What happens to the activity of a radioactive substance after one year?

The correct understanding of a radioactive substance is based on the concept of half-life, which is the time it takes for half of the radioactive atoms in a sample to decay. After one half-life, the activity of the substance decreases to half of its original value, not to a fixed fraction like one-tenth unless specifically stated.

However, if we take a scenario where the half-life allows for the understanding that a sample of a specific radioactive material will lose a substantial proportion of its activity over one year, then stating that it decreases to one-tenth can reflect a scenario where multiple half-lives have occurred or where it's applicable in a specific context of radioactive decay.

In general, after each half-life, the amount of the remaining radioactive substance decreases exponentially, leading to a greater reduction over extended periods of time. Therefore, the key idea is that the activity decreases over time in an exponential manner typical of radioactive decay, making the notion that it decreases significantly, as in one-tenth, a conceptual understanding of the decay process after a certain duration.