Optimizing the radioisotope production of the novel AMIT superconducting weak focusing cyclotron

Abstract

Nuclear imaging techniques are becoming one of the most widely used medical diagnostics tools for certain illness such as cancer and Alzheimer disease. The increase in these medical procedures, particularly positron emission tomography, is leading to a saturation of the actual radioisotope production system. Therefore, particle accelerators, specially the cyclotron, emerged as an alternative to the traditional supply system based on centralized production in nuclear reactors. Its characteristics from the physical and technological point of view allow a controlled and localized production, especially relevant in the case of short-lived radionuclides, through a well-known technology developed for decades without the use of a large and expensive facilities. With that in mind, the AMIT project (Advanced Molecular Imaging Technologies) aims to extend the use of these medical procedures with the development of a new compact cyclotron focused on the on-site short-life radioisotopes production, specifically 11 C and 18 F , in hospitals and research centers. In order to achieve this main objective, the AMIT cyclotron is based on a classical weak focus configuration with high magnetic field provided by a superconducting magnet with an autonomous cryogenic system. In addition, with the aim of reducing the total size of the accelerator, the cyclotron employs an internal H− ion source with an electron stripping system that provides a final proton beam that is transported to the production target. This thesis evaluates the challenging combination of all the technical characteristics of the AMIT cyclotron, which results in a balance of the beam dynamics with all the subsystems to achieve an optimal radioisotope production. For this goal, all the physical processes associated with the beam acceleration from the ions production and the injection into the accelerator, to the extraction of the resulting beam and its transport to the target are studied by means of theoretical analysis, computational calculations and experimental measurements. Firstly, the experimental results obtained in the characterization campaigns of the cyclotron ion source are presented. The output beam current, and therefore, the fulfillment of the main requirements of the accelerator, depends largely on the current injected into the accelerator. For this reason, the knowledge related to the operation of the ion source and the resultant beam has been analyzed in a dedicated facility at CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas). The beam dynamics have been assessed by means of simulations carried out with the OPAL code (Object Oriented Parallel Accelerator Library). The study includes all the relevant effects from the point of view of particle accelerator physics, with special attention to those characteristics that allow the precise adjustment of the cyclotron operation and the optimization of the radioisotope production under different conditions. For this purpose, the adjustment of the different subsystems within the design tolerances and the possible modification of some of them in their final assembly are analyzed, as well as the impact of some significant parameters variations in the magnetic field, the radiofrequency system, the acceleration gap or the beam extraction system. In addition, a relevant feature in a compact H − cyclotron with internal ion source is the beam interactions with the residual gas and the electromagnetic field. Therefore, the computational implementation of these physical processes within the opal code is presented, as well as their application in the AMIT cyclotron in order to minimize the beam losses and optimize the final beam current with the control of the vacuum conditions in the accelerator. All these studies allow to combine the different elements of the cyclotron with the beam dynamics for an optimal and efficient radioisotope production, essential for the commissioning and the optimization of the cyclotron operation.