GB2253070A - Optic fibre plates containing scintillators - Google Patents

Abstract

The resolution of a scintillator in the physical form of a thin sheet made from or carrying a layer of some suitable scintillating substance can never be better than the thickness of the sheet, and while thinner sheets give higher resolution they also result in less efficient conversion of the incident radiation into visible light. Even when the device is constructed as a fibre optic plate - made from a bundle of very fine fibres of scintillator material - the intrinsic conversion efficiency of the materials used to make the optical fibres is low. In this application. the scintillator capability is separated from the fibre-forming capability, by employing one material, e.g. PMMA, (10) to make the fibres and particles of a scintillator (11), eg. ZnS(Ag) or doped SrS mixed into that fibre-forming material, forming a fibre (13) therefrom, which is then clad (14), formed into a bundle (15), drawn and formed into a fibre optic plate (21). <IMAGE>

Description

SCINTILLATORS This invention relates to scintillators, and concerns in particular a novel form of fibre-optic plate for use in image intensifier systems and the like.

Broadly, a scintillator is a material that emits electromagnetic energy in the visible light part of the spectrum after having absorbed energy in some other part thereof. Thus, a scintillator may convert X-ray or ultra-violet energy (X-ray or UV photons) into visible light photons (it is a "down" converter, as high energy, high frequency radiation is converted into a lower energy forra) or it may convert infra-red energy (IR photons) into visible light photons (it is then an "up" converter, absorbing low energy, low frequency radiation and emitting a higher energy form).

A scintillator may take the physical form of a thin sheet, or plate, which is either made from some suitable scintillating substance or carries a thin layer of such a scintillating substance bound as a fine powder on its input suface. In either case the resolution of the device - the ability to distinguish between two adjacent points - can never be better than the thickness of the sheet Cas simple geometry can demonstrate). For higher resolution there is therefore needed thinner sheets and since for most useful equipment employing such a scintillator there will be required a resolution of 1 mm or better it follows that the sheet should be no more than 1 mm thick.Now, for efficient conversion of the incident radiation into visible light - for absorption of sufficient of that radiation without unacceptable re-absorption and/or scattering of the emitted visible light in the underlying depths either of the plate or of the scintillator particles thereon - there is an optimum thickness for the sheet Cor the layer), and unfortunately that thickness is usually far too large several millimetres - for the device to have anything like the sort of resolution wanted.

One very good method of overcoming this resolution versus conversion efficiency problem is to construct the device not as a continuous sheet of the scintillator material but instead as a fibre optic plate - a two-dimensional bundle of very thin optically transparent fibres made out of scintillator material and mounted side by side like bristles in a brush - for in such a plate the radiation passing through can (in theory) pass only along each fibre, so the resolution of the scintillator can never be worse than the diameter of each fibre no matter how long the fibres are. In this way it is easy to construct a scintillator plate in which the fibres are as long as is necessary (10 mm, say) for maximum conversion efficiency but also as thin as possible (50 micron, perhaps, or even 10 micron) for maximum resolution.

Some excellent scintillator systems have been built utilising this optical fibre technique, and both glassbased fibres (typically a cerium-doped glass core with a borosilicate cladding) and plastics-based fibres Ctypically a polyCvinyltoluene) core with a poly(methyl methacrylate) cladding) have been used to good effect in the construction of systems for converting X-rays, y-rays and charged-particle radiation Celectrons, alpha-particles and the like) into visible light.However, one problem associated with these systems - and, indeed, with any of the present-day conversion systems employing a fibre-optic scintillator - is that the intrinsic conversion efficiency of the materials used to make the optical fibres is pretty low, and less than 60% of that of anthracene Cthe classic scintillator sheet material for use in electron detection and/or conversion). Moreover, at the moment there are no up converter scintillators available in plastics or glass fibre optic form.

