In order for physicists to understand the reasons for beta decay, a new nuclear force had to be developed - a task accomplished by Enrico Fermi in 1934.

Of the three forms of radioactive decay – alpha, beta, and gamma – it was the middle form, beta, which was the most difficult for physicists to finally understand. While alpha radiation took advantage of an already-existing law of physics (the strong force which holds atomic nuclei together) and only needed the addition of a quantum mechanical explanation (which came in the form of “quantum tunneling” as proposed in 1928 by George Gamow) and gamma rays were well understood as being the manifestation of the “excess” energy accompanying the decaying process, to understand beta radiation would require the addition of an entirely new physical force.

To understand why this is a big deal, one must realize that even today there are only four known forces acting within the entire universe – Gravity, electromagnetism, strong nuclear and weak nuclear. This means that, just to understand this one phenomenon of beta decay, a rather huge element of physics had to be established. Beta decay, therefore, was a crucial step in the history of physics.

The Need for a New Force

It was the famed Italian physicist Enrico Fermi who first realized, in 1934 the need for a weaker version of the nuclear force in order to explain beta decay. On the surface, it doesn’t seem very difficult; after all, beta decay is made up of simple electrons, which aren’t bound by the strong force – so it doesn’t seem like much of a stretch that they might be able to escape from their “orbits” around atomic nuclei.

In his experiments involving beta decay, Fermi noticed that there were some aspects of the process which this way of thinking did not explain, including the fact that the electron did not seem to come from the normal cloud of electrons surrounding the atom’s nucleus (if it did, the atom would become ionized, being left with a positive net charge, which was not the case). Rather, the beta particle appeared to be an entirely new electron, created somewhere within the atomic nucleus specifically for this process.

In addition, it had been noted as early as 1911 that the energy emitted during beta decay in the form of the electron was often times less than the total change in the atom’s mass (which can be measured using Einstein’s equation - E = mc²). This appeared to be a contradiction to the law of conservation of energy, so there had to be some other outlet by which some other energy was released in beta decay which had not yet been measured.

The Weak Nuclear Force

The solution Fermi proposed was a rather radical one at the time. In his model of the weak interactions within atoms from 1934, Fermi postulated that what happened during beta decay was this: A neutron spontaneously decays into both an electron and a proton (thus, the electron emitted would carry with it a negative charge, giving the neutron a positive charge, thus making it into a proton – a common phenomenon in the theory of the strong nuclear force).

Neutrinos and the Weak Force

Along with this interaction, according to Fermi’s model, was an all new particle whose sole job was to carry away the extra energy, dubbed the antineutrino. This particle was important because it was in this particle that the “missing” energy from the beta decay was released, but had not been discovered yet by physicists due to a couple important reasons:

First – it is a neutral particle (as the charge within the nucleus was conserved by the release of the electron, and according to basic laws of physics, no new charge, either positive or negative, can be created), and the particle detecting tools at the time could only detect charged particles.

Second - it became clear that this was perhaps the most “ghost-like” particle ever theorized. A neutrino possesses the superhero-like ability to pass through almost any amount of matter without so much as striking a single atom.

Fermilab’s famous physicist Leon Lederman explained just how difficult these things were to detect:

Neutrinos float through vast thicknesses of matter unscathed because they obey only the weak force, whose short range reduces the probability of a collision enormously. It was estimated that to ensure a collision of a neutrino with matter would require a target of lead on light year thick! Quite an expensive experiment. However, if we use a very large number of neutrinos, the required thickness to see a collision every once in a while is correspondingly reduced.

So, the only way to detect a neutrino was to try and find a way to make the neutrino collide with something else and then measure the reaction. Again, we have a particle that we can never really see, beyond how it effects other particles.

While it would be some time before some of the kinks were worked out of the theory of the weak force and some of the more complicated aspects were fully understood (including the addition of three more particles – the W+, W- and the Z – whose job it was to “carry” the weak force between particles, which would come about in the sixties and win some Nobel prizes for some physicists.)

The work of Enrico Fermi and others in the 1930’s formed the backbone upon which the theories of the weak force would be built, and an important stepping stone it was. While it may seem to be a rather mundane physical phenomenon from a macroscopic perspective, the weak force and the consequent beta radiation is absolutely essential for life itself, as are the other three forces.

Thus, it was finally understood after this point how and why all three forms of radiation came about. Physicists could then begin to understand some of the even deeper mysteries of the atomic structure –which were a great many, to be sure.

References:

Gribbin, J. (1994). In Search of Schrodinger's Cat: Quantum Physics and Reality. New York, NY: Bantam Books.

Holzner, S. (2006). Physics for Dummies. Indianapolis, IN: Wiley Publishing, Inc.

Isaacs, A. (2003). Dictionary of Physics. London: Grange Books.

Lederman, L. (1993). The God Particle: If the Universe is the Answer, What is the Question? New York: Dell Publishing.

Asimov, I. (1966). Understanding Physics: 3 Volumes in 1. Barnes and Noble Books.