Patent Number: 062890719
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in detail with reference to the drawings attached. Referring to FIG. 1, there is a schematic sectional view of a positron emitter-generating unit for generating a positron emitter (radioisotope) in a liquid target by irradiating a liquid target with a beam of charged particles. The positron emitter-generating unit 10 is composed of three blocks, an upper block 12 and an intermediate block 13 both having a through hole through the blocks 12 and 13 and a lower block 14 with a concave part 18. These three blocks are secured to one another by screws with the alignment of the through holes and the concave part 18 being made sure. In the intermediate block 13, the upper and lower openings of the through hole are sealed with a metal foil 15 (e.g., a titanium foil) and 16 (e.g., a silver foil), respectively, to form a space 17 for containing a liquid target (i.e., a liquid target container). O-rings 13a, 13b and 14a are provided seal between the blocks 12, 13 and 14. A charged particle beam 11 enters an opening 12a of the upper block 12 and passes through the metal foil 15 (e.g., a titanium foil) and applied to the liquid target in the container 17. The concave part 18 of the lower block 14 is provided with cooling water feed pipes 19a and 19b connected thereto, so that the target solution heated by the irradiation with the charged particle beam 11 is cooled down with the cooling water in the concave part 18 fed through the pipes 19a and 19b. To the container 17 are connected a liquid target feed pipe 23 shown in FIG. 1, a liquid target feed pipe (not shown) which is connected to the container 17 in a direction perpendicular to the plane of the sheet of FIG. 1, and a gas feed pipe (not shown) for introducing a N.sub.2 gas into the container 17. In the positron emitter-generating unit 10, a valve 23a is closed to store the liquid target in the container 17. In the container 17, the liquid target is irradiated with the charged particle beam 11, whereby a positron emitter is generated in the liquid target. In this embodiment, water containing H.sub.2.sup.18 O and 2 ppm of NaF is used as the liquid target, and H.sub.2.sup.18 O in the container 17 is irradiated with a proton beam (i.e., the charged particle beam 11) accelerated to an energy level of 16 MeV with an accelerator, thereby generating .sup.18 F through a .sup.18 O(p,n).sup.18 F reaction. The irradiation with the proton beam 11 is performed, for example, for 30 minutes. Thereafter, the valve 23a is opened, and N.sub.2 gas is introduced into the container 17 through the gas feed pipe, whereby the .sup.18 F-containing H.sub.2.sup.18 O in the container 17 is transferred into a container 30 placed in another room. The container 30 is formed of a copper block with a semispherical cavity 31 on the inner surface of which rhodium plating 32 is applied. Referring to FIG. 2, there is a partially sectional view illustrating the process of preparing a positron source by causing to bind the positron emitter .sup.18 F contained in the liquid target 35 in the container 30 onto an end surface of the carbon member. The container 30 contains a solution 35 containing both .sup.18 F and 2 ppm of NaF which has been subjected to irradiation with the proton beam. The upper end of the carbon member 40 is held to a stand 46 by a plastic-made insulating holder 45. The carbon member 40 and the container 30 are connected to a constant-voltage power supply 47 so that the carbon member 40 is located on an anode side and the container 30 is located on a cathode side. It is preferably for the carbon member 40 to pass an electric current in the state that the bottom surface of the carbon member 40 is contacted with the solution 35 with the smallest possible surface contact area so that .sup.18 F is mostly bound to the bottom surface of the carbon member 40 and is bound to the side surface of the carbon member 40 as small as possible. Therefore, for instance, the carbon member 40 is first located above the liquid surface of the solution 35 in the container 30 and then lifted down slowly toward the liquid surface of the solution 35. When the contact of the bottom surface of the carbon member 40 with the liquid surface of the solution 35 is confirmed by the detection of the flow of electricity from the constant-voltage power supply 47, the carbon member 40 is further lifted down (for example by 0.1 mm), and then held to the stand 46. Thus, the bottom surface of the carbon member 40 is ensured to contact with the solution 35 while maintaining the smallest possible contact area. When an electric current from the constant-voltage power supply is passed through the carbon member 40 contacting with the solution 35, .sup.18 F in the solution 35 is concentrated near the carbon member 40 (an anode) and bound onto the carbon member 40. Thus, a positron source with a .sup.18 F(positron emitter)-rich end surface can be prepared. Referring to FIGS. 3A and 3B, there are schematic views of embodiments of a positron source according to the present invention. FIG. 3A shows a positron source prepared by the process illustrated in FIG. 2. In the positron source of FIG. 3A, a positron emitter .sup.18 F is bound onto one end surface 41 of a fine