The situation, then, is that the available and/or efficient scintillator energy converters are usable only in low resolution sheet form: the benefits of employing a fibre-optic structure are not being realised because either the scintillator materials are not provided in fibre form or the available fibre converters have an inadequate and unacceptable conversion efficiency. The invention proposes a possible solution to this, which is basically the surprisingly simple idea of separating the scintillator capability from the fibre-forming capability, and so employing one material to make the fibres and another, compatible with and mixed into that fibre-forming material (possibly as discrete particles therewithin), to provide the scintillator effect.In this way the scintillator material per Se may be chosen primarily for its efficiency at converting the relevant incident wave/particle energy into visible light radiation, while the fibre-forming material may be chosen for the ease with which it can be shaped and formed into transparent optical fibres (and thence into a face plate made therefrom).

In one aspect, therefore, this invention provides, for use in the construction of a scintillator device in the form of a fibre optic plate, an optical fibre, or precursor thereof, containing dispersed therewithin a scintillator material.

In a second aspect the invention provides, for use in a scintillator device, a fibre optic plate made up from a multiplicity of side-by-side lengths of the optical fibres of the invention.

The invention provides in its first aspect an optical fibre, or precursor thereof, and in a second aspect a fibre optic plate made therefrom. To appreciate this fully it is necessary to understand the processes used to make optical fibres and form them into a fibre optic plate.

Firstly, there is prepared, by any convenient technique, an elongate rod or block (usually circular in cross section) of the glass or plastics material to make up the core of the fibre. Such a block will typically be 30 mm in diameter, and in the present invention the core has the scintillator material dispersed therewithin. This block is then "drawn" out - commonly by controlled heating to its softening point, and then careful pulling so that it elongates and thins - down to a thin "wire" typically around 1 mm diameter (a 30 times reduction). This "wire" is provided with a suitable cladding coating of a material having a lower refractive index than the core material (this will result in the eventually-formed fibre being a "stepped index" fibre; the difference in the refractive index between the core and cladding materials being such as to prevent radiation travelling along the fibre from ".escaping" through the surface), and cut into handleable lengths, typically 100 mm or co. These lengths are then formed into a stack - a 10 x 10 stack, say - like bristles in a brush, and are "welded" together by controlled heat and pressure to form a rod-like bundle, and this bundle is itself drawn out, in the same way as was the original rod, to give a much thinner rod wherein the original individual "wires" are now drawn to a few tens of microns - typically, 30 micron (0.03 mm) - in diameter (another 30 times reduction), the bundle being (for example) about 10 times bigger CSQO micron; 0.3 mm).

Finally, this reduced, elongated, bundle is itself cut into handy lengths and stacked and bonded (just as the "wire" was before), this time into a quite thick bundle - 300 mm diameter, say - and this thick bundle, or super stack, is sawn into convenient lengths (around 5 mm > appropriate to the ab.-orption/conversion efficiency of the scintillator material dispersed within the core.

These lengths are the desired 5 mm-thick fibreoptic plates, and may be finished (by polishing or etching, say) and mounted in any appropriate way. The input surface of each plate may also be given an anti-reflection coating (which is transparent to the input radiation but is reflective to the scintillator output), so ensuring that that output is emitted on the output side of the plate. For an X-ray scintillator a suitable anti-reflection coating is a few hundred - 500, say - Angstroms of a light metal such as aluminium evaporated into place.

The expression "optical fibre" is intended to cover the "wire" into which the original block is drawn down (whether clad or unclad) as well as the bundle of fibres formed therefrom by the subsequent stacking, binding and drawing operations. The "precursors" to the optical fibre are, primarily, both the original blo itself, and the general mixture! of core material and scilsl.illator material prior to its being shaped into that block. The fibre optic plate is, of course, the sawn lengths made from the subsequently-produced second stack (whether thereafter "finished" or note.

