International GCSE Physics · Pearson Edexcel 4PH1 · Topic 7
Radioactivity and particles
Original exam-style questions with mark schemes. Questions marked “Paper 2 only” use content assessed only on Paper 2. Notes 7(b) →Notes 7(c) →
40questions
300marks
Freefull mark schemes
Original questions in the style of each exam boardEvery answer re-solved independently before publishingCreated by Evan Kotronis with AI assistanceUpdated 13 October 2026 · new questions added regularlyHow every question is checked →
1IGCSE-PHY-7-001Protons and neutrons from nuclide notationPapers 1 & 2Grade 4–51 mark
A nucleus of iron is represented by the symbol ⁵⁶₂₆Fe. How many neutrons are in this nucleus?
Show mark scheme
A (30);
B is incorrect because 26 is the atomic (proton) number, the number of protons
C is incorrect because 56 is the mass (nucleon) number, the total number of protons and neutrons
D is incorrect because 82 is the mass number added to the atomic number instead of subtracted
4IGCSE-PHY-7-004Sources of background radiationPapers 1 & 2Grade 4–57 marks
The pie chart shows the average contributions to the background radiation dose received by people living in one country.
(a)
State what is meant by background radiation.
(1)
(b)
Give the source that makes the largest contribution to background radiation in this country.
(1)
(c)
Many homes in one region are built on granite rock. Granite contains small amounts of uranium. Explain why people living in these homes receive a larger dose of background radiation from radon gas.
(2)
(d)
An airline pilot spends many hours each year flying at high altitude. Explain why the pilot receives a larger dose of background radiation than most people.
(2)
(e)
Use the pie chart to determine the percentage of the background radiation dose that comes from natural sources.
(1)
Show mark scheme
(a)
(ionising) radiation that is present all the time / everywhere (from natural and man-made sources);
Allow
radiation present without a radioactive source being used / in the environment
Do not accept
radiation from microwaves or mobile phones
(b)
radon (gas);
(c)
uranium in the granite decays (through a series of decays) to produce (more) radon gas;
radon (gas) is radioactive and collects in the homes / is breathed in, so the radiation is emitted inside the body/lungs;
Allow
more radon gas released from the ground under the homes for MP1
radon emits alpha particles inside the lungs for MP2
Do not accept
granite gives out cosmic rays
uranium is a gas
(d)
(more) cosmic rays (from space/the Sun);
less atmosphere above to absorb the radiation;
Allow
closer to space for MP2 if linked to less absorption
5IGCSE-PHY-7-005Practical: absorber needed to stop beta particlesPapers 1 & 2Grade 4–51 mark
A student uses a simulation to investigate the penetrating power of radiation. The simulated source emits only beta (β−) particles. The detector is 3 cm from the source. Which absorber, placed between the source and the detector, would reduce the count rate to background level?
Show mark scheme
B (5 mm of aluminium);
A is incorrect because paper stops alpha particles but most beta particles pass through it
C is incorrect because very thin aluminium foil only absorbs some of the beta particles; a few millimetres of aluminium are needed
D is incorrect because beta particles have a range of about a metre in air, so a few extra centimetres of air absorb very few of them
6IGCSE-PHY-7-006Food irradiation with gamma raysPapers 1 & 2Grade 4–58 marks
Some food is exposed to gamma rays from a cobalt-60 source to kill bacteria. This is called food irradiation. The food is sealed in its packaging before it is irradiated.
(a)
Describe the difference between irradiation and contamination.
(2)
(b)
Explain why the irradiated food does not become radioactive.
(1)
(c)
Explain how gamma rays kill bacteria.
(2)
(d)
Explain why gamma rays are used rather than alpha particles.
(2)
(e)
Cobalt-60 has a half-life of 5.3 years. Suggest why the food must be exposed to the source for a longer time each year to receive the same dose of radiation.
(1)
Show mark scheme
(a)
irradiation: (object) exposed to radiation from a source outside it / source not in contact;
contamination: radioactive material/substance is on or in the object;
Do not accept
contamination is when radiation gets into the object
(b)
no radioactive material/source gets onto/into the food / food is not contaminated;
Allow
the food only absorbs the radiation
Do not accept
gamma rays are not radioactive
food is sealed so radiation cannot get in
(c)
gamma rays are ionising / cause ionisation;
damages/kills cells/DNA (of the bacteria);
Allow
causes mutations in the bacteria for MP2
(d)
gamma (highly) penetrating so passes through packaging/food;
alpha would be absorbed/stopped by packaging/surface of food;
Allow
alpha has a range of only a few cm in air
Do not accept
alpha is not ionising
(e)
activity of the source decreases (with time) / fewer gamma rays emitted per second;
Allow
the cobalt-60 decays so there are fewer unstable nuclei left
8IGCSE-PHY-7-022Ionising ability and penetrating power comparedPapers 1 & 2Grade 4–51 mark
Alpha particles, beta (β−) particles and gamma rays are all ionising radiations. Which row identifies the radiation that is the most strongly ionising and the radiation that is the most penetrating?
Show mark scheme
D (most strongly ionising: alpha; most penetrating: gamma);
A is incorrect because it reverses the order: gamma is the weakest ioniser and alpha the least penetrating
B is incorrect because beta is in the middle for both properties
C is incorrect because gamma, not beta, is the most penetrating (it needs thick lead to absorb it)
9IGCSE-PHY-7-023Radiation badge worn by a hospital workerPapers 1 & 2Grade 4–58 marks
People who work with radioactive sources in a hospital wear a badge to monitor the radiation they receive. The diagram shows the front of the badge. Behind the four windows W, X, Y and Z is a light-proof packet containing a material that darkens when ionising radiation reaches it.
(a)
Name the material in the packet that darkens when ionising radiation reaches it.
(1)
(b)
Name one other device that can detect ionising radiation.
(1)
(c)
After one month the packet is developed. The region behind window Z (2 mm of lead) has darkened. Name the type of radiation that reached the packet behind window Z. Give a reason for your answer.
(2)
(d)
Another worker's packet has darkened only behind windows W and X. Name the type of radiation that this worker received. Give a reason for your answer.
(2)
(e)
Describe two ways in which ionising radiation can harm the worker's body.
