E.3.2 Types of radioactive decay

Types of Radioactive Decay: Alpha, Beta, and Gamma

You're holding a piece of uranium ore in your hand.
Though it appears unremarkable, deep within its atoms, a remarkable transformation is taking place.
The uranium nuclei are unstable, and they spontaneously emit particles and energy in a process called radioactive decay, which allows unstable nuclei to become more stable.

Alpha Decay: Emission of Helium Nuclei

Definition

Alpha decay

Alpha decay occurs when an unstable nucleus emits an alpha particle, which is essentially a helium nucleus made up of two protons and two neutrons ( $_{2}^{4} He$ ).

This emission reduces both the proton number ( $Z$ ) and the nucleon number ( $A$ ) of the parent nucleus, producing a new element.

Example

Uranium-235 Decay

Consider uranium-235 ( $_{92}^{235} U$ ), which undergoes alpha decay to form thorium-231 ( $_{90}^{231} Th$ ):

$_{92}^{235} U \to_{90}^{231} Th +_{2}^{4} He$

Here’s what happens:

The proton number ( $Z$ ) decreases by 2: $92 \to 90$ .
The nucleon number ( $A$ ) decreases by 4: $235 \to 231$ .

Energy Released During Alpha Decay

The energy released in alpha decay comes from the mass defect—the difference in mass between the parent nucleus and the products.
Using Einstein’s equation, $E = m c^{2}$ , you can calculate this energy.

Example

For instance, if the mass difference ( $Δ m$ ) is 0.0046 u, the energy released is:

$E = Δ m \cdot 931.5 MeV/u = 0.0046 \cdot 931.5 MeV \approx 4.3 MeV .$

This energy is carried away as the kinetic energy of the alpha particle and the recoiling daughter nucleus.

Tip

Alpha particles are highly ionizing but have low penetration power—they can be stopped by a sheet of paper or a few centimeters of air.

Beta Decay: Conversion of Neutrons or Protons

Beta Minus ( $β^{-}$ ) Decay

Definition

Beta minus decay

In beta minus decay, a neutron in the nucleus is converted into a proton, emitting an electron ( $β^{-}$ ) and an antineutrino ( $\bar{ν}$ ):

$n \to p + e^{-} + \bar{ν}$

This increases the proton number ( $Z$ ) by 1 while the nucleon number ( $A$ ) remains unchanged.

Example

Thorium-234 Decay

$_{90}^{234} Th \to_{91}^{234} Pa + e^{-} + \bar{ν}$

Here:

The proton number ( $Z$ ) increases by 1: $90 \to 91$ .
The nucleon number ( $A$ ) remains $234$ .

Beta Plus ( $β^{+}$ ) Decay

Definition

Beta plus decay

In beta plus decay, a proton is converted into a neutron, emitting a positron ( $β^{+}$ ) and a neutrino( $ν$ ):

$p \to n + e^{+} + ν$

This decreases the proton number ( $Z$ ) by 1 while the nucleon number ( $A$ ) remains unchanged.

Example

Sodium-22 Decay

$_{11}^{22} Na \to_{10}^{22} Ne + e^{+} + ν$

Why Do Beta Particles Have a Continuous Energy Spectrum?

Unlike alpha particles, which have a fixed energy, the electrons or positrons emitted in beta decay exhibit a continuous energy spectrum.
This puzzled scientists until Wolfgang Pauli proposed the existence of the neutrino—a nearly massless, neutral particle that shares the released energy with the electron or positron.

Note

The neutrino was later confirmed experimentally and is essential for conserving energy and angular momentum in beta decay.

Common Mistake

Do not assume that the emitted electron or positron pre-exists in the nucleus. These particles are created during the decay process.

Gamma Decay: Emission of High-Energy Photons

What is Gamma Decay?

Definition

Gamma decay

Gamma decay occurs when an excited nucleus releases excess energy by emitting a gamma ray, a high-energy photon.

Unlike alpha and beta decay, gamma decay does not alter the proton number ( $Z$ ) or the nucleon number ( $A$ ) of the nucleus.

Example

Cobalt-60 Decay

After undergoing beta decay, cobalt-60 ( $_{27}^{60} Co$ ) produces a daughter nucleus ( $_{28}^{60} Ni$ ) in an excited state. The nucleus then releases energy by emitting a gamma photon:

$_{28}^{60} {Ni}^{*} \to_{28}^{60} Ni + γ$

Properties of Gamma Rays

Gamma rays are massless and uncharged.
They are the most penetrating type of radiation, requiring thick lead or concrete to block.

Tip

Gamma decay often accompanies alpha or beta decay as the nucleus transitions to a more stable energy state.

The Role of Neutrinos and Antineutrinos in Beta Decay

Why Were Neutrinos Postulated?

In beta decay, the emitted electron or positron does not account for all the energy released.
To conserve energy and momentum, physicists hypothesized the existence of neutrinos and antineutrinos.

Properties of Neutrinos

Electrically neutral.
Extremely small mass (nearly zero).
Weakly interacting with matter, making them very difficult to detect.

Note

Neutrinos were experimentally detected in studies of solar neutrinos, confirming their existence and validating the principle of energy conservation in beta decay.

Theory of Knowledge

How does the discovery of neutrinos demonstrate the interplay between theoretical predictions and experimental evidence in scientific progress?

Reflection

Self review

What changes occur to the proton number ( $Z$ ) and nucleon number ( $A$ ) during alpha decay?
Why do beta particles have a continuous energy spectrum?
How do gamma rays differ fundamentally from alpha and beta particles?

Radioactive decay is more than just a fascinating natural process—it has practical applications in fields such as medicine (e.g., gamma rays in cancer treatment) and archaeology (e.g., carbon dating using beta decay).

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E.3.2 Types of radioactive decay

Types of Radioactive Decay: Alpha, Beta, and Gamma

Alpha Decay: Emission of Helium Nuclei

Uranium-235 Decay

Energy Released During Alpha Decay

Beta Decay: Conversion of Neutrons or Protons

Beta Minus (β−) Decay

Thorium-234 Decay

Beta Plus (β+) Decay

Sodium-22 Decay

Why Do Beta Particles Have a Continuous Energy Spectrum?

Gamma Decay: Emission of High-Energy Photons

What is Gamma Decay?

Cobalt-60 Decay

Properties of Gamma Rays

The Role of Neutrinos and Antineutrinos in Beta Decay

Why Were Neutrinos Postulated?

Properties of Neutrinos

Reflection

You've read 2/2 free chapters this week.

Introduction to Radioactive Decay

Beta Minus ( $β^{-}$ ) Decay

Beta Plus ( $β^{+}$ ) Decay