There are several types of accelerator, all of which can, in principle, be used for radionuclide production. The dominant one for radionuclide production is currently the cyclotron that was invented by Lawrence in the early 1930s. Cyclotrons were first installed in hospitals in the 1960s, but during the past two decades, hospital-based small cyclotrons yielding 10–20 MeV protons have become fairly common, especially with the rise of PET.
A cyclotron is composed of four systems:
The inside of a cyclotron , The ion source , is usually placed inside the vacuum and in the center (internal), but, in larger machines, can be external. The ions are then injected from the outside through a central hole in the magnet. The main idea of the ion source is to have a slow flow of gas that is made into plasma by an arc discharge. The desired ion species are extracted through a collimator and accelerated in a static electric field. There are several types of ion source with different operating characteristics. In modern accelerators, negative ions, protons, or deuterium with two orbit electrons are usually used. These facilitate extraction of the beam.
The ions leave the ion source with some velocity. Since the vacuum chamber is in a magnetic field, the ions move in a circular orbit. Inside the vacuum chamber, there are two electrodes, historically called the ‘Dees’ since the first ones have the shape of the letter D. These electrodes are hollow, with the electrodes called the acceleration gap. If a voltage is applied between the electrodes, the ions will experience the potential gradient when traversing the gap between the electrodes. If the voltage polarity is switched at the correct rate, the ions will be continuously accelerated when crossing the gap, thus resulting in an increase in the ions’ energy and velocity. As their velocity increases, the ions will move into a circular orbit of increasing radius. The time taken for the ions to return to the gap is independent of their radius in accelerators 30 MeV.
For the cyclotron to operate correctly, it is necessary for the frequency of the electric field across the Dees to be the same as the frequency of the circulating ions, so that the polarity changes upon each traversal of the ions across the Dees. In commercial accelerators, with high beam currents of several milliamperes, it is usual to have an internal target for radionuclide production located inside the chamber. In accelerators with lower beam currents 100 μA, such as those dedicated for PET hospital facilities, it is more common to extract the beam onto an external target system.
The modes of extraction depend upon whether positive or negative ions are accelerated. Extraction of positive ions is made by using a deflector that applies a static electric field which acts upon the particles when in the outer orbits. Some beam current is invariably lost in the process and the deflector often becomes quite radioactive. Modern proton/deuterium accelerators usually accelerate negative ions that are more easily extracted. In these systems, a thin carbon foil is used that will strip away the two orbit electrons. As a consequence, the particles suddenly change from negative to positive charge and are effectively bent out of the magnetic field with an almost 100% extraction efficiency and with little activation.
The extracted beam can either be transported further in a beam optical transport system or will hit a production target directly. The target is usually separated from the vacuum by metallic foils that are strong enough to withstand the pressure difference and the heat from the beam energy, as it is transferred and absorbed by the foils. The reason why two foils are used is that the heat produced by the beam passage has to be removed, which is facilitated by a flow of helium gas between the foils. Helium is preferred as the cooling medium since no induced activity will be produced in this gas.
Note: The extraction efficiency, operating conditions, and the presence of cooling mechanisms are key aspects of a cyclotron's operation for radionuclide production.