Method of manufacturing a scintillator and a scintillator layer thus manufactured

In order to manufacture a scintillator layer for a detector for the detection of electromagnetic radiation, transmitted by an object, which has a high spatial resolution and only a slight interaction between the scintillator elements, it is proposed to pour a molten mass of a radiation-absorbing metal, having a melting point below 350.degree. C., into intermediate spaces which extend vertically between neighboring scintillator elements.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a method of manufacturing a scintillator layer for
 a detector for the detection of electromagnetic radiation transmitted by
 an object, in which a scintillator layer for converting the radiation of a
 first energy level into radiation of a second energy level is provided on
 a photosensor layer for converting such radiation into an electric
 current, and in which the scintillator layer, comprising a plurality of
 scintillator elements, is provided with intermediate layers which extend
 in the vertical direction along the side faces of the scintillator
 elements.
 2. Description of Related Art
 Scintillator layers of detectors which are used, for example in computer
 tomographs, customarily consist of cadmium tungstate [CWO] (CdWO.sub.4),
 of yttrium gadolinium oxide [YGO] (Y,Gd).sub.2 O.sub.3 :E) or of
 gadolinium oxysulphides [GOS] (Gd.sub.2 O.sub.2 S:Pr). Such materials
 provide conversion of radiation of a first energy level into radiation of
 a second energy level, for example, the conversion of X-rays into visible
 light in the case of computer tomographs.
 The international patent application WO 95/04289 describes a detector with
 a scintillator layer which consists of a two-dimensional array of
 scintillator elements, i.e. a plurality of rows of scintillator elements
 arranged parallel to one another. The scintillator elements are formed by
 monocrystals. Between the scintillator elements there are provided optical
 separation layers which extend along the side faces of the scintillator
 elements. These layers are provided with a thickness of from 0.05 to 1
 .mu.m by metal deposition. The materials used for these layers are
 aluminium, tungsten, molybdenum, iron, chromium, nickel, gold, silver or
 copper.
 Since recently it is attempted to enhance the resolution, and hence the
 image quality, of X-ray examination devices, notably computer tomographs,
 by utilizing detectors comprising a larger number of detector elements or
 scintillator elements. This gain in respect of optical resolution,
 however, is accompanied by the drawback of increased crosstalk between
 neighboring scintillator elements, which increase is due to the crossing
 over of photons and X-ray quanta.
 Citation of a reference herein, or throughout this specification, is not to
 construed as an admission that such reference is prior art to the
 Applicant's invention of the invention subsequently claimed.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to propose a method of
 manufacturing a scintillator layer for a detector which has a higher
 resolution and in which the disturbing interaction between the
 scintillator elements is only insignificant.
 This object is achieved by means of the method which is characterized as
 described in the claims as well as by means of the scintillator layer
 which is characterized as described in the claims.
 The basic idea of the invention is to pour in a molten mass of a
 radiation-absorbing metal, having a melting point below 350.degree. C.,
 into the intermediate spaces between neighboring scintillator elements.
 The molten mass may consist of pure radiation-absorbing metals, preferably
 lead, bismuth and mercury which have melting points of 327.5, 271.3 and
 33.4.degree. C., respectively. Molten masses of lead and bismuth solidify
 at room temperature. Mercury is liquid at room temperature. The metal is
 retained by layers which enclose the scintillator layer as a whole.
 Preferably, use is made of metal alloys whose components can be selected
 from the following group of metals: bismuth and/or lead and/or zinc and/or
 tin and/or cadmium and/or mercury. Preferably, compositions of the
 components which correspond to the eutectic compounds are selected.
 The described choice of metals and metal alloys offers a particularly
 attractive low melting point in combination with a high absorptivity. The
 pouring method enables the formation of thin intermediate layers which do
 not unnecessarily reduce the active surface area of the scintillator
 element. Because of the choice of metals having melting points below
 350.degree. C., chemical reactions with the scintillator crystals or
 damage are avoided.
