Published: 18 April, 2007

Bombardment of nuclei is necessary for the creation of synthetic elements

How do you make synthetic elements?

An atom is the smallest part of an element that can exist alone or in combination with other elements. It is composed of a dense core called a nucleus, which contains protons (positively charged; #protons = atomic #) and neutrons (no charge) of roughly equal mass, and much smaller, negatively charged electrons that occupy the vast space about the nucleus. Most elements exist as mixtures of various isotopes (_{at. Number}Symbol^{at. Mass}) that have different numbers of neutrons. For example, carbon consists of a mixture of ₆C¹² (6 neutrons; 98.89%) and ₆C¹³ (7 neutrons, 1.11%).

An element not found in nature (i.e., synthetic) is synthesized by bombarding the nucleus of a target element with smaller particles such as protons, a-particles (He nuclei) or neutrons. Because of their positive charges, protons and a-particles need to be accelerated to enormous velocities to get them to collide and "react" with the positively charged nucleus of a target atom; the repulsion of like charges must be overcome! Since neutrons are not charged, the "repulsion problem" is circumvented, and they are very important reagents for synthesizing radioisotopes. All synthetic elements are radioisotopes, which are isotopes that undergo measurable nuclear decay. A target element is placed in a chamber in a nuclear reactor that is subject to a neutron flux in order to transmute it into another isotope or different element. There are fancier neutron sources, such a spallation sources where neutrons are themselves a product of a bombardment, but nuclear reactors are the only practical means of synthesizing useful quantities of radioisotopes. For example, ₄₃Tc^99m is produced from ₄₂Mo⁹⁸ in this manner. Tc turns out to be rather benign, and is injected in the body in various chemical forms to image the heart and detect certain cancers; it is "observed" by loss of gamma rays, and its use in medicine has saved thousands of lives.

Radioisotopes (some occur naturally!) are also produced as a byproduct of fission in nuclear reactors, and some can be harvested by separating them from others in a nuclear fuel rod. In a fission reaction, a fuel, typically ₉₂U²³⁵, ₉₂U²³⁸ or ₉₄Pu²³⁹ depending on the type of reactor, absorbs a neutron, becomes unstable, and splits into two products, releasing more neutrons and a tremendous amount of energy in the process. Because of separation difficulties and the danger of working with spent fuel rods, the synthesis of elements from fission is mostly limited to research purposes. However, neutrons generated through fission can be captured by ₉₂U²³⁸ to ultimately produce ₉₄Pu²³⁹, which is a fuel itself. This is essentially the way a breeder reactor -- a reactor that produces more fuel than it consumes -- operates.