Nuclear Physics
Radioactive decay, fission, fusion, and nuclear energy
Do Now: Most smoke detectors contain a tiny amount of a radioactive element. Why isn’t that dangerous for the people in your house?
It gives off alpha particles, which are stopped by a few centimeters of air or a sheet of paper. They never make it out of the detector. You’ll learn why in this unit.
Try It First
Pick a starting nucleus, then press a decay button. Watch the protons, neutrons, and notation change.
Lessons
NYS P-12 Science Learning Standards: HS-PS1-8 — Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.
The Smoke Detector
Many smoke detectors contain a tiny amount of americium-241, which emits alpha particles. The alpha particles ionize air in a small chamber, allowing a small current to flow. Smoke particles absorb the alpha particles, the current drops, and the alarm sounds. Americium-241 has a half-life of about 430 years.
Why is it safe to have an alpha source in a smoke detector at home?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. Each alpha particle from americium-241 leaves the nucleus with about 5.5 MeV of kinetic energy. Substitute numerical values into a mathematical representation that can be used to convert this energy to joules (1 eV = 1.60 × 10−19 J). Then calculate the energy.
C2. What is an alpha particle?
C3. An alpha particle loses about 35 eV of energy each time it ionizes an air molecule. Estimate the number of ions one 5.5 MeV alpha particle produces before it stops.
C4. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C5. Americium-241 (24195Am) emits an alpha particle. What is the daughter nucleus?
C6. A designer suggests replacing the americium-241 alpha source with a gamma-ray source. Identify one reason an alpha source works better for the detector and one safety reason the alpha source is the better choice.