 The radiation-absorbing layers have two functions. First of all, they serve
 as an optical separation layer in that they reflect photons arising during
 the conversion (optical crosstalk) back to the individual scintillator
 elements, so that they increase the signal strength. The intermediate
 layers extend in the vertical direction, i.e. transversely to or
 perpendicularly to the surface of the scintillator layer.
 On the other hand, such layers serve for the absorption of K fluorescence
 X-ray quanta (X-ray crosstalk). K fluorescence X-ray quanta or secondary
 X-ray quanta arise when the energy of the electromagnetic radiation or
 X-rays is not fully taken up by the scintillator elements. In the case of
 the described scintillator metals GOS and CWO such K fluorescence X-ray
 quanta amount to from 40 to 50% of the primary absorbed radiation. This is
 because the energy of the absorbed X-ray quanta exceeds the so-al led
 K-edge energy of the scintillator crystal. Only a part of the K
 fluorescence X-ray quanta then arising can be absorbed in the same
 scintillator element; a further part is emitted and a third part is
 absorbed by neighboring crystals. The proposed poured absorption layers
 prevent such X-ray crosstalk between neighboring scintillator elements.
 Known layers of materials such as molybdenum offer only an unsatisfactory
 optical separation function of this kind in conjunction with a high
 absorptivity for secondary X-rays. The same also holds for known optical
 separation layers of titanium dioxide embedded in epoxy resin. Therefore,
 such materials can be used only for one-dimensional detectors with a low
 spatial resolution; such one-dimensional detectors involve less crosstalk
 of X-ray quanta in comparison with two-dimensional detectors.
 The proposed pouring method enables optimum filling of the gaps between the
 scintillator elements. Fillings with a width of preferably 100 .mu.m can
 be realized in a fast, easily reproducible and inexpensive manner. The
 method is, therefore, very suitable for the manufacture of scintillators
 composed of a large number of scintillator elements in a flat arrangement.
 The pouring process is preferably performed in vacuum or in an inert gas
 atmosphere. This has a positive effect on the fluidity of the molten
 metal.
 The preparation of gaps along the side faces of the scintillator elements,
 and hence of a pattern of recesses to be filled with molten metal, is
 realized on the one hand by arranging monocrystals in such a manner that a
 minimum distance is maintained. On the other hand, a pattern of recesses
 can be mechanically formed in a scintillator layer, for example by sawing
 by means of an appropriate tool. This method enables the formation of a
 large number of individual scintillator elements. After the filling of the
 gaps or the pattern of recesses with the molten metal, a remaining
 non-filled edge zone of the scintillator layer can be mechanically
 removed.
 The proposed method of manufacturing a scintillator layer for a detector is
 particularly suitable for a two-dimensional scintillator layer, i.e. an
 array of scintillator elements with n rows and m columns, where n, m are
 numbers larger than 1. The method, however, is also suitable for the
 manufacture of the scintillator layer of one-dimensional or linear
 detectors.
 Assuming a two-dimensional detector or a cone beam detector provided with a
 scintillator array, a version of the method according to the invention is
 proposed in which the radiation-absorbing layers are poured into the gaps
 between the rows of scintillator elements whereas the radiation-absorbing
 layers are inserted as preformed thin layers or foils in the gaps between
 the columns of scintillator elements. The reverse situation is also
 feasible. The preformed metal layers are preferably made of lead,
 tantalum, tungsten or gadolinium. Whereas the insertion of preformed metal
 layers into the gaps of a linear scintillator for a one-dimensional
 detector would still be feasible in practice, such a method would be faced
 with excessive mechanical problems and problems in respect of time in the
 case of two-dimensional detectors. The dressing of linear segments and
 their flat arrangement is feasible, but involves technical problems
 because of the low hardness of the metals and the associated resilience of
 the layers during cutting.
 Some examples of preferred radiation-absorbing metal alloys and their
 melting points will be given hereinafter.
 1) An alloy consisting of 56.5% by weight of bismuth, 43.5% by weight of
 lead (binary eutectic).
 Melting point: 125.degree. C.; density: 10.42 g/cm3.