(2)
Show mark scheme
(a)
photographic film;
Allow
film
Do not accept
X-ray plate
paper
(b)
Geiger–Müller tube / GM tube / Geiger counter;
Allow
spark counter
cloud chamber
Do not accept
ammeter
radiation badge
(c)
gamma (rays);
(only) gamma can penetrate / pass through (2 mm of) lead;
Allow
MP2: alpha and beta are absorbed before reaching the lead / by thinner materials
Do not accept
beta (for MP1)
MP2 without naming gamma in MP1
(d)
beta (β−);
passes through plastic but is absorbed / stopped by (2 mm of) aluminium;
10IGCSE-PHY-7-024Potassium and calcium nuclei in the human bodyPapers 1 & 2Grade 4–57 marks
Potassium is an element found in foods such as bananas and in the human body. The table gives information about three nuclei. Nucleus / name: potassium-39, potassium-40, calcium-40 Number of protons / count: 19, 19, 20 Number of neutrons / count: 20, n, 20
(a)
Determine the value of n.
(1)
(b)
Write the nuclide symbol for a potassium-40 nucleus, showing its mass number and its atomic number.
(1)
(c)
State which two nuclei in the table are isotopes of the same element. Give a reason for your answer.
(2)
(d)
Potassium-40 decays by emitting a beta (β−) particle to form calcium-40. Describe what happens inside the potassium-40 nucleus when it emits the beta particle and state how the mass number and atomic number change.
(2)
(e)
Explain why potassium-40 in the human body is a source of background radiation.
(1)
Show mark scheme
(a)
21;
Do not accept
40
19
(b)
⁴⁰₁₉K;
Allow
40 written as top number and 19 as bottom number
Do not accept
¹⁹₄₀K
(c)
potassium-39 and potassium-40;
same number of protons / same atomic number but different number of neutrons / different mass number;
Allow
MP2 independent of MP1 if a correct definition of isotope is given
Do not accept
potassium-40 and calcium-40 (same mass number)
same mass number different proton number for MP2
(d)
neutron changes into a proton (and an electron is emitted);
mass number unchanged and atomic number increases by 1;
Allow
MP2: A stays 40, Z goes from 19 to 20
Do not accept
an electron from the outer shell is emitted
mass number decreases by 1
(e)
it is radioactive / unstable and is naturally present (in food and) inside the body (so it emits radiation all the time);
11IGCSE-PHY-7-025Background count in an underground laboratoryPapers 1 & 2Grade 4–57 marks
Some very sensitive experiments are carried out in a laboratory 1100 m underground in an old salt mine. Scientists use the same Geiger–Müller (GM) tube and counter to measure the background count rate in three places: A: a laboratory on the surface B: the underground laboratory C: inside the underground laboratory, with the GM tube surrounded by a thick lead box. Location / label: A, B, C Background count rate / counts per minute: 31, 9, 2
(a)
Give one source of background radiation from space and one source from the Earth.
(2)
(b)
Explain why the background count rate in location B is lower than in location A.
(2)
(c)
Suggest why the background count rate in location B is not zero.
(1)
(d)
Suggest why the lead box reduces the count rate further.
(1)
(e)
The scientists record the number of counts in each minute. The values are different each minute even though nothing is changed. Explain why.
14IGCSE-PHY-7-008Radium-226 decay chain equationsPapers 1 & 2Grade 6–710 marks
Radium-226 (²²⁶₈₈Ra) is found in some rocks. It decays in several stages. Part of its decay chain is: radium-226 → radon-222 → polonium-218 → lead-214 → bismuth-214
(a)
Radium-226 decays by emitting an alpha particle to form radon (Rn). Determine the mass number and the atomic number of the radon nucleus.
(2)
(b)
Lead-214 (²¹⁴₈₂Pb) decays by emitting a beta (β−) particle to form bismuth (Bi). Write a balanced nuclear equation for this decay.
(3)
(c)
Bismuth-214 nuclei often emit gamma radiation as well. State the effect of gamma emission on the mass number and the atomic number of a nucleus.
(1)
(d)
Some unstable nuclei emit a neutron. A nucleus has mass number A and atomic number Z. It emits one neutron. State, in terms of A and Z, the mass number and the atomic number of the nucleus formed.
(2)
(e)
Lead-214 and bismuth-214 have the same mass number. Explain why they are not isotopes of the same element.
(2)
Show mark scheme
(a)
(mass number =) 222;
(atomic number =) 86;
(b)
β written as 0 (top) and −1 (bottom);
Bi with mass number 214;
Bi with atomic number 83;
Allow
e for β
e.g. ²¹⁴₈₂Pb → ²¹⁴₈₃Bi + ⁰₋₁e
Do not accept
β with mass number 1
atomic number 81
(c)
neither changes / both stay the same;
Do not accept
mass number decreases by 1
(d)
(mass number =) A − 1;
(atomic number =) Z / unchanged;
Do not accept
Z − 1
A − 4
(e)
(they have) different atomic / proton numbers (82 and 83);
isotopes (of an element) have the same atomic / proton number but different mass numbers / numbers of neutrons;
Allow
MP1: lead-214 has 82 protons and bismuth-214 has 83 protons
MP2: they are different elements because the number of protons is different
15IGCSE-PHY-7-009Practical: penetrating power of radiation from a sourcePapers 1 & 2Grade 6–710 marks
A student investigates the radiation emitted by a sealed radioactive source using the apparatus shown. She first measures the background count rate with no source present: 25 counts per minute. She then places different absorbers between the source and the GM tube and records the count rate. Absorber / material: none, paper, 3 mm aluminium, 5 cm lead Count rate / counts per minute: 845, 838, 131, 28
(a)
State two variables that the student should keep constant.
(2)
(b)
Explain why the student measures the background count rate.
(2)
(c)
Determine the count rate due to the source when the 3 mm aluminium absorber is used.
(1)
(d)
Deduce which types of radiation the source emits. Use the results to justify your answer.
(4)
(e)
Suggest one safety precaution the student should take when using the source.
