Cambridge IGCSE Physics · Syllabus 0625 · Nuclear Physics
Background Radiation
What is Background Radiation?
Background radiation is the ionising radiation that is present in the environment at all times, detected even when no radioactive source has been deliberately introduced.
This definition is part of the Nuclear Physics chapter in Cambridge IGCSE Physics.
Background 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.
Common mistakes with Background Radiation
- Error Repair Common-Mistake Clinic Twenty-six errors, each with the sentence you can safely write instead and a check that it has stuck. This is a register: it is built to be scanned, so that you recognise a wrong claim when it turns up in your own writing. The full argument for each one — why it is wrong, and the model that replaces it — is in the section that teaches the concept, and the tier column points you at it. The tier column tells you which route each one matters on: a Core candidate can skip the Supplement rows. The wrong statements are quoted so that you recognise them; every quoted misconception below is false. The full misconception register for Topic 5 The false claim Tier Write this instead — the exam-safe sentence Retrieval check 1. "The nucleus occupies most of the atom." Core "Most of the atom is empty space; the nucleus is very small but contains nearly all the mass." Which two quantities does the model treat oppositely? Volume and mass. 2. "An ion forms by changing its nucleus." Core "An atom becomes a positive ion by losing electrons and a negative ion by gaining electrons; the nucleus is unchanged." An aluminium atom, \(13\) protons, \(14\) neutrons, forms a \(3+\) ion. Protons, neutrons, electrons? 13, 14, 10. 3. "Electrons and protons have equal mass because they have equal and opposite charge." Core "The proton and electron have equal and opposite relative charges, \(+1\) and \(-1\), but the electron's mass is far smaller." Which of the two does the syllabus ask you to state? The relative charges. 4. "Isotopes have different proton numbers." Core "Isotopes are atoms of the same element with the same proton number but different numbers of neutrons." Are \({}^{40}_{18}\mathrm{Ar}\) and \({}^{40}_{20}\mathrm{Ca}\) isotopes? No — different elements. 5. "The nucleon number counts the electrons too." Core "The nucleon number is the total number of protons and neutrons in the nucleus." Does \(A\) change when an atom becomes a \(2-\) ion? No — nor does \(Z\). 6. "The counter tells you which radiation it is." Core "The count rate was unchanged by paper but fell to background with a few millimetres of aluminium, so the source emits beta particles." What extra evidence identifies a radiation? Which absorbers stop it — and, on the Extended route, how it deflects in a field. 7. "Background radiation can be ignored." Supp "The background count rate was measured with the source removed and subtracted from each reading to give the corrected count rate." Corrected rate \(200\), background \(20\). What does the counter read? \(220\). 8. "Radioactive decay can be triggered on demand." Core "Radioactive decay is spontaneous: it is not caused or controlled by anything outside the nucleus." A source is heated to \(300\ ^{\circ}\mathrm{C}\). Effect on its activity? None attributable to the heating. 9. "An individual nucleus's decay time can be predicted." Core "The decay of an individual nucleus is random and cannot be predicted, but the behaviour of a large sample is statistically predictable." Give both halves from memory. Individual random; population predictable. 10. "Alpha is the most penetrating radiation." Core "Alpha is the most strongly ionising and the least penetrating; it is stopped by paper or a few centimetres of air." Unaffected by paper, stopped by \(3\ \mathrm{mm}\) of aluminium. Which radiation? Beta. 11. "Penetration and ionisation mean the same thing." Core "Gamma has the weakest ionising effect but the greatest penetrating ability, so it passes through packaging while alpha cannot." Which property makes alpha dangerous inside the body? Its strong ionising effect. 12. "Gamma is completely stopped by thin lead." Core "Thick lead or concrete greatly reduces the intensity of gamma radiation; it is not completely absorbed." Is a thin lead sheet adequate gamma shielding? No — only a small fraction is absorbed. 