📖 In-Depth Theory
What Is Background Radiation?
BACKGROUND RADIATION is low-level ionising radiation that is present everywhere in the environment at all times — from natural and artificial sources.
It is always present — even when no radioactive source is in the lab.
SOURCES OF BACKGROUND RADIATION:
NATURAL SOURCES (~85% of total in UK):
RADON GAS (~50%): naturally occurring radioactive gas from uranium in rocks. Seeps into buildings. Major health risk in granite areas (e.g. Cornwall).
GAMMA RAYS FROM GROUND AND BUILDINGS (~15%): radioactive isotopes in rocks (granite) and building materials.
COSMIC RAYS (~10%): high-energy particles from space. More at high altitude (pilots receive more).
FOOD AND DRINK (~10%): small amounts of naturally occurring radioactive isotopes (e.g. ¹⁴C, ⁴⁰K).
ARTIFICIAL SOURCES (~15% of total):
MEDICAL: X-rays, CT scans, nuclear medicine.
NUCLEAR INDUSTRY: small amounts from nuclear power stations and waste.
NUCLEAR WEAPONS TESTING: historical fallout still present.
Measuring and Correcting for Background
WHY IT MATTERS FOR EXPERIMENTS:
All measurements of radioactive sources include background radiation.
If not corrected, measured activity appears higher than the true source activity.
CORRECTING FOR BACKGROUND:
1. Measure the count rate without any source present (background count rate).
2. Measure the count rate with the source present.
3. Subtract background: corrected count rate = measured count rate − background count rate.
EXAMPLE:
Background count rate: 25 counts per minute.
Measured count rate with source: 175 counts per minute.
Corrected count rate from source: 175 − 25 = 150 counts per minute.
This correction is important for accurate half-life calculations and activity measurements.
VARIATION IN BACKGROUND:
Background radiation varies from place to place (geology, altitude).
Background radiation varies slightly over time (cosmic ray intensity fluctuates).
The count rate of the background should be measured over a long time period and averaged.
Health Risks and Radiation Dose
RADIATION DOSE is measured in SIEVERTS (Sv) or millisieverts (mSv).
The dose accounts for both the amount of radiation and its biological effect.
UK AVERAGE ANNUAL DOSE: approximately 2.7 mSv.
Majority from radon gas and medical procedures.
FACTORS AFFECTING PERSONAL DOSE:
Location: granite areas → more radon → higher dose.
Occupation: pilots, astronauts, nuclear workers → higher doses.
Medical procedures: X-rays, CT scans add to dose.
Altitude: more cosmic radiation at high altitude.
RADON GAS RISK:
Radon decays in lungs → alpha particles emitted → highly ionising → increases lung cancer risk.
Ventilating buildings in high-radon areas reduces exposure.
Radon test kits available for homes in affected areas.
BENEFIT vs RISK:
Medical uses (imaging, treatment) involve weighing the benefit of diagnosis/treatment against radiation dose risk.
Regulated exposure limits protect workers.
⚠️ Common Mistake
Background radiation must be SUBTRACTED before any half-life calculations. Using the uncorrected count rate gives a half-life that appears longer than the true value because the count rate never falls to zero. Background should be measured BEFORE introducing any source.