Alpha, Beta, and Gamma Radiation (VCE SSCE Physics): Revision Notes

Alpha, Beta, and Gamma Radiation

Introduction to radioactivity

In 1896, Henri Becquerel discovered radioactivity. Three years later, Ernest Rutherford began studying how radioactive emissions interact with materials. By testing thin metal foils, he identified three distinct types of radiation based on their ability to penetrate matter. These were named alpha (α), beta (β), and gamma (γ) radiation, using the first three letters of the Greek alphabet in order of increasing penetration ability.

Unstable atomic nuclei spontaneously emit these different types of radiation as they transform into more stable forms. Understanding these three radiation types is fundamental to nuclear physics.

Properties of the three radiation types

The three types of natural nuclear radiation have very different properties in terms of their composition, charge, mass, speed, penetrating power, and ionizing ability.

When Rutherford first discovered these radiations, their exact nature was unknown. He simply named them according to how easily they were absorbed by materials, from least penetrating (α) to most penetrating (γ).

Ionization

All three types of nuclear radiation carry enough energy to ionize atoms they encounter. Ionization means removing or adding electrons to a neutral atom, creating an ion.

Key points about ionization:

• Positive ions form when electrons are removed from a neutral atom
• Negative ions form when electrons are added to a neutral atom
• α-, β-, and γ-radiation can all ionize atoms they come into contact with

Alpha particle radiation

Composition and properties

An alpha (α) particle consists of two protons and two neutrons bound together. This is identical to a helium nucleus. Because it contains two protons, an α-particle carries a charge of +2 (or +3.2 × 10⁻¹⁹ coulombs).

Alpha particles are relatively massive compared to other forms of radiation. They travel at speeds around 30,000 km s⁻¹, which is approximately 10% of the speed of light.

Penetration and ionizing power

Alpha particles have very limited penetrating ability:

• They travel only a few centimetres in air
• They are stopped by a sheet of paper
• They cannot penetrate human skin

Despite their poor penetration, α-particles are extremely effective at ionizing atoms. Because they are relatively massive and fast-moving, they can ionize a large number of atoms over their short range before losing all their kinetic energy.

Transmutation through alpha decay

Transmutation is the conversion of one element into another through changes in the atomic nucleus. Alpha particle emission causes transmutation because the parent nucleus loses two protons and two neutrons.

When a nucleus emits an α-particle:

• The mass number (A) decreases by 4
• The atomic number (Z) decreases by 2
• A different element is formed (the daughter nucleus)

Worked example: energy conversion

Applications of alpha radiation

Despite their low penetrating power, α-particles have several useful applications:

• Smoke detectors: Americium-241 ionizes air in smoke detectors. When smoke enters, it reduces the ionization current, triggering the alarm
• Static eliminators: Polonium-210 α-particles neutralize static electric charges on equipment
• Cancer treatment: α-emitters can be used in targeted radiotherapy
• Power generation: Plutonium-238 in radioisotope thermoelectric generators produces heat from α-decay, which is converted to electricity

Beta radiation

Composition and properties

Beta (β) particles are high-speed electrons (β⁻) or positrons (β⁺) emitted from the nucleus. Each β-particle carries either one negative charge (electron: -1.6 × 10⁻¹⁹ coulombs) or one positive charge (positron: +1.6 × 10⁻¹⁹ coulombs).

Beta particles typically travel at speeds up to 90% of the speed of light (0.9c or 270,000 km s⁻¹). They are much lighter than α-particles but faster.

Penetration and ionizing power

Beta particles have moderate penetrating ability:

• They can travel up to 1 metre in air
• They are absorbed by a few millimetres of aluminium
• They can penetrate paper and several millimetres of skin

Their ionizing capacity is considerably less than α-radiation, but their greater penetration makes them dangerous at closer distances (within a metre externally) and very dangerous if ingested internally.

Beta minus (β⁻) decay

In β⁻-decay, an electron is emitted from inside the nucleus. This seems paradoxical since nuclei contain only protons and neutrons, not electrons. The explanation is that a neutron transforms into a proton, emitting an electron in the process:

$^1_0\text{n} \rightarrow ^1_1\text{p} + ^0_{-1}\text{e} + \text{energy}$

Or using β notation:

$^1_0\text{n} \rightarrow ^1_1\text{p} + ^0_{-1}\beta + \text{energy}$

This transformation is an example of the weak nuclear force in action.

