Adhesive plastic scintillator

Abstract: An adhesive plastic scintillator which can be attached onto a solid support medium, either by melting it in and/or on a solid support medium when the plastic scintillator is capable of being changed between solid and liquid phases, whichever is desired, by temperature control, or by deposition from a solution of said adhesive plastic scintillator. The plastic scintillator comprises fluorescent substances and optionally energy transfer compounds for converting radiation energy into light energy. The plastic scintillator remains transparent upon transition from the liquid to the solid state, and upon the same transition adheres to the solid support medium.

Description: Field of the Invention. The present invention relates to scintillation counting and more particularly to the integration of a plastic scintillator onto a support medium. Background of the Invention. Liquid scintillation counting and automated instruments known as liquid scintillation counters are widely utilized to analyze samples containing radioactively labelled substances. Typically, a sample in solution is mixed with a liquid scintillator, commonly referred to as a cocktail, and the light events produced from the sample and cocktail mixture are detected according to their energy and number of events. The light events occur when the energy of the particles, emitted from the radioactive isotope component of the sample in solution, is transferred to the molecules of liquid scintillator. This produces a light emission of a specific energy range which is characteristic of the radioactive isotope. Detecting both the energy and number of light events in a particular energy range provides the information necessary to construct a spectrum. Using this information the radioactive species can be quantitatively analyzed. Liquid scintillation counting and automated instruments to perform liquid scintillation counting have been widely discussed in a multitude of publications and patents. Scintillation counting of liquid samples has certain disadvantages attributable to the nature of the liquid solution used. One is a phenomenon known as quench. Quench commonly refers to an effect in the scintillation process of a chemical or optical nature which results in loss of light events or reduction in light emission energy. The chemical nature of the solution in which the sample and scintillator are mixed and the colour of the liquid sample solution are the causative agents. The result is inefficiency in the ability of the liquid scintillation counter to accurately count the particle disintegrations of the isotopes, and therefore interference with sample analysis. Another disadvantage is that after analysis the liquid produced by mixing the radioactive sample with the cocktail must be disposed of. The regulations and controls governing the disposal of liquid radioactive materials are particularly rigorous. Due to the volume of liquid radioactive materials that require disposal of, the costs can be considerable. In many cases a solid material having a radioactive nature is easier to dispose of and incurs less expense. Plastics are often used as such solid scintillation materials and previously mentioned in the literature are the thermosetting plastics which include polystyrene, polyvinyltoluene, various acrylic polymers and copolymers. The following patents further illustrate the use of plastics as scintillation materials. US Patent No. 3,010,908 issued Nov. 28, 1961 discloses the use of dialkylstyrene polymers as the primary absorber in a solid solution scintillation counting composition. US Patent Nos. 2,985,593 and 3,356,616 disclose styrene-derived monomers polymerized or copolymerized with vinyl or methacrylate monomers to form the solvent for a solid solution scintillation counting composition. US Patent No. 3,457,180 issued July 1969 discloses as the solvent for a solid solution scintillator copolymerized para-vinyltoluene and methylmethacrylate. US Patent No. 3,513,102 discloses a fluorescent coating in which a fluor and a copolymer of an acrylate and styrene are dissolved in an organic solvent, and the solution is emulsified in an aqueous dispersion of a hydrophillic colloid. The copolymer is not derived from a latex, but is a solution polymer isolated, redissolved and blended by high-speed milling for dispersion in a gel binder. US Patent No. 3,886,082 issued May 27, 1975 discloses an example of one such plastic scintillator material. The scintillator employs acrylic polymers and copolymers as the host plastic and bis(0-methylstyryl)-benzene, perylene, tetraphenylbutadiene, diphenyl anthracene, bis(-phenyloxazolyl benzene) and dimethyl bis(phenyl oxazolyl benzene) as the fluorescent additive. US Patent No. 4,180,479 issued December 25, 1979 discloses the use of various stilbene derivatives as fluorescent agents in scintillators. US Patent No. 4,495,084 discloses plastic scintillators in which a scintillating substance is incorporated into a matrix resin which comprises a copolymer of a styrene type compound and various unsaturated copolymers including unsaturated esters. US Patent No. 3,068,178 discloses plastic scintillators based on polystyrene and polyvinyltoluene. More recently there have been further advances in the field of solid scintillators. US Patent No. 4,713,198 describes the preparation of a poly-methylpentene thermoplastic scintillator capable of functioning at high temperatures. International Patent application WO 90/16002 describes a detection material that is solid at room temperature but optionally meltable to fluid. This material is composed of a low molecular weight