19.9: Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with another particle. The first manmade nucleus was produced in Ernest Rutherford’s laboratory in 1919 by a transmutation reaction, the bombardment of one type of nuclei with other nuclei or with neutrons. Rutherford bombarded nitrogen-14 atoms with high-speed α particles from a natural radioactive isotope of radium and observed protons being ejected from the reaction. The product nucleus was identified as oxygen-17 in 1925 by Patrick Blackett.

To reach the kinetic energies necessary to produce transmutation reactions, devices called particle accelerators are used. These devices use magnetic and electric fields to increase the speeds of nuclear particles. In all accelerators, the particles move in a vacuum to avoid collisions with gas molecules. When neutrons are required for transmutation reactions, they are usually obtained from radioactive decay reactions or from various nuclear reactions occurring in nuclear reactors.

Many artificial elements have been synthesized and isolated, including several on a large scale via transmutation reactions. The elements beyond element 92 (uranium) are called transuranium elements. These elements were all discovered via transmutation reactions, although elements 93 and 94, neptunium and plutonium, were subsequently found in nature as uranium decay products.

Neptunium-239 was created by bombarding uranium-238 with neutrons. The reaction creates unstable uranium-239, with a half-life of 23.5 minutes, which then decays into neptunium-239. Neptunium-239 is also radioactive, with a half-life of 2.36 days, and it decays into plutonium-239.

Plutonium is now mostly formed in nuclear reactors as a byproduct during the decay of uranium. Some of the neutrons that are released during U-235 decay combine with U-238 nuclei to form uranium-239; this undergoes β⁻ decay to form neptunium-239, which in turn undergoes β⁻ decay to form plutonium-239.

Nuclear medicine has developed from the ability to convert atoms of one type into other types of atoms. Radioactive isotopes of several dozen elements are currently used for medical applications. The radiation produced by their decay is used to image or treat various organs or portions of the body, among other uses.

This text is adapted from Openstax, Chemistry 2e, Section 21.4: Transmutation and Nuclear Energy.

Nuclear transmutation is the conversion of one element into another, which is possible via radioactive decay, nuclear fusion, and nuclear fission.

Additionally, Ernest Rutherford demonstrated that nitrogen-14, when hit with a fast-moving alpha particle, produces a proton along with a different nuclide, which, a few years later, was identified as oxygen-17 by Patrick Blackett. 

The condensed notation of the process lists, in order, the target nucleus, the bombarding and ejected particles, and the product nucleus. Neutrons and alpha particles are common bombarding particles in transmutation processes. 

The elements with atomic numbers greater than 92 are called transuranium elements. These elements are common targets of transmutation experiments because they are entirely synthetic except for neptunium and plutonium, which are also produced naturally in uranium decay chains.

For example, neptunium-239 is generated in specialized nuclear reactors by bombarding uranium-238 with fission neutrons. Being electrically neutral, neutrons encounter no electrostatic repulsion from the nuclei, so fission speeds are sufficient for this transmutation. The radioactive neptunium-239 subsequently decays into plutonium-239. 

In further experiments, plutonium-239 is hit with high-speed alpha particles to yield curium, which has an atomic number of 96. Unlike neutrons, alpha particles must overcome electrostatic repulsion exerted by the positively charged target nuclei and thus require greater kinetic energy.

Notably, the electrostatic repulsion is greater with larger nuclei like plutonium-239 than with smaller nuclei like the nitrogen-14 used in the Rutherford and Blackett experiments.

Particle accelerators, which include linear accelerators and cyclotrons, impart the desired high speeds to charged nuclear particles.

A multistage linear accelerator has a series of tubes of increasing lengths and alternating polarities. An oscillating electrical potential rapidly switches the polarities so that the charged particles are alternately attracted and repelled by each tube.

The particle accelerates as the tubes get longer, ultimately reaching speeds that can exceed 90% of the speed of light. In a cyclotron, alternating voltage instead accelerates the particle in a spiral path.

Particle accelerators can bombard nuclei even with other relatively large nuclei, like bombardment of lead-208 with a beam of zinc-70. The product transuranium element, copernicium-277, generates thirteen transuranium elements through its major decay chain, which eventually leads to bismuth-209.