METHOD AND APPARATUS FOR DETECTING RADIOISOTOPES IN FLUIDS

Abstract: Apparatus for detecting radioisotopes in fluids (e.g. in gas/liquid chromatography) comprises a bundle of scintillating optical fibres (12) extending along the interior of a conduit (10), along which a fluid may flow, and having the ends (14, 16) thereof passing through the side of the conduit (10) to light detecting means (18, 20). Other embodiments and methods of use are described.

Description: "Method and Apparatus for Detecting Radioisotopes in F ui ds" Description The present invention reLates to the detection of radioisotopes in fluids and is appli cable, for example, to any requirement for measuring the time of arrival of a radi oa ct i ve Ly LabeLLed sLug of fLuid at a fixed point in a system. Such a requirement exists in the fieLd of gas/Liquid chromatography and Fig. 1 schematicaLLy i LLustrates a conventional radioisotope detection technique employed in known chromatography columns. Such columns are widely used to analyse radioisotope- labelled molecules (such as proteins) contained in a liquid carrier flow on the basis of their time of transit down a long narrow conduit 2. After transit down the conduit 2 the different molecular species arrive at a detector zone 4 at different times where they are monitored by a scinti llation counting technique. In known columns the detector zone 4 comprises a packed 3+ powder of crystalline phosphor (commonly P47 Ce doped yttrium si licate) or powdered scinti l lating glass (ie. glass doped with scinti lla ing material) . The light output from the detector zone 4 is monitored through the glass wall of the conduit 2 by a phot omu 11 p 1 i e r 6 or the li ke. Disadvantages associated with this technique are, fi rstly, that the powder packing density and powder particle size distribution are limited by the need to avoid clogging up the column or creating a back pressure and, secondly, that much of the emitted light from the detector zone 4 is lost by internaL scatter and seLf-absorpt i on within the powder or gLass grains, so reducing the sensitivity of the apparatus. It is an object of the present invention to obviate or mitigate the aforesaid disadvantages. According to a first aspect, the invent on provides a method of detecting radioisotopes contained within a fluid, wherein at least one scintillating optical f bre is i mersed within sa d fluid and the light e itted from at least one end of said fibre is monitored by light detect ng means. Preferably, a plurality of fibres is used and the light emitted from each end of the fibres is mom " tored . Preferably also, the diameter of the fibres is selected according to the type and energy of radiation emitted from the rad oisotope being used and the scintillating mater al with which the fibres are doped is selected to suit the spectral characteristics of the light detecting means. The fibres are preferably also unclad. The fluid is preferably also flo " wed pa ' st the fibres and fibres are pre'ferab.l-y or ented either substant ally along the direction of flow of the fluid or transversely to said direction of flow. Preferably also, light detecting means are pro ded at either en ~ d of the fibres. The " outputs from the light detecting means may be summed to g ve an output of amplitude proportional to the amount of radioacti ity in contact with the fibres, or, in the case where the fibres are oriented transversely to the direct on o*f fluid flow, the outputs from the light detecting means are used to determine the transverse position from which a pulse of light is emitted. According to a second aspect, the invention provides an apparatus for detecting radioisotopes ' contained within a fluid comprising at least one sc nt llating optical fibre having at least one end thereof coupled to light detect ng means. Preferably, said apparatus includes a plurality of fibres coupled to said light detecting means. Preferably also, both ends of said- fibres are coupled to light detecting means. Alternati ely, one end of said fibres is coupled to light detecting means and the other end thereof is mirrored. Preferably also, said fibres are unclad and the ends thereof are fused together. Preferably also, the outputs from said light- detecting means are connected to summing means. Preferably also, said fibres are Located within a conduit along which said fluid is flowed, the ends of said fibres passing through the walls of the conduit and coupled to said light detecting means externally thereof. Preferably also, the fibres within said conduit are oriented substantially along the di rection of flu-id flow. Alternatively, the fibres are oriented substantially transversely to said direction of fluid flow. Embodiments of the invention wi ll now be described, by way of ex-ample only, with reference to the accompanying drawings, in which:- Fig. 2 is a schematic drawing of apparatus embodying the present invention; and Fig. 3 is a schematic drawing of an alternative embodiment of the invention. Referring now to Fig. 2, a carrier fluid containing radioisotopes is passed along a conduit 10 which might form part of a chromatography column or the like, past detector means comprising a bundle of optical f bres 12 doped with scinti llating material. The bundle of fibres 12 extends along part of the Length of the conduit 10 and the ends 14, 16 thereof pass through the wall of the conduit 10 and are coupled to Light detecting means such as photomu Lt i p I i e r s 18, 20 using optical coupling fluid. The ends 14, 16 of the bundle 12 are preferably fused together ^ to simplify installation in the wall of the conduit 10 and to eliminate leakage problems. ' The electrical output signals from the photomu Iti pI ers 18, 20 may be combined by a summation amplifier 22. Thus, as a slug of fluid containing radioisotopes passes the bundle of fibres 12, the radiation emitted therefrom will enter the fibres 12 and cause ' the scintillating material to fluoresce. The light thus emitted will be channeled along the fibres 12 to the photomu Iti pI i e rs 18, 20 so that the output from the summation amplifier 22 w ll be directly proportional to the radioactivity of the fluid nstantaneously in contact w th the fibres12. Optical fibres are normally clad with a layer of material of lower refractive index than the fibre itself in order to ensure total internal reflection of the l ght pass ng along the fibre..