(1)
Show mark scheme
(a)
any two from:
distance between source and GM tube;
time for each count / measurement;
same source;
same GM tube / counter;
Allow
same orientation of source
Do not accept
type of absorber
(b)
background radiation is (always) present / detected as well as the source radiation;
subtract (background) from readings to get count rate due to source;
Allow
to correct the readings
(c)
106 (counts per minute);
Do not accept
131
(d)
no (significant) alpha because paper causes (almost) no change in count rate;
beta because 3 mm aluminium causes a large decrease;
gamma because a (corrected) count rate of about 106 counts per minute remains after the aluminium;
(thick) lead reduces count to about background, consistent with gamma (which is absorbed by thick lead);
Allow
845 → 838 is within random variation for MP1
Do not accept
alpha because count drops with paper
(e)
handle with tongs / long tweezers / hold at arm's length / store in lead box when not in use / limit time of exposure / do not point at people;
16IGCSE-PHY-7-010Half-life from an activity–time graphPapers 1 & 2Grade 6–79 marks
The graph shows how the activity of a sample of a radioactive isotope X changes with time.
(a)
State what is meant by the half-life of a radioactive isotope.
(2)
(b)
Use the graph to determine the half-life of isotope X. Show on the graph how you obtain your answer.
(2)
(c)
Calculate the time, in days, for the activity of the sample to fall from 800 Bq to 25 Bq.
(3)
(d)
Isotope Y has a half-life of 6 hours. A sample of Y also has an activity of 800 Bq at the start. Describe how the graph for Y would compare with the graph for X.
(2)
Show mark scheme
(a)
time (taken);
for activity / count rate / number of (undecayed) nuclei to halve;
Allow
half the (unstable) nuclei to decay for MP2
Do not accept
half the time for the sample to decay
time for the mass of the sample to halve (unqualified)
(b)
evidence on graph e.g. line from 400 Bq to curve and down to time axis;
17IGCSE-PHY-7-011Choosing isotopes for industrial usesPapers 1 & 2Grade 6–79 marks
The table gives information about four radioactive isotopes, P, Q, R and S. Isotope / label: P, Q, R, S Radiation emitted / type: alpha, beta, gamma, gamma Half-life / time: 430 years, 28 years, 15 hours, 30 years
(a)
A smoke alarm contains a radioactive source. The radiation ionises the air between two metal plates so that a small current flows. Smoke absorbs the radiation and the current falls. State which isotope should be used in the smoke alarm and give two reasons for your choice.
(3)
(b)
A factory makes paper. A source is placed above the moving paper and a detector below it, to monitor the thickness of the paper. Explain why isotope Q is the most suitable.
(3)
(c)
Engineers need to find a leak in a water pipe buried 1 m underground. They add a small amount of a radioactive isotope to the water and use a detector at the ground surface. Explain which isotope they should use.
(3)
Show mark scheme
(a)
P;
any two from:
alpha is (highly) ionising (so ionises the air);
alpha absorbed by smoke / short range / cannot escape the alarm;
long half-life so activity (nearly) constant / does not need replacing;
Allow
alpha stopped by paper/skin so safe outside alarm
Do not accept
P because it has a short half-life
(b)
any three from:
beta (partly) absorbed by paper;
(so) change in thickness gives (measurable) change in count rate;
alpha would be (completely) stopped by paper / gamma would pass straight through (with little absorption);
long half-life so count rate does not decrease (noticeably) due to decay / does not need replacing often;
Do not accept
beta passes through paper without absorption
(c)
R;
gamma can penetrate (1 m of) soil/ground to reach the detector;
short half-life so activity falls quickly / water does not stay radioactive for long / less risk to people;
Allow
S rejected because half-life too long for MP3
Do not accept
alpha or beta because more ionising so easier to detect
18IGCSE-PHY-7-012Fission of uranium-235 into xenon and strontiumPapers 1 & 2Grade 6–77 marks
The diagram shows one way in which a uranium-235 nucleus can undergo fission. The equation for this fission is: ¹₀n + ²³⁵₉₂U → ¹⁴⁰₅₄Xe + ⁹⁴₃₈Sr + x ¹₀n
(a)
Determine the value of x.
(1)
(b)
State the form in which most of the energy released by the fission is first carried.
(1)
(c)
The daughter nuclei produced in fission are radioactive. Explain why this makes used fuel rods from a nuclear reactor hazardous.
(2)
(d)
Explain why the number of fissions per second could increase very rapidly if every neutron released went on to be absorbed by another uranium-235 nucleus.
19IGCSE-PHY-7-013Pool-type research reactor: moderator, control rods and shieldingPapers 1 & 2Grade 6–79 marks
The diagram shows a pool-type research reactor. The core, containing uranium fuel rods and control rods, sits at the bottom of a deep pool of water. The water in the pool acts as the moderator and also as shielding.
(a)
Explain why a moderator is needed in the reactor.
(2)
(b)
Scientists can stand safely on the platform above the pool while the reactor is operating. Explain the role of the 6 m of water above the core.
(2)
(c)
Explain what happens to the rate of fission when the control rods are lowered further into the core.
(3)
(d)
The water in the pool slowly warms up while the reactor is operating. Explain why.
(2)
Show mark scheme
(a)
slows down neutrons;
(slow neutrons) more likely to be absorbed by U-235 / cause (further) fission;
20IGCSE-PHY-7-014Deuterium–tritium fusion in an experimental reactorPapers 1 & 2Grade 6–79 marks
Scientists are developing fusion reactors. The diagram shows the fusion reaction used in one experimental reactor. ²₁H + ³₁H → ⁴₂He + X The fuel is heated to about 150 million °C.
(a)
Identify particle X. Give its mass number and its atomic number.
(2)
(b)
The total mass of the products is slightly less than the total mass of the deuterium and tritium nuclei. State what this decrease in mass is accompanied by.
(1)
(c)
Explain why the fuel must be at a very high temperature for fusion to happen.
(3)
(d)
Give two differences between nuclear fusion and nuclear fission.
(2)
(e)
State where fusion is the natural source of energy.
