What is meant by saying that radioactive decay is a random process?

It is impossible to predict which nucleus will decay next or when any particular nucleus will decay, and every undecayed nucleus has the same constant probability of decaying in the next second.

What is meant by saying that radioactive decay is a spontaneous process?

The decay of a nucleus is not triggered by anything outside it and cannot be influenced by external conditions such as temperature, pressure or chemical combination.

Explain why radioactive decay follows an exponential law.

The number of nuclei decaying per second is proportional to the number of undecayed nuclei remaining, so as the number remaining falls the rate of decay falls in the same proportion, which is the defining condition for exponential decay.

What shape is the graph of the number of undecayed nuclei against time for a radioactive sample?

It is a curve that starts at the initial number and falls steeply at first, then less and less steeply, approaching the time axis without ever reaching it.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

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The Nucleus and Radioactive Decay

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The Nucleus and Radioactive Decay

The random and exponential nature of radioactive decay

3 things to know
Unit A2 1

Overview

Saying that radioactive decay is a random process, saying that radioactive decay is a spontaneous process and why radioactive decay follows an exponential law.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why can a temperature rise of several hundred degrees not speed up a radioactive decay?

Key term

Give experimental evidence that radioactive decay is random.

Key term

Why does a large sample of a radioactive nuclide give a smooth decay curve even though each individual decay is random?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by saying that radioactive decay is a random process?
Model answer
It is impossible to predict which nucleus will decay next or when any particular nucleus will decay, and every undecayed nucleus has the same constant probability of decaying in the next second.
2
What is meant by saying that radioactive decay is a spontaneous process?
Model answer
The decay of a nucleus is not triggered by anything outside it and cannot be influenced by external conditions such as temperature, pressure or chemical combination.
3
Give experimental evidence that radioactive decay is random.
Model answer
Repeated counts of a steady source over equal intervals of time give values that fluctuate about a mean rather than repeating exactly, and the size of the fluctuation is what would be expected of a random process.
4
Why does a large sample of a radioactive nuclide give a smooth decay curve even though each individual decay is random?
Model answer
With an enormous number of nuclei present the statistical fluctuations are a negligible fraction of the total, so the average behaviour predicted by the constant decay probability is followed very closely.
5
Explain why radioactive decay follows an exponential law.
Model answer
The number of nuclei decaying per second is proportional to the number of undecayed nuclei remaining, so as the number remaining falls the rate of decay falls in the same proportion, which is the defining condition for exponential decay.
6
What shape is the graph of the number of undecayed nuclei against time for a radioactive sample?
Model answer
It is a curve that starts at the initial number and falls steeply at first, then less and less steeply, approaching the time axis without ever reaching it.

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