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The ratio of protons to neutrons in the nucleus is also important to stability. The lighter
elements are usually stable, with a ratio of about 1:1. As elements get heavier, the ratio
reaches about 2:3, increasing the chances of instability. Whenever the ratio shifts from a
stable configuration to an unstable one (for whatever reason), the nucleus attempts to
regain stability by emitting radiation.
When an unstable atom emits radiation, it is said to decay. The half-life of a radionuclide
is the average amount of time that it takes for half of the atoms in a sample to decay.
Half-lives can vary from fractions of a second to millions of years. When an atom decays,
it does not necessarily become stable. Rather, it is quite common for decay to be a series
process, in which a given atom might decay dozens of times in different ways before
becoming stable. Radon gas is a common example of this, with a very long decay chain.
Radioactive decay can be associated with several different types of radiation.
Photons (x-rays and gamma rays). Photons are (nearly) massless “bundles” of energy.
The energy determines the wavelength, which can be classified into various categories for
convenience. Radio waves, visible light, ultraviolet light, x-rays, and gamma rays are all
photons of various energies. X-rays and gamma rays differ only in their origin. An
electron losing energy yields an x-ray photon, while loss of energy from a nucleus
exciting produces a gamma photon. X-rays and gamma rays with the same energy are
otherwise identical.
Gamma or x-rays are often released to remove residual energy that remains after other
forms of decay (e.g., alpha or beta particles). For example, 60Co, a common industrial
and medical radionuclide, decays by beta emission, while simultaneously giving off two
powerful gamma rays during its transformation into the stable element 60Ni.
Beta particles (electrons). One of the ways that an unstable nucleus can adjust its
proton:neutron ratio is to emit an electron, effectively transforming a neutron into a
proton and an electron. This electron is then ejected from the nucleus as a “beta particle,”
which in all respects is identical to any other electron. Beta particles can cause damage to
tissue directly or through secondary processes.
Bremstrahlung. When an electron decelerates (or turns), it loses energy in the form of
photons, which is a process called “bremstrahlung.” The quicker the deceleration, the
greater the energy imparted to the photons. Electron paths tend to curve near heavy
nuclei, so that electrons lose energy when interacting with dense matter. This is one of
the reasons that lightweight materials are preferred as protection against beta radiation.
For example, in old televisions, leaded glass was used for the picture tube. When the
electrons from the electron guns hit the back of the screen, they produced bremstrahlung
that were roughly equivalent to weak x-rays. Newer televisions are designed to produce
minimal bremstrahlung.
Positrons. Some nuclei (e.g., 12N or 124I) emit an anti-electron or positron, converting a
proton to a neutron. The ejected positron is identical to an electron but has a +1 charge
rather than a –1 charge. Because it is an anti-particle to an electron, the two will
annihilate when they interact. The annihilation completely converts both particles to
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