A scientist measures the activity of a radioactive source using a detector and counter.

First, they measure the background radiation count rate when the source is not present. Then, they measure the total count rate with the radioactive source in place.

The table shows the scientist's results:

$\begin{array}{lc}Measurement& Count\; rate\; (cpm)\\ Background\; radiation\; only& 24\\ With\; radioactive\; source\; present& 984\end{array}$

How can the scientist determine the actual activity due only to the radioactive source?

Tick ($✓$) one box.

• Add the background count rate to the measured count rate.
• Subtract the background count rate from the measured count rate.
• Multiply the measured count rate by the background count rate.
• Divide the measured count rate by the background count rate.

Calculate the actual activity of the radioactive source in counts per second (cps).

Activity = $\dots \dots \dots \dots \dots$ counts per second

Iodine-131 is a radioactive isotope used to treat thyroid cancer.

Iodine-131 has a relatively short half-life of 8 days. Explain why a short half-life is important for the patient.

Using Iodine-131 to treat thyroid cancer can cause temporary side effects such as nausea and dry mouth, and patients must temporarily avoid close contact with others. Explain why patients are still treated with Iodine-131 despite these problems.