Companion to the reading passage Glowing in the Dark: Marie Curie and the Atom That Wouldn't Sit Still. Answer each question, then click Check. Multiple-choice and number questions grade themselves; for written answers, compare yours with the model answer and checklist, then score yourself honestly.
In 1898, Marie and Pierre Curie discovered polonium and radium and named the process they observed “radioactivity.” Today, many homes contain a radioactive source inside an ionization smoke detector. The detector holds a tiny sample of americium-241, which has a half-life of about 432 years. Each americium-241 nucleus that decays emits an alpha particle. The alpha particles knock electrons off air molecules between two charged metal plates, so a small, steady current flows across the gap. When smoke enters, smoke particles absorb the charged air molecules, the current drops, and the alarm sounds. Each decay of an americium-241 nucleus releases about 9.0 × 10−13 joule of energy as the nucleus rearranges into a more stable nucleus.
1. Which row correctly describes an alpha particle?
2. Substitute numerical values into a mathematical representation that can be used to determine the mass converted into energy when one americium-241 nucleus decays. Then calculate the mass.
Hospitals use the radioactive isotope technetium-99m to make images of organs such as the heart and bones. A small amount is injected into a patient, and it emits gamma rays that pass out of the body and are recorded by a gamma camera. A medical physicist measured the activity (the number of decays per second) of a technetium-99m sample over one day. The results are shown in Table 1.
Table 1: Activity of a Technetium-99m Sample
| Time (h) | Activity (MBq) |
|---|---|
| 0 | 800 |
| 6.0 | 400 |
| 12.0 | 200 |
| 18.0 | 100 |
| 24.0 | 50 |
Data are representative values for practice. 1 MBq = 1 × 106 decays per second.
3. A doctor makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response with evidence from Table 1.
4. Which graph best represents the activity of the technetium-99m sample as a function of time?
Engineers and doctors choose isotopes by their half-lives. A smoke detector source must keep working for the ten-year life of the detector without being replaced. A medical tracer must last long enough to be prepared, injected, and imaged, but the patient’s exposure to radiation should be kept as low as possible. Technetium-99m is made at or near the hospital shortly before use because it decays so quickly.
5. Place a check mark next to three statements that are correct.
6. Describe one trade-off doctors face when choosing a tracer isotope with a half-life of 6 hours instead of an isotope with a half-life of 6 days.