cylindrical carbon member 40 in a high density. FIG. 3B shows an alternative embodiment of a positron source of the present invention, in which a fine cylindrical positron source 40a is applied with an insulating coating 42 at a part of the side surface near its one end. The application of the insulating coating 42 serves to prevent the bonding of the positron emitter .sup.18 F onto the side surface of the carbon member 40 even when the carbon member 40 is immersed in the solution 35 relatively deeply upon the passage of electric current in the process as shown in FIG. 2. Thus, the .sup.18 F binds onto the end surface 41a exclusively. In the positron source according to the present invention, a positron emitter .sup.18 F binds uniformly onto an end surface 41 or 41a of the carbon member 40 or 40a, respectively, without any carrier and the thickness of the positron emitter .sup.18 F bound onto the end surface is negligible. Therefore, the positron from the positron emitter .sup.18 F can be emitted from the small surface area of the carbon member 40 (which is almost a point source) efficiently without any influence of scattering or absorbance. Then, the binding efficiency of the positron emitter .sup.18 F onto the carbon member is examined. Water (1 ml) containing H.sub.2.sup.18 O (purity: 90%) and 2.mu.g of NaF is used as a liquid target. The liquid target is irradiated with a proton beam which is accelerated to an energy level of 16 MeV. After the irradiation, the liquid target is transferred to a semi-spherical container (void volume: 1 ml) of 8 mm in radius as shown in FIG. 2 and a carbon member 40 is set as shown in FIG. 2. The carbon member 40 used is a graphite rod which is prepared by working a high-purity graphite for spectrometry purpose into a cylindrical rod of 5 mm or 3 mm in diameter and 3 cm in length. The graphite rod is provided with a copper terminal on one end, and the other end is polished to give a smooth surface. The graphite rod is mounted to a plastic holder 45 and arranged so that the center of the end surface is aligned with the center of the container 30, and then connected to a constant-voltage power supply 47 to pass electric current. The voltage applied is varied from 70V to 180V in 10V intervals and the period of time for passing electric current is set at 5, 10 and 20 minutes. The intensity of the gamma ray of 0.511 MeV emitted from the graphite rod is measured with a semiconductor detector. As a control sample, the liquid target (1 ml) is irradiated with the proton beam, applied on an aluminum foil, dried, and then measured on the intensity of the gamma ray of 0.511 MeV emitted from the control sample in the, same manner. The measured value for the graphite rod is compared with that for the control sample to determine the binding efficiency relatively. Referring to FIG. 4, there is a graph illustrating the time course of the binding efficiency of .sup.18 F onto a 3 mm.phi. graphite rod at the electrodeposition voltage of 120V, in which the time for passing the electric current is plotted as abscissa and the binding efficiency as ordinate. As shown in FIG. 4, it is found that the binding efficiency of 50% or higher can be achieved by passing electric current for 20 minutes or 30 minutes. In the examination, graphite rods of 3 mm and 5 mm in diameter are used. However, other carbon materials having excellent conductivity and satisfactory material strength (e.g., glassy carbon) may also give the similar results. Although the diameters of the carbon member used in the tests is 3 mm and 5 mm, diameters of less than 3 mm (e.g., less than 1 mm) may also be employed. It will be obvious that the cross section of the carbon member is not particularly limited, such as a square, hexagonal or circular shape. Referring to FIG. 5, there is a sectional view of an embodiment of a slow positron beam-generating unit with the positron source according to the present invention. One end of a vacuum container 72 with a step 73 is double sealed with a reinforcing titanium foil 75 and a moderator 76, in the front of which a grid 77 is provided. The grid 77 is applied with a voltage of about -30V from a power supply 78. The moderator 76 is composed of a tungsten foil of about 10 .mu.m thick. A positron source 50 with a positron emitter .sup.18 F bonded onto its one end is engaged in the step 73 of the vacuum container 72 so that the positron source 50 is aligned in the right place against the moderator 76. The positron emitted from the positron emitter present at the end surface of the positron source 50 is ejected to the vacuum container 72 through the titanium foil 75. Then, the positron enters the moderator 76 to be slowed down. The slowed positron is then accelerated through the electric field generated by the grid 77 and transferred to a place where the positron beam is to be used as a slow positron beam 71 along the magnetic field generated by a coil 79. Referring to FIGS. 6 and 7, there are a schematic illustration of an embodiment of an automated system for supplying a positron source according to the present invention, and a connection diagram illustrating a general set-up for driving the system. The automated system for supplying a positron source comprises a rotary table 80 on which a plurality of