 2) An Alloy consisting of 55.0% by weight of bismuth, 43.0% by weight of
 lead, 2.0% by weight of zinc (ternary eutectic).
 Melting point: 124.degree. C.; density: 10.32 g/cm3.
 3) An alloy consisting of 52.5% by weight of bismuth, 32.0% by weight of
 lead, 15.5% by weight of tin (ternary eutectic).
 Melting point: 96.degree. C.; density: 9.69 g/cm3.
 4) An alloy consisting of 51.7% by weight of bismuth, 40.2% by weight of
 lead, 8.1% by weight of cadmium (ternary eutectic).
 Melting point: 91.5.degree. C.; density: 10.24 g/cm3.
 5) An alloy consisting of 49.5% by weight of bismuth, 27.3% by weight of
 lead, 13.1% by weight of tin, 10.1% by weight of cadmium (quaternary
 eutectic, Lipowitz's metal).
 Melting point: 71.degree. C.; density: 9.57 g/cm3.
 6) An alloy consisting of 50.0% by weight of bismuth, 25.0% by weight of
 lead, 12.5% by weight of tin, 12.5% by weight of cadmium (Wood's metal).
 Melting point: 70.degree. C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The detector consists of a scintillator layer 1 and a photodiode array 2
 which is situated therebelow. The photodiode array 2 is connected to
 amplifiers and, via the amplifiers, to multiplexers (diagrammatically
 represented by subsequent layers). The signals of the multiplexers are
 applied to an arithmetic unit via an analog-to-digital converter.
 The scintillator layer 1 itself consists of a plurality of scintillator
 elements (in this case 1a, 1b, 1c). Intermediate layers 3a, 3b of
 radiation-absorbing metals, having a melting point below 350.degree. C.,
 are poured into a pattern of recesses along the side faces of the
 scintillator elements. The scintillator layer is completely enclosed by a
 reflection or protection layer (4a, 4b).
 When the detector is exposed to X-rays, the X-ray quanta (y) in the
 individual scintillator elements (in this case shown in lb by way of
 example) are converted into photons. The intermediate layers 3a, 3b then
 serve as optical separation layers in that they reflect the photons back
 to the individual elements. Any K fluorescent X-ray quanta (y') occurring
 are absorbed by the intermediate layers 3a, 3b so that undesirable X-ray
 crosstalk is avoided.
 The scintillator layers manufactured in conformity with the method
 according to the invention are suitable for use in all known detectors.
 Also included are, for example so-called flat solid-state X-ray detectors
 with large electronic circuits. Detectors of this kind may also be
 referred to as X-ray sensor matrices and are known, for example from
 European patent application 0 440 282 A2. The detectors provided with the
 proposed scintillator layer are used notably in computer tomographs.
 FIG. 2 shows diagrammatically a computer tomograph provided with a
 multi-line detector. In a circular portal frame or gantry 31 there are
 arranged the X-ray source 32 and the multi-line detector 33 which is
 mounted so as to face the tube. The X-ray tube 32 projects a pyramidal
 X-ray beam 34 through the patient 35 and onto the multi-line detector 33.
 For the purpose of examination the patient 35 is displaced through the
 rotating gantry 31 on a table 36.
 The detector array 33 is arranged at a distance r from the focus of the
 X-ray tube 32. During a complete revolution of the gantry 31 the X-ray
 beam 34 irradiates the patient 35 within the plane of the gantry from
 different angles .phi. relative to the perpendicular. A cross-sectional
 image 37 of the irradiated region of the patient is calculated by means of
 these projections.
 The detector array 33 is composed of a plurality of detector elements which
 are arranged in a plurality of rows. These rows extend parallel to one
 another in the direction of the axis of rotation (z direction). The
 detector array includes the photodiode array 2.
 All references cited herein are incorporated herein by reference in their
 entirety and for all purposes to the same extent as if each individual
 publication or patent or patent application was specifically and
 individually indicated to be incorporated by reference in its entirety for
 all purposes.