Types of Radiation

This lesson covers the three main types of nuclear radiation — alpha ($\alpha$α), beta ($\beta$β) and gamma ($\gamma$γ) — including their nature, penetrating power, ionising ability and behaviour in electric and magnetic fields. This is a core part of the Radioactivity section of the Edexcel GCSE Combined Science specification (1SC0).

---

Why Do Nuclei Emit Radiation?

Some atomic nuclei are unstable — they have too many or too few neutrons relative to protons. To become more stable, the nucleus emits radiation in a process called radioactive decay. This is a random and spontaneous process:

• Random — it is impossible to predict when a particular nucleus will decay.
• Spontaneous — it is not triggered by external conditions (temperature, pressure, chemical reactions).

---

The Three Types of Radiation

Property	Alpha ($\alpha$α)	Beta ($\beta$β)	Gamma ($\gamma$γ)
What is it?	2 protons + 2 neutrons (a helium nucleus)	A high-speed electron emitted from the nucleus	An electromagnetic wave (high-energy photon)
Symbol	$^{4}_{2}\text{He}$24​He or $\alpha$α	$^{\;\;0}_{-1}\text{e}$−10​e or $\beta$β	$\gamma$γ
Charge	+2	−1	0 (no charge)
Mass	4 (relative)	Very small (≈ 1/1836)	0 (no mass)
Speed	Slow (≈ 5–10% of speed of light)	Fast (up to 90% of speed of light)	Speed of light ($3 \times 10^8$3×108 m/s)
Ionising power	Strongly ionising	Moderately ionising	Weakly ionising
Penetrating power	Low — stopped by paper or a few cm of air	Moderate — stopped by a few mm of aluminium	High — only significantly reduced by thick lead or several metres of concrete
Deflected by fields?	Yes — towards the negative plate (positive charge)	Yes — towards the positive plate (negative charge)	No — not deflected

graph LR
    subgraph "Penetrating Power"
    direction LR
    Source["Radioactive<br/>source"] --> Paper["📄 Paper"]
    Paper -->|"α stopped"| Aluminium["🔲 Aluminium<br/>(few mm)"]
    Aluminium -->|"β stopped"| Lead["🟫 Thick lead /<br/>concrete"]
    Lead -->|"γ reduced"| Beyond["Beyond"]
    end

Exam Tip: Remember the mnemonic: Alpha is absorbed by pAper, Beta is blocked by aBout a few mm of aluminium, and Gamma needs thick lead or concrete. Ionising power is the reverse of penetrating power — the most ionising radiation is the least penetrating.

---

Ionisation

Ionisation is the process of removing (or adding) electrons from atoms, turning them into ions. Radiation causes ionisation when it interacts with atoms in the material it passes through.

Why Is Alpha the Most Ionising?

• Alpha particles are large and heavily charged (+2).
• They interact strongly with atoms they pass, pulling electrons away.
• Because they lose energy quickly through many ionisation events, they are stopped by just a few centimetres of air or a sheet of paper.

Why Is Gamma the Least Ionising?

• Gamma rays have no charge and no mass.
• They interact with atoms far less frequently.
• They pass through most materials, losing energy gradually — hence they are the most penetrating.

---

Alpha Decay

During alpha decay, a nucleus emits an alpha particle ($^{4}_{2}\text{He}$24​He):

$^{A}_{Z}X \rightarrow ^{A-4}_{Z-2}Y + ^{4}_{2}\text{He}$ZA​X→Z−2A−4​Y+24​He

• The mass number decreases by 4.
• The atomic number decreases by 2.
• A new element is formed.

Example

$^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}$88226​Ra→86222​Rn+24​He

Radium-226 decays to radon-222 by emitting an alpha particle.

---

Beta Decay

During beta decay, a neutron in the nucleus changes into a proton and an electron. The electron is ejected at high speed as a beta particle:

$^{A}_{Z}X \rightarrow ^{\;\;A}_{Z+1}Y + ^{\;\;0}_{-1}\text{e}$ZA​X→Z+1A​Y+−10​e

• The mass number stays the same.
• The atomic number increases by 1 (one more proton).
• A new element is formed.

Example

$^{14}_{6}\text{C} \rightarrow ^{14}_{7}\text{N} + ^{\;\;0}_{-1}\text{e}$614​C→714​N+−10​e

Carbon-14 decays to nitrogen-14 by emitting a beta particle.

Exam Tip: In beta decay the mass number does not change because a neutron turns into a proton — the total number of nucleons (protons + neutrons) stays the same.

---

Gamma Emission

Gamma rays are often emitted alongside alpha or beta decay, when the daughter nucleus is in an excited state and needs to release excess energy.

• Gamma emission does not change the mass number or atomic number.
• It is a release of electromagnetic energy, not a particle.

$^{A}_{Z}X^* \rightarrow ^{A}_{Z}X + \gamma$ZA​X∗→ZA​X+γ

(The asterisk * indicates an excited state.)

---

Deflection in Electric and Magnetic Fields

When radiation passes between charged plates (an electric field):

Radiation	Deflection
Alpha ($\alpha$α)	Deflected towards the negative plate (attracted to opposite charge); deflection is small because alpha particles are heavy
Beta ($\beta$β)	Deflected towards the positive plate; deflection is large because beta particles are light
Gamma ($\gamma$γ)	Not deflected — no charge

graph TD
    Pos["+  Positive plate"] --- Space["  "]
    Space --- Neg["−  Negative plate"]
    Space --- Alpha["α: slight deflection towards −"]
    Space --- Beta["β: large deflection towards +"]
    Space --- Gamma["γ: straight through"]

Exam Tip: In a magnetic field, alpha and beta are deflected in opposite directions (because they have opposite charges). Gamma is unaffected. The amount of deflection depends on the charge-to-mass ratio — beta particles are deflected much more than alpha particles.

---

Summary Table

Property	Alpha ($\alpha$α)	Beta ($\beta$β)	Gamma ($\gamma$γ)
Identity	Helium nucleus	Electron	EM wave
Charge	+2	−1	0
Mass (relative)	4	≈ 0	0
Ionising power	Strong	Moderate	Weak
Penetrating power	Low (paper)	Medium (aluminium)	High (lead/concrete)
Deflection	Small, towards −	Large, towards +	None

---

Common Misconceptions