What is physical half-life?
Physical half-life is the time required for half of the radioactive nuclei in a sample to undergo decay. It is a fixed property of a given radionuclide and does not depend on external conditions such as temperature, pressure, or chemical form.
Physical half-life is the time required for half the radioactive nuclei in a sample to decay, and is determined solely by the intrinsic decay probability of the isotope.
Physical half-life reflects the probability of decay of individual nuclei. Although it is impossible to predict when a specific atom will decay, large populations of atoms decay in a predictable way. After one half-life, half the original activity remains; after two half-lives, one quarter remains; after three half-lives, one eighth remains, and so on.
Physical half-life determines how long a radionuclide remains radioactive and therefore influences both imaging duration and radiation dose.
Understanding the physics
Radioactive decay is a random process at the level of individual atoms, but the probability of decay per unit time is constant for a given radionuclide. This constant probability gives rise to exponential decay.
If N0 represents the initial number of radioactive nuclei, the number remaining at time t is:
N(t) = N0e-λt
where λ is the decay constant, representing the probability of decay per unit time.
The half-life (T1/2) is related to the decay constant by:
T1/2 = ln 2 / λ
This relationship shows that half-life is simply another way of expressing the decay probability.
Importantly, physical half-life depends solely on nuclear structure. It cannot be altered by chemical reactions, physical state, or environmental factors. This distinguishes nuclear processes from chemical decay, which can be influenced by external conditions.
In nuclear medicine, radionuclides are selected with half-lives appropriate for the intended application. They are long enough to complete imaging, but short enough to minimise radiation exposure.
Where this matters clinically
Physical half-life determines how quickly activity falls during a nuclear medicine study. It affects imaging time, radiation dose, logistics of radiopharmaceutical delivery, and waste handling. For example, Tc-99m has a physical half-life of approximately 6 hours, making it well suited to diagnostic imaging.