The specific activity of cobalt 60 is dependent on what factor?

The specific activity of cobalt-60 is primarily determined by the time the material has been in the reactor. This is because specific activity refers to the amount of radioactive substance present in a given mass, which changes over time due to the radioactive decay process. Cobalt-60 is produced by irradiating cobalt-59 in a nuclear reactor, and as it continues to be exposed to neutron flux, it undergoes activation and subsequently decays over time.

As time progresses, the quantity of cobalt-60 increases until it reaches a certain level when it comes out of the reactor; thereafter, the decay continues, which reduces the specific activity. The longer it remains in the reactor, the more cobalt-60 isotopes are created and later decay products may also form, affecting the overall radioactivity measured. Therefore, the specific activity is intricately linked to the duration of exposure in the reactor environment.

In contrast, temperature changes, physical size of the source, and type of radiation do not influence the specific activity in the same direct manner as time spent in the reactor. Temperature might affect the activity indirectly by modifying the reaction rates, but time is the central factor in determining how much cobalt-60 is available for measurement.