Radioactivity: natural, artificial and its units
Radioactivity is the spontaneous emission of alpha, beta or gamma radiation by unstable nuclei. It can be natural or artificial (induced).
Discovery of radioactivity
In 1896, the French scientist Henri Becquerel kept some uranium salt near a photographic plate wrapped in black paper, in a dark drawer. The plate got fogged, as if light had fallen on it. The uranium was giving out invisible, penetrating rays on its own. In 1898, Marie Curie and Pierre Curie found two new, more strongly radioactive elements: polonium and radium. Becquerel and the Curies shared the Nobel Prize in Physics in 1903.
What is radioactivity?
The nucleus is the tiny centre of an atom, made of protons and neutrons. Some nuclei are unstable. They break up on their own and give out alpha, beta or gamma radiation. This is radioactivity, and the breaking up is called radioactive decay or disintegration. It is spontaneous: heating, pressure or chemical reactions cannot speed it up or slow it down. Elements that show it are radioactive elements, such as uranium, thorium, radium and polonium. In nature, elements with an atomic number above 83 are radioactive. The activity of a sample is the number of its nuclei that decay each second.
Natural vs artificial radioactivity
The phosphorus-30 formed is radioactive. It decays as ₁₅P³⁰ → ₁₄Si³⁰ + ₊₁e⁰, giving out a positron (a particle like an electron but with a positive charge).
Units of radioactivity
| Unit | Value | Notes |
|---|---|---|
| becquerel (Bq) | 1 disintegration per second | SI unit; named after Henri Becquerel |
| curie (Ci) | 3.7 × 10¹⁰ disintegrations per second | Older unit; about the activity of 1 g of radium |
| rutherford (Rd) | 10⁶ disintegrations per second | Named after Ernest Rutherford |
| roentgen (R) | X-rays or gamma rays that produce a charge of 2.58 × 10⁻⁴ C in 1 kg of air | Unit of exposure to X-rays and gamma rays, not of activity |
Worked example: converting units
A radioactive sample has an activity of 7.4 × 10¹⁰ Bq. Express it in curie and in rutherford.
- 1 Ci = 3.7 × 10¹⁰ Bq, so activity in Ci = (7.4 × 10¹⁰) ÷ (3.7 × 10¹⁰) = 2 Ci.
- 1 Rd = 10⁶ Bq, so activity in Rd = (7.4 × 10¹⁰) ÷ 10⁶ = 7.4 × 10⁴ Rd.
2 Ci; 7.4 × 10⁴ Rd
The power stations at Kalpakkam near Chennai and Kudankulam in Tirunelveli district run on uranium, a natural radioactive element. Hospitals use artificial radioisotopes made in reactors to test and treat diseases.
1 marks: “The SI unit of radioactivity is ___.” becquerel (Bq).
2 marks: “What is artificial radioactivity? Who discovered it?” Radioactivity induced in a stable light element by bombarding it with particles. Discovered by Irene Curie and Frederic Joliot in 1934.
4 marks: “Distinguish between natural and artificial radioactivity. Define the curie and the becquerel.” Natural: spontaneous, heavy elements (Z above 83), cannot be controlled. Artificial: induced by bombardment, light elements, can be controlled. 1 Ci = 3.7 × 10¹⁰ decays per second; 1 Bq = 1 decay per second.
Becquerel Began it (1896); the Curies Continued (radium, polonium); Joliot-Curie Jump-started artificial radioactivity (1934).