METHOD OF MEASURING RADIATION INTENSITY

Abstract: A method of measuring radiation intensity which com­ prises steps of; placing a sample containing a radioactive substance in a radiation-measuring container with a built-in stimulable phosphor for a given period of time to cause said container to absorb at least a portion of radiation energy emitted by said radioactive substance; irradiating said container with an electromagnetic wave to release the radiation energy stored in said container as stimulated emission; and photoelectrically detecting the stimulated emission to measure radioactivity of said sample. The radiation-measuring container having a built-in stimulable phosphor is also disclosed.

Description: METHOD OF MEASURING RADIATION INTENSITY AND RADIATION-MEASURING CONTAINER USED THEREFOR BACKGROUND OF THE INVENITON FIELD OF THE INVENTION The present invention relates to a method of measuring intensity of a radiation emitted by a radioactive substance and a radiation-measuring container used therefor, and more particularly, to a method of measuring radiation intensity by utilizing a stimulable phosphor and a radiation-measuring container used therefor. DESCRIPTION OF THE PRIOR ART Heretofore, as methods of measuring the intensity of radiation of a radioactive substance (substance containing a radioisotope), namely radioactivity thereof, there are known, for example, scintillation counting methods using a substance (so-called scintillator) capable of absorbing a portion of radiation energy emitted by the radioactive substance and emitting light (fluorescence). In this method, the radioactivity of radioactive substance is measured by causing the scintillator to absorb a radiation emitted by the sample, detecting light emitted by the scintillator by means of a photomultiplier and counting it as electric pulse. For example, there is well known a liquid scintillation couting method wherein a liquid scintillator composed of a solute (fluorescent agent) dissolved in an organic solvent is introduced into a liquid sample to detect radioactivity of the liquid sample. In the case of measuring a low-energy radiation such as a-rays or rays using the above-mentioned scintillation counting method, the radiation emitted by a sample is measured by mixing (dissolving or suspending) the sample with the liquid scintillator in a measuring container. On the other hand, in the case of measuring a highenergy radiation such as y-rays, the radiation emitted by a sample is usually measured by placing the scintillator outside the sample container. As a special measuring container (namely vial) for y-ray measurement, there has been developed a measuring container for counting liquid scintillation, wherein the liquid scintillator is sealed up in the side and bottom walls of the container. The scintillation counting method is applied, for instance, to the measurement of radioactivity of the tissue of an organism and/or substances originating from organism which contains a radioactive substance. It is generally difficult to dissolve such a sample in a solvent so that the sample is burnt in an oxygen-containing atmosphere to convert 14C in the sample into 14002 which is then absorbed by an appropriate absorbent, and a liquid scintillator is introduced thereinto to measure radioactivity of the sample. The measurement of 14C02 produced by a chemical reaction such as an enzyme reaction is also conducted in the same manner as mentioned above. In a radio-gas chromatography, the radioactivity of a sample is measured after dissolving the collected gaseous sample in a solution containing a scintillator. When the sample is a solid sample from which a strong radiation such as y-rays radiates, the radioactivity of the sample is measured using a well-type scintillation counter composed of a combination of a solid scintillator with a photomultiplier and fixing the sample together with the container in the cavity (well) of the scintillator. By means of this well-type scintillation counter, the measurement of radioactivity of a sample in the form of a liquid or a gas sealed in a tube can be conducted in a similar manner. As described above, the scintillation counting method is a useful means for measuring the radioactivity of samples. Particularly, the liquid scintillation counting method has many advantages in that even when a radiation from a radioactive substance is a feeble one such as arays or rays, the radioactivity thereof can be measured, so that said method is widely used for the measurement of radioactivity of samples. The mechanism of scintillation in the liquid scintillation counting method is described as follows: A solvent molecule dissolving a solute (fluorescent agent) is initialy excited by a radiation emitted by a radioactive substance in a liquid sample, and then a solute molecule (scintillator) is excited through impingement of the excited solvent molecule on the solute molecule, or the like. In the course of transferring of the radiation energy