containers 30a-30f are mounted, and a rotary member 90 to which the same numbers of carbon members 40a-40f as that of the containers are removably mounted. Each of the containers 30a-30f is manufactured by forming a semispherical cavity on a copper block and plating the inner surface of the cavity with rhodium. The rotary table 80 is capable of rotating in a 360-degree arc by the aid of a pulse motor 81. The rotary member 90 is capable of rotating in a 360-degree arc by the aid of a pulse motor 91. The rotary member 90 is also capable of up-and-down movement by the aid of a pulse motor 92. The pulse motors 81, 91 and 92 are driven by motor drivers 95, 96 and 97, respectively, that are controlled by a computer 106 through an interface 105. The constant-voltage power supply 100 is connected to the rotary plate 80 (negative side) and the rotary member 90 (positive side) through phospher bronze-made brushes 83 and 84, respectively. Between the power supply 100 and the rotary member 90 is provided a liquid surface-detection circuitry 101. The output of the liquid surface-detection circuitry 101 is input into the computer 106 through the interface 105. In the apparatus, there are determined Position A where the solution is supplied to the container and Position B where electric current is passed through the solution. At Position A, a solution containing a positron emitter .sup.18 F is supplied into a container 30a from a positron emitter-generating unit as shown in FIG. 1 through a liquid target feed pipe 23. After the supply of the .sup.l8 F-containing solution into the container 30a is completed, the pulse motor 81 is driven to rotate the rotary table 80, so that the container 30a moves to Position B which is positioned underneath the carbon member 40a mounted on the rotary member 90. Next, the pulse motor 92 is driven to move down the rotary member 90 slowly. Then, the carbon member 40a mounted on the rotary member 90 also moves down slowly toward the solution in the container 30a. When the carbon member 40a (at a positive potential) contacts with the liquid surface of the solution in the container 30a (at a negative potential), an electricity of about a few mA flows. The liquid surface-detection circuitry 101 detects the generated a micro-current by a photocoupler and sends it as a liquid surface-detection signal to the computer 106 through a ultra-compact relay. When the computer 106 receives the signal, it operates a driver 97 so that the carbon member 40a further moves down by about 0.1 mm. Thereafter, an electric current is passed through the liquid with the constant-voltage power supply 100 at 90V for 20 minutes to cause to bind the positron emitter .sup.18 F onto one end of the carbon member 40a. Thus, a positron source can be prepared. Once the positron source is prepared, the pulse motor 92 is driven to elevate the rotary member 90 upward, whereby the positron source (carbon member 40a) is also moved upward of the container 30a. The pulse motor 91 is also driven to move the carbon member 40a to the position opposed to the positron source-receiving section (step) 73 of the positron beam generating unit. Thereafter, the pulse motor 92 is driven to move the rotary member 90 upward by a predetermined distance, so that the carbon member 40a is attached to the positron source-receiving section (step) 73 of the positron beam generating unit. Using this sequence of operations, a slow positron beam 71 can be generated from the positron beam generating unit. The sequence of operations is performed automatically under computer control. The half-life of the positron emitter .sup.18 F is about 110 minutes. Therefore, the positron source (i.e., carbon member 40a) can generate a positron beam for about two hours. When the intensity of the positron beam 71 is decreased, a solution which contains a positron emitter .sup.18 F prepared as described above in the positron emitter-generating unit as shown in FIG. 1 is supplied to a next container 30b on the rotary table 80 through the liquid target feed pipe 23. Then, the positron emitter .sup.18 F in the container 30b is bound onto a carbon member 40b and supplied to the positron beam-generating unit 110. By these operations, for instance, a 20 minute passage of electric current at Position B and a subsequent two hour positron beam generation can be performed repeatedly. In this case, for instance, if the system is provided with six containers 30 and six carbon members 40, a continuous running for 12 hours becomes possible, and if the system is provided with 12 containers 30 and 12 carbon members 40, a continuous running for 24 hours becomes possible. The solution after the passage of electric current is recovered through a recovery pipe 109. As stated above, according to the present invention, a positron source capable of generating positrons of high intense efficiently from a small surface area which is almost a point source, can be prepared. Using the system of the present invention as described above, the positron source can be supplied to a positron beam-generating unit automatically. The invention has been described in detail with reference to various embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the invention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.