Induced nuclear reactions: processes – systems – and elements – Nuclear transmutation – By charged particle bombardment
Reexamination Certificate
1999-09-01
2001-09-11
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Nuclear transmutation
By charged particle bombardment
C376S202000, C250S493100
Reexamination Certificate
active
06289071
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a positron source capable of generating a positron beam of high intensity, a method of preparing the positron source, and an automated system for supplying the positron source.
RELATED BACKGROUND ART
Slow positron beams have been commonly used in positron microscopes, for research in physical properties and for crystal defect evaluation of the surfaces or interfaces of semiconductors and metallic materials, and recently have become useful more and more. At present, slow positron beams are generated by emitting from positron emitters (radioisotopes), or by ejecting positrons that are generated through pair creation with a braking radiation into a moderator to be slowed down the positrons. A positron emitter is often prepared by irradiating a solid target (e.g., aluminum or boron nitride) with a beam of charged particles (e.g., protons) accelerated with a cyclotron or the like; thus a positron emitter can be generated in the solid target. A braking radiation is usually generated by irradiating a heavy metal target with an electron beam accelerated with a linear accelerator or the like.
Upon the utilization of positron beams, a strong point source for a positron emitter is required. Various approaches have been proposed for increasing the intensity of positron beams, such as the improvement in moderator efficiency and the use of a stronger positron source. As a moderator, one formed of a tungsten foil which is annealed at 2000° C. is currently used. However, such moderator cannot achieve an efficiency of the order of 10
−4
or more. Although many efforts are being made to improve moderators, drastic and practical improvements could hardly be expected. On the other hand, for preparing a strong positron source, the use of a large-scale and expensive device is needed.
In the preparation process for a strong positron source using a solid target, there is a serious problem that heat generated during the passage of a large electric current should be removed. The process also has another problem as follows. A solid target is placed nearby a moderator for the purpose of causing to emit positrons from a positron emitter generated in the target and increasing the incident efficiency of positrons generated through pair creation with a braking radiation into the moderator. When such solid target is irradiated with an electron beam or an ion beam, the moderator sustains a radiation damage or is radioactivated by a secondary radiation other than the positrons. In order to overcome this problem, it is proposed an approach for avoiding the influence of the secondary radiation during the irradiation of the target, which comprises: irradiating a solid target at a place a distance away from a moderator thereby generating a positron emitter; transferring the irradiated solid target to the place where the moderator is placed; and ejecting a beam of positrons emitted from the positron emitter in the solid target into the moderator. However, such approach is not practical. This is because the use of a solid target usually needs a cooling device for removing heat generated as a result of the irradiation and, therefore, if a solid target is to be transferred, the system as a whole will inevitably become a large scale due to the integration of the cooling device. In the process utilizing a braking radiation generated with an electron beam, it is impossible in principle to separate a heavy metal target and a moderator. Moreover, in this process, it is necessary to automate the supply of a positron source to a positron beam-generating unit for the purpose of avoiding the harmful irradiation exposure of operators.
SUMMARY OF THE INVENTION
Under these situations, the present invention is made. That is, the object of the present invention is to provide a positron source capable of generating a positron beam of high intensity without damaging a moderator, a method of preparing the positron source, and an automated system for supplying the positron source.
The present inventors have found that the positron source can be prepared using a liquid target containing H
2
18
O [
18
O(H
2
O)] as a target for generating a positron emitter, by irradiating the liquid target with a proton beam to generate a positron emitter
18
F through a
18
O(p,n)
18
F reaction, and causing to bind the
18
F onto a carbon member to trap the
18
F on the carbon member. This finding leads the accomplishment of the present invention.
Therefore, the present invention provides a positron source comprising a carbon member having
18
F bound onto the surface thereof. The-carbon member is preferably made of graphite or glassy carbon. The carbon member preferably has a rod-like or strip-like geometry onto an end of which
18
F is bound.
The present invention also provides a method of preparing a positron source comprising: irradiating a liquid target containing H
2
18
O with a beam of charged particles to generate
18
F; and passing an electric current through the liquid target using a carbon member as an anode to cause to bind the
18
F onto the surface of the carbon member. The liquid target may contain a small amount of natural fluorine ions, for example, by the addition of a fluoride of an alkali metal which is soluble in the liquid target and is a strong electrolyte (e.g., NaF, NaHF
2
and KF).
The reason for the pre-addition of a small amount of natural fluorine ions to a liquid target [
18
O(H
2
O)] is as follows. The number of the
18
F atoms generated through a nuclear reaction in the liquid target is at most 3.5×10
15
atoms, which corresponds to only 1.1×10
−8
g in terms of the weight of fluorine atoms. Such extremely trace amount of
18
F atoms might result in insufficient current for electrodeposition. In order to prevent this problem, natural fluorine ions are added to the liquid target at a concentration of 2&mgr;/ml so that the number of the
18
F atoms becomes about 100 times greater than that without natural fluorine ions. This ensures the chemical behavior of the generated
18
F as F
31
in an aqueous solution (a liquid target). Since the amount of the fluorine ions added is very small, it is necessary for the fluorine ions to be added to the liquid target prior to the irradiation.
In the present invention, it is preferable that the carbon member (i.e., an anode) have a rod-like or strip-like geometry and an electric current be passed through the liquid target while contacting an end surface of the carbon member with the liquid target so that the
18
F is concentratedly bound onto the end surface of the carbon member. It has not been made clear yet whether the bonding of the
18
F onto the surface of the carbon member is via a direct bonding between the
18
F and a carbon atom in the carbon member (e.g., generation of a C-F bonding) or via intercalation of the
18
F into a graphite-type crystal structure of the carbon member (i.e., formation of an intercalation compound).
The present invention also provides an automated system for supplying a positron source comprising: means for moving a container with a solution containing
18
F to the position where an electric current is to be passed through the solution; means for passing an electric current through the solution at that position using a carbon member as an anode; and means for transferring the carbon member after the passage of the electric current to a positron beam-generating unit. In this system, the solution containing
18
F is fed to a container placed in another room, and an electric current is then passed through the solution at that place. This system may further comprise means for recovering the solution after the passage of electric current.
The present invention further provides an automated system for supplying a positron source comprising: a rotary table for rotating a container mounted thereon; means for supplying a solution containing
18
F into the container; first drive means for rotationally driving the rotary table so that the container moves between th
Fujiwara Ichiro
Goto Akira
Hyodo Toshio
Ikegami Yuji
Ito Yoshiko
Jordan Charles T.
Mun K. Kevin
Riken
Rosenthal & Osha LLP
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