METHOD OF SCINTILLATION COUNTING USING A SOLID SCINTILLATOR

Abstract: A method of scintillation counting utilizes a solid scintillator dispersed on a counting support (16), wherein a sample material is deposited on the counting support over the solid scintillator to produce light emissions in response to particles emitted by a radio-active substance labelling said sample.

Description: METHOD OF SCINTILLATION COUNTING USING A SOLID SCINTILLATOR Field of the Invention This invention relates to measurement of a radioactively labeled sample material by an automated scientific instrument for radioisotope detection and measurement, such as for example a liquid scintillation counter, which detects light emissions generated from a scintillator material responsive to the radioactive substance labeling the sample. Background of the Invention Liquid scintillation counting and automated instruments known as liquid scintillation counters are widely utilized to analyze samples having radioactively labeled substances. Generally, a sample in solution is mixed with a liquid scintillator, commonly referred to as a cocktail, and light events produced from the sample and cocktail mixture are detected according to their energy and frequency. The light events are caused when particles, emitted from the radioactive isotope labeling a select substance of the sample in solution, are received by a molecule of liquid scintillator in solution. This produces a light emission having an energy characteristic of the radioactive particle received. Detecting the energy of the light events and number of light events in a particular energy range provides an assemblage of information known as a spectrum from which the select substance of the sample, that material which is radioactively labeled, 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 possesses some characteristic disadvantages due to the nature of the liquid solution which is being utilized; 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. It is in part due to- the chemical nature of the solution in which the sample and scintillator are mixed and in part due to the coloring of the liquid sample solution. The result is inefficiency in the ability of the liquid scintillation counter to accurately count the particle disintegrations of isotopes identifying the investigated material in the sample, thus, interfering with sample analysis. Another disadvantage is the use of "a liquid in which the radioactive sample and scintillator material, i.e., cocktail, are intermixed. Following analysis this solution and the vial in which it is held -must be disposed of. However, regulations relating to disposal of radioactive materials impact disposal and control the method in which disposal may be accomplished. These regulations are particularly rigorous for liquid radioactive materials. Generally, samples are of a sufficient volume that specialized disposal methods must be followed which are quite costly. In many cases a solid material having a radioactive nature is treated differently. Summary of the Invention A method of scintillation counting a radioactively labeled sample is presented which utilizes a solid scintillator and requires no liquid solution to mix a sample material and scintillator to perform scintillation counting. The solid scintillator is dispersed on a support material referred to herein as a counting support. The material of which the counting support is made may be a number of substances including glass, metal, plastics and fibrous materials such as those used in common filters including glass fibers compatible with the scintillation material and its process of deposition. The solid scintillator material is either adhered to, bonded to or embedded on or in the counting support so that it is uniformly dispersed across its surface area and securely held. A volume of sample material is deposited on the counting support over the solid scintillator and dried if in liquid form. The counting support containing the scintillator and sample material is then placed in a common scintillation counter to detect light events produced by interaction of the radioactive isotope labeling the sample with the scintillator. The information obtained is used to analyze the sample. Preferably the counting support is transparent so that light events can be detected by both of two photodetectors commonly used in a liquid scintillation counter, when counting support is interposed between them. However, the counting support may be opaque if it is positioned in a manner to permit light emission from the scintillator to be received by the photodetectors. Positioning of the counting support in the counting chamber of a scintillation counter has not been found to be critical in obtaining usable count data. Description of the Drawings Fig. 1 illustrates a counting chamber including two photodetectors found in a liquid scintillation counter in which a transparent counting support is interposed between the photo detectors. Fig. 2 illustrates a counting chamber including two photo detectors found in a liquid scintillation counter in which a typical vial for containing a liquid scintillation solution which has a solid scintillator bound to its interior surface is positioned within the chamber. Detailed Description of the Invention A typical liquid scintillation counter generally has a counting chamber in which a sample vial is placed for detecting light events emitted by the liquid scintillator/sample solution. At least two photodetectors, generally comprising photomultiplier tubes, are positioned to receive light emissions from the sample vial when it is placed in the counting chamber. This is generally shown in Figs. 1 and 2 by the photodetectors 10 and 12 directed to the interior of a counting chamber 14 which is sealed from ambient light. With the invention presented herein, a liquid scintillation counter is used to analyze a sample interacting with a solid scintillator which is supported on a sample counting support such as transparent support 16 shown in Fig. 1. A typical liquid scintillation counter having an ability to analyze a sample placed on a counting support with a solid scintillator as described herein, such as support 16, may be any one of a number of modern liquid scintillation counters manufactured by various companies such as Beckman Instruments, Inc., Packard Instruments, Inc., and others. As an example, a Model 3801 or 5801 Liquid Scintillation Counter presently manufactured