What is radioactivity and how is it related to atomic physics?

Radioactivity is the process by which unstable atomic nuclei lose energy by emitting radiation. It is a fundamental concept in atomic physics, which studies the structure and behavior of atoms. In the IB MYP Science curriculum, understanding radioactivity helps students grasp how elements transform and the implications of these changes in both natural and artificial processes.

What are the different types of radioactive decay?

There are three main types of radioactive decay: alpha decay, beta decay, and gamma decay. Alpha decay involves the emission of an alpha particle, which consists of two protons and two neutrons. Beta decay occurs when a neutron in an unstable nucleus is transformed into a proton, emitting a beta particle (electron or positron). Gamma decay involves the release of gamma rays, which are high-energy photons, from an excited nucleus. These processes are key topics in the IB MYP Science curriculum, providing insight into atomic transformations.

How is radioactivity measured and what units are used?

Radioactivity is measured using instruments like Geiger-Müller counters and scintillation detectors. The activity of a radioactive substance is measured in becquerels (Bq), which indicates the number of decays per second. Another unit, the curie (Ci), is also used, where 1 Ci equals 3.7 x 10^10 decays per second. Understanding these measurements is essential for students in the IB MYP Science program to quantify and analyze radioactive processes.

What are some real-world applications of radioactivity?

Radioactivity has numerous applications in various fields. In medicine, it is used in diagnostic imaging and cancer treatment through techniques like PET scans and radiotherapy. In industry, radioactive isotopes are used for material testing and quality control. Additionally, radioactivity plays a crucial role in carbon dating, which helps determine the age of archaeological finds. These applications highlight the practical importance of radioactivity, a key topic in the IB MYP Science curriculum.

What safety precautions are necessary when handling radioactive materials?

Handling radioactive materials requires strict safety measures to protect against harmful radiation exposure. Key precautions include using shielding materials like lead, maintaining a safe distance, minimizing exposure time, and wearing protective clothing. Proper storage and disposal of radioactive materials are also crucial. Understanding these safety protocols is vital for students studying radioactivity in the IB MYP Science curriculum, ensuring they are aware of both the benefits and risks associated with radioactive substances.

Why is "Believing radioactive samples eventually drop to zero activity at a fixed time." a common mistake in Radioactivity?

Why it happens: Half-life sounds like a definite endpoint. How to avoid it: Each half-life HALVES what remains. ¼, ⅛, ¹⁄₁₆ ... never quite zero, but becomes negligible.

Why is "Saying alpha is the most dangerous form of radiation in all situations." a common mistake in Radioactivity?

Why it happens: Alpha is described as 'most ionising'. How to avoid it: Alpha is the MOST IONISING when it hits cells — but the LEAST PENETRATING. Outside the body, paper stops it. Alpha is dangerous mainly when INGESTED or INHALED.

Why is "Saying lead stops gamma rays completely." a common mistake in Radioactivity?

Why it happens: Lead is shown blocking gamma in textbooks. How to avoid it: Lead REDUCES gamma intensity — thicker lead reduces it more. No reasonable thickness stops it 100%.

Atomic physicsIB MYP Sciences (Years 1-5)

Radioactivity

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Radioactivity — IB MYP Sciences: alpha, beta, gamma, half-life and uses

Some atomic nuclei are unstable and decay, emitting radiation. This MYP Sciences note covers the three main types of radiation (alpha, beta, gamma), what they do to matter, how half-life works, and the uses and dangers of radioactive sources.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Name the three types of radiation and state their properties (mass, charge, penetration, ionisation).
MYP Sciences A — Define half-life and use a decay curve to read it off.
MYP Sciences A — List sources of background radiation.
MYP Sciences C — Calculate remaining activity after a given number of half-lives.
MYP Sciences D — Evaluate uses and risks of radioactivity (medical, industrial, energy).

Three types of radiation: alpha, beta, gamma

Different particles or waves — different mass, charge and penetrating power.

An unstable nucleus emits one of three main types of radiation:

Type	Symbol	Made of	Mass	Charge	Stopped by
Alpha	α	2 protons + 2 neutrons (He nucleus)	4 u	+2	A sheet of paper / skin
Beta	β	Fast-moving electron from the nucleus	~0	−1	A few mm of aluminium
Gamma	γ	High-energy EM wave (photon)	0	0	Thick lead / concrete (reduced, not stopped)

Ionising power (how much the radiation damages atoms by knocking electrons off):

α (highest — heavy, slow, double-charged → bumps into many atoms)
β (medium)
γ (lowest — but penetrates deeply, so still dangerous)

Penetration is the reverse order: γ penetrates most, α least.

