Radioactivity
Radioactivity
Radioactivity is a process by which unstable atomic nuclei lose energy by emitting radiation. This phenomenon was discovered by Henri Becquerel in 1896 while investigating phosphorescence in uranium salts. The term itself was coined by Marie Curie, who, along with her husband Pierre, conducted pioneering research in radioactivity.
Types of Radioactive Decay
Radioactive decay can occur in several ways. The most common types of decay are alpha decay, beta decay, and gamma decay.
Alpha Decay ($\alpha$-decay)
In alpha decay, an atomic nucleus emits an alpha particle (consisting of two protons and two neutrons), which is essentially a helium-4 nucleus. This process decreases the mass number by 4 and the atomic number by 2.
Example: Uranium-238 decaying to Thorium-234.
$$ {}^{238}{92}U \rightarrow {}^{234}{90}Th + {}^{4}_{2}He $$
Beta Decay ($\beta$-decay)
Beta decay is of two types: beta-minus ($\beta^-$) and beta-plus ($\beta^+$) decay.
- In $\beta^-$ decay, a neutron is transformed into a proton, and an electron and an antineutrino are emitted.
- In $\beta^+$ decay, a proton is converted into a neutron, and a positron and a neutrino are emitted.
Example ($\beta^-$): Carbon-14 decaying to Nitrogen-14.
$$ {}^{14}{6}C \rightarrow {}^{14}{7}N + e^- + \bar{\nu}_e $$
Example ($\beta^+$): Fluorine-18 decaying to Oxygen-18.
$$ {}^{18}{9}F \rightarrow {}^{18}{8}O + e^+ + \nu_e $$
Gamma Decay ($\gamma$-decay)
Gamma decay involves the emission of gamma rays from an excited nucleus to a lower energy state. Gamma rays are high-energy photons and do not have mass or charge.
Example: Technetium-99m decaying to Technetium-99.
$$ {}^{99m}{43}Tc \rightarrow {}^{99}{43}Tc + \gamma $$
Differences Between Decay Types
| Decay Type | Emission | Change in Atomic Number | Change in Mass Number | Penetrating Power |
|---|---|---|---|---|
| Alpha | Helium nucleus (2 protons, 2 neutrons) | -2 | -4 | Low |
| Beta-minus | Electron and antineutrino | +1 | 0 | Moderate |
| Beta-plus | Positron and neutrino | -1 | 0 | Moderate |
| Gamma | High-energy photon | 0 | 0 | High |
Radioactive Decay Law
The rate at which a radioactive substance decays is proportional to the number of undecayed nuclei. This is described by the radioactive decay law, which is mathematically expressed as:
$$ \frac{dN}{dt} = -\lambda N $$
Where:
- $N$ is the number of undecayed nuclei,
- $\lambda$ is the decay constant (probability of decay of a nucleus per unit time),
- $t$ is time,
- $\frac{dN}{dt}$ is the rate of decay.
Integrating this equation gives us the number of nuclei remaining after time $t$:
$$ N(t) = N_0 e^{-\lambda t} $$
Where $N_0$ is the initial number of nuclei.
Half-Life
The half-life of a radioactive isotope is the time taken for half of the radioactive nuclei to decay. It is denoted by $T_{1/2}$ and is related to the decay constant by:
$$ T_{1/2} = \frac{\ln(2)}{\lambda} $$
Activity
The activity of a radioactive substance is the number of decays per unit time and is given by:
$$ A = \lambda N $$
The SI unit of activity is the becquerel (Bq), which is one decay per second. Another commonly used unit is the curie (Ci), which is $3.7 \times 10^{10}$ decays per second.
Examples of Radioactive Isotopes and Their Uses
- Carbon-14: Used in radiocarbon dating to determine the age of archaeological samples.
- Uranium-235: Used as fuel in nuclear reactors and in nuclear weapons.
- Iodine-131: Used in medical diagnostics and treatment, particularly for thyroid conditions.
- Technetium-99m: Widely used in medical imaging.
Safety and Protection
Radioactive materials can be hazardous due to their ionizing radiation, which can damage living tissue. Safety measures include:
- Time: Limiting the time of exposure.
- Distance: Increasing distance from the source.
- Shielding: Using materials like lead or concrete to absorb radiation.
Understanding radioactivity is crucial for various applications in medicine, industry, and research. It is also essential for handling radioactive materials safely and for protecting the environment from radioactive contamination.