Answer to Question #843 Submitted to "Ask the Experts"

Q

While doing some research about ionizing radiation, I was looking at beta particles. Since beta particles do not originate from the electron shell, but from the nucleus of an atom, one of my sources claimed that a neutron decays into an electron and a proton. Later on in my research I also read how beta particles can be both positively and negatively charged! (Source: http://www.epa.gov/radiation/ionize.htm) To begin with, I would like to ask you if the first two points are true. If they are not, then that would be sufficient of an answer to me. If they are, then I would like to follow up with this wild guess. If an electron is ejected from a nucleus and a neutron is turned into a proton, does it mean that if a positive electron (positron) is ejected from a neutron, the neutron will turn into an antiproton? And to take this a step farther, if the above is true, would it mean that a proton and antiproton in the nucleus would annihilate each other? And what consequences could that possible have on an atom?!

A

Thank you for the interesting question. To start with the easier of the questions, beta particles, or beta radiation, are indeed electrons that are emitted from the nucleus, not from the orbital electron shells. The beta particles can be either negative or positive. We call the positive beta particle a positron, and technically, it is antimatter or, more appropriately, it is the antiparticle of the negative beta particle. In addition to the beta, an antineutrino is emitted during negative beta emission and a neutrino from positive beta emission. We sometimes call these emissions a decay or disintegration, but whatever we call it, they are nuclear processes. All matter is, of course, made up of atoms and all atoms are made up of smaller parts, which we like to think of as discrete particles in the nucleus called protons and neutrons, with electrons orbiting the nucleus. The protons repel each other because they have like charges, but all the protons and neutrons are held together in the nucleus. This is the Standard Model of the atom. But, there are many other particles that we have to include to understand the beta-radiation emission. Protons and neutrons themselves are made up of smaller particles, called quarks. A proton is made up of two "up" quarks and one "down" quark. A neutron is made up of one "up" quark and two "down" quarks. The "up" and "down" are just names for these two types of quarks, but there are many other ones too. The up quark has a two-thirds electrical charge and the down quark a negative one-third charge. As you can probably start to tell, atoms are not as simple as we like to think and the nucleus is a complex place, where forces interact with particles and matter and energy change back and forth. There are four forces in nature: electromagnetic, strong nuclear, weak nuclear, and gravity. Gravity is perhaps the most familiar force to us, but it is not strong enough to be considered for nuclear particles. But the other three forces are of some importance. Electromagnetic forces are also familiar; they are responsible for binding the electrons to the nucleus to form electrically neutral atoms. Atoms combine to form molecules or crystals because of electromagnetic effects due to their charged substructure. Most everyday forces, such as the support of the floor or friction, are due to the electromagnetic forces in matter that resist displacement of atoms or electrons from their equilibrium positions in the material. In particle processes the forces are described as due to the exchange of particles; for each type of force there is an associated carrier particle. The carrier particle of the electromagnetic force is the photon; gamma ray is the name given to a photon from a nuclear transition. For distances much larger than the size of an atomic nucleus, the remaining two forces have only tiny effects—so we never notice them in everyday life. But we depend on them for the existence of all the stuff from which the world is made, and for the decay processes that make some types of matter unstable. The strong force holds quarks together to form hadrons; its carrier particles are whimsically called gluons because they so successfully "glue" the quarks together. The binding of protons and neutrons to form nuclei is a residual strong interaction effect due to their strongly interacting quark and gluon constituents. Leptons have no strong interactions. Weak interactions are the only processes in which a quark can change to another type of quark, or a lepton to another lepton. They are responsible for the fact that all the more massive quarks and leptons decay to produce lighter quarks and leptons. That is why stable matter around us contains only electrons and the lightest two quark types (up and down). The carrier particles of weak interactions are the W and Z bosons. Beta decay of nuclei was the first observed weak process: in a nucleus where there is sufficient energy a neutron becomes a proton and gives off an electron and an antineutrino. This decay changes the atomic number of the nucleus. Beta radiation is the name given to the emerging electron. When a neutron decays into a proton, one of the down quarks ends up becoming an up quark, and, in the process, a negatively charge electron and antineutrino are formed. The electron and neutrino are a lepton, antilepton pair. This pair is emitted by a down quark as it transforms itself into an up quark. (Actually, the down quark emits a W-particle which "decays" into the lepton, antilepton pair, but that is not essential to the fact that lepton number is conserved). In a stable nucleus, neutrons do not decay. A free neutron, or one bound in a nucleus that has an excess of neutrons, can decay by emitting a beta particle. Sharing the energy with the beta particle is a neutrino. The neutrino has little or no mass and is uncharged, but, like the photon, it carries momentum and energy. The source of the energy released in beta decay is explained by the fact that the mass of the parent isotope is larger than the sum of the masses of the decay products. Mass is converted into energy just as Einstein predicted. While you did well with the beta particle emission, you are not correct with your guess on the positron emission. No antiproton is formed. In the simplest terms, when a positron is emitted from the nucleus, the result is that a proton is "made" into a neutron, the positron is emitted along with the neutrino. Perhaps this makes little sense when you look at both processes, the negative and positive beta emissions. But, the nuclear processes involved are similar and the resulting masses and energy are all conserved during it. I hope this helps you understand the beta emissions. Bruce Busby, RSO