IB Physics SLTopic 5 — The Atomic & Nuclear WorldPaper 1 & 2corrected count rate~7 min read
Background Radiation
Point a Geiger counter at nothing in particular and it still ticks. That’s because we’re all bathed, all the time, in a low level of radiation coming from rocks, food, space, and even our own bodies. It’s harmless at these levels — but when you’re measuring a radioactive source in the lab, you have to subtract this ever-present hum to get the true reading.
📘 What you need to know
Background radiation is the ionising radiation always present in the environment
Its sources split into natural (radon gas, cosmic rays, rocks, food) and artificial (medical, nuclear waste, fallout, accidents)
Radon gas from rocks and soil is the single largest contributor; nuclear waste and fallout together add up to well under 1%
Background level is low and harmless, but varies from place to place (e.g. more radon over certain rocks)
To measure a source accurately, subtract the background: corrected count rate = source reading − background reading
Count rate is measured in counts per minute (cpm) or counts per second (cps); divide cpm by 60 to get cps
Accuracy improves by repeating readings and counting over longer times
What Is Background Radiation?
Background radiation is simply the ionising radiation that is always there in the environment around us, whether or not anyone brings a radioactive source into the room. It comes from a whole spread of sources, which physicists group into two families: natural sources that have always existed, and artificial (human-made) sources that are far smaller contributors.
Radon gas is the biggest single source of background radiation, and natural sources overall far outweigh human-made ones. Nuclear waste and fallout make up only a tiny slice.
Natural Sources
Nature supplies most of the background dose. The main players are:
Radon gas — radioactive uranium sits in tiny amounts inside rocks and soil everywhere. As it decays it produces radon, a gas that seeps up into the air (and into buildings made of stone or brick). Radon is an alpha emitter, tasteless and odourless, and is the biggest single contributor.
Cosmic rays — the Sun and distant events like supernovae fire high-energy particles at Earth. When they smash into molecules high in the atmosphere they produce gamma radiation that reaches the ground.
Rocks and buildings — the same radioactive elements in the ground are in the stone and concrete around us.
Food and drink — some foods carry naturally radioactive elements. Bananas, for example, contain potassium-40. The amounts are minuscule and no cause for concern.
uranium in rocks
→ decays to →
radon gas in air
→ biggest →
background source
Artificial Sources
Human-made sources add only a small fraction of the total. They include nuclear medicine (X-rays, CT scans, radioactive tracers, radiotherapy), nuclear waste from power stations, fallout from past nuclear weapons testing, and rare nuclear accidents like Chernobyl. Medical uses are the largest artificial contributor; waste and fallout are tiny under normal conditions but would rise sharply near weapons testing or a serious accident.
Correcting for Background
Here’s the practical bit that comes up in calculations. Because background radiation is always present, a Geiger counter placed near a source records the source plus the background. To get the true reading from the source alone — the corrected count rate — you first measure the background with no source present, then subtract it from the reading with the source there.
Convert units if needed — divide counts per minute by 60 to get counts per second
Improve accuracy by repeating and averaging, and by counting over a longer time
Quick recap: background radiation is always present, mostly from natural sources (radon is the biggest); to measure a source properly you subtract the background reading to get the corrected count rate, then divide by 60 to swap cpm for cps.
WE 1
A student measures 33 counts in one minute with no source present. With a radioactive source in place, the counter records 372 counts in one minute. Calculate (a) the corrected count rate in counts per minute, and (b) the corrected count rate in counts per second.
Part (a) — corrected count rate (cpm)
background = 33 cpm, source reading = 372 cpm
corrected = source − background
corrected = 372 − 33 = 339339 counts per minutePart (b) — convert to counts per second
divide by 60 (60 seconds in a minute)
339 ÷ 60 = 5.655.65 counts per second
WE 2
(a) Explain why a background reading must be taken before measuring a source. (b) State two ways the student could make the count-rate measurements more accurate.
Part (a) — why subtract background
background radiation is always present in the environment
so the counter detects the source AND the background together
subtracting the background gives the true count rate from the source alone
→ needed to get an accurate, corrected readingPart (b) — improving accuracy
repeat the readings and take an average
count over a longer period of timeBoth reduce the effect of the random fluctuations in the count rate.
💡 Top tips
Always subtract the background before quoting a source’s count rate — corrected = source reading − background
cpm → cps: divide by 60. Watch which unit the question wants; mixing them up is an easy mark to drop
Radon is the biggest source — if asked for the largest contributor to background radiation, name radon gas from rocks and soil
Natural beats artificial. Nuclear waste and fallout are a tiny slice; most background is natural
⚠ Common mistakes
Forgetting to subtract the background, so the source’s count rate comes out too high
Assuming nuclear power or weapons are the main source of background radiation — they contribute very little; radon and other natural sources dominate
Muddling cpm and cps — remember there are 60 seconds in a minute, so cps is the smaller number
Saying background radiation is dangerous — at normal levels it’s harmless; it just has to be accounted for in measurements
Up next: Alpha, Beta & Gamma Particles. Now you know the radiation that’s always around us, we’ll look closely at the three types a nucleus can emit — what each one is, how ionising and penetrating it is, and how it changes the nucleus behind it.
Want this to actually click before the exam?
Book a free meeting and let’s work through the tricky bits together.