plastic, a hot melt copolymer and a paraffin wax. US Patent Application 07/499,434 refers to a solid-liquid reversible scintillator used for solid support sample counting. This scintillator is composed of fluors, paraffin and p-xylene and is fluid above 40°C but reverts to a translucent waxy solid upon cooling. French Patent No. 1,590,762 describes the use of polyolefin resins and solvents to form gels which can be used as scintillation materials. These materials are solid-liquid reversible. International Patent application WO 89/02088 describes the use of an inorganic solid scintillator which is attached to a solid support medium by a binder material. US Patent No. 4,692,266 issued Sept. 8, 1987 describes a dry solid scintillator counting composition for the detection of radiative substances in a liquid. US Patent No. 3,491,235 describes a method for producing fluorescent layers by dispersing organic solution of fluorescent compounds in aqueous colloid solution, coating and drying. Japanese Patent Publication Sho 63-101787 describes multilayer scintillators made by piling up either mixed monomolecular films consisting of radiation absorbing compounds and compounds emitting ultraviolet, visible or infrared radiation, or monomolecular films consisting of radiation absorbing compounds and separate monomolecular films consisting of compounds emitting ultraviolet, visible or infrared radiation. The layers are deposited from a solution of the compounds in chloroform. US Patent No. 4,258,001 describes an element for analysis or transport of liquid, which contains a structure comprising a plurality of heat-stable, organo-polymeric particles non-swellable in and impermeable to the liquid, and an adhesive concentrated at particle surface areas contiguous to adjacent particles bonding the particles into a coherent, three-dimensional lattice that is non-swellable in the liquid. Interconnected void spaces among the particles provide for transport of the liquid. The prior art scintillators have the disadvantage that it is not possible to cohesively bond them onto plastic support media. Said plastic support media can be polystyrene, polyvinylchloride, polyethylene, polypropylene, other polyolefins, acrylonitrile copolymers and combinations of these. Said plastic support media can also be clear, translucent, white or black or a combination of these. The said plastic support media can be fabricated into microplates, petri dishes, culture flasks, test tubes and stand-alone single cups. As well as plastic support media, other media e.g. glass and metals are suitable host support media. Further disadvantages of the prior art scintillators are that they do not possess the properties necessary for producing a scintillating plastic coating on a solid support medium. Summary of the Invention. It is therefore an object of this invention to provide a plastic scintillator which can be attached from a liquid state onto a solid support medium. It is a further object of this invention to provide a plastic scintillator which can be melted in and/or on (abbreviated herein as melted-in-on) a solid support medium. It is a further object of this invention to provide a plastic scintillator which is capable of becoming fluid at a temperature low enough to prevent distortion of the solid support media. It is a still further object of this invention to provide a plastic scintillator which acts as a hot melt adhesive. A still further object of this invention is to provide a plastic scintillator which remains transparent upon transition from liquid to solid state. A still further object of this invention is to provide a plastic scintillator which has sufficient solubility for the fluorescent agents and thus demonstrates good scintillation properties. These objects are achieved by the articles of manufacture and methods for preparing an article of manufacture as set forth in the claims. Further, this invention relates to the use of alpha-methylstyrene polymer or alpha-methylstyrene/vinyltoluene copolymer or a (low molecular weight) styrene polymer to provide the scintillator plastic material which will melt-in-on a solid support medium. Primary, and optionally, secondary scintillating agents are added to the host plastic such as wavelength shifters and energy transfer compounds. This invention also relates to a method for producing a coating of the plastic scintillator composition, which comprises melting the host plastic, adding and mixing the fluorescent agents and any other additives while the composition is maintained at a temperature above the melting point, and cooling the composition. Furthermore the invention relates to a method for producing the melted-in-on plastic scintillator bonded to the solid support medium. This is effected by judicious selection of application temperature which ensures that the solid support medium retains its structural integrity. It is a further object of this invention to provide a plastic scintillator which can be attached or bound to a solid support medium from a solution in a suitable solvent. It is a further object of this invention to provide a plastic scintillator which is capable of dissolving in suitable solvent media. A further object of this invention is to provide a method of bonding/attaching the plastic scintillator to a solid support medium by contacting the solid support medium with a solution of the plastic scintillator in a suitable solvent followed by evaporation of the solvent. Furthermore, the solution of the plastic scintillator may also be applied by spraying, e.g. using conventional spray-can technology. The solid support medium can be made of plastic or any