond overlayer 16, then the first doped layer 14 comprises a 10B doped sublayer 20 preferentially restricted to a layer 5-15 Microm thick immediately adjacent to the detector grade diamond overlayer 16. The overall thickness of this first doped layer 14 can be increased, and thus its performance as one of the electrical contacts to the detector grade diamond overlayer 16 improved, by using other dopants, for example "B or other dopant

substantial fraction of the neutron flux and provide adequate detection thereof.In this configuration the exposed surface of the device 10 is the surface 22 of the thick support layer 12, which provides extremely good protection to the structure of the device 10 behind. These structures behind are usually further protected by encasing or potting into a suitable casing. A second doped layer may not be required in this configuration or may be used as part of a second contact (not shown) to the detector grade diamond overlayer 16. In the second configuration, 10B doped diamond sublayer 20 once again lies in the first doped layer 14. If the neutrons impinge on the major face 26 opposed to that of the support layer 12, passing through the detector grade diamond overlayer 16 prior to contacting the first doped layer 14, then again the first doped layer 14 comprises a 10B doped sublayer 20 preferentially immediately adjacent to the detector grade diamond overlayer 16. In this configuration the need to restrict the 10B doped layer 20 to 15 Microm or less is less important, since the alpha particles detected are those backscattered and thus passing only through the thickness of 10B layer already traversed by the neutron beam.Again, the overall thickness of this first doped 14 can be increased, and thus its performance as one of the electrical contacts to the detector grade diamond overlayer 16 improved, by using other dopants, for example "B or other dopant elements. Sufficient 10B is present in the 10B doped sublayer 20 to capture a substantial fraction of the neutron flux and provide adequate detection thereof. In this configuration the exposed surface of the device is the second doped layer if used or other elements of the second contact to the detector grade diamond overlayer 16, which may require additional packaging for protection. This is not generally of great concern, since the penetrating power of neutrons is substantial. The second doped layer may not be required in this configuration or may be used as part of the second contact to the detector grade diamond overlayer 16. A third configuration is one where the 10B doped diamond layer lies in the second doped layer (not shown). If the neutrons impinge from the major face 22 of the support layer 12, they pass through the first doped layer 14 preceding the detector grade diamond overlayer 16, the detector grade overlayer 16 itself and then a second doped layer wherein at least the first part of this second doped layer in contact with the detector grade diamond overlayer 16 is doped with 10B. As with the second configuration discussed above, it is not necessary to restrict the 10B to a thin layer, for example 15 Microm thick. There may, however, be for example cost benefits in doing so, the remaining thickness of the second doped layer comprising for example natural isotopic abundance B or some other dopant.In this configuration, as in the first configuration, the exposed surface of the device is the surface 22 of the thick support layer 12, which provides extremely good protection to the structure of the device behind. The structures behind this face may, however, be further protected by encasing or potting into a suitable casing. The disadvantage of this configuration is the need for two doped layers, the first of which is substantially 10B free. A further variant in this configuration is that the 10B may not be included in a diamond layer, but may be present in another form such as metallic 10B. This metallic 10B layer may beneficially be separated from the high purity diamond overlayer 16 by a very thin interfacial layer, preferably < 3 Microm thick and more preferably < 1 Microm thick, which acts to form an ohmic contact between the high purity layer 16 and the metallic 10B layer.Examples would be a metallisation layer or a thin doped diamond layer. The advantage of using 10B in another solid form other than as a dopant in diamond is that the concentration is not so limited, enhancing absorption of the neutrons and proportionately reducing the absorption of the alpha particles because of the lower penetration of the neutrons into this layer.

again lies in the second doped layer. If the neutrons impinge on the major face 26 opposed to that of the support layer 12, passing through the second doped diamond layer into the detection grade diamond overlayer 16, then again the second doped layer comprises a 10B doped layer preferably 5-15 Microm thick and immediately adjacent to the detector grade diamond overlayer 16. Other design considerations are the appropriate elements of those in the first to third configurations discussed above. A fifth configuration may also be appropriate in some circumstances. As a result of the preferred backscatter behaviour of the alpha particles generated by the 10B atom, the second and third configurations above offer significantly higher sensitivity to neutrons than do the first and fourth configurations, and such directional sensitivity may be used to characterise the direction of neutron flux. Where a detector specifically needs to detect neutrons from all directions, or show minimal directional sensitivity, or for other reasons, then both the first and second doped layers may have a layer of 10B doping immediately adjacent to the detector grade overlayer 16. Under such circumstances the thickness of the respective 10B doped layers

proportion of backscattered alpha particles from the doped layers adjacent the detector grade diamond overlayer 16. The neutron detector 10 of the invention is manufactured at a relatively low fabrication cost. The diamond synthesis can be by any known method of CVD synthesis, but it is preferable to use microwave synthesis to obtain the degree of control required between the various layers. Example process steps are set out schematically in Figures 2a - 2e. Those skilled in the art will understand that similar structures with equivalent function can be manufactured in a variety of ways. Similar detector structures are possible using single crystal diamond. The issues of grain size are however not relevant to single crystal diamond so there is no benefit from this aspect in growing the device on thick layers. Further, single crystal diamond does not provide the large detector areas or low costs that polycrystalline diamond can provide. In addition, polycrystalline diamond, because of the presence of {111} growth sectors which generally have a higher impurity/dopant uptake than growth sectors such as the {100}, can contain substantially higher concentrations of 10B than {100} growth sector single crystal diamond, enabling a higher density of 10B atoms in the thin layer adjacent to the high purity layer, and increasing the electrical conductivity of the doped layers. A layer of diamond of standard quality is first grown to a thickness typically in excess of 500 Microm and polished into a flat plate 30 (Figure 2a). The plate 30 is then remounted in the synthesis reactor with the face 32 with large grain size exposed and a layer 34 of B doped diamond synthesised onto it (Figure 2b), with the grain structure of layer 34 forming an epitaxial relationship with that of face 32 of plate 30. The layer 34 may be one that

