RadioGraphics, Vol 18, 1247-1256, Copyright © 1998 by Radiological Society of North America
Radioactive decay
MW Groch
Department of Nuclear Medicine, Northwestern University Medical School and Northwestern Memorial Hospital, Chicago, IL 60611, USA.
When a parent radionuclide decays to its daughter radionuclide by means of
alpha, beta, or isomeric transition, the decay follows an exponential form,
which is characterized by the decay constant lambda. The decay constant
represents the probability per unit time that a single radioatom will
decay. The decay equation can be used to provide a useful expression for
radionuclide decay, the half-life, the time when 50% of the radioatoms
present will have decayed. Radiotracer half- life has direct implications
in nuclear imaging, radiation therapy, and radiation safety because
radionuclide half-life affects the ability to evaluate tracer kinetics and
create appropriate nuclear images and also affects organ, tumor, and
whole-body radiation dose. The number of radioatoms present in a sample is
equal to the activity, defined as the number of transitions per unit time,
divided by the decay constant; the mass of radioatoms present in a sample
can be calculated to determine the specific activity (activity per unit
mass). The dynamic relationship between the number of parent and daughter
atoms present over time may lead to radioactive equilibrium, which takes
two forms-- secular and transient--and has direct relevance to
generator-produced radionuclides.
