lesson
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Right now, a single banana on your kitchen counter emits roughly 15 radioactive particles every second from naturally occurring potassium-40. How do physicists measure this invisible rate of atomic disintegration?
Radioactive Activity and the Becquerel
The activity of a radioactive source is the total number of unstable atomic nuclei that decay per second.
We measure activity in Becquerels (Bq), where 1 Bq=1 decay per second. In 1896, French physicist Henri Becquerel discovered spontaneous radioactivity while investigating uranium salts, giving this fundamental unit its name.
📊Interactive/animated visual showing a radioactive sample in a lead box. Above it, an animated counter displays 'Decays: X' and 'Activity: 5 Bq (5 decays/sec)'. Inside the sample, distinct purple nucleus circles flash red one-by-one and eject a small yellow particle ray every 0.2s. A clean label below shows: '1 Becquerel (Bq) = 1 decay per second'. Minimal UI with a modern card style, #1e2945 dark text, white card background, light-gray container, subtle blue (#22b7ff) accent badge.
If a sample has an activity of 600 Bq, exactly 600 of its unstable nuclei break apart every second.
If thousands of nuclei decay every second, why does a detector held near the sample only catch a tiny fraction of them?
Count Rate vs. True Activity
A Geiger-Müller (GM) tube is an instrument that detects ionizing radiation by generating an electrical pulse each time a particle enters its chamber.
The count rate is the number of radiation pulses recorded by the GM tube per unit time, typically written in counts per second (cps) or counts per minute (cpm).
📊A 2D cross-section diagram showing why Count Rate is less than Activity. In the center, a source radiates particles in a full 360-degree sphere (represented by 12 radial dotted arrows). A cylindrical Geiger-Müller (GM) tube is positioned to the right, intercepting only 2 of the 12 arrows. A counter on the GM tube reads 'Count Rate: 2 cps' while the source label reads 'Total Activity: 12 Bq'. Clear callout text explains: 'Radiation spreads in all directions + detector efficiency < 100%'. Light gray background, white card container, dark blue text (#1e2945).
The measured count rate is always significantly smaller than the source's true activity because radiation spreads outward in all directions, and the GM tube's window intercepts only a small geometric area.
Even if you remove the radioactive sample entirely from the room, the GM counter will still click every few seconds — so where is this extra radiation coming from?