other suitable material, such as glass or metal. It is also possible to use plastic-coated or metal-coated support media. Description of the preferred embodiments. As indicated above this invention relates to plastic scintillators and a method of producing a melted-in-on coating, a coating from solution and a coating from spraying, onto a solid support medium. According to the present invention it has been found that alpha-methylstyrene/vinyltoluene copolymer finds particular advantage as a plastic scintillator because the material has a desirable melting point (75°C), retains its transparency upon transition from liquid to solid state, has good scintillation properties, is a hot melt adhesive and is soluble in organic solvents. Alpha-methylstyrene/vinyltoluene copolymer is a known thermoplastic and is available commercially. For example Piccotex 75 is available from Hercules B.V. in The Netherlands. Examples of suitable alpha-methylstyrene polymers are Kristalex F100 (having a weight-average molecular weight M w of 1400, a number-average molecular weight M n of 800 and a Z-average molecular weight M z of 2350) and Kristalex F85 (M w = 1200, M n = 750 and M z = 1950) from Hercules B.V. in The Netherlands. An example of a suitable low molecular weight styrene polymer is Piccolastic A75 from Hercules B.V. in The Netherlands. For use in the present invention the preferred embodiment is processed to provide the scintillation properties described. In the method for forming the product of the invention, the resin of the preferred embodiment is either subjected to an elevated temperature to melt it and form a plastic melt, or dissolved in a suitable solvent medium. Solvents suitable for such a medium include aromatic hydrocarbons, aliphatic hydrocarbons, chlorinated hydrocarbons, ethers, esters and nitroparaffins. Fluors useful in the successful practice of the present invention include any of the organic fluors well-known in the scintillation counting art which are compatible with the host plastic. Generally, suitable organic fluorescent compounds may be selected, for example, from those described as "organic fluors" and "organic scintillators" in Organic Scintillation Detection, E. Schram and R. Lombaert, Elsevier Publishing Co., 1963. Useful wavelength shifters (i.e. secondary fluors) are also well-known in the scintillation counting art. Preferred among these classes of materials are 2,5-diphenyloxazole (PPO) as the primary fluor and either bi(0-methylstyryl)benzene (bis-MSB) or 9,10-diphenylanthracene or 9,10-dimethylanthracene as the secondary fluor. The primary fluor is preferably present in the range 0.01 to 5.0 wt%. The secondary fluor is preferably present in the range 0.001 to 0.5 wt%. Energy transfer compounds which enhance the scintillation properties are optional and include compounds like mono- and dialkylnaphthalenes, naphthalene, anthracene, and durene. They are added in the range 0.01 to 15 wt%. After completion of the addition of fluorescent agents and the optional additives the melt is cooled to room temperature. The plastic scintillator can now be readied for integration onto the solid support medium in various ways. The solid plastic scintillator can be ground to a fine white powder or granulated into regular size granules. Alternatively the plastic scintillator can be hot melt extruded and chopped into regular sized pellets. A further alternative, for larger sized solid support media, is to re-melt the plastic scintillator and pour the plastic melt onto or into the support medium. For small support media the powdered or granular plastic scintillator is weighed into the desired receptacle and carefully heated to the required temperature which will be specific for each solid support medium. The plastic scintillator is maintained at this temperature for such a period as to ensure that all the plastic scintillator is transformed into a homogeneous melt. Upon cooling to ambient temperature the melted-in-on plastic scintillator is firmly attached to the solid support medium. A yet further alternative is the integration of the plastic scintillator onto the solid support medium using a solution of the plastic scintillator (in the range of from 5 to 80 wt%) in a suitable solvent medium. After dissolving the host plastic, the fluorescent agent and the optional additives in the desired solvent medium, the desired receptacle is filled with the plastic scintillator solution and then emptied again. A layer of plastic scintillator solution remains on the internal surface and as the solvent evaporates, a solid layer of plastic scintillator is attached to the internal surface. The present invention is illustrated in the following examples, where parts are by weight unless otherwise indicated. However, the invention is not to be considered as limited thereto. Example 1 The preferred embodiment is made by a straightforward melting, mixing and cooling process. To 100 gms of plastic (Piccotex 75) is added 0.9 gms 2,5-diphenyl oxazole (PPO) and 0.1 g bis(methylstyryl)-benzene (bis-MSB). This mixture is heated at 110°C-120°C for 1 hour. When the plastic melt has formed it is mixed thoroughly to ensure complete homogeneity. After mixing the plastic melt is cooled to ambient temperature and then broken down to a fine white powder. This procedure produces the plastic scintillator. Example 2 The plastic scintillator powder produced by the process disclosed in Example 1 is weighed into the wells of a polystyrene microplate. The polystyrene microplate is of the 96 well configuration and 50 mgm are weighed into each well. A Dynatech Microfluor 96 well plate obtained from Dynatech (USA) was