- In the present application, however, the refractive index of the fluid in contact with the fibres 12 will always be less than that of the fibres 12. For this reason total nternal reflection of the light emitted w thin the fibres 12 will occur n any case, so that the usual cladding may be omitted. This is particularly desirable in the case of low-energy radioactive labelling isotopes such as carbon - 14 (decay energy: 0.156 MeV) or tr tium - 3 (decay energy: 0.018 MeV) since any cladding would absorb most of the radiation before reaching the scintillating material of the fibres 12, thereby reducing the sensiti ity of the detector. Fibres for use with radioisotopes of higher energy, above, say, 0.5 MeV (such as phosphorus - 32 - decay energy: 1.710 MeV), may be clad if desired since proportionately little energy loss will occur in the cladd ng. The provision of a photomu 1t p Ii e r 18, 20 at each end of the bundle 12 ensures that all of the light emitted within the fibres is collected and so the sensitivity of the detector maximised. One of the photomu 1t i pi i ers 18, 20 could, however, be omitted, and those ends of the fibres 12 mirrored. In the embodiment of Fig. 2, the orientation of the fibres 12 along the length of the conduit 10 provides high sensitivity to low levels of radiation, since a given slug of fluid wi ll be in contact with the fibres 12 over an extended period of time, however the positional resolution along the axis ' of the conduit 10 wi ll be relatively poor. Fig. 3 shows an alternative configuration wherein the fibres 24 are arranged in a transverse plane across the conduit 26. In this case the positional resolution of the system wi ll be enhanced but its sensitivity wi ll be relatively poorer. Numerous other configurations are, of course, possible including helical arrangements, lattices etc. The detectors of Figs. 2 and 3 may be applied advantageously in chro atographic analysis, replacing the packed powder of Fig. 1. Sensitivity is improved since all the light. emitted in the fibres is coupled to the external detectors so t hat losses by scattering and absorption are eliminated. Additionally, there is intimate contact between the fluid and the fibres and there is minimal disturbance of the fluid flow along the conduit. The creation of back pressure in the conduit is also avoided. The scinti llating material with which the fibres are doped may be selected to emit in the spectral bands best suited to the characteristics of the external- phot odetec tors . For example, cerium wi ll emit in the 3+ 3+ . blue, terbium in the yel low and europium in the red. The diameter of the fibres themselves may also be selected according to the type and energy of the radiation emitted from the particular radioactive label to be used in order to ensure that substantially all the radiation is absorbed within the fibre. For example, with the beta-emitting radioisotope phosphorus-32 a diameter of approximately 1 mm will give virtually 100% absorption. The present technique is applicable to any requirement for measuring the time of arrival of a radioactively labelled slug of fluid at a f ed point in a system, gas/liquid chromatography merely being an example of one such application. The transverse arrangement of the fibres 24 in Fig. 2 having photomuIti pi ers 28, 30 at either end thereof might also be used to monitor the transverse position of a labelled slug of fluid arriving at the detector using anysuitable technique such as time-ampl tude-conversion since light emitted from a point along the length of the fibres 24 will arrive at the opposite ends thereof at different times. Such an ' arrangement might thus be used in fl ' ow-prσfile analysis currently requiring complex and expensive laser anemo etry techniques. The fibres themselves would normally be of glass which is resistant to chemical attack in fluids other than hydrofluoric acid and will withstand temperatures of at least 300 C. Fibres formed from plastics materials may also be used in some applications, but their suitab lity is limited by their poorer resistance to heat and chemical attack, Lσwer refractive index and generally poor optical qualities. The evolution of gases from plastics materials may also be undesirable in ultra-clean systems. The invention thus prov des a method and apparatus for detect ng radioisotopes in fluids having significant advantages over existing techniques as noted above. The scintillating materials from which the fibres are formed will detect all forms of radioactive labelling including alpha, beta and gamma radiation, X-rays and neutrons. They also offer the potential for a high count rate capability; for example, the decay time of a luminescent pulse from cerium is 200 nanoseconds so that the -9 -1 potential count rate is 1/(200x10 ) seconds or 5 MHz.

