Radioactive decay is a natural process where an unstable atomic nucleus loses energy by emitting radiation, transforming into a more stable state. Beta decay represents one of the fundamental types of this nuclear transformation. It is a process of nuclear transmutation, meaning the original element changes its identity to become a different one.
Defining the Transformation
Beta decay occurs in nuclei that possess an unbalanced ratio of neutrons to protons. A nucleus with an excess of neutrons, or one with too many protons, seeks stability by converting one type of nucleon into the other. This conversion is the defining characteristic of beta decay at the nuclear level.
The transformation involves a neutron changing into a proton or a proton changing into a neutron. This internal nuclear change ejects a high-energy particle, known as a beta particle, and results in a new element. The mass number of the atom, which is the total count of protons and neutrons, remains the same because one nucleon is simply changing its identity within the nucleus. However, the atomic number, which is the count of protons, shifts up or down by one, signifying the creation of a new element.
The Three Forms of Beta Decay
The imbalance within the nucleus can be corrected through three distinct processes, all categorized as beta decay. The most common form is beta-minus (\(\beta^-\)) decay, which occurs in nuclei with an excess of neutrons. In this process, one neutron is converted into a proton, an electron (the beta particle), and an electron antineutrino, increasing the atomic number by one.
The second form is beta-plus (\(\beta^+\)) decay, which is the mechanism used by proton-rich nuclei to achieve stability. Here, a proton converts into a neutron, a positively charged particle called a positron (the beta particle), and an electron neutrino, which decreases the atomic number by one. The positron is the antimatter equivalent of an electron, possessing the same mass but a positive charge.
The third process is electron capture (EC), which is an alternative decay route for proton-rich nuclei. Instead of emitting a positron, the nucleus captures an electron from the atom’s innermost electron shell, combining it with a proton to form a neutron. This conversion reduces the atomic number by one, achieving the same result as \(\beta^+\) decay. The vacancy left by the captured electron leads to the subsequent emission of characteristic X-rays or Auger electrons.
The Role of the Weak Force and the Neutrino
The entire process of beta decay is mediated by the Weak Nuclear Force. This force is responsible for the fundamental change of one type of subatomic particle, or quark, into another. Specifically, in \(\beta^-\) decay, a down quark within the neutron changes into an up quark, transforming the neutron (two down, one up quark) into a proton (one down, two up quarks).
This transformation is physically carried out by the exchange of a massive subatomic particle, the W boson, which exists only momentarily. The W boson then immediately decays into the emitted beta particle and its corresponding neutrino or antineutrino. This mechanism explains how particles that did not exist in the nucleus beforehand are spontaneously created and ejected.
The existence of the neutrino was first hypothesized to solve a problem involving the conservation of energy and momentum. Scientists observed that the emitted beta particles had a continuous spectrum of kinetic energy, meaning they did not all have the same energy. Wolfgang Pauli suggested that a third, unobserved particle—the neutrino or antineutrino—must be carrying away the missing energy and momentum in each decay event. The neutrino is an extremely light, electrically neutral particle that interacts with matter so weakly that it can pass through the entire Earth undetected.
Practical Uses of Beta Emitters
The particles released during beta decay are utilized in a variety of applications, particularly in medicine and science. In medical imaging, \(\beta^+\) emitters like Fluorine-18 are used in Positron Emission Tomography (PET) scanning. The emitted positron quickly encounters an electron in the body, leading to an annihilation event that produces detectable gamma rays, which map biological processes.
Beta-minus emitters are effective in targeted radiation therapy because the electrons they emit have a short range in tissue. Isotopes such as Iodine-131 are used to treat thyroid cancer, as the iodine naturally accumulates in the thyroid tissue, allowing the beta particles to destroy cancerous cells locally. In scientific research, the \(\beta^-\) emitter Carbon-14 is the foundation of radiocarbon dating. The predictable decay rate of Carbon-14 allows scientists to accurately determine the age of organic materials spanning tens of thousands of years.