22.3 Radioactivity | Nuclear Physics | Physics 12

What is Radioactivity?

Radioactivity is the spontaneous disintegration of unstable atomic nuclei, accompanied by the emission of ionizing radiation. The three main types of radiation emitted are:

Radiation	Symbol	Nature	Charge	Mass
Alpha	$α$	Helium nucleus ( $_{2}^{4} He$ )	$+ 2 e$	$4 u$
Beta	$β$	Electron or positron	$- e$ or $+ e$	$\approx 0$
Gamma	$γ$	Electromagnetic photon	$0$	$0$

Spontaneous Nature of Radioactive Decay

Radioactive decay is described as spontaneous because:

The decay process occurs without any external trigger.
It is independent of physical conditions such as temperature, pressure, or chemical environment.
It is purely an internal nuclear event — the instability lies within the nucleus itself.

Example: Heating a radioactive sample, placing it under high pressure, or changing its chemical compound does not alter its rate of decay or half-life.

This is fundamentally different from chemical reactions, which can be sped up or slowed down by changing temperature or adding a catalyst.

Random Nature of Radioactive Decay

Radioactive decay is described as random because:

It is impossible to predict exactly which nucleus in a sample will decay next.
It is impossible to predict the exact moment a particular nucleus will decay.
Each nucleus decays independently of its neighbours.
Only the probability of decay per unit time (the decay constant $λ$ ) can be determined.

Analogy: Like flipping a coin — you cannot predict the outcome of a single flip, but for a large number of flips you can predict that approximately half will be heads.

Because of this randomness, the count rate from a Geiger counter fluctuates randomly around a mean value, even for a constant source. These fluctuations are direct evidence of the random nature of decay.

Statistical Nature

Because individual decays are random, the law of radioactive decay ( $\frac{Δ N}{Δ t} = - λ N$ ) is a statistical law — it is only accurate for a large number of nuclei. For a small sample, fluctuations become significant.

The decay constant $λ$ represents the probability of decay per unit time for a single nucleus. It is a fixed property of the isotope and does not change as the sample ages.

Key Points Summary

Spontaneous → independent of external conditions (temperature, pressure, chemical state)
Random → cannot predict which nucleus decays or when; only probability is known
Statistical → the exponential decay law emerges from the random behaviour of large numbers of nuclei
The decay constant $λ$ is constant for a given isotope regardless of age or environment

What is Radioactivity?

Radioactivity is the spontaneous disintegration of unstable atomic nuclei, accompanied by the emission of ionizing radiation. The three main types of radiation emitted are:

Radiation	Symbol	Nature	Charge	Mass
Alpha	$α$	Helium nucleus ( $_{2}^{4} He$ )	$+ 2 e$	$4 u$
Beta	$β$	Electron or positron	$- e$ or $+ e$	$\approx 0$
Gamma	$γ$	Electromagnetic photon	$0$	$0$

Spontaneous Nature of Radioactive Decay

Radioactive decay is described as spontaneous because:

The decay process occurs without any external trigger.
It is independent of physical conditions such as temperature, pressure, or chemical environment.
It is purely an internal nuclear event — the instability lies within the nucleus itself.

Example: Heating a radioactive sample, placing it under high pressure, or changing its chemical compound does not alter its rate of decay or half-life.

This is fundamentally different from chemical reactions, which can be sped up or slowed down by changing temperature or adding a catalyst.

Random Nature of Radioactive Decay

Radioactive decay is described as random because:

It is impossible to predict exactly which nucleus in a sample will decay next.
It is impossible to predict the exact moment a particular nucleus will decay.
Each nucleus decays independently of its neighbours.
Only the probability of decay per unit time (the decay constant $λ$ ) can be determined.

Analogy: Like flipping a coin — you cannot predict the outcome of a single flip, but for a large number of flips you can predict that approximately half will be heads.

Statistical Nature

The decay constant $λ$ represents the probability of decay per unit time for a single nucleus. It is a fixed property of the isotope and does not change as the sample ages.

Key Points Summary

Spontaneous → independent of external conditions (temperature, pressure, chemical state)
Random → cannot predict which nucleus decays or when; only probability is known
Statistical → the exponential decay law emerges from the random behaviour of large numbers of nuclei
The decay constant $λ$ is constant for a given isotope regardless of age or environment