(1)
Show mark scheme
(a)
neutron;
mass number 1 AND atomic number 0;
Allow
¹₀n for both marks
Do not accept
proton
electron
(b)
(a release of) energy;
Allow
kinetic energy of products
Do not accept
mass is destroyed with nothing produced
(c)
(both) nuclei are positive(ly charged) so (electrostatically) repel;
high temperature gives (nuclei) high kinetic energy / high speed;
so nuclei can get close enough to fuse / overcome repulsion;
Do not accept
electrons repel
so nuclei can split
(d)
any two from:
fusion joins (light) nuclei whereas fission splits a (heavy) nucleus;
fusion needs very high temperature/pressure but fission does not;
fission is started by absorbing a neutron, fusion is not;
fission produces radioactive daughter nuclei whereas the helium produced by fusion is not radioactive;
Allow
fission products are radioactive waste, fusion products (here) are not
21IGCSE-PHY-7-027Fraction of nuclei decayed during storagePapers 1 & 2Grade 6–71 mark
A hospital stores waste containing iodine-131 in a shielded store. Iodine-131 has a half-life of 8 days. The waste is stored for 24 days. What fraction of the iodine-131 nuclei that were present at the start have decayed during the 24 days?
Show mark scheme
C (7/8);
A is incorrect because 1/8 is the fraction that remains undecayed after 3 half-lives, not the fraction that has decayed
B is incorrect because it treats 3 half-lives as leaving 1/(2 × 3) = 1/6 of the nuclei, so 1 − 1/6 = 5/6 have decayed
D is incorrect because 3/4 is the fraction decayed after only 2 half-lives (16 days)
22IGCSE-PHY-7-028Identifying unknown particles in nuclear equationsPapers 1 & 2Grade 6–79 marks
Each nuclear equation below contains one unknown particle. Equation 1: ²¹⁰₈₄Po → ²⁰⁶₈₂Pb + X Equation 2: ¹⁴₆C → ¹⁴₇N + Y Equation 3: ¹⁴₇N + ¹₀n → ¹⁴₆C + W (Equation 3 shows how carbon-14 is made in the upper atmosphere when neutrons produced by cosmic rays collide with nitrogen nuclei.)
(a)
Identify particle X. Show how you used the equation.
(2)
(b)
Identify particle Y. Explain how charge is conserved in equation 2.
(2)
(c)
Identify particle W. Show how you used the equation.
(2)
(d)
A nucleus with mass number A and atomic number Z emits one alpha particle and then two beta (β−) particles. Show that the final nucleus is an isotope of the original element and state its mass number in terms of A.
(3)
Show mark scheme
(a)
mass number 4 and atomic number 2 (from 210 − 206 and 84 − 82);
alpha particle / ⁴₂He / helium nucleus;
Allow
MP2 for ⁴₂α
Do not accept
helium atom
MP2 with incorrect numbers in MP1
(b)
beta (β−) particle / (fast) electron / ⁰₋₁e;
charge on left (6) = 7 + (−1) on right;
Allow
MP2: beta has charge −1 so total charge is 6 on both sides
Do not accept
positron / β+
Y identified as a neutron
(c)
mass number 1 and atomic number 1 (from 14 + 1 − 14 and 7 + 0 − 6);
proton / ¹₁p / ¹₁H / hydrogen nucleus;
Allow
MP2 for ¹₁H⁺
Do not accept
neutron
hydrogen atom
MP2 with incorrect numbers in MP1
(d)
alpha: mass number decreases by 4 and atomic number decreases by 2;
two betas: atomic number increases by 2 / returns to Z, mass number unchanged;
same atomic number so isotope (of same element) with mass number A − 4;
Allow
ECF within the chain
a worked example with numbers that is clearly generalised
23IGCSE-PHY-7-029Practical: range of alpha particles in airPapers 1 & 2Grade 6–79 marks
A student uses a computer simulation to investigate how far alpha particles travel in air. The simulated alpha source produces a narrow, parallel beam of alpha particles aimed at the GM tube, set up as shown. The student changes the distance d between the source and the GM tube and records the count rate. Distance d / cm: 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 Count rate / counts per minute: 1260, 1240, 1220, 1190, 610, 32, 29, 31
(a)
State the independent variable and the dependent variable in this investigation.
(2)
(b)
Estimate the range of the alpha particles in air.
(1)
(c)
Explain why the count rate is not zero when d is 6.0 cm or more.
(2)
(d)
Explain, in terms of their nature, why alpha particles can travel only a short distance through air.
(2)
(e)
The student sets d to 2.0 cm and places a sheet of paper between the source and the GM tube. Predict the count rate. Give a reason for your answer.
(2)
Show mark scheme
(a)
(independent) distance (d) between source and GM tube;
(dependent) count rate;
Do not accept
time (as dependent variable)
(b)
any value from 5.0 cm to 6.0 cm;
Allow
about 5 cm / between 5 and 6 cm
Do not accept
4 cm or less
8 cm
1 cm
(c)
(GM tube detects) background radiation;
alpha particles do not reach the GM tube / are all absorbed by the air (beyond about 5 cm);
Allow
MP1: count of about 30 cpm is background from rocks / cosmic rays
Do not accept
source emits gamma
(d)
alpha particles are (highly / strongly) ionising / have a large charge (+2) and mass;
lose (kinetic) energy quickly (in collisions with air particles);
Allow
MP2: each ionisation takes energy from the alpha particle
Do not accept
alpha particles are slowed by gravity
alpha particles are weakly ionising
(e)
about 30 counts per minute / (close to) background count;
24IGCSE-PHY-7-030Radiotherapy using gamma beams from several directionsPapers 1 & 2Grade 6–79 marks
Gamma rays can be used to treat a tumour deep inside a patient's head. The diagram shows three narrow beams of gamma rays aimed at a tumour from different directions.
(a)
Explain why gamma rays are used for this treatment rather than alpha or beta particles.
(2)
(b)
Explain why the tumour is treated using several beams from different directions instead of one more intense beam.
(3)
(c)
During the treatment the patient's head is held in a fixed position by a frame. Explain why.
(2)
(d)
The radiographer leaves the treatment room while the beams are switched on. Explain why.
