IB Physics HL Topic 7 — Atomic, Nuclear & Particle Physics Paper 1 & 2 corrected count rate ~14 min read

Background Radiation

Radiation isn’t just something that comes from a lab source — it’s everywhere, all the time. The rocks beneath your feet, the food you eat, cosmic rays from space, even your own body all give off a low, steady level of radiation called background radiation. It’s harmless at these levels, but it matters hugely for experiments: if you’re measuring a source, the background is always sneaking into your readings, so you have to subtract it out to get the true count.

📚 What you need to know

Background radiation is the ionising radiation always present in the environment
It comes from natural sources (radon gas, cosmic rays, food, rocks) and artificial sources (medical, nuclear waste, fallout)
Radon gas from rocks is by far the largest contributor; nuclear waste and fallout are less than 1%
Background must be accounted for when measuring a source
The corrected count rate = measured count rate − background count rate
Count rate can be measured in counts per minute (cpm) or counts per second (cps)
Accuracy improves by repeating readings and measuring over a long time

What is background radiation?

Background radiation is defined simply as the ionising radiation present in the environment. It’s low-level and everywhere, and it never switches off. To make sense of it, we split its sources into two groups: natural (there whether humans exist or not) and artificial (created by human activity).

Don’t let “radiation is everywhere” scare you — the everyday background level is tiny and completely normal. The reason we care about it in physics isn’t danger; it’s accuracy. Every reading you take of a real source secretly includes some background, so you always have to subtract it to find what the source alone is doing.

Where it comes from

In the UK, the single biggest slice is radon gas, which seeps out of rocks and building materials. The rest is a mix of cosmic rays, medical procedures, food, and a very small amount from nuclear activity.

Radon gas dominates the UK background. “Other” (~1%) includes nuclear waste and fallout — far smaller than people often assume.

Natural sources

These would exist even without any human activity:

Radon gas — radioactive elements like uranium occur naturally in rocks and soil; uranium decays into radon, an alpha-emitting gas that can build up indoors. It’s tasteless, colourless and odourless, so only a Geiger counter reveals it.
Cosmic rays — the Sun and events like supernovae send high-speed particles into the atmosphere, producing gamma radiation when they collide with air molecules.
Food and drink — some foods carry naturally radioactive elements, such as potassium-40 in bananas. The amount is minuscule.
Carbon-14 — all living things contain a tiny, constant amount, replenished while alive.

Artificial sources

These come from human technology:

Nuclear medicine — X-rays, CT scans, tracers and radiotherapy all use radiation.
Nuclear waste — contributes little to general background, but is hazardous to those handling it.
Nuclear fallout — radioactive material thrown into the air by nuclear explosions; currently very low, but rises where weapons are tested.
Nuclear accidents — rare events like Chernobyl release large doses locally.

Correcting for background

Whenever you measure a source in the lab, your detector picks up the background as well as the source. To find the true reading from the source alone, you first measure the background with no source present, then subtract it. The result is the corrected count rate.

Corrected count rate corrected count rate = measured count rate − background count rate

Measure
background
(no source)

then

Measure
with source
present

subtract

Corrected
count rate

WE 1

A Geiger counter records 24 counts per minute with no source present. With a source in place it records 285 counts per minute. Calculate the corrected count rate (a) in counts per minute and (b) in counts per second.

(a) Step 1 — subtract the background corrected = 285 − 24 = 261 cpm 261 counts per minute (b) Step 2 — convert to counts per second 261 ÷ 60 = 4.35 cps 4.35 counts per second Always subtract the background BEFORE converting units. To go from cpm to cps, divide by 60 (there are 60 seconds in a minute).

WE 2

A student measures the count rate at different distances from a source and finds it settles to a constant 15 counts per minute far from the source. What does this constant value represent, and why?

Step 1 — what happens far from the source? Far away, the air absorbs the source’s radiation before it reaches the detector. Step 2 — interpret the constant reading The count rate stops falling and levels off, so this leftover reading isn’t coming from the source. Background count = 15 counts per minute When the count rate becomes constant with distance, whatever’s left must be background radiation only — the source is no longer reaching the detector.

⚛ Working a background question

Need the corrected rate? Subtract background from the measured rate.
cpm to cps? Divide by 60.
Reading gone constant with distance? That constant value is the background.
Want more accuracy? Repeat readings and take averages; measure over a longer time.
Asked for the biggest source? Radon gas from rocks (UK).

💡 Top tips

Always subtract the background first, then convert units.
cpm → cps: divide by 60.
Radon gas is the largest UK background source; fallout/waste is under 1%.
A count rate that goes constant with distance tells you the background level.
Improve accuracy by repeating readings and measuring over a long time.

⚠ Common mistakes

Forgetting to subtract the background from the source reading
Converting to cps before subtracting the background
Thinking nuclear waste/fallout is a large part of background — it’s under 1%
Assuming background is dangerous — everyday levels are harmless
Dividing cpm by the wrong number — use 60, not 100

Quick recap: Background radiation is the low-level ionising radiation always around us, mostly from natural sources (radon gas is biggest) plus smaller artificial ones. When measuring a source you must find the corrected count rate = measured − background. Convert cpm to cps by dividing by 60, and improve accuracy by repeating readings over a long time.

Now that you can strip the background out of a reading, you’re ready to study the radiation itself. When an unstable nucleus decays, it fires out one of three types of radiation — each with a different charge, range, and stopping material. Next page: Alpha, Beta & Gamma Radiation.