the layer. Another alternative is that the 10B layer 34 can be grown thicker

less, but this is not normally required. At this stage the outer edge 36 (for example a 2 mm wide strip) of the disc is masked, for example by using metal (Figure 2c). This masking may be as a coating applied to the edge 36 or as a structure 38 placed over the edge 36 of the disc in the synthesis reactor. A layer 40 of high purity detector grade diamond, of large grain size because of its location on the growth face 42 of the 10B layer 34, is then grown, with this layer 40 exhibiting a sufficiently high charge collection distance to obtain the efficiency required for the detector to operate (Figure 2d). By introducing the masking 38 in the previous step mechanically within the reactor, or by using subsequent etching to reveal the doped diamond layer 34 sufficient to make contact to it, it is possible to make the transition from the 10B doped layer 34 to the detector grade layer 40 a continuous process. Layer 40 may be grown directly to final thickness, or it may be grown to a greater thickness and polished back. Finally mask 38 is removed and contact is made to the doped diamond layer 34 via a contact layer 44 and to the top 46 of the detector grade layer 40 via contact layer 48, typically using wire bonds 50, connecting the device to external circuitry such as that known in the art to detect the charge carriers generated in the detector grade layer 40 enabling the device to operate (Figure 2e). These contacts can for example be prepared by sputter coating to form a Ti/Pt/Au contact pad, using methods well known in the art, and then attaching wire bonds 50. After fabrication of the diamond layers 34 and 40 and the support layer 30, it is possible to remove the support layer 30. For example, using implantation release methods such as those described in US 5,334,283 and US 5,587,210 would enable the support layer 30 to be removed and made available for further use. To provide mechanical support for the device after releasing the diamond support layer 30 it is also possible to optionally first complete the metallisation and contacting stages and then encapsulate the side away from the diamond support layer 30 with a material that provides a degree of mechanical support. It is however preferred that the support layer 30 remains attached and this provides mechanical support to the final assembly. In operation the rear face 52 of the device and any non-diamond electrodes and attaching wires are generally enclosed or encapsulated. Where the front face presented to the neutron flux is face 54 this is normally unencapsulated. Where the front face of the device presented to the neutron flux is face 52 this is either generally a 10B free doped diamond contact layer or else provided with some form of encapsulation. Typical fields applied to polycrystalline detector layers are of the order of 10 kV/cm. For the 20-50 Microm thick detector grade layer 16,40 typically used in this invention, to obtain this electric field a voltage of 200 V - 500 V needs to be applied. However, where a thin layer is used in this detector it is also possible to operate satisfactorily at lower applied fields, for example 3 kV/cm - 10 kV/cm, enabling the applied voltage to be reduced to as low as 60 V. These applied voltages compare with voltages in excess of 1 kV required for certain alternative types of detector. The key features of this invention are that it provides a means of fabricating large area detectors which operate at low applied voltages and which are sufficiently high performance for a range of applications. Applications include the monitoring of neutron radiation for example in geological studies or well logging. Additional benefits of the design include that it has a relatively low

to neutrons and thus does not attenuate the signal, the device is solid state and very robust, and the exposed face of the device is generally diamond, providing a chemically inert and abrasion resistant surface.

Claims (6)

CLAIMS:

1. A neutron detector comprising: a support layer of polycrystalline diamond material providing a large grain polycrystalline surface for further layers to be grown onto; a first doped, electrically conductive diamond layer grown on the support layer; an overlayer of diamond grown on the first doped layer, the diamond overlayer being of a quality sufficient for the detection of alpha-particles; and optionally, dependent on the configuration of the detector, a second doped diamond layer grown onto the detector grade diamond layer, wherein the first doped layer and/or the second doped layer, when

2. A neutron detector according to claim 1, wherein at least one contact layer or counter electrode is applied to an exposed surface of the detector grade diamond or to the second doped diamond layer, if present, and a further contact made to the first doped diamond layer.

3. A neutron detector according to claim 1 or claim 2, wherein the support layer is grown as a thick diamond layer with a minimum thickness of greater than 200 Microm.

4. A neutron detector according to any one of the preceding claims, wherein the doped diamond layers are doped with one or more of 10B, 11B or optionally other dopants selected from P, S and the like.

5. A neutron detector according to any one of the preceding claims, wherein the diamond overlayer is a high purity CVD diamond layer.

6. A process for producing a neutron detector, the process including the steps of: growing a support layer of diamond to a thickness in excess of 200 Microm, polishing it into a plate, and exposing a surface thereof having large grain size diamond particles; growing a first doped layer of diamond on the exposed surface of the support layer; masking an edge or surface of the first doped diamond layer; growing an overlayer of high purity detector grade diamond on the exposed surface of the first doped diamond layer; optionally, growing a second doped layer of diamond on the exposed surface of the detector grade diamond overlayer; and providing at least one contact on the exposed surface or a portion of the exposed surface of the detector grade diamond overlayer, or on the exposed surface or a portion of the exposed surface of the second doped layer, where present, and at least one further contact on an exposed surface or a portion of an exposed surface of the first doped diamond layer.