(2)
Show mark scheme
(a)
gamma (very) penetrating / can pass through skin and skull to reach tumour;
alpha and beta would be absorbed (by skin / bone) before reaching tumour;
Do not accept
gamma is the most ionising
(b)
(all) beams pass through / cross at the tumour;
tumour receives dose from all the beams / large total dose;
healthy tissue receives dose from only one beam / small dose so less damage to healthy cells;
Allow
MP3: healthy cells are less likely to be killed or mutated
Do not accept
the beams cancel out outside the tumour
(c)
(if the head moved) the beams would no longer cross at / all pass through the tumour;
(so) healthy tissue would receive a larger dose / tumour would receive a smaller dose;
Allow
MP2: more healthy cells would be damaged / killed
Do not accept
to stop the patient becoming contaminated
so the gamma rays do not bend
(d)
to avoid (repeated) irradiation (from the gamma rays);
(which) can damage cells / cause mutations / cancer;
Allow
MP1: radiographer works with many patients so dose would build up
25IGCSE-PHY-7-031Plutonium-238 power source for a space probePapers 1 & 2Grade 6–79 marks
Space probes that travel far from the Sun use a power source containing plutonium-238. Plutonium-238 decays by emitting alpha particles. The alpha particles are absorbed inside the power source, which heats up. Thermocouples convert some of this thermal energy into electricity. Plutonium-238 has a half-life of 88 years. At launch, the thermal power of the source is 2000 W.
(a)
State what is meant by an activity of 1 becquerel (Bq).
(1)
(b)
Explain why an alpha emitter is suitable for this power source.
(2)
(c)
The engineers state that the thermal power will be 1000 W when the probe is 88 years old. Explain why.
(2)
(d)
The thermocouples have an efficiency of 6.0%. Calculate the electrical power output of the source 176 years after launch.
(3)
(e)
Polonium-210 is also an alpha emitter. Its half-life is 138 days. Suggest why polonium-210 would not be suitable for a mission lasting many years.
(1)
Show mark scheme
(a)
one (nucleus) decays per second / one decay per second;
Allow
one disintegration per second
Do not accept
one particle per second detected
one count per second
(b)
alpha absorbed (easily) within the source / short range / highly ionising;
(so) energy transferred as heat to the source / little radiation escapes (to damage equipment);
Do not accept
alpha is the most penetrating
(c)
88 years is one half-life so activity / number of decays per second halves;
energy per decay is unchanged (so power halves);
Allow
MP1: half of the plutonium-238 nuclei have decayed
26IGCSE-PHY-7-032Correcting a student's description of fissionPapers 1 & 2Grade 6–78 marks
A student writes this description of nuclear fission in a reactor. "A fast-moving proton is fired at a nucleus of uranium-238. The nucleus absorbs it and splits into two smaller nuclei, which are stable. Two or three electrons are also released. Most of the energy released is carried away by gamma rays. The electrons are then absorbed by other uranium nuclei, which causes a chain reaction."
(a)
Identify four errors in the student's description. For each error, write a correct version.
(4)
(b)
In one fission, the two smaller nuclei are barium-141 (atomic number 56) and krypton-92 (atomic number 36). Krypton-92 decays by emitting a beta (β−) particle. Determine the mass number and the atomic number of the nucleus formed.
(2)
(c)
In this fission, three neutrons are released. Assume that every neutron released causes one more fission. The first fission is generation 1. Determine the number of fissions in generation 4.
(2)
Show mark scheme
(a)
any four from:
proton should be neutron;
fast-moving should be slow-moving (neutron);
uranium-238 should be uranium-235;
daughter / smaller nuclei are radioactive / unstable (not stable);
neutrons are released (not electrons) / neutrons cause the chain reaction;
most energy is carried as kinetic energy of the daughter nuclei / fission products (not gamma rays);
Allow
each correction must replace the error, e.g. 'neutron not proton'
'slow-moving neutron' as a single correction scores MP1 and MP2
27IGCSE-PHY-7-033Practical: modelling half-life with dicePapers 1 & 2Grade 6–711 marks
Students use dice to model radioactive decay. They throw 600 dice. Every die that lands showing a six is removed (it has "decayed"). They count the dice remaining and throw them again. They repeat this several times. Number of throws / count: 0, 1, 2, 3, 4, 5, 6, 7, 8 Number of dice remaining / count: 600, 502, 415, 349, 292, 240, 201, 170, 139
(a)
Explain why throwing dice is a good model for radioactive decay.
(2)
(b)
Plot a graph of the number of dice remaining against the number of throws on the grid.
(3)
(c)
Draw a curve of best fit.
(1)
(d)
Use your graph to determine the half-life of the dice, in throws.
(2)
(e)
Another group repeats the experiment with only 60 dice. Explain why their results are less likely to fit a smooth curve.
(2)
(f)
Suggest how the model could be changed to represent an isotope with a longer half-life.
(1)
Show mark scheme
(a)
cannot predict which die will show a six / random (like decay of a nucleus);
each die has the same probability / chance (of 1/6) of being removed each throw;
Allow
MP2: fixed fraction (about 1/6) removed each throw like a constant fraction of nuclei decaying in equal times
Do not accept
the dice are radioactive
(b)
suitable linear scales chosen so that the plotted points occupy at least half of the grid in both directions;
axes labelled with the quantities (number of throws, number of dice remaining);
all points plotted correctly to within ½ small square;
Allow
no units needed on either axis as both quantities are numbers
Do not accept
awkward scales, e.g. 3 or 30 per large square (reject for MP1)
non-linear scale
(c)
single smooth curve of best fit passing close to all the points;
Allow
ECF from plotting
Do not accept
straight line of best fit
dot-to-dot ruled lines
(d)
method shown on graph / reading of throws at 300 dice (or at 150 compared with 300);
3.8 throws (accept 3.6 to 4.0);
Allow
ECF from candidate's curve
Do not accept
4 (from reading throw 4 in table only, without graph)
(e)
(decay / throwing) is random;
with fewer dice the random variations / fluctuations are large compared with the number remaining;
Allow
MP2: small numbers mean results are less predictable / less reliable
Do not accept
60 dice decay faster
(f)
use dice with more faces / remove only when a particular number shows on a die with more sides / smaller chance of removal each throw;
Allow
remove a die only when two dice thrown together both show a six
28IGCSE-PHY-7-034Radium luminous paint and modern tritium markersPapers 1 & 2Grade 6–79 marks
About 100 years ago, the numbers on some watch faces were painted with paint containing radium-226 so that they glowed in the dark. Radium-226 emits alpha particles. It has a half-life of 1600 years. The workers who painted the numbers often pointed their brushes by licking them. Many of these workers later became seriously ill. People who only wore the watches were at much lower risk.