from the solvent molecule to the solute molecule, there occur other phenomena such that the energy is transferred between the solvent molecules by interaction between the solvent molecule in the excited state and that in the ground state, or that the energy is transferred from the solvent molecule in the excited state to the other solute molecule than a scintillator by interaction therebetween before the scintillator is excited. The transfer of energy takes place not only through the interaction between molecules such as impingement, but also through such a phenomenon that the scintillator absorbs light emitted by the excited solvent molecule or other excited solute molecule. However, in the course of the energy-transfer procedure, there also occurs such a quenching phenomenon that the excitation energy is absorbed by a portion of the solvent molecules or other solute molecules so as to be converted into heat, etc., or that the light emitted by the scintillator is absorbed by a light-absorbing substance contained in the sample. The liquid scintillator, which is essential to said liquid scintillation counting method, is expensive and has to be isolated and refined to re-use it. Usually, it is difficult to recover the scintillator in a high purity so that it is not generally re-used and the measuring cost is increased thereby. Further, there are problems in handling thereof. For example, the used scintillator containing the radioisotope may cause a problem in the disposal stage. Other problems reside in that the solvents allowed to use in conjunction the solutes (fluorescent agent) are generally limited to certain organic solvents so that there is difficulty in choosing a solvent for a sample and specific procedure has to be taken in preparing a sample in the case that the sample is sparingly soluble in the solvent. Since the scintillation mechanism of the liquid scintillator is complicated as stated above, the counting efficiency (that is, intensity of radioactivity to be detected) is apt to decrease by the quenching effect due to impurities contained in a sample or the sample per se. For example, light emitted by the scintillator is liable to be quenched by the oxygen dissolved in the liquid scintillator, or to be absorbed (i.e. quenched) by a colorant in the case that the sample solution is colored thereby. In the case that a sample is sparingly soluble, it is not easy to prepare a sample solution in a homogeneous phase, and the unhomogeneous phase thereof causes internal absorption of a radiation emitted from the sample. For this reason, it is necessary to accurately determine the counting efficiency of the sample by making correction for quenching caused by the above-mentioned various phenomena, and this makes measuring operation complicated. Further, there is a disadvantage that it is substantially difficult to accurately measure the radioactivity of the sample even if the correction for quench ing is made. In order to prevent the counting efficiency from being lowered by quenching effect of contaminants, impurities and colored substances contained in the sample, the sample has to be carefully prepared and high skill and much experience are required for operators. The pretreatment of the sample to remove the contaminants is of importance to the measuring operation. In the conventional scintillation counting method, the measurement of radioactivity of a sample is conducted by a real time operation. Namely, it is necessary to continuously measure light emitted by the scintillator for a given time (for example, for several minutes to several ten minutes) after the sample is introduced into the scintillator solution. If the intensity of radiation from the sample is low, the measuring time (i.e., counting time) extends for a long time so that it can be hardly said that the measuring efficiency and the handling efficiency of measuring apparatus are sufficiently high. Therefore, in the case that a great number of samples are involved, for instance, the case that a great number of samples obtained from liquid chromatography are involved, it is difficult to treat such a great number of samples once and the waiting period accordingly extends for many hours so that it disadvantageously takes a long time until the results are be obtained. Particularly, in the case that the half-life of radioisotope in the sample is short, it becomes difficult to measure the radioactivity thereof. Further, in the case that the radiation intensity is low, the measurement becomes more difficult. This means that an apparatus (for example, dark current drift of photomultiplier) to be used must be stable over a long period of time. To keep the stability of apparatus, the apparatus becomes expensive, or much skill and experience to adjust the apparatus are required. SUMMARY OF THE INVENTION The present inventor has made studies to solve the above-mentioned problems associated with the conventional scintillation counting method, particularly the liquid