Beta plus (β⁺) decay

In β⁺-decay, a positron is emitted when a proton transforms into a neutron:

$^1_1\text{p} \rightarrow ^1_0\text{n} + ^0_{+1}\text{e} + \text{energy}$

Or using β notation:

$^1_1\text{p} \rightarrow ^1_0\text{n} + ^0_{+1}\beta + \text{energy}$

The neutrino

When physicists measured the energy of β-particles, they discovered that not all the initial energy could be accounted for in the products. This appeared to violate energy conservation.

The complete β⁻-decay process is:

$^1_0\text{n} \rightarrow ^1_1\text{p} + ^0_{-1}\beta + \text{neutrino}$

The complete β⁺-decay process is:

$^1_1\text{p} \rightarrow ^1_0\text{n} + ^0_{+1}\beta + \text{neutrino}$

The neutrino carries away the "missing" energy, ensuring energy conservation.

Applications of beta radiation

The medium penetrating power of β-particles provides useful applications:

• Thickness detectors: Quality control for thin materials like paper
• Cancer treatment: Strontium-89 for treating eye and bone cancers
• Medical imaging: Fluorine-18 as a tracer in positron emission tomography (PET)

Gamma radiation

Nature and properties

Gamma (γ) radiation is fundamentally different from α- and β-radiation. It does not consist of particles but rather is a form of very short wavelength electromagnetic energy, similar to X-rays.

Gamma rays can be considered as discrete packages of energy called photons. They travel at the speed of light (300,000 km s⁻¹ or c) and carry no electrical charge.

Penetration and ionizing power

Gamma radiation has the highest penetrating ability of the three radiation types:

• Several centimetres of lead do not stop all γ-radiation
• They can easily penetrate the human body and reach internal organs

Gamma decay process

Gamma emission typically occurs after α- or β-decay. When a nucleus emits an α-particle or β-particle, it may remain in an excited (unstable) energy state. The nucleus then releases excess energy by emitting a γ-ray photon.

Since γ-emission releases energy without ejecting particles:

• The mass number (A) remains unchanged
• The atomic number (Z) remains unchanged
• No transmutation occurs

Comparison with X-rays

Both γ-rays and X-rays are part of the electromagnetic spectrum and their wavelength ranges overlap. They have similar properties but originate from different parts of the atom:

Applications of gamma radiation

Gamma-emitting radioisotopes are the most widely used radiation sources due to their strong penetrating power. The three most useful γ-emitters are caesium-137, technetium-99m, and cobalt-60.

Cobalt-60 applications:

• Sterilization of medical equipment
• Food pasteurization through irradiation
• Thickness gauges in industry
• Industrial radiography
• Cancer treatment using a gamma "knife"

Caesium-137 applications:

• Flow measurement in industrial processes
• Investigation of underground strata (oil, coal, gas exploration)
• Soil moisture-density measurement at construction sites
• Cancer treatment

Technetium-99m applications:

• Medical diagnostic imaging (brain, bone, liver, spleen, kidney)
• Blood flow studies
• Most widely used medical diagnostic radioisotope

Nuclear transformations

What is a nuclear transformation?

A nuclear transformation is the conversion of one nuclide into another. This can occur through:

• Natural radioactive decay
• Artificial bombardment with nuclear particles

Both α-radiation and β-radiation cause transmutation because they change the number of protons in the nucleus, creating a different element. Gamma radiation does not cause transmutation.

Conservation laws in nuclear equations

Summary table of nuclear transformations

The table below summarizes the four main types of nuclear decay:

Type	Nuclear equation	Mass number change	Atomic number change
Alpha decay	$^A_Z\text{X} \rightarrow ^4_2\text{He} + ^{A-4}_{Z-2}\text{Y}$	Decreases by 4	Decreases by 2
Beta decay	$^A_Z\text{X} \rightarrow ^0_{-1}\text{e} + ^A_{Z+1}\text{Y}$	Unchanged	Increases by 1
Positron emission	$^A_Z\text{X} \rightarrow ^0_{+1}\text{e} + ^A_{Z-1}\text{Y}$	Unchanged	Decreases by 1
Gamma decay	$^A_Z\text{X}^* \rightarrow ^0_0\gamma + ^A_Z\text{X}$	Unchanged	Unchanged

Artificial nuclear transformations

Nuclear transformations can also be induced artificially by bombarding elements with nuclear particles.

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