Charged particles, in contrast to neutrons, cannot directly penetrate the nucleus due to the electrostatic repulsion imposed by the Coulomb barrier, which is the energy barrier caused by the positive charge of the nucleus repelling other positively charged particles. To initiate a nuclear reaction, these charged particles must possess sufficient kinetic energy to overcome this barrier. Unlike neutrons, which are electrically neutral and can diffuse into the nucleus without such energy considerations, charged particles require acceleration to high velocities. This acceleration enables them to reach kinetic energies that surpass the Coulomb barrier, thereby opening access to a broader spectrum of nuclear reaction channels compared to the fast neutrons typically utilized in nuclear reactors. Some advanced particle accelerators demonstrate the potential of using charged particles such as protons (p), deuterons (d), tritium nuclei (3He), and alpha particles (4He, also known as α) to achieve practical and cost-effective nuclear reactions. These accelerators provide alternative opportunities for generating energy and producing isotopes, emphasizing their relevance in both scientific research and industrial applications.
Various nuclear reactions produce 123I, and these reactions can be performed at relatively low particle energies. Some of the reactions are:
In the first reaction, where five neutrons are expelled, the proton energy requirement is about 5 × 101 MeV = 50 MeV, which is the most energy demanding. The rule of thumb is that about 10 MeV are required per expelled particle. All the other reactions typically require 20 MeV or less.
One of the advantages of accelerator production is the ease of finding nuclear reactions where the product is a different element from the target. Since different elements can be separated chemically, the product can usually be of high specific radioactivity, which is important when labeling biomolecules.
A technical difference between reactor and accelerator irradiation is that in the reactor, the particles come from all directions, but in the accelerator, the particles have a particular direction. The number of charged particles is often smaller and is usually measured as an electric current in microamperes (1 μA = 6 × 1012 protons/s but 3 × 1012 alpha/s because of the two charges of the α particle).
| Proton Energy (MeV) | Accelerated Particles | Used For |
|---|---|---|
| < 10 | Mainly single particle, p or d | PET |
| 10–20 | Usually p and d | PET |
| 30–40 | p and d, 3He and 4He may be available | PET, commercial production |
| 40–500 | Usually p only | National centres with several users |
One challenge with accelerator production is that charged particles are stopped more efficiently than neutrons. For example, 16 MeV protons are stopped in 0.6 mm of Cu. A typical production beam current of 100 μA hitting a typical target area of 2 cm2 will put 1.6 kW in a volume of 0.1 cm3, which can evaporate most materials if not efficiently cooled.
In addition, the acceleration of the beam occurs in a vacuum, but the target irradiation takes place at atmospheric pressure or in gas targets at 10–20 times overpressure. To separate the vacuum from the target, the beam has to penetrate foils that absorb some of the particle energy, and these foils will also become strongly activated.