Understanding Nuclear physics: IGCSE Physics Explained

Get the basics locked down first

Nuclear physics scares students. It shouldn't. The topic is compact, the maths is straightforward, and the mark schemes are predictable. If you know your particle charges, can sketch a decay equation, and understand half-life, you're already ahead of most candidates sitting the IGCSE Physics (0625) paper.

Here's what you need to do: learn the structure of the atom cold, memorise the three types of radiation and their properties, practise half-life calculations until they're automatic, and understand why radioactive sources are both useful and dangerous. That covers roughly 90% of what Cambridge asks on this topic.

The nuclear model of the atom: IGCSE essentials

Every atom has the same basic layout. A small, dense, positively charged nucleus sits at the centre. Negatively charged electrons orbit around it. The nucleus contains two types of particle: protons (positive) and neutrons (neutral). Almost all the atom's mass is concentrated in the nucleus. The electrons occupy a comparatively huge volume of space around it, but they contribute almost nothing to the total mass.

Relative charges and masses

These numbers must be memorised. No formula sheet will save you here. Proton is +1, neutron is 0, electron is -1. Proton and neutron each have relative mass 1. The electron's mass is so tiny it's treated as zero for nuclear calculations.

Proton number (Z) and nucleon number (A)

Two numbers define any nucleus:

• Proton number (Z), also called the atomic number: the number of protons in the nucleus. This defines the element. Change Z and you change the element entirely.
• Nucleon number (A), also called the mass number: the total number of protons and neutrons. So the number of neutrons is simply A - Z.

Standard notation writes the nucleon number as a superscript and the proton number as a subscript before the element symbol. For carbon-14: ¹⁴₆C tells you there are 6 protons and 14 - 6 = 8 neutrons.

Quick check: An atom of aluminium has A = 27 and Z = 13. How many neutrons? Answer: 27 - 13 = 14 neutrons. If you hesitated, practise ten more of these tonight.

Ions

A neutral atom has equal numbers of protons and electrons. Lose an electron and the atom becomes a positive ion (more protons than electrons). Gain an electron and it becomes a negative ion. The nucleus doesn't change during ionisation. Only the electron count shifts.

Isotopes

Isotopes are atoms of the same element with the same proton number but different nucleon numbers. That means same number of protons, different number of neutrons. Carbon-12 and carbon-14 are both carbon (Z = 6), but carbon-12 has 6 neutrons while carbon-14 has 8. Their chemical behaviour is identical because chemistry depends on electrons, not neutrons. But their nuclear behaviour differs, and that's what makes some isotopes radioactive.

Radioactivity: what it is and where it comes from

Some nuclei are unstable. They have too many or too few neutrons relative to their protons, or they simply carry too much energy. These unstable nuclei decay spontaneously, emitting radiation in the process. You can't speed it up, slow it down, or stop it. It's a random process that no chemical or physical change can influence.

That last point matters for the exam. Radioactive decay is spontaneous and random. "Spontaneous" means it happens without any external trigger. "Random" means you can't predict exactly when a particular nucleus will decay. You can only predict the overall rate for a large sample.

Background radiation

Radiation isn't just in laboratories. It's everywhere. Background radiation comes from natural sources (rocks, soil, cosmic rays from space, radon gas from the ground) and artificial sources (medical X-rays, nuclear fallout, industrial waste). Any experiment measuring radioactivity must account for background radiation by subtracting it from the measured count rate. Cambridge examiners test this regularly.

The three types of radiation

Unstable nuclei can emit three types of radiation. Each has different properties, different penetrating power, and different ionising ability. You need all of this for the exam.

The pattern is simple: the heavier the particle, the more ionising but less penetrating it is. Alpha particles are big and slow. They smash into air molecules constantly, transferring energy at every collision, so they run out of energy quickly. Gamma rays are massless photons. They slip through matter with few interactions, which is why they penetrate so far but ionise weakly.

Exam tip: If a question asks you to identify the type of radiation from an absorption experiment, think about what stops it. Paper stops it? Alpha. Aluminium stops it? Beta. Only thick lead reduces it? Gamma. This three-step test answers most identification questions.

Nuclear decay equations

When a nucleus emits radiation, it transforms. You need to write balanced nuclear equations for alpha and beta decay.

Alpha decay: The nucleus loses 2 protons and 2 neutrons. The nucleon number drops by 4 and the proton number drops by 2.

Example: Radium-226 undergoes alpha decay.