scintillation counting method and discovered that said problems can be solved or reduced by using a measuring container with a built-in stimulable phosphor in measuring the intensity of radiation from a radioactive substance in a sample. The present invention provides in one aspect a method of measuring radiation intensity which comprises steps of: placing a sample containing a radioactive substance in a radiation-measuring container with a built-in stimulable phosphor for a given period of time to cause said container to absorb at least a portion of radiation energy emitted by said radioactive substance; irradiating said container with an electromagnetic wave to release the radiation energy stored in said container as stimulated emission; and photoelectrically detecting the stimulated emission to measure radioactivity of said sample. The present invention provides in another aspect a radiation-measuring container used for carrying out the above-mentioned method. BRIEF DESCRIPTION OF DRAWINGS Figures l-(a) to 1-(e) are vertical cross-section views diagrammatically showing embodiments of the measuring container with a built-in stimulable phosphor according to the present invention. DETAILED DESCRIPTION OF THE INVENITON The stimulable phosphor used in the present invention has the property of emitting light (giving stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after absorbing a radiation. Accordingly, the radioactivity of a sample can be measured in such a manner that a measuring container with a built-in stimulable phosphor is caused to absorb a radiation radiated from a radioactive substance in a sample, and then the container is irradiated with an electromagnetic wave (stimulating rays) such as visible light or infrared rays to release the radiation energy stored in the container in proportion to the applied radiation dose'as light emission (stimulated emission), which is photoelectrically detected and converted into an electric signal. According to the method of the present invention, the conventional scintillator is not required in the measurement of the radioactivity of a sample, and the operation of mixing (dissolving or suspending) the scintillator with the sample in a container is not either required, although said operation has to be performed in the conventional liquid scintillation counting method. In the method of the present invention, the measurement can be made merely by introducing the sample containing a radioactive substance into a measuring container having a built-in stimulable phosphor so that it is not necessary to separate the phosphor from the sample and to refine it after use. Accordingly, the measuring container of the present invention can be repeatedly used and the cost for one measurement can be reduced. The measuring container is very easy to handle, because it can be made of plastic material etc. A solvent is not necessarily employed in the present invention, but if desired, an appropriate solvent can be employed, which is different from the conventional liquid scintillation counting method. Accordingly, the selection of a solvent and the preparation of a sample solution required in the use of the liquid scintillator are not always required in the method of the invention. The sample is not necessarily dissolved in a solvent to prepare a liquid sample, and even when the sample is in the form of a solid or a gas, the radioactivity thereof can be easily measured by introducing the sample into the measuring container. Further, the above-mentioned quenching phenomena, particularly such as the phenomenon of quenching of the emitted light does not occur in the present invention. Hence, it is not necessary to make complicated quenching correction (determination of counting efficiency) for measuring the radioactivity of the sample, and the radioactivity thereof can be accurately measured without being greatly influenced by measuring conditions, etc. Accordingly, the measuring operation is simplified in this respect. Since it is not in particular necessary to remove impurities, etc. contained in the sample, the pretreatment of the sample as stated hereinbefore is not required, and much skill and attention based on experience are not required in the course of the preparation of the sample. The measurement of the radioactivity of the sample can be easily made still in this respect. The measurement of radiation intensity in the present invention basically comprises a step of storing radiation energy from a sample in the measuring container with a built-in stimulable phosphor, and a step of detecting stimulated emission corresponding to the radiation energy stored in said container. It is also possible to completely separate these two steps from each other. Storing the radiation energy in the measuring container is conducted by placing the sample in the container for a certain period of time, so that accuracy of measurement can be increased by choosing preferred storing time. Reading out the stimulated emission