Patent Number: 
Section: claims

1. A method of producing a useful short lived radioisotope for medical or industrial applications from a first target isotope, the method comprising the steps of:providing a first buffer region around a neutron source for providing a first reduction in neutron energy by inelastic scattering;providing an activation region around the first buffer region, said activation region being made of heavy elements of lead and/or bismuth;distributing a material containing said first target isotope throughout the whole volume of the activation region, the inner buffer region and the neutron source being devoid of said first target isotope;activating the neutron source to emit a neutron flux such that neutrons of said neutron flux are captured by the first target isotope to produce said useful short-lived radio-isotope for medical or industrial applications; andrecovering said useful short-lived radioisotope from the exposed material for use in medical or industrial applications;wherein multiple elastic collisions between the neutrons in the neutron flux and the heavy elements in the activation region result in an enhanced neutron flux in the activation region; and a rate of progressive decrease in neutron energy such that neutron capture efficiency in said first target isotope is enhanced by resonance neutron capture. 2. A method according to claim 1, further comprising the step of providing a neutron moderator surrounding the activation region where the exposed material is distributed. 3. A method according to claim 2, further including the step of providing a second buffer region, made of said heavy elements free of the exposed material, located between the moderator and the activation region where the exposed material is distributed. 4. A method according to claim 2, wherein the moderator is made of carbon or deuterated water. 5. A method according to claim 4, wherein the moderator is made of carbon, and has a thickness of the order of 5 to 10 cm. 6. A method according to claim 1, wherein the neutron source consists of a central region of the lead and/or bismuth medium, which is bombarded with a high-energy charged particle beam to produce neutrons by spallation. 7. A method according to claim 6, wherein the lead and/or bismuth of said central region is in liquid phase, and is circulated by natural convection along a circuit including a heat exchanger and an auxiliary heater. 8. A method according to claim 1, wherein the neutron source consists of a beryllium or lithium target bombarded with a charged particle beam. 9. A method according to claim 1, wherein the neutron source is a radioactive source. 10. A method according to claim 1, wherein the neutron source consists of a spallation target bombarded with a high-energy charged particle beam. 11. A method according to claim 1, wherein the exposed material comprises 127I as said first isotope, which produces the useful radio-isotope 128I by capturing neutrons from the flux. 12. A method according to claim 11, wherein the exposed material is an iodine compound to be administered to patients after the neutron exposure. 13. A method according to claim 1, wherein the exposed material comprises 98Mo as said first isotope, which produces 99Mo by capturing neutrons from the flux, said 99Mo being allowed to decay into the useful radio-isotope 99mTc. 14. A method according to claim 13, wherein the exposed material comprises a phosphomolybdate complex salt which, after the neutron exposure, is absorbed in an alumina matrix from which the 99mTc is extracted after the decay of a substantial portion of the 99Mo. 15. A method according to claim 1, wherein the exposed material comprises 130Te as said first isotope, which produces 131Te by capturing neutrons from the flux, said 131Te decaying into the useful radio-isotope 131I. 16. A method according to claim 15, wherein the exposed material comprises metallic tellurium, which is melted after the neutron exposure so as to volatilise the iodine content thereof. 17. A method according to claim 1, wherein the exposed material comprises a fissile element as said first isotope, which produces fission fragments by capturing neutrons from the flux, said useful isotope being a radio-isotope extracted from said fission fragments. 18. A method according to claim 1, wherein the exposed material comprises 124Xe as said first isotope, which produces 125Xe by capturing neutrons from the flux, said 125Xe decaying into the useful radio-isotope 125I. 19. A method according to claim 1, wherein the exposed material comprises a semiconductor material, and the useful isotope is a doping impurity within said semiconductor material, which is obtained from neutron captures by a first isotope of the semiconductor material. 20. A method according to claim 19, wherein the semiconductor material consists of silicon, with 30Si as said first isotope producing 31Si by capturing neutrons from the flux, said 31Si decaying into 31P as an electron-donor doping impurity. 21. A method according to claim 19, wherein the semiconductor material consists of germanium, with 70Ge as said first isotope producing 71Ge by capturing neutrons from the flux, said 71Ge decaying into 71Ga as an electron-acceptor doping impurity, and also with 74Ge producing a smaller amount of 75Ge by capturing neutrons from the flux, said 75Ge decaying into 75As as an electron-donor doping impurity.