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Radioactive decay is the spontaneous transformation of an unstable atomic nucleus into a more stable nucleus by emitting radiation.

An unstable nucleus contains too much energy or an imbalance between protons and neutrons. To become stable, it releases excess energy or particles.

During radioactive decay:

• The nucleus changes
• Radiation is emitted
• A new element or isotope may form

The original unstable nucleus is called the parent nucleus. The new nucleus formed after decay is called the daughter nucleus.

• Spontaneous Process: Happens naturally without any external cause. No physical or chemical process can stop or speed it up significantly.
• Random Process: Cannot predict exactly when a particular nucleus will decay. Some decay quickly while others take thousands of years. However, overall rate can be predicted accurately for a large number of atoms.
• Unaffected by External Conditions: Does not depend on temperature, pressure, chemical state, or electric/magnetic fields.

This makes radioactive decay very reliable for scientific measurements.

Example: Uranium-238 is written as ²³⁸₉₂U

• Atomic number Z = 92 (protons)
• Mass number A = 238
• Neutrons = A − Z = 146

Alpha decay occurs when a heavy nucleus emits an alpha particle.

An alpha particle consists of 2 protons and 2 neutrons — identical to a helium nucleus: ⁴₂He or α

During alpha decay:

• Mass number decreases by 4
• Atomic number decreases by 2

Properties of Alpha Radiation:

• Strong ionizing power
• Low penetration ability (stopped by paper or skin)
• Moves relatively slowly
• Dangerous inside the body, less dangerous outside

Beta decay occurs when a neutron changes into a proton or a proton changes into a neutron.

Beta Minus Decay (β⁻)

A neutron changes into: a proton, an electron, and an antineutrino. The electron is emitted as beta radiation.

Properties: Medium ionizing power, medium penetration (stopped by thin aluminum), travels close to speed of light.

Beta Plus Decay (β⁺)

A proton changes into a neutron, a positron, and a neutrino.

During beta decay:

• Mass number stays the same
• Atomic number increases by 1 (β⁻) or decreases by 1 (β⁺)

Gamma decay occurs when a nucleus releases excess energy in the form of electromagnetic radiation.

Gamma radiation is not a particle — it's high-energy EM radiation similar to X-rays but with higher energy: ⁰₀γ

During gamma decay:

• Mass number remains the same
• Atomic number remains the same

Properties of Gamma Radiation:

• Low ionizing power
• Very high penetration ability (requires thick lead or concrete)
• Travels at the speed of light
• Very useful in medicine, dangerous in high doses

Radiation can remove electrons from atoms. This process is called ionization.

• Alpha radiation produces strong ionization (large, slow particles)
• Gamma radiation produces weak ionization (no charge)

Ionization is important because radiation damage in living tissue is mainly caused by ionization.

Two important rules must always be followed:

1. Conservation of Nucleon Number: Total mass number must be equal on both sides
2. Conservation of Charge: Total atomic number must be equal on both sides

High activity = many atoms decaying per second. Low activity = fewer atoms decaying.

The decay constant (λ) represents the probability of decay per unit time.

• Large λ → fast decay, short lifetime
• Small λ → slow decay, long lifetime

A half-life graph shows number of nuclei or activity versus time.

• The curve decreases smoothly — exponential decay curve
• Equal time intervals reduce the quantity by half
• Decay slows down with time
• Never becomes exactly zero

Radiation is always present in the environment — called background radiation.

Sources include:

• Cosmic rays from space
• Radioactive rocks
• Soil and air
• Food and water
• Human body

Scientists subtract background radiation from measurements to obtain accurate results.

Radioactive decay connects several important areas of physics:

• Nuclear structure
• Energy production
• Medical physics
• Environmental science

Understanding radioactive decay helps explain how atoms change and how nuclear energy is produced. It also allows scientists to measure time on a geological scale and understand processes inside stars and the Earth.