is conducted by irradiating the measuring container with an appropriate light (light having a wavelength within the region of stimulation wavelength for the stimulable phosphor enclosed in the container) and causing the stimulable phosphor to emit light. By counting the amount of the emission, the intensity of radiation from the sample can be determined. Since the stimulable phosphor instantaneously emits light upon irradiation with stimulating rays, the period for photometric measurement of the stimulated emission (i.e. measuring time of radiation intensity) can be set independently of the radiation intensity, so that the measuring time can be greatly shortened. For example, a period of several seconds to several ten seconds is sufficiently employed as the measuring time. Therefore, a great number of samples can be measured in one lot so that the working efficiency of the appara tus is increased, and the number of the measurement per a unit time can be increased. Further, the storing operation for a great number of samples can be simultaneously made by using a plurality of the measuring containers, that is, the measurement of these samples can be made under the same conditions. This means that the radioactivities of a great number of samples can be measured with high accuracy under the same conditions even when a radioisotope having a short half-life and a feeble radiation is used. Further, when a plurality of the measuring containers are used with only one measuring apparatus in the present invention, the measuring efficiency can become substantially equal to that obtained in the case that a plurality of measuring apparatuses are used together in the conventional method. The following illustrates the radiation-measuring container of the present invention in more detail. The radiation-measuring container of the present invention comprises a housing part for a sample and a lid part. The lid is not always necessary and may be used according to the shape of the container, the type of the sample and measuring conditions. The shape of the container may be in any form of a cylindrical, prismatic and bagged shape, but the cylindrical or prismatic shape is generally preferred from the viewpoint of the convenience for detecting radiation energy stored in the container. The stimulable phosphor built in the container of the present invention is not always contained in the whole portion of the container, and it may be contained within a part of the container. As typical embodiments of the radiation-measuring container of the present invention, there may be mentioned the following: (1) Radiation-measuring container having te stimulable phosphor built in the whole portion of the housing part thereof; (2) Radiation-measuring container having the stimulable phosphor built in at least a part of the elide wall thereof; (3) Radiation-measuring container having the stimulable phosphor built in at least a part of the bottom thereof; and (4) Radiation-measuring container having the stimulable phosphor built in at least a part of the lid thereof. In the case that the surface area of the part having the bult-in stimulable phosphor is increased with respect to the container of the present invention, the ability of capturing of a radiation from a sample and the accuracy of measured value can be improved, and the detection of a trace amount of a radioactive substance in the sample or the detection of a substance exhibiting a low radiation intensity becomes possible. On the other hand, from the viewpoint of convenience of detecting radiation energy, it is preferred to use an embodiment wherein the stimulable phosphor is built in a part of the container. In such a case, it is desirable that the phosphor is built in a part which is to be in contact with the sample. Further, it is preferred to use a measuring container wherein the distance between the part having the built-in phosphor and the radioactive substance (radiation source) in the sample is as short as possible from the viewpoint of the range of a radiation from the sample. Therefore, it is desirable to choose the shape of the container according to the amount, the condition and the radiation intensity of a sample to be measured and measuring conditions in the measurement of the content of the radioactive substance in the present invention. Figure 1 is vertical section views diagrammatically showing the embodiments of the cylindrical measuring container of the present'invention wherein the container has a built-in stimulable phosphor at each area represented by the shaded portion, and wherein: (a) shows a radiation-measuring container having the stimulable phosphor built in a dispersed state in the whole portion of the housing part thereof, (b) shows a radiation-measuring container having the stimulable phosphor built in a dispersed state in the whole portion of the side wall thereof, (c) shows a radiation-measuring container having the stimulable phosphor built in a dispersed state in a portion