An isotope is the structural unit of a chemical element whose atomic nucleus contains a specific number of protons and neutrons which are known as nucleons. There are two types of isotope configurations in the nucleus, stable and radioactive. Stable isotopes emit no radiation whereas radioactive isotopes emit radiation during a transformation within the nucleus to achieve a more stable configuration of nucleons in the nucleus. This transformation process is known as radioactive decay.
Most isotopes in nature, such as sodium, have been around since the beginning of the universe and are stable. However there a few radioisotopes in nature that are radioactive and have very long half-lives such as uranium-238. Uranium-238 is a naturally occurring radioisotope and has a half-life of 4.5 billion years! There are a few other sources of naturally occurring radioisotopes such as cosmic radiation originating from the sun and the earth’s atmosphere. The rest of the radioisotopes encountered in the world are man-made. When a combination of neutrons and protons, is produced artificially, the atom will usually be unstable and is called a manmade radioactive isotope or radioisotope. Overall there are some 1800 radioisotopes. At present, there are up to 200 radioisotopes used on a regular basis, and most must be produced artificially.
Radioisotopes aree manufactured in several ways. The two most common methods are via a nuclear reactor or a particle accelerator. Production using a reactor requires nuclear fuel such as highly enriched uranium-235. The fission or “splitting” of the uranium atom leads to production of large amounts of neutrons. This neutron “flux” can either be used to further split more uranium atoms or the neutrons can be focused onto stable target materials to make radioactive isotopes using the neutrons. While reactor technology for radioisotope production is well proven, there is a global focus to move away from reactors for radioisotope production, as uranium-235, is also of great concern with respects to applicability in weapons of mass destruction. Another drawback of nuclear reactors is their mechanism of operation which require elaborate safety systems to prevent the nuclear fuel from overreacting and a meltdown occurring.
Production of radioisotopes using a particle accelerator utilizes charged particles such as protons (a positively charged hydrogen atom). These protons are accelerated using magnets and alternating electrical fields. These particles can achieve very high velocities (some up to 75% of the speed light) before they smash into stable target atoms. When this collision occurs the target nucleus takes up the high energy proton and de-excites by emitting particles and radiation to form radioisotopes. These man-made radioisotopes then further de-excite to form more stable configurations by emitting radiation. This radiation can be in the form of a gamma ray or a particle such as a positron. The new daughter nucleus is now more stable than its parent. However, daughter nucleuses themselves can also be radioactive and this cycle can continue until the nucleus reaches a stable state.
Particle accelerators are broken down into two categories-linear accelerators (LINACS) and cyclotrons. Linear accelerators are generally straight “linear” machines which have a large footprint and occupy large tracts of land. Cyclotrons are much more compact machines and even the largest machines can fit inside a moderate sized warehouse. LINACS are capable of achieving proton energies of hundreds of MeV. Cyclotrons are limited to less than 100 MeV. The most useful proton energies for radioisotope production are less than 150 MeV as it allows for maximization of product radioisotopes while minimizing coproduction of radioactive waste isotopes.
In either case, particle accelerators are a much safer solution for radioisotope production versus reactors as there is no special nuclear material to manage as the fuel source . As particles accelerators rely solely on electricity for operation, a simple electrical disconnect is all that is required to terminate radioisotope production. This simple disconnect is not an option in taking a reactor out of service. The reactor is a self-sustaining machine which must be operated with a high level of oversight and an equal amount of oversight maintained when the reactor is taken out of service.
The Positron solution to the radioisotope shortage in the US and as part of our commitment to producing radioisotopes in the most environmentally responsible manner is the high energy/high current cyclotron. This is the safest method of isotope production and over time can produce a suite of radiosiotopes which will provide radiopharmaceutical solutions to not only cardiac imaging but many other maladies.
For more information about Positron's 70 MeV Cyclotron project click here.