Penetration of the three types: α stops at paper, β at aluminium, γ is reduced by lead.

α = helium nucleus; β = electron from nucleus; γ = EM wave.
Penetration: γ > β > α.
Ionising: α > β > γ.
Stopped by paper / aluminium / lead respectively.

Half-life

Time for half the radioactive atoms in a sample to decay.

Radioactive decay is random for an individual atom — you can't predict exactly when one will decay. But across a large sample the average behaviour is highly predictable.

Half-life is the time it takes for HALF the radioactive nuclei in a sample to decay (or, equivalently, for the count rate or activity to halve).

After each half-life, the activity halves: 100 → 50 → 25 → 12.5 ...

Half-lives range hugely: carbon-14 has a half-life of 5 700 years (used in carbon dating); iodine-131 has 8 days (medical tracer); uranium-238 has 4.5 billion years.

Worked example. A radioactive source has activity 800 Bq. Its half-life is 2 hours. What is its activity after 6 hours?

6 hours = 3 half-lives. Activity halves three times: 800 → 400 → 200 → 100. So 100 Bq.

Half-life = time for HALF the nuclei (or activity) to decay.
Decay is random per atom; predictable in large samples.
Each half-life: activity ÷ 2.

Uses and dangers

Used in medicine, industry and dating. All ionising radiation carries risk.

Background radiation is the low-level radiation we are exposed to all the time. Sources include rocks (radon gas), cosmic rays from space, food (e.g. bananas contain ⁴⁰K), and medical procedures. Average annual dose: 2-3 mSv.

Uses of radioactive sources:

Medical tracers: a short half-life isotope (e.g. technetium-99m) injected into the patient, followed by a gamma camera, shows up problems like tumours or blocked arteries.
Cancer radiotherapy: targeted gamma rays kill cancer cells.
Sterilising: surgical instruments and some foods are exposed to gamma rays to kill bacteria.
Smoke alarms: contain a tiny americium source. Smoke disturbs the alpha particle flow, triggering the alarm.
Carbon dating: living things absorb ¹⁴C from the atmosphere; once they die, the ¹⁴C decays with a half-life of 5 700 years. Measuring the remaining ratio of ¹⁴C dates the sample.
Nuclear power: controlled nuclear fission releases heat to drive turbines.

Dangers:

All ionising radiation can damage DNA in living cells, leading to cancer.
Acute high doses cause radiation sickness.
Protection methods: SHIELDING (lead, concrete), DISTANCE (inverse-square law), TIME (limit exposure), CONTAINMENT (keep sources sealed).

Background radiation ≈ 2-3 mSv per year on average.
Uses: medical (tracers, therapy), industrial (sterilisation, smoke alarms), scientific (carbon dating), power.
Protection: SHIELDING, DISTANCE, TIME, CONTAINMENT.

How it’s examined

Criterion A tests recall of the three radiations and their properties. Criterion C is heavy on half-life calculations — make sure you can use a decay graph to read off a half-life. Criterion D often asks for an evaluation of medical or industrial use.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Radioactivity

Step-by-step solutions to past-paper-style questions on radioactivity, written exactly the way a tutor would explain them at the board.

