Half-life (A-level only) Flashcards – AQA A-level Physics (7408)

What is the physical half-life $T_P$ of a radionuclide?

The time taken for its number of undecayed nuclei or its activity to fall to half its initial value through radioactive decay.

How predictable is radioactive decay for an individual nucleus?

It is random.

How does a large population of radioactive nuclei decrease over successive half-lives?

It decreases exponentially: half of the remaining nuclei decay in each half-life.

How is physical half-life related to the radionuclide and its biological removal?

It is characteristic of the radionuclide and does not depend on biological removal.

What is the biological half-life of a radionuclide in the body?

The time for its amount to fall by half through biological removal alone.

What is the effective half-life of a radionuclide in the body?

The time for its amount or activity to fall by half through biological removal and radioactive decay combined.

For constant exponential removal and decay rates, how are effective half-life $T_E$, biological half-life $T_B$, and physical half-life $T_P$ related?

$\frac{1}{T_E}=\frac{1}{T_B}+\frac{1}{T_P}$

How does a radionuclide’s effective half-life compare with its physical and biological half-lives?

It is shorter than either the physical or the biological half-life.