of the side wall thereof, (d) shows a radiation-measuring container having the stimulable phosphor built in a dispersed state in a whole portion of the bottom thereof, and (e) shows a radiation-measuring container having the stimulable phosphor built in a dispersed state ina portion of the lid thereof. The above five kinds of the embodiments are provided only to illustrate certain typical cases, but are not to be construed as limiting the container of the present invention. For example, the containers of the above (a) to (d) may have no lid. Further, in the above (b) to (e) the stimulable phosphor is not always fixed to the containers but may be detachably provided within the container. The stimulable phosphor, as described hereinbefore, give stimulated emission when irradiated with stimulating rays after exposure to a radiation. From the viewpoint of practical use, the stimulable phosphor is desired to give stimulated emission in the wavelength region of 300 - 500 nm when excited with stimulating rays in the wavelength region of 400 - 850 nm. Examples of the stimulable phosphor employable in the radiation-measuring container of the present invention include: SrS:Ce,Sm, SrS:Eu,Sm, ThO2:Er, and La202S:Eu,Sm, as described in U.S. Patent No. 3,859,527; ZnS:Cu,Pb, BaO xAQ203:Eu, in which x is a number satisfying the condition of 0.8 < x < 10, and M2+0 xSiO2:A, in which M2+ is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn, Cd and Ba, A is at least one element selected from the group consisting of Ce, Tb, Eu, Tm, Pb, Tt, Bi and Mn, and x is a number satisfying the condition of 0.5 < x < 2.5, as described in U.S. Patent No. 4,326,078; (Ba1-x-y,Mgx,Cay)FX:aEu2+, in which X is at least one element selected from the group consisting of C± and Br, x and y are numbers satisfying the conditions of O < x+y < 0.6, and xy ss 0, and a is a number satisfying the condition of 10'6 < a < 5x10 2, as described in Japanese Patent Provisional Publication No. 55(1980)-12143; LnOX: :xA, in which Ln is at least one element selected from the group consisting of La, Y, Gd and Lu, X is at least one element selected from the group consisting of Cl and Br, A is at least one element selected from the group consisting of Ce and Tb, and x is a number satisfying the condition of O < x < 0.1, as described in the above-mentioned U.S. Patent No. 4,236,078; (Ba1-x,MIIx)FX:yA, in which MII is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn and Cd, X is at least one element selected from the group consisting of Cl, Br and I, A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er, and x and y are numbers satisfying the conditions of O < x # 0.6 and O < y # 0.2, respectively, as described in Japanese Patent Provisional Publication No. 55(1980)-12145; MIIFX.xA:yLn, in which MII is at least one element selected from the group consisting of Ba, Ca, Sr, Mg, Zn and Cd; A is at least one compound selected from the group consisting of BeO, MgO, CaO, SrO, BaO, ZnO, Al2O3, Y203, La203, In203, SiO2, TiO2, ZrO2, GeO2, SnO2, Nb205, Ta205 and ThO Ln is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, Sm and Gd; X is at least one element selected from the group consisting of Cl, Br and I; and x and Z are numbers satisfying the conditions of 5x10-5 < x < 0.5 and 0 < y < 0.2, respectively, as described in Japanese Patent Provisional Publication No. 55(1980)-160078; (Ba1-x,MIIx)F2.aBaX2:yEu,zA, in which MII is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cl, Br and I; A is at least one element selected from the group consisting of Zr and Sc; and a, x, y and z are numbers satisfying the conditions of 0.5 # a # 1.25, 0 # x # 1, 10 ¯ y < 2x101, and O < z # 10 , respectively, as described in Japanese Patent Provisional Publication No. 56(1981)116777; (Ba1-x,MIIx)F2.aBaX2:yEu,zB, in which MII is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of CL, Br and I; and a, x, y and z are numbers satisfying the conditions of 0.5 < a < 1.25, 0 < x < 1, 10-6 < y < 2x10-1, and 0 < z < 2x10-1, respectively, as described in Japanese Patent Provisional Publication No. 57(1982)-23673; (Ba1-x,MIIx)F2.aBaX2:yEu,zA, in which MII is at least one element selected from the group consisting of Be, Mg, Ca, Sr, Zn and Cd; X is at least one element selected from the group consisting of Cl, Br and I; A is at least one element selected from the group consisting of As and Si; and a, x, y and z are numbers satisfying the conditions of 0.5 < a < 1.25, 0 < x < 1, 10 < y # 2x10-1, and O < z # 5x10-1, respectively, as described in Japanese Patent Provisional Publication No. 57(1982)23675; MIIIOX:xCe, in which MIII is at least one trivalent metal selected from the group consisting of Pr, Nd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Bi; X is at least one element selected from the group consisting of CL and Br; and x is a number satisfying the condition of O < x < 0.1, as described in Japanese Patent Provisional Publication No. 58(1983)-69281; Ba1¯xMx/2Lx/2FX:yEu2+, in which M