1Identifying the radiation
Getting started• α β γ
Question
A radioactive source emits radiation that passes through a sheet of paper but is stopped by a few mm of aluminium. Which type is it?
Step-by-step solution
1. Step 1
  Paper would stop α — so it's not α.
2. Step 2
  Aluminium stops β. Gamma would pass through aluminium.
3. Step 3
  Therefore the radiation is BETA.
Answer
Beta (β).
2Activity after half-lives
Building confidence• half-life
Question
A source has activity 480 Bq. Its half-life is 1.5 hours. Find the activity after 4.5 hours.
Step-by-step solution
1. Step 1
  Number of half-lives = 4.5 / 1.5 = 3.
2. Step 2
  Activity halves 3 times: 480 → 240 → 120 → 60.
Answer
60 Bq.
3Reading half-life from a decay curve
Building confidence• graph reading
Question
A decay curve shows activity dropping from 200 Bq to 50 Bq in 6 days. What is the half-life?
Step-by-step solution
1. Step 1
  200 → 100 → 50: two halvings. So 6 days covers 2 half-lives.
2. Step 2
  Half-life = 6 / 2 = 3 days.
Answer
3 days.
4Choosing a medical tracer
Stretch• medical use
Question
A doctor needs to choose between two isotopes for a medical tracer: isotope A (half-life 10 minutes, alpha emitter) and isotope B (half-life 6 hours, gamma emitter). Which is more suitable and why?
Step-by-step solution
1. Step 1
  Alpha is highly ionising — would damage the patient's tissue inside the body. Gamma can leave the body for detection and damages less.
2. Step 2
  Half-life of 10 minutes is too short — there isn't enough time to perform the scan. 6 hours is short enough that the source decays away soon after the scan but long enough to be useful.
3. Step 3
  Therefore isotope B is more suitable.
Answer
Isotope B (6-hour, gamma) is better — gamma can escape the body for imaging and is less ionising, and 6 hours gives time to do the scan but doesn't keep irradiating the patient long-term.

Key Formulae — Radioactivity

The formulae you need to memorise for radioactivity on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Activity after n half-lives
$A_n = A_0 / 2^n$
$A_n$
activity after n half-lives
$A_0$
starting activity
$n$
number of half-lives that have passed
When to use
Quick way to find remaining activity when an exact number of half-lives have passed.

Key Definitions and Keywords — Radioactivity

Definitions to memorise and the exact keywords mark schemes credit for radioactivity answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Radioactivity
Examiner keyword
The spontaneous emission of radiation from an unstable nucleus as it decays.
Alpha (α) particle
Examiner keyword
A helium nucleus (2 protons + 2 neutrons). Heavy, +2 charge, stopped by paper. Highly ionising.
Beta (β) particle
Examiner keyword
A high-energy electron emitted from the nucleus. −1 charge, stopped by a few mm of aluminium.
Gamma (γ) radiation
Examiner keyword
A high-energy electromagnetic wave. No charge, no mass, reduced (not stopped) by lead.
Ionising radiation
Examiner keyword
Radiation with enough energy to knock electrons off atoms, damaging cells and DNA.
Half-life
Examiner keyword
The time for half the radioactive nuclei in a sample to decay (or for the activity to halve).
Activity
Number of nuclei decaying per second. Unit: becquerel (Bq) = 1 decay/s.
Background radiation
Examiner keyword
The low-level radiation we are exposed to from natural and medical sources (rocks, cosmic rays, food, scans). ~2-3 mSv/year on average.

Common Mistakes and Misconceptions — Radioactivity

The traps other students keep falling into on radioactivity questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Believing radioactive samples eventually drop to zero activity at a fixed time.
Why it happens
Half-life sounds like a definite endpoint.
How to avoid it
Each half-life HALVES what remains. ¼, ⅛, ¹⁄₁₆ ... never quite zero, but becomes negligible.
✕Saying alpha is the most dangerous form of radiation in all situations.
Why it happens
Alpha is described as 'most ionising'.
How to avoid it
Alpha is the MOST IONISING when it hits cells — but the LEAST PENETRATING. Outside the body, paper stops it. Alpha is dangerous mainly when INGESTED or INHALED.
✕Saying lead stops gamma rays completely.
Why it happens
Lead is shown blocking gamma in textbooks.
How to avoid it
Lead REDUCES gamma intensity — thicker lead reduces it more. No reasonable thickness stops it 100%.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Radioactivity — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Radioactivity

Short Study Notes in the form of Slides

Short Study Notes — Radioactivity

Detailed Notes

What you’ll learn

How it’s examined

1Identifying the radiation

2Activity after half-lives

3Reading half-life from a decay curve

4Choosing a medical tracer

Activity after n half-lives

Radioactivity

Alpha (α) particle

Beta (β) particle

Gamma (γ) radiation

Ionising radiation

Half-life

Activity

Background radiation

✕Believing radioactive samples eventually drop to zero activity at a fixed time.

✕Saying alpha is the most dangerous form of radiation in all situations.

✕Saying lead stops gamma rays completely.

Practice questions

Past paper style quiz

Assess your understanding

Radioactivity — frequently asked questions