Knowledge of the types and properties of radiation emitted by changes in atomic nuclei, as well as radiation exposure, is essential for modern citizens.
Basic Knowledge
Types and characteristics of radiation:
| Type | What it is | Charge | Penetrating power | Ionizing effect |
|------|------|------|--------|----------|
| α ray | Helium nucleus ⁴He | +2 | Low (stopped by paper) | High |
| β ray | High-speed electron | −1 | Medium (stopped by thin aluminum) | Medium |
| γ ray | Electromagnetic wave | 0 | High (stopped by lead or thick reinforced iron) | Low |
・X ray is also an electromagnetic wave (essentially the same as a γ ray, but produced by a different mechanism)
Half-life: The time required for the number of atoms of a radioactive element to become half its original amount. It is unique to each element.
Amount remaining after t half-lives: N = N₀ × (1/2)ᵗ
Units of radiation:
・Bq (becquerel): Number of atoms that decay per second
・Gy (gray): Absorbed dose (energy absorbed by a substance [J/kg])
・Sv (sievert): Equivalent dose (taking effects on living organisms into account)
α ray (alpha ray) (helium nucleus; strong ionizing effect and low penetrating power)
β ray (beta ray) (high-speed electron; medium penetrating power)
γ ray (gamma ray) (high-energy electromagnetic wave; greatest penetrating power and weak ionizing effect)
Half-life (half-life) (time required for the number of atoms in a radioactive nuclide to become half)
Sievert (Sv) (magnitude of the effects of radiation on the human body; unit of equivalent dose)
Nuclear fission and nuclear fusion (a heavy nucleus splits or light nuclei combine and release energy)
Further Exploration (Background and Significance)
The annual natural radiation exposure of Japanese people is approximately 2.1 mSv (world average: 2.4 mSv). One chest X ray is approximately 0.06 mSv, and one CT scan is approximately 2–20 mSv. No definite health effects have been confirmed below 100 mSv.
Radioactive carbon isotope ¹⁴C (half-life: 5730 years) exists in living organisms at a certain proportion while they are alive, but it is no longer replenished after death and decreases. Estimating age from the remaining percentage is called radiocarbon dating.
Nuclear fusion power generation is a technology that fuses deuterium and tritium into helium to obtain a large amount of energy. Because its fuel, including deuterium obtained from seawater, is virtually unlimited and it produces less radioactive waste than nuclear fission, it is called the “energy of dreams.” However, confining plasma at high temperature and high density is technologically difficult, and research toward practical use is advancing at ITER (International Thermonuclear Experimental Reactor).
- α ray: Lowest penetrating power and greatest ionizing effect
- γ ray: Greatest penetrating power and lowest ionizing effect
- Half-life: A unique period for each element
- After n half-lives: N₀ × (1/2)ⁿ
- Bq: Number of decays per second
- Sv: Amount of effect on the human body
- Nuclear fusion: Hydrogen→helium (the Sun and future power generation)
Points to Note (Common Confusions)
① Penetrating power and ionizing effect show an opposite tendency. Greater penetrating power → weaker ionizing effect. ② Both γ rays and X rays are electromagnetic waves and are essentially the same. Their causes of production (nuclear decay vs. electron braking) differ. ③ Distinguish radiation (particles and electromagnetic waves with energy), radioactivity (the ability to decay, expressed in Bq), and radioactive material (a material that emits radiation). ④ The half-life formula is an exponential function. After 2 half-lives, 1/4 remains; after 3, 1/8 remains.
Practice
- Among α rays, β rays, and γ rays, identify the one with the greatest penetrating power.
- What fraction of the original amount of a radioactive element with a half-life of 10 days remains after 30 days?
- State the unit that expresses the magnitude of radiation’s effects on the human body.