by Beckman Instruments, Inc. of Fullerton, California is capable of performing scintillation counting according to the invention described herein. These instruments and the publications distributed by Beckman describing them, exemplify and describe the design and construction of an instrument necessary to perform this method. The scintillation counting method set forth herein comprises counting a radioactively labeled sample material by placing the sample material on a counting support which has a dry solid scintillator dispersed about its surface. The sample may be in a liquid, semi- liquid or dried form, however, it generally will be in solution with a volatile liquid and dried to leave only the solid elements of the sample remaining in intimate contact with the solid scintillator. This is important to obtain an efficient interaction between the particles of the radioactive isotopes labeling select sample material and the scintillator to produce light emission. After deposition of a sample onto the sample counting support, the counting support containing the solid scintillator and sample is placed in the counting chamber of a typical liquid scintillation counting instrument and detected in a manner in which common liquid scintillation analysis is performed, as exemplified by the above referenced liquid scintillation instruments. It is found that positioning of the counting support within the counting chamber of the liquid scintillation instrument is not a critical characteristic to obtain accurate analysis of the sample material. However, advantages may be obtained by placing the counting support within the chamber in a manner in which light emissions are directed toward the photodetectors comprising the liquid scintillation instrument, and in particular where the counting support is made of a transparent material advantages are obtained by interposing the counting support between the photo detectors. The counting support may be comprised of any material which is inert and nonreactive to the solid scintillator, to the materials utilized in the deposition of scintillator onto the support, and to the sample and sample carrier materials. Glass and fibrous materials commonly including glass utilized in laboratory filters are preferable. Plastics, metals and others are clearly adequate. The counting support illustrated in Fig. 1 is flat, such as a flat glass slide commonly used in laboratories. The shape of the counting support, however, is not a limitation in the scintillation counting method presented herein, and many differing shapes may be utilized for the counting support to provide various advantages in containing sample material or scintillator, or permitting easy placement within the liquid scintillation instrument counting chamber. The solid scintillator material dispersed on the sample counting support is generally deposited on the surface of the counting support by bonding or other methods of attraction which are appropriate for the selected solid scintillator and the material from which the sample counting support is made. The solid scintillator may, however, be embedded into the material from which the counting support is made if it is porous or it may be physically embedded onto the surface of the material from which the counting support is made, e.g., embedded in a filter material. Counting supports which may be utilized with the method of scintillation counting presented are described in a copending patent application filed concurrently herewith for SAMPLE COUNTING SUPPORT WITH SOLID SCINTILLATOR FOR USE IN SCINTILLATION COUNTING in the names of Stephen W. Wunderly and Joseph F. Quint, the inventors. Typical solid scintillators which are deposited on ' a counting support are any one or combination of the following: calcium fluoride doped with europium, CaF2(Eu), zinc sulfide doped with silver ZnS(Ag), yttrium silicate doped with cerium Y2Siθg (Ce), lithium glass doped with cerium, anthracene,. PPO, . (2,5-diphenyloxazole) polystyrene or poly(vinylaryl) functions doped with common liquid scintillation fluors. Other solid scintillation materials may be utilized and the method described herein is not limited to any select solid scintillation compound. Use of the smallest size particles of solid scintillator, preferably in a range of 2-10 microns, obtains the best counting efficiency. A solid scintillator may be deposited on the sample counting support by preparing a slurry of a solid scintillator and a binding agent in water. For example, 20 milligrams of solid scintillator may be mixed with 5 milligrams of binder in 10 milliliters of water. The binder may be any number of substances such as, for example, a binder used to bind silica gel to hard surface backings. The slurry of solid scintillator and binder may be coated on the surface of the counting support to obtain a proper thickness of scintillator when dried. For example, a thickness such as the final weight of solid scintillator and binder is 70 milligrams, plus or minus 50 milligrams over a 25 millimeter diameter area is adequate. Deposition of the solid scintillator on the support should be as uniform as possible and the particle size of the solid scintillator should be as fine as possible. Fig. 2 shows the use of a typical sample vial used in liquid scintillation counting, which has had the interior surface, either the floor , walls or both, coated with a solid scintillator into which a sample may be placed to obtain light events for detection. Sample material is generally mixed in a solution with volatile material which readily evaporates or vaporizes so that the sample material, which must be nonvolatile, comes into intimate contact with the dry solid scintillator when it is dried. This provides highly efficient interaction between the radioactive isotope labeling the sample material and the solid scintillator. Sample material may be deposited directly onto the surface of the sample counting support having the solid scintillator deposited thereon. Sample deposition can be accomplished either by spotting, dabbing, blotting or absorbing the sample material directly on one or both sides of the counting support. A simple pipetting technique would be used. • In the case in which the counting support is made using a filter material sample deposition is accomplished by utilizing the counting support in a standard filter -process having the sample precipitate captured on the surface of the filter while sample solution is