How can scientists measure the half-life of a radioactive substance?

Scientists measure the half-life of a radioactive substance using different techniques depending on the duration of the decay process.1

• Seconds to years: For substances with half-lives in this range, scientists measure the intensity of radiation at successive times. By plotting the logarithm of the decay rate against time, they create a straight-line decay curve; the time interval required for this value to decrease by half is the half-life.1
• Longer than several years: Because the decrease in activity is too slow to measure directly over a reasonable period, scientists determine the decay constant ($\lambda$) by measuring the specific activity of a known mass of the isotope. The half-life ($T_{1/2}$) is then calculated using the relationship $T_{1/2}=0.693/\lambda$.21
• Microseconds to nanoseconds: These are measured using electronic coincidence experiments, where the time interval between the detection of a start pulse (formation) and a stop pulse (decay) is plotted semi-logarithmically to determine the slope.1
• Intermediate ranges: For half-lives between 100 microseconds and one second, specialized techniques such as using rapidly rotating drums or moving tapes with detectors are employed.1