Patent Application
20140029709
There are many industrial and clinical applications requiring a high flux of thermal neutrons. A neutron is considered to be thermal when it is in thermal equilibrium with the surrounding materials. Thermal neutrons have a Maxwellian distribution of energies and can be generally considered to have a kinetic energy less than 1 eV (electron-volt). Examples of industrial applications include neutron radiography and Prompt Gamma Neutron Activation Analysis (PGNAA). Some examples of clinical applications include production of radioactive stents used in the prevention of restenosis following arterial intervention, such as balloon angioplasty, and production of short lived radioisotopes used in radiation synovectomy or brachytherapy.
Hampering the continued development of these applications is often the lack of a suitable neutron source. The highest thermal neutron fluxes are produced in nuclear research reactors. These facilities, however, are few in number and often lack the clinical environment necessary for medical research. Other types of neutron sources include radioisotope sources, fusion sources, cyclotrons, and ion accelerators. Much work has gone into the development of these neutron sources with many variations in each category. However, a neutron source that has a high thermal flux suitable for installation in industrial or clinical environments is not generally available. Furthermore, the cost of many of these systems is beyond the reach of many institutions that could make use of the technology.
Another known method of producing neutrons is with an electron accelerator fitted with an x-ray converter and a photoneutron target. In one system, a high power (1 MW) continuous current electron accelerator is used to generate a 30 MeV electron beam, which is incident on a Tungsten target of the x-ray converter. The resulting bremsstrahlung photons are then directed to a tank of heavy water, thereby producing high energy neutrons (up to 14 MeV). While this system may maximize the photoneutron yield, the energy of these neutrons is too high to be thermalized effectively. Such high energy photons and neutrons also requires a massive thickness of biological shielding. Moreover, the high power electron accelerator would make the system relatively large, extremely expensive to build and to operate, and would stretch the technical expertise of a typical radiology department. These types of electron accelerators are primarily used for research and do not have the reliability required for use in a clinical setting.
The present invention is directed to an apparatus for generating thermal neutrons and includes an electron accelerator for generating an electron beam and a converter for converting the electron beam into photons. A receiving device is provided for receiving the photons and includes a material which provides a photoneutron target for the photons, for producing high energy neutrons in a photonuclear reaction between the photons and the photoneutron target, and for moderating the high energy neutrons to generate the thermal neutrons. The electron beam has an energy level that is sufficiently low as to enable the material to moderate the high energy neutrons resulting from the photonuclear reaction.
Turning now to
The LINAC 12 of the invention is preferably a commercially available, repetitively pulsed type used, for example, in hospitals for photon radiotherapy. The LINAC 12 has an electron beam energy from approximately 5 to approximately 30 MeV, but preferably in the range of approximately 5-15 MeV, and an electron beam current of approximately 0.1 to 1 mA or 1 to 10 kW for a 10 MeV electron beam.
Turning to
When the electron beam 14 is incident on the front surface 22 of the converter 16, bremsstrahlung photons are produced as the electrons slow down in the converter. This process is most efficient in producing photons when the electrons are stopped in a material of high atomic number, such as Ta or W, for example, used in the preferred embodiment. Experiments have shown that the x-ray converter 16 fitted to a 10 MeV LINAC 12 converts approximately 17% of the electron beam 14 power into photons. This figure rapidly increases with electron energy. The maximum photon production occurs when the converter 16 thickness is approximately 30% to 50% of the incident electron range evaluated using the CSDA method. Electrons that have penetrated further than 50% of the CSDA range typically have too little energy to create bremsstrahlung photons.
Turning now to
In the preferred embodiment, the reflector 26 has a thickness of approximately 30 cm to 60 cm, and can be any neutron reflecting material such as, for example, graphite, light water, heavy water, polyethelene or other polymer, or lead. The thickness of the reflector may vary depending on the size of the photoneutron target (tank) 24 and the reflector 26 material. A different reflector 26 material may be used on the top or bottom of the tank 24 than on the radial side of the tank. The sample delivery tube 28 is a pneumatic type tube which carries a sample (not shown) to be irradiated with thermal neutrons into and out of the neutron generating tank 24. The sample delivery tube 28 should be large enough to carry the item to be irradiated. This will vary depending on the application. The sample delivery tube 28 should also be waterproof and generally resistant to absorption of neutrons. Polyethylene or crystal polystyrene are examples.
In operation, a sample (not shown) to be irradiated with thermal neutrons is injected into the neutron generating tank 24 using the sample delivery tube 28. The LINAC 12 is set by the control device 20 to generate an electron beam having the desired energy level, which is converted into photons by the x-ray converter 16. The photons are injected into the tank 24, where neutrons are produced through a photonuclear reaction with heavy water. A photonuclear reaction occurs when a photon has sufficient energy to overcome the binding energy of the neutron in the nucleus of an atom. In the reaction the photon is absorbed by the nucleus and a neutron is emitted with relatively high energy. In the present invention, neutrons are produced in a photonuclear reaction in deuterium, 2H (which is an isotope of hydrogen having a mass number of 2) found in heavy water, 2H2O. Deuterium has a low photonuclear threshold energy of 2.23 MeV. Thus, photons created from the LINAC 12 having electron energies preferably in the range of approximately 5-15 MeV are sufficient to cause a photonuclear reaction in heavy water and generate high energy neutrons. The high energy neutrons are then slowed down, or moderated, to thermal energies by heavy water. Because of its small neutron absorption cross section and low effective atomic mass, heavy water functions also as a moderator. The thermal neutrons are then captured by the sample, and the radioactive sample is then removed from the tank 24 through the delivery tube 28, and used in various therapies.
From the foregoing description, it should be understood that a thermal neutron generator has been shown and described which has many desirable attributes and advantages. The neutron generator includes a readily available low energy electron generator, which makes the present invention suitable for installation in industrial or clinical environments.
While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.
Various features of the invention are set forth in the appended claims.
This application claims the benefit of U.S. Provisional Application No. 60/289,356, filed May 8, 2001.
| Number | Date | Country | |
|---|---|---|---|
| 60289356 | May 2001 | US |