(a)
Explain why the workers who licked their brushes were at much greater risk than people who only wore the watches.
(3)
(b)
Radium is chemically similar to calcium, so radium taken into the body collects in the bones. Explain why radium in the bones remains a hazard for the rest of the person's life.
(2)
(c)
Radium-226 decays to form radon-222, which is a radioactive gas. Radon-222 also emits alpha particles. Collectors are advised not to keep large numbers of old radium watches in a closed cupboard in a bedroom. Suggest why.
(2)
(d)
Modern watches use small sealed glass tubes containing tritium gas. Tritium emits low-energy beta particles that cannot pass through the glass. Explain why these watches are much safer for the wearer.
(2)
Show mark scheme
(a)
radium taken into the body / internal contamination;
alpha is (strongly) ionising / all energy absorbed by nearby cells (inside the body);
outside the body alpha is absorbed by the watch glass / air / (dead layer of) skin;
Allow
MP3: alpha cannot penetrate skin
Do not accept
alpha is very penetrating
the watches are irradiated so become radioactive
(b)
long half-life / activity hardly falls in a human lifetime;
radium stays in the body / bones so (nearby) cells are irradiated continuously;
Do not accept
radium becomes more radioactive with time
(c)
radon gas escapes from the paint and builds up / becomes concentrated in a closed space (not ventilated);
radon is breathed in so alpha particles are emitted inside the lungs / body (contamination), damaging cells / causing cancer;
Allow
MP1: ventilation would disperse the radon
MP2: alpha particles ionise cells in the lungs
Do not accept
the watches make the cupboard radioactive by irradiation
alpha particles pass through the cupboard door
(d)
sealed so tritium cannot get onto / into the body / no contamination;
beta particles absorbed by glass / do not reach wearer so no irradiation;
29IGCSE-PHY-7-015Scheduling technetium-99m doses in a hospitalPapers 1 & 2Grade 8–911 marks
Technetium-99m is a gamma emitter with a half-life of 6.0 hours. It is injected into patients as a medical tracer. A hospital receives a sample of technetium-99m at 06:00. Its activity at 06:00 is 1600 MBq. Two patients are booked for scans that day: patient 1 at 12:00 needs a dose with an activity of 500 MBq at the time of injection patient 2 at 18:00 needs a dose with an activity of 200 MBq at the time of injection. Each dose is taken from the sample just before it is injected. A technician claims that the sample is enough for both patients.
(a)
Evaluate the technician's claim. Use calculations to support your answer.
(4)
(b)
Calculate the activity of the technetium in patient 1 at 12:00 the next day. Assume none of the technetium leaves the patient's body.
(3)
(c)
After the injection, patient 1 is contaminated. Explain why the patient is advised to avoid close contact with young children for about a day.
(2)
(d)
Explain why an isotope with a half-life of 6 hours is more suitable as a tracer than one with a half-life of 6 minutes or one with a half-life of 6 years.
(2)
Show mark scheme
(a)
(activity at 12:00 =) 800 (MBq);
(remaining after dose) 800 − 500 = 300 (MBq);
(at 18:00) 150 (MBq);
150 < 200 (MBq) so claim is incorrect / not enough;
Allow
alternative: 400 MBq needed at 12:00 for patient 2 so 500 + 400 = 900 > 800 for MP2–MP4
ECF for MP4 from a candidate's value
Do not accept
claim correct because 1600 > 700
(b)
(12:00 to 12:00 =) 24 h = 4 half-lives;
500 ÷ 2⁴ / 500 ÷ 16;
31 (MBq);
Allow
31.25 / 31.3 (MBq)
e.g. 500 → 250 → 125 → 62.5 → 31.25
Do not accept
125 (MBq) (500/4)
(c)
(radioactive material inside patient) emits gamma (rays) that can irradiate people nearby;
activity falls to (about) 1/16 / 31 MBq after a day so risk reduced;
Allow
children's cells divide rapidly so more at risk of damage/mutation for MP1 if linked to irradiation
Do not accept
patient is irradiated so children become radioactive
(d)
6 minutes: activity too low (by the time it is prepared/injected/scanned) / decays too quickly;
6 years: patient stays radioactive for a long time / larger total dose to patient;
30IGCSE-PHY-7-035Gamma radiography of pipeline weldsPapers 1 & 2Grade 8–911 marks
Engineers check welds in steel pipelines using gamma rays from an iridium-192 source. The gamma rays pass through the pipe wall and reach a sheet of photographic film on the other side. When a source is new, its activity is 1600 GBq. The exposure time needed for each photograph is inversely proportional to the activity of the source. The table shows the exposure time needed as the source gets older. Age of source / days: 0, 37, 74, 111 Exposure time needed / min: 15, 21, 30, 42
(a)
Explain why a gamma source is used rather than a beta source.
(2)
(b)
A weld contains a small crack filled with air. Explain how the crack shows up on the developed film.
(2)
(c(i))
Use the data in the table to determine the half-life of iridium-192. Explain your reasoning.
(2)
(c(ii))
Company rules state that an exposure time of more than 1 hour must not be used, so a source is replaced as soon as it needs an exposure time of 1 hour. The company starts with a new source and must carry out inspections every day for one year (365 days). Determine the number of new sources the company needs to buy during the year, including the first one.
(3)
(d)
When not in use, the source is kept inside a thick container made of tungsten, a very dense metal. Explain why.
(2)
Show mark scheme
(a)
gamma can penetrate / pass through (thick) steel (to reach the film);
beta would be absorbed / stopped by the steel;
Do not accept
gamma is more ionising
(b)
more gamma rays pass through the crack / less absorption at the crack;
film is darker at the crack (than around it);
Allow
MP2: a dark line / mark on the film
Do not accept
crack shows as a lighter region
(c(i))
exposure time doubles (15 → 30 min or 21 → 42 min) so the activity has halved;
half-life = 74 days;
Allow
MP1: activity at 74 days = 1600 × 15 ÷ 30 = 800 GBq, half of 1600 GBq
Do not accept
37 days (from 15 → 21 min)
half-life = 74 days with no reasoning (MP2 only)
(c(ii))
60 min is 4 × 15 min so activity has fallen to ¼ / 400 GBq;
2 half-lives = 148 days (life of each source);
365 ÷ 148 = 2.5 so 3 sources;
Allow
ECF from half-life in (c)(i)
MP3: new sources needed on day 0, day 148 and day 296, so 3
31IGCSE-PHY-7-039Decay sequence on a neutron–proton chartPaper 2 onlyGrade 6–712 marks
The chart shows part of a radioactive decay sequence. Each nucleus is plotted by its number of protons Z and its number of neutrons N. The sequence starts at nucleus P, which is uranium-235 (²³⁵₉₂U). Each arrow shows one alpha or beta (β−) decay.