passed therethrough. The sample counting support with sample would then be dried before counting. Once sample has been deposited on the sample counting support and the volatile carrier solvent removed, the counting support then is placed into the counting chamber of a typical liquid scintillation counter. A scintillation counting process is performed on the counting support under normal conditions and as the liquid scintillation counter would normally operate, with the exclusion of any quench measurement or compensation operations. The liquid scintillation counter should have its coincidence gate circuitry modified to vary the coincidence time period or. window to match the larger decay times generally exhibited by solid scintillators as compared to liquid scintillators. This circuitry permits light events to be measured only when more than one photodetector in a multiple photodetector system receives a light emission within the time period a coincidence window specified. • The time period of a coincidence window should approximate the decay time of the particular solid scintillator used, and may be optimized by optimizing the function E 2 /B, where E = counting efficiency and B = background. The equation E B is a measure of the effectiveness of a scintillation counting instrument to measure disintegrations of the radioactive isotope labelling the sample material. As an example, if ytrium silicate is used as a solid scintillator, the coincidence gate period may be 50-100 nanoseconds, preferably 100 nanoseconds. If PPO were utilized as a solid scintillator the coincidence gate period may be 5 nanoseconds. Use of this method of scintillation counting has been shown to be as efficient as common liquid scintillation counting, and without the detrimental effects of quench found in a liquid scintillation sample/scintillator mixture. Additionally, the sample counting support may be easily disposed of. Since the volume of radioactive material it contains after analysis is very small and when it is not controlled. It may be thrown away. This eliminates most disposal costs associated with scintillation counting analysis. Furthermore, the sample may be recovered from the sample counting support for further analysis.

Claims: What is claimed is: 1. A method of counting activity of a radioactively labeled sample material with an automatic scintillation counter comprising the steps of: first, placing a discrete sample having a radioactively labeled sample material on a sample counting support having a solid scintillator dispersed about the surface of said support, said solid scintillator being responsive to particles emitted by the radioactive substance labeling said sample to produce light emissions in response thereto, such that emitted particles are received by the solid scintillator and light emissions are produced having an energy characteristic of said particles, second, positioning said counting element in a counting chamber of a liquid scintillation counter, in the presence of photodetectors responsive to light emissions produced by said solid scintillator, measuring the activity of said labeled sample material with said liquid scintillation counter by detecting the frequency and energy of light emissions from said solid scintillator. 2. A method of counting activity of a radioactively labeled sample material with a liquid scintillation counter comprising the steps of: first, placing a discrete sample having a radioactively labeled sample material in a sample counting vial having a solid scintillator dispersed about the interior surface of said vial including the floor of said vial, said solid scintillator being responsive to particles emitted by the radioactive substance labeling said sample to produce light emissions in response thereto, such that emitted particles are received by said solid scintillator and light emissions are produced having an energy characteristic of said particles, second, positioning said counting support in a counting chamber of a liquid scintillation counter, in the presence of photodetectors responsive to light emission produced by said solid scintillator, measuring the activity of said labeled sample material with said liquid scintillation counter by detecting the frequency and energy of light emissions from said solid scintillator. 3. The method of claim 1 in which said sample counting support has solid scintillator dispersed in a thin layer about the surface of said support. 4. The method of claim 2 in which said counting vial has solid scintillator dispersed in a thin layer about the floor of said vial. 5. The method of claim 1 in which said radioactively labeled sample is spotted on said sample counting support. 6. The method of claim 4 in which said radioactively labeled sample is spotted on the floor of said counting vial. 7. The method of claim 1 in which said radioactively labeled sample material is deposited on said sample counting support by a filtration process in which said counting support comprises a filter material through which the solution containing the sample material is flowed. 8. The method of claim 1 in which said counting support is transparent. 9. The method of claim 8 in which said sample counting support is positioned in said counting chamber such that it is interposed between photodetectors. 10. A method of counting activity of a radioactively labeled sample material with a liquid scintillation counter comprising the steps of: placing a discrete sample having a radioactively labeled sample material on a porous sample,counting support having a solid scintillator embedded within said support, said solid scintillator being responsive to particles emitted by the radioactive substance labeling said sample to produce light emissions in response thereto, such that emitted particles are received by said solid scintillator and light emissions are produced having an energy characteristic of said particles, positioning said counting element in a counting chamber of a liquid scintillation counter in the presence of photodetectors responsive to light emissions produced by said solid scintillator, measuring the activity of said labeled sample material with said liquid scintillation counter by detecting the frequency and energy of light emissions from said solid scintillator.

Inventors: QUINT JOSEPH F, WUNDERLY STEPHEN W, QUINT, JOSEPH, F., WUNDERLY, STEPHEN, W.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1988
Main classification: G01T1
IPC codes: G01T1