Half-life, activity and background radiation
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Flip to reveal answersDefine the half-life of a radioactive sample.
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Question
Define the half-life of a radioactive sample.
Answer
The **time** for the activity (or count rate, or number of undecayed nuclei) to fall to **half** its value.
Question
What is activity, and its unit?
Answer
The number of nuclei that **decay each second**. Unit: the **becquerel (Bq)**, where 1 Bq = 1 decay per second.
Question
What is count rate?
Answer
How many decays a **detector records each second** (clicks per second). It is always ≤ the activity.
Question
What is background radiation?
Answer
Radiation a detector picks up **even with no source** (from rocks, soil, cosmic rays). It must be **subtracted** to get the true source count.
Question
How do you find the true count rate from a source?
Answer
**Measured count rate − background count rate**. Always correct for background **before** halving.
Question
Count rate after n whole half-lives?
Answer
Start value **× (1/2)ⁿ**. So 1, 2, 3 half-lives leave 1/2, 1/4, 1/8 of the start.
Question
How do you find the number of half-lives that have passed?
Answer
**n = total time ÷ half-life.** Then halve the start value n times.
Question
Does radioactive decay ever reach exactly zero?
Answer
No — the count rate keeps **halving** and flattens out, but in theory never reaches zero.
Question
Two samples have the same half-life; what happens to their activity ratio over time?
Answer
It **stays the same** — both halve by the same factor each half-life, so the ratio is unchanged.
Question
A source reads 84 s⁻¹, background 4 s⁻¹, half-life 2 h. Measured rate after 4 h?
Answer
Source 84 − 4 = 80; 4 h = 2 half-lives → 80 → 40 → 20; add background → **24 counts s⁻¹**.
Question
Why is radioactive decay called 'random'?
Answer
You **cannot predict** when any one nucleus will decay; only the **average** behaviour (the half-life) is fixed.
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Radioactive decay
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