The material used for the fibre, and the material used for the scintillator, may be of almost any sort known or suggested for use for that purpose (naturally there may in each case be employed more than one such material - thus, a mixture of two or more core materials, and/or a mixture of two or more scintillator materials dispersed therewithin, but normally the fibre will essentially be one of each).Most conveniently, though, the Cor each) fibre material will be a plastics substance - a natural or synthetic resin - rather than a glass (it may be difficult to find a scintillator that is both compatible with the glass and table enough to be incorporated therewithin), and will be one that is easy to handle Cto shape and form - especially to draw) and having good optical transmission characteristics.A much preferred core material is in fact polyCmethyl methacrylate), referred to hereinafter as "pmna" The (or each) scintillator will advantageously be an inorganic material (organic scintillators are likely to be either incompatible with the core material or unstable under the forming conditions needed), and will of course be chosen for its ability to convert the relevant incoming radiation - X-rays, y-rays, ss-rays (electrons), UV, IR and so on - to visible light. A classic particle converter (recognised for its ability to convert electrons to visible light photons) is zinc silver sulphide, ZnS(Ag), a composition of zinc sulphide with around 0. 1 wt% silver, which has a conversion efficiency three times that of anthracene.A typical IR converter is doped strontium sulphide CSrS) - this can be optimised for IR up conversion. These materials will each conveniently be employed as a fine powder with a particle size of from 1 to 5 micron, and so will be dispersed, conveniently uniformly, within the core material as discrete particles.

The amount of scintillator material to be employed depends upon the particular scintillator and core materials employed (and the exact amounts best suited to each pair of materials can easily be determined empirically), but will generally be around 1 to 5% by volume of the core material. It should be noted, though, that because, unlike in present-day scintillator plates where a layer of scintillator powder is bound to the input surface, in the invention the cintillator material is dispersed much more "dilutedly" through the length of each fibre, so the conversion efficiency can be increased Cby adding more scintillator) without such a significant and undesirable increase in absorption, etc, of the emitted radiation (as the mathematics can show, in an optical fibre 10 microns in diameter containing dispersed therein a number of 5 micron scintillator particles, it would take eight such particles per unit area to block as much light as one per unit area in a conventional powder-coated plate) The incorporation of the scintillator material into the core material may be carried out at different stages in the forming of that latter material. For example, some suitable plastics polymers (such as pmma) are thermoplastics, that can be rendered fairly fluid by heating after polymerisation is complete, so that the scintillator can be incorporated into a melt of the core material.In other cases - and, indeed, in the case of pmma as well - it may be possible (and desirable) to add the scintillator to a pre-polymer mix rather than to the final polymer, and then to effect the polymerisation so that the scintillator ends up in situ within the polymer as it is formed. For pmma, then, the chosen scintillator can be dispersed within the methyl methacrylate monomer before it is polymerised.

At some stage in the preparation of the individual optical fibres it will be desirable, or even necessary, to provide the fibre with a suitable cladding. The choice of appropriate cladding materials, and their application to the core, is well understood, and need not be commented on further at this time, save to say that the core and cladding materials should have similar mechanical properties., particularly their thermal expansion coefficients and viscosity/temperature curves, so that the fabrication is as homogenous as possible. A typical cladding for pmma fibres is fluorinated pmma.

Manufacturing a fibre-optic plate from the formed optical fibres is by cutting, stacking, bonding, drawing, re-stacking, re-bonding, and sawing (and finishing), as outlined above. The details of this process are themselves well known in the Art, and for the most part need no further comment here. However, it may be worth saying a little about the finishing of the plate. The finishing may involve providing an anti-reflection coating (as mentioned hereinbefore) appropriate to the type of radiation being input to the plate. As explained, an X-ray scintillator might have a metal - aluminium - coating. An IR-scintillator, on the other hand, might have an interference coating, typically of magnesium fluoride CMgF2), of /2 Ck is the wavelength of the visible output radiation; a SrS IR scintillator is receiving IR light at a wavelength of around 1.2 micron, to which the coating is k/4, and thus transparent, and emitting visible light at around 0.6 micron) to which the coating is reflective.

