Cambridge IGCSE Physics · Syllabus 0625 · Nuclear Physics
Ionising Radiation
What is Ionising Radiation?
Ionising radiation is radiation with enough energy to remove electrons from atoms, forming ions. Alpha particles, beta particles and gamma radiation are all ionising, which is why they can be detected and why they can damage living cells.
This definition is part of the Nuclear Physics chapter in Cambridge IGCSE Physics.
Ionising Radiation in context
Ionising nuclear radiation is measured with a detector connected to a counter. The detector registers one count each time an ionising event occurs inside it, and the counter adds those counts up over a timed interval. Dividing the total by the time gives the count rate, in counts/s or counts/minute. Some counts are always recorded even with no source present: this is background radiation, the ionising radiation present in the environment at all times. The sources that contribute significantly to it are radon gas in the air, rocks and buildings, food and drink, and cosmic rays.
Examiner tips on Ionising Radiation
- Command Words Examiner-Language Guidance The command word tells you the shape of the answer required. Getting the shape wrong leaves the question unanswered even when the physics is right — a beautiful explanation does not answer "State" if it never states the thing. Command words as they apply to Topic 5 Command wordWhat the answer must containWorked shape from this topic StateThe fact, expressed in clear terms. No justification is asked for, so do not spend time on one."State the relative charge of a proton." → "\(+1\)." Do not write a paragraph. KnowRecall of the stated fact. The syllabus uses this verb for several Core statements, and it is examined exactly like "State"."Know the sources of background radiation." → radon gas in the air, rocks and buildings, food and drink, cosmic rays. DefineThe formal definition, in the standard wording, including every qualifying phrase."Define half-life." → "The time taken for half the nuclei of that isotope in any sample to decay." Dropping "of that isotope" weakens it. DescribeWhat happens, in the right order. No causes required unless asked."Describe how radioactive materials are stored safely." → sealed, in labelled shielded containers, in a locked store, with a record kept. ExplainCause and mechanism, not just outcome. Look for the word "because" in your own answer — if it is missing, you have probably described rather than explained."Explain why beta deflects more than alpha." → "Because its mass is far smaller, so the same order of force produces a much larger acceleration." CalculateWorking, substitution, answer, unit. Show the relationship before the numbers."Calculate the count rate." → show count ÷ time, then the answer with its unit stated as counts/s or counts/minute. DetermineUsually means "get it from the data or the graph". Show which values you read off and where."Determine the half-life from the graph." → mark the two points used and show the time interval between them. IdentifyThe name of the thing: naming, selecting or recognising it is what is asked for. Quoting the evidence you used is good practice and is what a follow-up "explain" would want."Identify the radiation." → "Beta — the count rate was unchanged by paper but fell to background with \(3\ \mathrm{mm}\) of aluminium." UseApply the stated relationship or notation to the case in front of you. The syllabus uses it for nuclide notation, count rate and decay equations."Use decay equations to show beta emission." → write the full equation and check both ledger rows. SuggestApply known physics to a situation you have not been taught directly. There may be more than one acceptable answer, but it must be justified."Suggest why a shorter half-life is preferred for a substance given to a patient." → the activity falls away soon after the procedure, limiting continuing exposure. CompareStatements about both things, on the same property. "A is strong" is not a comparison."Compare alpha and gamma." → "Alpha is more strongly ionising than gamma, but gamma is far more penetrating than alpha." DeduceA conclusion drawn from the information available, together with the step in the data that forces it. Do not assert the conclusion on its own."Deduce the daughter nuclide." → subtract \(4\) from \(A\) and \(2\) from \(Z\), then name the element with that proton number. Phrase bank: what to write, and what to avoid writing Replace the left column with the right column Avoid — vague or wrongWrite instead — precise and creditableWhy "Alpha is the strongest.""Alpha is the most strongly ionising and the least penetrating.""Strong" does not name a property, and the two properties run in opposite orders. "Lead stops gamma.""Thick lead greatly reduces the intensity of gamma radiation."Gamma is absorbed gradually; "stops" is an overstatement. "The count rate is 300.""The count rate is \(300\) counts/minute."A count rate without its time unit is not a rate. "Be careful with the source."Core: "Handle it with tongs and return it to its shielded container immediately." Supplement: add "…which increases the distance and reduces the exposure time."Core wants the action; Supplement wants the action and the quantity it changes. "Wear protective clothing.""Keep the source in its shielded container until it is needed."A laboratory coat absorbs essentially no beta or gamma; it names no quantity that changes. "The atom loses a proton to become an ion.""The atom loses an electron to become a positive ion; the nucleus is unchanged."Ion formation is an electron event. "The nucleus splits into two identical halves.""The nucleus splits into two daughter nuclei, usually of unequal size and not the same every time."Fission products vary between events. "Radiation is dangerous.""Ionising radiation can cause cell death, mutations and cancer, and the risk increases with the dose received."Name the effects and the dependence. "The source ran out.""The activity fell until the count rate was too small to measure reliably."Sources do not run out; their activity falls. "It gets deflected in the field.""With the upper plate positive, it is deflected downwards towards the negative plate."A direction is meaningless without a defined field. "Half the atoms disappear.""Half the nuclei of that isotope decay, forming daughter nuclei which remain in the sample."Nothing disappears; it changes. "Energy is released, so mass is created.""The products have slightly less total mass than the reactants, and that difference is released as energy."The mass change runs the other way, and the syllabus wants it stated qualitatively. One habit that separates Core and Extended answers. Where a Core answer names something — the radiation, the effect, the precaution — an Extended answer names it and then names the physical quantity behind it: the charge, the kinetic energy, the corrected count rate, the exposure time. If your Extended answer could have been written by a Core candidate, it is probably not yet answering the question. Before you write a single Topic 5 sentence, ask three things: which particle or emission am I talking about; which property am I using (ionising, penetrating, charge, mass, half-life); and which number changed (\(A\), \(Z\), count rate, time)? An answer that names all three is almost always creditable. An answer that names none of them almost never is.