Alpha particles are emitted from a radioactive substance by way of quantum tunneling and is governed by a statistical look at the strong nuclear force.

After having realized that the radiation emitted from certain atoms (generally those with massive nuclei, like Uranium or Thorium) existed in three very distinct forms – Alpha particles (Helium nuclei), Beta particles (escaping electrons) and gamma rays (high intensity, short wavelength electromagnetism) – work on the subject was nowhere near complete. Still left to be determined was perhaps the most important question of all: Why radiation occurred in the first place.

Spontaneous Radioactive Decay

As Ernest Rutherford witnessed, all three types of radiation seemed prone to jumping randomly out of particles, which made it very difficult to either fully measure or fully predict.

The emission of these particles from an atom at random (and truly, random is just the right word to use for the phenomenon) is what is known in science as radioactive decay. It only occurs in certain isotopes of certain atoms (an isotope is a form of an element containing the same number of protons but a different number of neutrons), which are known as radionuclides (such as Carbon-14, Flourine-18 and Thallium-201, just to name a few). It only occurs in these certain isotopes because they consist of a number of protons/neutrons within their nucleus which together become very unstable.

While there are certain forces in place within an atom to hold the thing together (specifically the strong nuclear force), sometimes it is only by a fine thread that everything is kept relatively stable.

There remained, however, things that Rutherford was still unable to explain about radiation, specifically spontaneous alpha decay. While certain isotopes of certain elements can indeed be particularly unstable, and more prone to radioactivity than others, it was still apparent that the force which held the elements of the nucleus together was far too strong to explain how a piece of the nucleus can simply “jump ship” so suddenly. It was also known, however, that this force, though incredibly strong, also seemed to only be able to cover very tiny distances (roughly the distance of the atomic nucleus.

Quantum Tunneling and Decay

It was in 1928 that Ukrainian Physicist George Gamow finally solved this mystery in the form of “quantum tunneling” – a phenomenon which utilizes the highly statistical aspects of quantum mechanics.

What Gamow realized was that, like everything else in the quantum world, even this basic principle is governed by laws of probability – while it is highly improbable that any of these parts of the nucleus (called nucleons) could ever possess the power to free themselves from the bond of the strong force, there is a probability, though a very small one, that just such a thing could spontaneously occur (even though it might seem against the laws of physics), and a particle may suddenly and inexplicably find itself freed from the restrictive clutches of the nuclear forces.

It may be against common sense, but this is how it works. In this way, a piece of alpha radiation can “tunnel” its way through a seemingly impenetrable barrier and escape from its nuclear prison.

Technically speaking, the idea of quantum tunneling is something akin to saying that if one was to throw themselves into a steel wall enough times, and with enough force, eventually they’d spontaneously find themselves standing on the other side, having spontaneously tunneled their way through, while the wall itself remains intact. This is essentially what protons and neutrons do on a regular basis.

What Makes an Isotope Unstable?

The reason that radioactive decay really only effects large nuclei is due to the short range of the strong force. If a nucleus gets large enough, such as in a uranium atom, the strong force is not quite far-reaching enough to be as effective in holding everything together successfully, giving the nucleons a much higher probability of tunneling their way free of its grip, thus making the atom very unstable. Furthermore, it is the energy released when an unstable nucleus finally breaks apart which creates gamma rays.

So, in essence, alpha radiation (and the gamma radiation which is a bi-product) is governed by the strong nuclear force (even though this is the force which, ironically, is meant to keep everything together).

Beta particles, on the other hand, are a different story altogether, as they are actually governed by the Weak Nuclear Force – a force whose job it is to “undermine” the strong nuclear force and steal energy away from the atomic nucleus.

For an introduction to beta radiation, click here.

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.