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

Check: nucleon numbers balance (226 = 222 + 4). Proton numbers balance (88 = 86 + 2). Always verify both.

Beta decay: A neutron inside the nucleus converts into a proton and emits a high-speed electron. The nucleon number stays the same and the proton number increases by 1.

Example: Carbon-14 undergoes beta decay.

¹⁴₆C → ¹⁴₇N + ⁰_-1e

Check: 14 = 14 + 0. And 6 = 7 + (-1). Both sides balance.

Gamma emission: The nucleus releases energy as a gamma ray. No particles are lost, so both A and Z remain unchanged. Gamma emission often accompanies alpha or beta decay.

Half-life

Half-life is the time taken for half the radioactive nuclei in a sample to decay. Equivalently, it's the time for the count rate (or activity) to fall to half its original value. Different isotopes have wildly different half-lives: uranium-238 has a half-life of 4.5 billion years, while radon-220 has a half-life of about 56 seconds.

Half-life is constant for a given isotope. It doesn't change with temperature, pressure, or how much of the substance you have. After one half-life, half the original nuclei remain. After two half-lives, a quarter remain. After three, an eighth. The decay curve is exponential.

Worked example 1: Counting half-lives

A radioactive sample has an initial activity of 800 counts per minute. The half-life is 3 hours. What is the activity after 9 hours?

1. Number of half-lives = 9 / 3 = 3
2. After 1 half-life: 800 / 2 = 400 counts per minute
3. After 2 half-lives: 400 / 2 = 200 counts per minute
4. After 3 half-lives: 200 / 2 = 100 counts per minute

The activity after 9 hours is 100 counts per minute.

Worked example 2: Finding half-life from a graph

A decay graph shows the activity dropping from 1200 Bq to 150 Bq over 6 days. Find the half-life.

1. Count the halvings: 1200 → 600 → 300 → 150
2. That's 3 half-lives to go from 1200 to 150
3. Half-life = 6 days / 3 = 2 days

Exam tip: When reading a half-life from a graph, pick a starting value on the y-axis, find where it halves, and read the time interval from the x-axis. Don't start from the very first data point if it looks unreliable. Pick a clean value.

Worked example 3: Background radiation correction

A Geiger counter records 360 counts per minute near a radioactive source. The background count rate is 40 counts per minute. The half-life of the source is 2 hours. What will the Geiger counter read after 6 hours?

1. Corrected initial count rate = 360 - 40 = 320 counts per minute (from the source only)
2. Number of half-lives = 6 / 2 = 3
3. After 3 half-lives: 320 / 8 = 40 counts per minute (from the source)
4. Total reading = 40 + 40 = 80 counts per minute

The background count doesn't decay. You subtract it before halving, then add it back at the end. Forgetting this step is one of the most common errors on IGCSE nuclear physics questions.

Uses and dangers of radioactivity

The properties of each radiation type determine its practical applications. Match the radiation to the job by thinking about penetration and ionisation.

Uses

• Medical tracers: Gamma-emitting isotopes with short half-lives are injected into patients. A gamma camera outside the body detects where the tracer accumulates, revealing blockages or tumours. Gamma is used because it passes through body tissue to reach the detector.
• Treating cancer (radiotherapy): Gamma rays are directed at cancerous cells to destroy them. The source is positioned carefully to minimise damage to surrounding healthy tissue.
• Sterilising medical equipment: Gamma radiation kills bacteria on surgical instruments without heating them. The high penetrating power means sealed packages can be sterilised from outside.
• Thickness monitoring in industry: Beta radiation is used to monitor the thickness of paper or metal sheets during manufacturing. If the sheet gets too thick, fewer beta particles pass through to the detector, and the rollers adjust automatically. Alpha would be stopped completely, and gamma would pass through regardless of small thickness changes, so beta is the right choice.
• Smoke detectors: A small alpha source ionises air between two plates, allowing a tiny current to flow. Smoke particles absorb the alpha radiation, reducing the current, and triggering the alarm. Alpha works here because its strong ionising power creates the current, and its short range means it's safely contained inside the detector.
• Carbon dating: Living organisms absorb carbon-14 from the atmosphere. When they die, the carbon-14 decays with a half-life of 5730 years. Measuring how much remains tells scientists how old the specimen is.