is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; L is at least one trivalent metal selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In and TL; X is at least one halogen selected from the group consisting of Cl, Br and I; and x and y are numbers satisfying the conditions of 10 2 < x < 0.5 and 0 < y ( 0.1, respectively, as described in Japanese Patent Provisional Publication No. 58(1983)-206678; 2+ BaFX'xA:yEu , in which X is at least one halogen selected from the group consisting of Cl, Br and I; A is at least one fired product of a tetrafluoroboric acid compound; and x and y are numbers satisfying the conditions of 10 6 < x < 0.1 and 0 < y < 0.1, respectively, as described in Japanese Patent Provisional Publication No. 59(1984)-27980; BaFX.xA:yEu2+, in which X is at least one halogen selected from the group consisting of Cl, Br and I; A is at least one fired product of a hexafluoro compound selected from the group consisting of monovalent and divalent metal salts of hexafluoro silicic acid, hexafluoro titanic acid and hexafluoro zirconic acid; and x and y are numbers satisfying the conditions of 10-6 < x < 0.1 and 0 < y < 0.1, respectively, as described in Japanese Patent Provisional Publication No. 59(1984)-47289; BaFX xNaX':aEu2+, in which each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I; and x and a are numbers satisfying the conditions of O < x #.2 and 0 < a < 0.2, respectively, as described in Japanese Patent Provisional Publication No. 59(1984)-56479; MIIFX.xNaX':yEu2+:ZA, in which MII is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br and I; A is at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co and Ni; and x, y and z are numbers satisfying the conditions of O < x < 2, 0 < y # 0.2 and O < z < 10-2, respectively, as described in Japanese Patent Provisional Publication No. 59(1984)56480; and MIIFX.aMIX'.bM'IIX"2.cMIIIX"'3.xA:yEu2+, in which M is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca; MI is at least one alkali metal selected from the group consisting of Li, Na, K, Rb and Cs; M'II is at least one divalent metal selected from the group consisting of Be and Mg; M111 is at least one trivalent metal selected from the group consisting of AQ, Ga, In and Ti; A is at least one metal oxide; X is at least one halogen selected from the group consisting of C, Br and I; each of X', X" and X"' is at least one halogen selected from the group consisting of F, C, Br and I; a, b and c are numbers satisfying the conditions of O < a < 2, 0 < b < 10 2, o < c < 10 2 and a+b+c > 10 6; and x and y are numbers satisfying the conditions of O < x < 0.5 and 0 < y < 0.2, respectively, as described in Japanese Patent Application No. 57(1982)184455. The above-described stimulable phosphors are given by no means to restrict the stimulable phosphor employable in the present invention. Any other phosphor can be also employed, provided that the phosphor gives stimulated emission when excited with stimulating rays after exposure to a radiation. Example of materials of the measuring container of the present invention include glass, quartz and plastics such as polyethylene, polypropylene, nylon, polyfluoroethylene, etc. The radiation-measuring container of the present invention can be formed by incorporating the stimulable phosphor into the above-mentioned material. The size, thickness and shape of the measuring container, and the amount and area of the stimualble phosphor to be built-in can be determined according. to the condition and amount of samples to be measured and the measuring conditions. If desired, the measuring container of the present invention may be in the form of a small bag composed of a polyester film containing a stimulable phosphor. The measuring container may have the built-in stimulable phosphor, said phosphor (in the form of particles) being dispersed in a binder. Examples of the binder include: natural polymers such as proteins (e.g. gelatin), polysaccharides (e.g. dextran) and gum arabic; and synthetic polymers such as polyvinyl butyral, polyvinyl acetate, nitrocellulose, ethylcellulose, vinylidene chloride-vinyl chloride copolymer, polymethyl methacrylate, vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate butyrate, polyvinyl alcohol, and linear polyester. Particularly preferred are nitrocellulose, linear polyester, and a mixture of nitrocellulose and linear polyester. Examples of the solvent employable for dispersing the stimulable phosphor particles in the binder include lower alcohols such as methanol, ethanol, n-propanol and n-butanol; chlorinated hydrocarbons such as methylene chloride and ethylene chloride; ketones such as acetone, methyl ethyl ketone and methyl isobutyl ketone; esters of lower alcohols with lower aliphatic acids such as methyl acetate, ethyl acetate and butyl acetate; ethers such as dioxane, ethylene glycol monoethylether and ethylene glycol monoethyl ether; and mixtures of the above-mentioned compounds. The ratio between the binder and the phosphor in the