What does the half-life of an XRF source refer to?

The half-life of an XRF (X-ray fluorescence) source refers to the time needed for the radioactive material within the source to decay by half its original quantity. This concept is critical in understanding how long a radioactive source remains effective and how its radioactivity diminishes over time.

In practical terms, knowing the half-life helps in assessing safety protocols and determining how long a source can be effectively used before its intensity decreases to a point where it is no longer practical for its intended use. The half-life is directly related to the source's radioactivity and impacts safety measures regarding handling and storage.

The other options do not accurately describe the concept of half-life. The obsolescence of the source or the time until it must be replaced pertains to different factors, such as technological advancements or wear and tear, rather than a specific measure of radioactivity. Similarly, while understanding the safe handling of a source is important, the half-life specifically refers to the decay process, not the overall safety timeline.