Claims: C l a s 1. A method of detecting radioisotopes contained within a fluid wherein at least one scinti llating optical fibre is immersed within said fluid and the light emitted from at least one end of said fibre is monitored by light detecting means. 2. A method as claimed in claim 1 wherein a plurality of fibres is immersed in said fluid. 3. A method as claimed in claim 1 or claim 2 wherein the light emitted from each end of said fibre or fibres i s moni tored . 4. A method as claimed in any of claims 1, 2 or 3 wherein the diameter of sa d fibre or fibres is selected according to the type and energy of radiation emitted from the radioisotope being used such that substant ally all of the rad ation is absorbed within the fibre. 5. A method as claimed in any preceding claim wherein the s int llating material with which the fibres are doped is selected to suit the spectral characteristics of the light detecting means. 6. A method as claimed in any preceding claim wherein said fibres are unclad. 7. A method as claimed in any preceding claim wherein the flu d is flowed past said fibres. 8. A method as claimed in cla m 7 wherein said fibres are oriented substantially along the direction of flow of said fluid. 9. A method as claimed in claim 7 wherein said fibres are oriented substantially transversely to the direct on of flow of said fluid. 10. A method as claimed in any preceding claim wherein light detecting means are provided at either end of said fibres and wherein the outputs from each of said light detecting means are summed to give an output of amplitude proportional to the amount of radioactivity in contact with the fibres. 11. A method as claimed in claim 9 wherein light detecting means are provided at either end of said transversely oriented fibres and wherein the outputs of said light detecting means are used to determine the transverse position from which a pulse of light is emitted. 12. Apparatus for detecting radioisotopes contained within a fluid comprising at least one scinti llating optical fibre having at least one end thereof coupled to light detecting means. 13. Apparatus as claimed in claim 12 including a plurality of fibres coupled to said light detecting means . 14. Apparatus as claimed in claim 12 or claim 13 wherein both ends of said fibres are coupled to light detect i ng .means . 15. Apparatus as claimed in claim 12 or claim 13 wherein one end of said fibres is coupled to light detecting means and the other end thereof is mirrored. 16. Apparatus as claimed in any of claims 13 to 15 wherein the ' ends of said fibres are fused together. 17. Apparatus as claimed in any of claims 12 to 16 wherein said fibres are unclad. 18. Apparatus as claimed in claim 14 wherein the outputs from said light-detecting means are connected to summi ng means . 19. Apparatus as claimed in any of claims 12 to 18 wherein said fibres are located within a conduit along which said fluid is flowed, the ends of said fibres passing through the walls of the conduit and coupled to said light detecting means externally of said conduit. 20. Apparatus as claimed in claim 19 wherein the fibres within the conduit are oriented substantially along the direction of fluid flow. 21. Apparatus as claimed in claim 19 wherein the fibres within the conduit are oriented substantially transversely to the direction of fluid flow. 22. A method of detecting radioisotopes substantially as hereinbefore descr bed with reference to the accompanying drawings. 23. Apparatus for detecting radioisotopes substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: SPOWART ALEXANDER ROLLO, SPOWART, ALEXANDER, ROLLO
Applicants: SPOWART ALEXANDER ROLLO, SPOWART, ALEXANDER, ROLLO
Application year: 1988
Main classification: G01T5
IPC codes: G01N30, G01T1, G01T5, G01T7