Radioactive Decay: Alpha, Beta, Gamma

Alpha Decay ($\alpha$)

• An alpha particle is emitted: $^4_2\text{He}$ (2 protons + 2 neutrons = helium nucleus)
• The nucleus loses 2 protons and 2 neutrons
• Mass number decreases by 4, atomic number decreases by 2

$^A_Z X \rightarrow ^{A-4}_{Z-2} Y + ^4_2 \alpha$

Example: Uranium-238 → Thorium-234 + alpha particle $^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^4_2\alpha$

Beta Decay ($\beta^-$)

• A neutron in the nucleus changes into a proton and an electron
• The electron is emitted as a beta particle: $^0_{-1}\beta$
• Mass number stays the same, atomic number increases by 1

$^A_Z X \rightarrow ^A_{Z+1} Y + ^0_{-1} \beta$

Example: Carbon-14 → Nitrogen-14 + beta particle $^{14}_6\text{C} \rightarrow ^{14}_7\text{N} + ^0_{-1}\beta$

Gamma Emission ($\gamma$)

• After alpha or beta decay, the nucleus may still have excess energy
• It releases this energy as a gamma ray — a high-energy electromagnetic wave
• No change in mass number or atomic number
• Often occurs alongside alpha or beta decay

Key Relationship

More ionising → less penetrating (and vice versa). Alpha particles are large and charged — they interact strongly with atoms (high ionisation) but lose energy quickly (low penetration).

Ionisation is the process where radiation knocks electrons out of atoms, creating ions.

• Alpha: strongly ionising because of large charge and mass — interacts with many atoms in a short distance
• Beta: moderately ionising
• Gamma: weakly ionising — passes through most atoms without interacting

Ionisation can damage living cells and DNA, potentially causing cancer or mutations.

We are constantly exposed to low levels of radiation from natural and artificial sources:

Natural sources (~85%):

• Radon gas (from rocks, especially granite)
• Cosmic rays (from space)
• Rocks and soil
• Food and drink (e.g., potassium-40 in bananas)

Artificial sources (~15%):

• Medical (X-rays, CT scans, radiotherapy)
• Nuclear power and weapons testing
• Industrial uses

• Geiger-Müller (GM) tube and counter — clicks or counts per second
• Photographic film — darkens when exposed to radiation (film badges for workers)
• Cloud chamber — shows tracks of ionising radiation

• Keep distance from sources (inverse square law for gamma)
• Minimise time of exposure
• Use shielding (lead, concrete)
• Handle sources with tongs (never directly)
• Point sources away from people
• Store in lead-lined containers

• Radioactive decay is random and spontaneous
• Alpha (α): $^4_2$He, +2, stopped by paper, most ionising
• Beta (β): electron, −1, stopped by aluminium, moderate ionising
• Gamma (γ): EM wave, 0, stopped by thick lead, least ionising
• More ionising = less penetrating
• Background radiation comes from natural and artificial sources