used. This plate is made of polystyrene and is stable up to 85°C. After dispensing the plastic scintillator into each well the plate is heated at 80°C for 1 hour and then cooled to ambient temperature. Example 3 To 90 gms plastic (Piccotex 75) is added 10 gms 2,6-di-isopropylnaphthalene, 0.9 gms 2,5-diphenyloxazole and 0.1 gms bis-(methylstyryl)benzene (bis-MSB). This mixture is heated at 110°C-120°C for 1 hour. When the plastic melt has formed it is mixed thoroughly to ensure complete homogeneity. After mixing, the plastic melt is cooled to ambient temperature and then broken down to a fine white powder. This procedure produces the plastic scintillator containing the optional energy transfer compound. This plastic scintillator can be processed by the process disclosed in Example 2. Example 4 The plastic scintillator produced by the process disclosed in Example 1 can be heated to a plastic melt and used to replace the liquid or solid scintillator used in some large detectors. Upon cooling to ambient temperature the large detector is now allowed a greater degree of positional orientation than is possible with a liquid scintillator. A pourable plastic melt offers advantages over a cast or machined solid plastic scintillator. Example 5 To 61.4 gms plastic (Piccotex 75) is added 0.31 gms 2,5-diphenyloxazole, 0.06 gms bis-(methylstyryl)benzene and 38.25 gms n-heptane. This mixture is stirred at room temperature until all the components have dissolved. This procedure produces a plastic scintillator solution containing fluorescent agents. Example 6 The plastic scintillator solution produced by the process disclosed in Example 5 is dispensed into the wells of a polystyrene microplate which is of the 96 well configuration. Each well is filled with approximately 250 µl of plastic scintillator solution and then emptied again. The residual layer which adheres to the inner surface of the wells is allowed to stand for 2 hours at ambient temperature (20°C) during which time the solvent evaporates. The final traces of solvent are removed by further heating at 40°C for a further 2-4 hours. The invention has been defined in detail with particular reference to certain embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims: Article of manufacture for analyzing samples containing radioactively labelled substances, comprising a solid support medium and an adhesive plastic scintillator composition, wherein the plastic scintillator composition includes an adhesive resin and at least one fluorescent agent for converting radiation energy into light energy, obtainable by heating the plastic scintillator composition to a temperature not lower than the melting point of said composition so that said plastic scintillator composition is transformed into a homogeneous melt; attaching said melt to a solid support medium; and cooling said melt attached to said solid support medium so that the plastic scintillator composition is changed into a solid phase and forms a coating on the solid support medium. Article of manufacture, comprising a solid support medium and an adhesive plastic scintillator composition, wherein the plastic scintillator composition includes an adhesive resin and at least one fluorescent agent for converting radiation energy into light energy, obtainable by dissolving the plastic scintillator composition in a suitable solvent medium to obtain a transparent solution; adding the solution to a solid support medium; removing sufficient of the solution to leave a residual transparent layer; allowing said residual layer to solidify by evaporation of the solvent, whereby the plastic scintillator composition is attached to the solid support medium in the form of a transparent coating. The article of manufacture of claim 1, wherein the coating is transparent. The article according to any one of the claims 1-3, wherein the resin of the plastic scintallator composition is composed of styrene polymer, α-methylstyrene polymer or α-methylstyrene / vinyltoluene copolymer. The article of any one of the preceding claims, wherein the plastic scintillator composition comprises an energy transfer compound added to enhance the scintillation properties. The article of any one of the claims 1-5, being a microplate. A method for preparing an article of manufacture according to any of the claims 1 or 3-6, comprising a solid support medium and a coating of a plastic scintillator composition, comprising the steps of heating a plastic scintillator composition to a temperature not lower than the melting point of said composition so that said plastic scintillator composition is transformed into a homogeneous melt; attaching said melt to a solid support medium; and cooling said melt attached to said solid support medium so that the plastic scintillator composition is changed into a solid phase and forms a coating on the solid support medium. A method for preparing an article of manufacture according to any of the claims 2-6, comprising a solid support medium and a transparent coating of a plastic scintillator composition, comprising the steps of dissolving a plastic scintillator composition in a suitable solvent medium to obtain a transparent solution; adding the solution to a solid support medium; removing sufficient of the solution to leave a residual transparent layer; allowing said residual layer to solidify by evaporation of the solvent, whereby the plastic scintillator composition is attached to the solid support medium in the form of a transparent coating.

Inventors: THOMSON JAMES, THOMSON, JAMES
Applicants: PACKARD INSTR BV, PACKARD INSTRUMENT B.V.
Application year: 1993
Main classification: G01T1
IPC codes: G01T1