Dangers and safety precautions

Radiation damages living cells. It can kill cells outright or damage DNA, potentially causing mutations and cancer. IGCSE Physics examiners expect you to distinguish between irradiation (exposure to radiation from outside) and contamination (getting radioactive material on or inside your body). The danger depends on the type of exposure:

• External exposure: Gamma is the biggest threat because it penetrates the body. Alpha is harmless externally because skin stops it.
• Internal exposure (ingested or inhaled): Alpha becomes the most dangerous. Its strong ionising power causes massive damage to surrounding tissue at close range.

Safety precautions when handling radioactive sources:

1. Use tongs or long-handled tools. Never handle sources directly.
2. Keep exposure time as short as possible.
3. Maximise distance from the source. Intensity falls with the square of distance.
4. Use shielding appropriate to the type of radiation.
5. Store sources in lead-lined containers when not in use.
6. Point sources away from the body.

Nuclear fission and fusion (Extended)

Extended IGCSE candidates need to understand two processes that release nuclear energy.

Nuclear fission

Fission is the splitting of a large, unstable nucleus (like uranium-235) into two smaller nuclei, plus additional neutrons and a large amount of energy. A neutron strikes the uranium nucleus, making it unstable. It splits, releasing two or three more neutrons. These neutrons can then hit other uranium nuclei, causing them to split too. This is a chain reaction.

In a nuclear power station, control rods (usually boron or cadmium) absorb excess neutrons to keep the chain reaction at a steady rate. Without control, the reaction accelerates exponentially. That's a bomb. With control, it's a power source.

Nuclear fusion

Fusion is the joining of two small, light nuclei to form a larger nucleus. This is what powers the Sun: hydrogen nuclei fuse to form helium, releasing enormous energy. Fusion requires extremely high temperatures (millions of degrees) to overcome the electrostatic repulsion between the positively charged nuclei. On Earth, sustaining these conditions long enough for practical energy generation remains an engineering challenge, though several experimental reactors are making progress.

Both fission and fusion release energy because the products have a lower total mass than the reactants. The "missing" mass converts to energy according to Einstein's equation E = mc².

Common mistakes and how to avoid them

Self-check questions

Answer each one fully before checking below.

1. An atom has 11 protons, 12 neutrons, and 11 electrons. State the proton number, nucleon number, and overall charge of the atom.
2. Thorium-232 (²³²₉₀Th) undergoes alpha decay. Write the balanced nuclear equation and identify the daughter element (proton number 88 is radium).
3. A radioactive source has an activity of 6400 Bq. After 20 minutes the activity is 400 Bq. Calculate the half-life.
4. Explain why alpha radiation is used in smoke detectors but not in medical tracers.
5. Describe two safety precautions you should take when handling a radioactive source in a school laboratory.

Answers: (1) Z = 11, A = 11 + 12 = 23, charge = 0 (equal protons and electrons). (2) ²³²₉₀Th → ²²⁸₈₈Ra + ⁴₂He (A: 232 = 228 + 4, Z: 90 = 88 + 2). (3) 6400 → 3200 → 1600 → 800 → 400: that is 4 half-lives in 20 minutes, so half-life = 20 / 4 = 5 minutes. (4) Alpha strongly ionises air, creating the current needed for the detector circuit, and its short range means it stays safely inside the detector housing. For medical tracers, the radiation must pass through body tissue to reach an external detector, so gamma is needed because it has high penetrating power. (5) Handle the source with tongs (never bare hands). Keep the source at arm's length and pointed away from the body. Keep exposure time to a minimum.

Exam strategy for nuclear physics

Nuclear physics appears on IGCSE Papers 2 and 4 almost every session. The questions fall into predictable categories, and preparation is straightforward if you target each one.

Definitions: Know proton number, nucleon number, isotope, and half-life word-for-word. These are recall questions. Either you know the definition or you don't. No partial credit for vague wording.

Decay equations: Practise balancing nuclear equations until the arithmetic is instant. Check both A and Z balance every time. Examiners give the element for the daughter product in the periodic table data, so use it.

Half-life calculations: The key is recognising how many half-lives have passed. For graph questions in IGCSE Physics, read carefully. For numerical questions, divide the total time by the half-life. If the answer doesn't come out as a whole number of half-lives, check whether you've accounted for background radiation.

Application questions: These ask why a particular type of radiation suits a particular job. The answer always connects the radiation's properties (penetration, ionisation, half-life) to the requirements of the application. Don't just name the property. Explain why it matters for that specific use.

Spend your revision time on half-life problems and decay equations. These carry the most marks, and they're the areas where practice pays off fastest. The theory you can read once and remember. The calculations need repetition until the method is automatic.