dispersion may vary according to the kind of the phosphor employed, etc. Generally, the ratio therebetween is within the range of from 1 : 1 to 1 : 100 (binder : phosphor, by weight), preferably from 1 : 8 to 1 : 40. The dispersion may contain a dispersing agent to assist the dispersibility of the phosphor particles therein, and also contain a variety of additives such as a plasticizer for increasing the bonding between the binder and the phosphor particles in the phosphor layer. Examples of the dispersing agent include phthalic acid, stearic acid, caproic acid and a hydrophobic surface active agent. Examples of the plasticizer include phosphates such as triphenyl phosphate, tricresyl phosphate and diphenyl phosphate; phthalates such as diethyl phthalate and dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethyl glycolate and butylphthalyl butyl glycolate; and polyesters of polyethylene glycols with aliphatic dicarboxylic acids such as polyester of triethylene glycol with adipic acid and polyester of diethylene glycol with succinic acid. The measuring container of the present invention may be a layer structure comprising a support made of a plastic material such as polyethylene terephthalate and a layer (phosphor layer) composed of a stimulable phosphor dispersed in a binder. Namely, the container may be prepared by heat-treating a sheet comprising a support and a phosphor layer to form a container. Alternatively, the measuring container of the present invention may be in a structure wherein a molded article composed of a stimulable phosphor dispersed in a binder is put in a portion of the container composed of the aforementioned material. It is desirable that the detachable molded article is covered with a transparent polymer material such-as polyethylene or polyethylene terephthalate to protect chemically and physically the phosphor. In the present invention, it is not always necessary to introduce a sample directly into the container. The sample is placed in a separate container, the sample together with said container is inserted into the measuring container of the present invention, and the measurement may be made. The following illustrates the operation of the measurement of radiation intensity according to the present invention using the radiation-measuring container having the built-in stimulable phosphor. Samples to be measured in the present invention may be in any form of a solid, liquid and gas. However, in the case that the sample is a gas or a volatile substance, it is necessary to measure it in a state where the measuring container is sealed up. In the case that the sample is a liquid, it is preferred to be in a homogeneous phase to enhance the accuracy of measurement. As solvents for a sample solution, any solvent immiscible with the material of the container may be used. In the case that the sample is a solid, a powdered form is preferred. Therefore, the samples of the present invention are not limited to liquids, and it is possible to measure the radioactivity of solid substances such as the tissue of radioactively labeled organism and substances originating from the organism, said substances being difficultly solubilized. It is also possible to measure the radioactivity of volatile samples. In order to determine the radiation intensity of the radioactive substance with high accuracy, however, the sample is preferably a liquid or a solid. Any radiation such as a-rays, rays, y-rays, proton beam, neutron beam, light beam, meson beam and cosmic rays radiated by the radioactive substance in the sample can be measured. Namely, any radiation radiated by radionuclide can be measured. A sample is first introduced into the measuring container having the built-in stimulable phosphor in a dark room, and at least a portion of radiation energy emitted by a radioactive substance in the sample is caused to be absorbed by the stimulable phosphor in the container. The exposure time varies depending on the intensity of radioactivity of the radioactive substance in the sample, the amount and density of said radioactive substance, the shape of the measuring container and the luminance of stimulated emission of the phosphor. The exposure time generally ranges from several seconds to several minutes. After exposure to a radiation emitted by the sample for a certain time, the measuring container in which the radiation energy is stored is subjected to a detecting operation. The detecting operation may be carried out using the container containing a sample as such. Alternatively, the sample is taken out of the container, which is then washed and subjected to the detecting operation. Preferably, the detecting operation is conducted after the sample is taken out of the container. It is also preferred to conduct the detecting operation in a dark room. The detecting of the measuring container with the above built-in stimulable phosphor storing the radiation energy emitted by the sample is carried out as follows: the container is