(a)
Explain how the chart shows that the decay from P to Q is an alpha decay.
(3)
(b)
Identify the type of decay from Q to R. Justify your answer using the chart.
(2)
(c)
R is an isotope of protactinium (Pa) and S is an isotope of actinium (Ac). Write a balanced nuclear equation for the decay of R to S.
(3)
(d)
Nucleus W is radium-223. It is injected into patients with cancer that has spread to their bones. Radium collects in the bones and emits alpha particles. Explain why an alpha emitter is suitable for this treatment.
(3)
(e)
Nucleus W often also emits a gamma ray. Explain why gamma emission is not shown as an arrow on the chart.
(1)
Show mark scheme
(a)
Z / proton number decreases by 2 (92 → 90);
N / neutron number decreases by 2 (143 → 141);
(so) loses 2 protons and 2 neutrons / mass number decreases by 4 = alpha particle (helium nucleus);
Do not accept
mass number decreases by 2
(b)
beta (β−);
Z increases by 1 and N decreases by 1 / neutron changes into a proton;
Allow
MP2: mass number unchanged (231) while Z goes from 90 to 91
Do not accept
alpha
Z decreases by 1
(c)
²³¹₉₁Pa (correct numbers on Pa);
²²⁷₈₉Ac (correct numbers on Ac);
⁴₂He / ⁴₂α as the other product;
Allow
ECF for MP3 if numbers on Pa and Ac are consistent with candidate's alpha
Do not accept
⁰₋₁e as the other product
²³¹₉₁Pa → ²²⁷₉₁Ac + ⁴₂He (atomic number of Ac not changed)
(d)
alpha (strongly) ionising so damages / kills cancer cells;
short range so (only) cells close to the radium / tumour are affected / healthy organs further away receive little dose;
(little radiation leaves the body so) others are not irradiated / low dose to staff and family;
Allow
MP2: alpha cannot travel far through bone or tissue
Do not accept
alpha is penetrating so reaches the bones from outside
alpha makes the cancer cells radioactive
(e)
gamma has no charge and no mass so N and Z / proton and neutron numbers are unchanged;
Allow
gamma ray is electromagnetic radiation so the nucleus stays at the same point on the chart
32IGCSE-PHY-7-016Neptunium-237 decay series: counting alpha and beta decaysPaper 2 onlyGrade 8–911 marks
Neptunium-237 (²³⁷₉₃Np) is an artificial radioactive isotope produced in nuclear reactors. It decays through a long series of alpha and beta (β−) decays to bismuth-209 (²⁰⁹₈₃Bi). Bismuth-209 has such an extremely long half-life that it can be treated as stable.
(a)
The first decay in the series is an alpha decay to protactinium (Pa). Write a balanced nuclear equation for this decay.
(3)
(b)
The protactinium nucleus then decays by beta (β−) emission to an isotope of uranium. Determine the mass number and the atomic number of this uranium nucleus.
(2)
(c)
The series from neptunium-237 to bismuth-209 contains only alpha and beta (β−) decays. Determine the number of alpha decays and the number of beta decays in the series.
(4)
(d)
Determine how many fewer neutrons a bismuth-209 nucleus has than a neptunium-237 nucleus.
(2)
Show mark scheme
(a)
α written with 4 (top) and 2 (bottom);
Pa with mass number 233;
Pa with atomic number 91;
Allow
⁴₂He for α
Do not accept
α with 2 on top and 4 underneath
(b)
(mass number =) 233;
(atomic number =) 92;
Allow
ECF from (a) for Pa values
(c)
change in mass number = 237 − 209 = 28;
number of alpha decays = 28 ÷ 4 = 7;
7 alpha decays reduce atomic number by 14 / to 79;
33IGCSE-PHY-7-017Half-life from count-rate data with backgroundPaper 2 onlyGrade 8–911 marks
A student uses a GM tube and counter to measure the count rate from a sample of a short-lived radioactive isotope. Before the sample is brought into the laboratory she measures the background count rate as 40 counts per minute. Her measured count rates are shown in the table. They have not been corrected for background radiation. Time / min: 0, 2, 4, 6, 8, 10 Measured count rate / counts per minute: 760, 494, 326, 220, 153, 111
(a)
State why the measured count rate would never fall to zero, however long the student waited.
(1)
(b)
Determine the half-life of the isotope. Show your working.
(4)
(c)
Explain why using the measured count rates, without correcting them, would give a value for the half-life that is too large.
(2)
(d)
Predict the measured count rate at a time of 15 minutes.
(3)
(e)
Explain why the measured count rates do not lie exactly on a smooth curve.
(1)
Show mark scheme
(a)
background radiation (always present);
Do not accept
the source never completely decays (on its own)
(b)
subtract 40 from (any) readings e.g. 720 at 0 min;
(corrected) 180 at 6 min / 360 between 2 and 4 min;
two half-lives in 6 min / evidence of halving twice;
3.0 (min);
Allow
values from a correctly plotted corrected graph
180 s
Do not accept
about 3.3 min from uncorrected data for MP4
(c)
background is a constant addition to each reading / measured value includes background;
so measured rate halves only after source rate has fallen below half / takes longer than one half-life;
(d)
(15 min =) 5 half-lives / 720 ÷ 2⁵;
22.5 (counts per minute due to source);
(add background) 62.5 / 62 / 63 (counts per minute);
Allow
ECF from (b) where their half-life gives a whole number of half-lives in 15 min
e.g. 720 → 360 → 180 → 90 → 45 → 22.5; 22.5 + 40
Do not accept
22.5 as final answer (background not added back) for MP3
34IGCSE-PHY-7-018Managing radioactive waste from a nuclear power stationPaper 2 onlyGrade 8–96 marks
A nuclear power station produces two main kinds of radioactive waste. Low-level waste: used protective clothing, gloves and cleaning materials. It has a low activity and contains mostly isotopes with short half-lives. High-level waste: used fuel rods. They have a very high activity, contain radioactive daughter nuclei from fission (some with half-lives of thousands of years) and are very hot when removed from the reactor.