A 5 mm (5,000 micron) thick plate made in this way, with the individual fibres thereof around 50 micron diameter and loaded with 5% scintillator, is equivalent in conversion efficiency to a single sheet device that is 250 micron (0.25 mm) thick - a fivefold improvement in resolution! It is also possible to use the super stack as an image conduit, or to effect some sort of image manipulation, rather than to cut it up into individual fibre optic plates. For example, if the super stack is itself carefully drawn out into an hour-glass, or apexto-apex double pyramid shape, that is then cut in half across the narrowest portion to form two "pyramids", then each such pyramid can be used as an image reducing or enlarging device (depending on which end is the input, and which the output).Again, if the super stack is carefully bent into a 90 (or other) curve then it becomes a conduit for channelling images around corners.

This type of image manipulation has, in fact, already been tried with "ordinary" glass fibre-optic super stacks (without scintillator material incorporated therein), and has a number of interesting uses.

Naturally, the invention extends to, at one extreme, a composition of a suitable core material having dispersed therein a scintillator material, and to, at the other extreme, a photon or particle counter, or an imaging device such a television or photographic camera with or without an image intensifier, constructed using a fibre-optic plate itself including optical fibres having a scintillator material dispersed within their core material.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying Drawings in which: The Figure shows a "cartoon" sequence depicting the formation of optical fibres containing dispersed scintillator material, and the construction therefrom of a fibre-optic plate.

The process depicted in the Figure speaks for itself. Briefly, however: A) A mixture of core material (10) and scintillator (11) is cast as a thick elongate rod (12).

B) This rod is heated and drawn, to produce a long thin "wire" (13).

C) After cladding (14), the wire 13 is cut into suitable lengths.

D) Many of these lengths 14 are bundled together to form a stack C15).

E) The stack 15 is bonded together by heat and pressure (P) in a mould (16).

F) The bonded stack 15 is then itself drawn into a "super wire" C17), and this too is cut into suitable lengths (18).

G) The lengths 18 are themselves bundled and bonded into a "super stack" (19).

H) The super stack 19 is sawn (20) into thin plates (21).

I) Each plate 21 is finished, and is then ready for use.

Claims (12)

1. For use in the construction of a scintillator device in the form of a fibre optic plate, an optical fibre, or precursor thereof, containing dispersed therewithin a scintillator material.

2. An optical fibre, or a precursor thereof, as claimed in Claim 1, wherein the core material used for the fibre is a natural or synthetic resin.

3. An optical fibre, or a precursor thereof, as claimed in Claim 2, wherein the core material is poly(methyl methacrylate).

4. An optical fibre, or a precursor thereof, as claimed in any of the preceding Claims, wherein the (or each) scintillator is an inorganic material.

5. An optical fibre, or a precursor thereof, as claimed in Claim 4, wherein the scintillator is zinc silver sulphide, ZnS(Ag), or doped strontium sulphide (SrS)

6. An optical fibre, or a precursor thereof, as claimed in any of the preceding Claims, wherein the amount of scintillator material employed is from 1 to 5% by volume of the core material.

7. An optical fibre, or a precursor thereof, as claimed in any of the preceding Claims and substantially as described hereinbefore.

8. For use in a scintillator device, a fibre optic plate made up from a multiplicity of side-by-side lengths of the optical fibres as claimed in any of the preceding Claims.

9. A fibre-optic plate as claimed in Claim 8, provided with an anti-reflection coating appropriate to the type of radiation being input to the plate.

10. A fibre-optic plate as claimed in Claim 9 and for use as an X-ray scintillator and incorporating an aluminium coating, or for use as an IR-scintillator and incorporating an interference coating of magnesium fluoride (MgFz).

11. A fibre-optic plate as claimed in any of Claims 8 to 10 and substantially as described hereinbefore.

12. A photon or particle counter, or an imaging device such a television or photographic camera with or without an image intensifier, constructed using a fibre-optic plate as claimed in any of Claims 8 to 11.