irradiated with light having a wavelength within the region of stimulation wavelength for the stimulable phosphor to emit light (stimulated emission) in the amount proportional to the stored radiation energy, and then the stimulated emission is detected by a photodetector such as photomultiplier. As this photodetector, there is employed a detector provided with a filter which transmits only light in the wavelength region of stimulated emission. The stimulated emission detected by the photodetector is converted to an electric signal (electric pulse) which is then amplified by an amplifier and input into a pulse height analyzer where noises from said photodetector and caused by the incorporation of natural radioactivity are removed. The electric pulse from which noises have been removed is input into a counting circuit to count it and the counted value is input into a data processing circuit. In the data processing circuit, the intensity of radioactivity is calculated according to detecting efficiency (luminance efficiency of stimulated emission) previously input and exposure time, and the amount or concentration of the radioactive substance contained in the sample is also calculated by inputting the intensity of radioactivity per 1 mole of the radioactive substance. The resulting data are transmitted to a display device and a recorder. In this way, the intensity of radioactivity of the sample and/or the amount or concentration of the radioactive substance can be represented as digital data. However, the above-described method (detecting method) for detecting radiation energy stored in the measuring container is given by no means to limit the method therefor. After the radiation-measuring container having the built-in stimulable phosphor is washed with a solvent, the container is irradiated with light to erase the remaining energy therein and so treated container can be re-used. By using the radiation-measuring container of the present invention with a built-in stimulable phosphor, the quenching phenomenon caused by the internal absorption of emission in the conventional scintillation counting method, or the quenching phenomenon caused by the external absorption of emission caused by the indirect contact of the sample with a scintillator can be eliminated. This means that the complicated operation for calculating the counting efficiency can be eliminated and the radiation intensity emitted by a radioactive substance in a sample can be easily calculated. According to the present invention, it is further possible to follow the change a sample with time, in the case that the sample is one having a reactivity such as producing a gaseous radioactive substance by chemical reaction.

Claims: CLAIMS: 1. A method of measuring radiation intensity which comprises steps of: placing a sample containing a radioactive substance in a radiation-measuring container with a built-in stimulable phosphor for a given period of time to cause said container to absorb at least a portion of radiation energy emitted by said radioactive substance; irradiating said container with an electromagnetic wave to release the radiation energy stored in said container as stimulated emission; and photoelectrically detecting the stimulated emission to measure radioactivity of said sample. 2. The method of measuring radiation intensity as claimed in claim 1, wherein the radiation-measuring container has a built-in stimulable phosphor being dispersed in a binder. 3. The method of measuring radiation intensity as claimed in claim 1, wherein the radiation-measuring container is a plastic container with a built-in stimulable phosphor. 4. The method of measuring radiation intensity as claimed in claim 1, wherein the radiation-measuring container is a glass or quartz container with a built-in stimulable phosphor. 5. The method of measuring radiation intensity as claimed in any one of claims 1 to 4, wherein the.stimula- ble phosphor is a divalent europium activated alkaline earth metal fluorohalide phosphor. 6. A radiation-measuring container having a builtin stimulable phosphor to measure intensity of a radiation emitted by a sample containing a radioactive substance. 7. The radiation-measuring container as claimed in claim 6, wherein said measuring container has a built-in stimulable phosphor being dispersed in a binder. 8. The radiation-measuring container as claimed in claim 6, wherein said measuring container is a plastic container with a built-in stimulable phosphor. 9. The radiation-measuring container as claimed in claim 6, wherein said measuring container is a glass or quartz container with a built-in stimulable phosphor. 10. The radiation-measuring container as claimed in any one of claims 6 to 9, wherein the stimulable phosphor is a divalent europium activated alkaline earth metal fluorohalide phosphor.

Inventors: SHIRAISHI, HISASHI C/O FUJI PHOTO FILM CO. LTD.
Applicants: FUJI PHOTO FILM CO., LTD.
Application year: 1984
Main classification: G01T1
IPC codes: G01T1, G01T7