13. "Alpha ionises most because it has the most energy." Supp "Alpha has the largest charge, so it exerts the largest force on nearby electrons, and being massive it moves slowly and gives up its kinetic energy over a very short path, so it makes the most ion pairs per unit length." Which factor explains why gamma ionises least? Charge — it has none. 14. "Deflection directions in a field diagram can be assigned freely." Supp "With the upper plate positive, the alpha is deflected downwards towards the negative plate and the beta upwards, and further because its mass is much smaller." Swap the plate polarities. What changes? Both charged paths reverse; gamma still goes straight. 15. "Most alpha particles hit the nucleus." Supp "Most alpha particles passed straight through undeflected; a very small number were deflected through large angles because they passed close to a nucleus." Name the force and its sense. Electrostatic; repulsive. 16. "A nucleus is unstable because it is radioactive." Supp "This isotope is radioactive because its nucleus has an excess of neutrons and/or is too heavy for the repulsion between its protons to be balanced." Carbon-12 is stable and carbon-14 is not. Which cause applies? Excess neutrons — carbon-14 has two more, and neither nucleus is heavy. 17. "A beta particle is an orbital electron that escaped." Supp "In beta decay a neutron in the nucleus changes into a proton and an electron; the electron is emitted as the beta particle." If beta were an orbital electron, what would happen to \(Z\)? Nothing — contradicting the observed change of element. 18. "Gamma emission changes \(A\) or \(Z\)." Supp "In gamma emission neither the nucleon number nor the proton number changes; the nucleus loses surplus energy." Alpha then gamma: total change in \(A\) and \(Z\)? \(-4\) and \(-2\). 19. "Fission and fusion are identical." Supp "Fission splits a heavy nucleus after it absorbs a neutron; fusion joins two light nuclei. In both, the products have slightly less total mass and energy is released." Which one needs a very high temperature, and why? Fusion — both nuclei are positive and repel. 20. "The equation balances, so no mass can have been lost." Supp "The nucleon number and proton number are conserved, but the total mass of the products is slightly less than that of the reactants, and the difference is released as energy." In \({}^{235}\mathrm{U} + \mathrm{n}\), how many nucleons before and after? \(236\) both times — and mass has still fallen. 21. "Half the sample's mass disappears each half-life." Core "Half-life is the time for half the nuclei of that isotope in the sample to decay; the daughter nuclei remain in the sample." After one half-life, has the sample lost half its mass? No. 22. "After a few half-lives no nuclei remain." Core "The activity continues to halve and approaches zero, but does not reach exactly zero after a finite number of half-lives." Fraction after \(5\) half-lives? \(\tfrac{1}{32}\), about \(3.1\%\). 23. "The raw count rate always halves." Supp "Background was subtracted from each reading before the half-life was determined, because only the corrected count rate halves." Background \(30\), corrected \(240\), \(T_{1/2} = 4\ \mathrm{h}\). Counter reading at \(8\ \mathrm{h}\)? \(90\). 24. "Food irradiation makes the food radioactive." Supp "The food is exposed to gamma radiation which kills bacteria; the food does not become radioactive, because no radioactive material is added." Is a patient radioactive after an external gamma scan? No — they were exposed to radiation, not given radioactive material. 25. "External alpha radiation is always harmless." Supp "An alpha source is a relatively low hazard from outside the body, but alpha-emitting material inside the body is especially damaging because of its strong ionising effect." Why does preventing ingestion matter more than shielding for an alpha emitter? Shielding handles the external hazard easily; nothing helps once it is inside. 26. "A radioactive source can be described as completely safe." Core "The precautions reduce the dose received to a very low level, so the risk is small; the source is not without risk." Rewrite "gamma is dangerous, alpha is safe". "Gamma is the greater external hazard, alpha the greater internal one; neither is without risk." Four things that are not on this register, because they are not on the syllabus. If you have met cloud chambers or spark counters as detection methods, chain reactions, nuclear reactors, moderators, control rods or coolant, or carbon-14 dating, those come from a different syllabus. They are not errors — they are simply outside Topic 5, so no mark is available for them and no mark is lost by not knowing them.
Examiner tips on Background 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.