Radioactive Decay
Appendix 1: Radioactive Decay
Radioactivity can be defined as spontaneous nuclear events that result in the transformation of one element into a different, stable element. Upon decay, various types of subatomic particles and energy will be released. Half-life described the time in which one half of the original radioactivity of a sample has decayed.
Alpha particles are massive, highly energetic fragments emitted from the nucleus of a radioactive atom when the neutron to proton ratio is too low. It is a positively charged helium nucleus, consisting of two protons and two neutrons. Gamma rays (energy of very short wavelength) are also released in this type of atomic disintegration. Alpha particles, due to their size, have a very limited ability to penetrate matter, including clothing and skin. Exposure to alpha radiation from external sources poses a minimal radiation hazard. However alpha particles may cause severe damage to cells when deposited internally and therefore ingestion, injection or inhalation is extremely hazardous.
Beta particles are electrons ejected from beta-unstable radioactive atoms. The particle has a single negative electrical charge (-1.6 x 10-19 C) and a very small mass (5.5 X 10-4 amu). Beta decay occurs in those isotopes that have a surplus of neutrons and are emitted when neutrons disintegrate into protons. Beta particles do not penetrate the body core but can produce significant radiation damage to the skin and eyes.
Gamma rays are electromagnetic radiations that are emitted from the nuclei of excited atoms following radioactive transformations. Following alpha or beta decay processes, gamma emission is the mechanism by which a nucleus loses energy in going from a high energy excited state to a low energy stable state. X-rays are electromagnetic radiations generated outside the atomic nucleus. Both X-rays and gamma rays are highly penetrating and can produce whole body radiation doses.
Bremsstrahlung ("breaking radiation") is radiation produced by the rapid deceleration of high energy beta particles when they interact with the electric fields surrounding atomic nuclei. The energy of the resulting X-rays is related to the energy of the incident beta particles as well as the electric field strength, which is greater in nuclei of high atomic number. For this reason lead is not an appropriate shielding material for beta emitting isotopes. Plexiglass shielding, composed of atoms with low atomic number, minimizes the energy and intensity of the bremsstrahlung.
Holding material for decay
The equation to use for calculating the amount of activity of a given radioactive substance left after a given length of time is as follows:
N=N0e-0.693(t/T1/2)
where:
N=the activity left;
No=the activity at time 0
t=the time elapsed since time 0
T1/2=the half life of the isotope
As a general rule of thumb, the time necessary for holding material so that it falls within the scheduled quantities for disposal is 10 half-lives.