(a)
Discuss the dangers of these two kinds of radioactive waste and how the risks from storing and disposing of each kind can be reduced.
(6)
Show mark scheme
(a)
Indicative content:
MP1. ionising radiation damages / kills cells or tissue;
MP2. causes mutations (in DNA) / cancer;
MP3. risk of contamination if waste leaks into groundwater / is ingested or inhaled;
MP4. high-level waste remains hazardous for thousands of years because of long half-lives;
MP5. high-level waste first stored under water to cool it (water also shields);
MP6. then sealed in glass / steel containers to prevent leaks;
MP7. buried deep underground in geologically stable rock / away from water supplies;
MP8. low-level waste compacted and sealed in drums / buried in shallow (lined) sites;
MP9. short half-lives mean low-level waste activity falls quickly / becomes safe sooner;
MP10. thick shielding (concrete / lead / water) and remote handling reduce irradiation of workers;
MP11. sites monitored over long periods / security to prevent access;
Allow
award 1 mark per MP to a maximum of 6, then use the levels to confirm the mark
vitrification for MP6
Do not accept
waste can be made safe by neutralising it chemically
waste becomes harmless after one half-life
Level 0 (0)
No rewardable content.
Level 1 (1–2)
Isolated statements about dangers OR about storage, with little linkage to the type of waste.
Level 2 (3–4)
Dangers and some risk-reduction methods are described; some linkage to the properties of at least one kind of waste (activity, half-life or heat).
Level 3 (5–6)
Dangers explained and risk-reduction methods for BOTH kinds of waste discussed, clearly linked to their activity, half-life and heat; logically structured.
35IGCSE-PHY-7-019Neutron balance in a reactor operating at constant powerPaper 2 onlyGrade 8–912 marks
A nuclear reactor has a thermal power output of 3.0 × 10⁹ W. Each fission of a uranium-235 nucleus releases 3.2 × 10⁻¹¹ J of energy. On average, each fission releases 2.5 neutrons.
(a)
State the relationship between power, energy transferred and time taken.
(1)
(b)
Calculate the number of fissions that take place each second.
(3)
(c)
For the reactor to operate at a constant power, on average exactly one neutron from each fission must cause another fission. The other neutrons are absorbed by the control rods and other materials or escape from the core. Calculate the number of neutrons per second that do not cause fission.
(3)
(d)
The operators notice that the power output is slowly increasing. Explain, in terms of neutrons, why the power is increasing and what the operators should do to the control rods to return the reactor to a constant power.
(3)
(e)
Even after the control rods have stopped the chain reaction, the core continues to need cooling for some time. Suggest why.
(2)
Show mark scheme
(a)
power = energy (transferred) ÷ time (taken);
Allow
P = E/t
power = work done ÷ time
Do not accept
P = E × t
(b)
(energy each second =) 3.0 × 10⁹ (J);
3.0 × 10⁹ ÷ 3.2 × 10⁻¹¹;
9.4 × 10¹⁹ (per second);
Allow
9.375 × 10¹⁹
MP1 and MP2 can be awarded together for the correct division
Do not accept
9.6 × 10⁻² (multiplied)
(c)
(neutrons not causing fission per fission =) 2.5 − 1 = 1.5;
1.5 × 9.4 × 10¹⁹;
1.4 × 10²⁰ (per second);
Allow
ECF from (b)
1.41 × 10²⁰
Do not accept
2.3 × 10²⁰ (2.5 × 9.4 × 10¹⁹) for MP3
(d)
on average more than one neutron from each fission is causing a further fission (so the fission rate rises);
lower / push the control rods further into the core (so they absorb more neutrons);
until on average exactly one neutron per fission causes another fission / then hold the rods in that position;
Allow
more neutrons are available to cause fission for MP1
(rods) absorb more neutrons for MP2 only if linked to lowering
Do not accept
control rods slow the neutrons down
raise the control rods
the moderator absorbs the neutrons
(e)
fission products / daughter nuclei are radioactive;
(their) decay continues to release energy / heat;
Allow
radioactive decay is a source of energy for MP2
daughter nuclei still emit radiation, which transfers energy to the thermal store of the core for MP2
36IGCSE-PHY-7-020Practical simulation: count rate against aluminium thicknessPaper 2 onlyGrade 8–910 marks
A student uses a computer simulation to investigate the radiation from a source. The simulated GM tube is 2 cm from the source. Aluminium sheets of different total thickness are placed between the source and the GM tube. The background count rate in the simulation is 30 counts per minute. The graph shows the student's results. The count rates have not been corrected for background radiation.
(a)
Explain why the count rate decreases rapidly as the thickness of aluminium increases from 0 to 3 mm.
(2)
(b)
Explain how the graph shows that the source also emits gamma radiation.
(2)
(c)
Determine the thickness of aluminium that halves the count rate due to the beta particles alone. Assume that the source does not emit alpha particles. Show your working.
(3)
(d)
Describe an additional measurement the student could make with the simulation to check that the source does not emit alpha particles.
(2)
(e)
Give one advantage of using a simulation rather than a real radioactive source for this investigation.
(1)
Show mark scheme
(a)
beta (particles) absorbed by aluminium;
thicker aluminium absorbs more / fewer beta (particles) reach the GM tube;
Do not accept
gamma absorbed by thin aluminium
(b)
count rate levels off at about 210 (counts per minute) which is well above background (30);
gamma (rays) not (significantly) absorbed by (a few mm of) aluminium;
Allow
constant count rate of about 180 above background for MP1
(c)
beta count rate at 0 mm = 1170 − 210 = 960 (counts per minute);
half = 480 so total reading of 690 (counts per minute) used;
0.6 (mm);
Allow
0.55–0.65 (mm)
ECF: thickness read from graph at candidate's total
Do not accept
about 1.2 mm (halving the total 1170 to 585)
(d)
measure the count rate with a sheet of paper (between source and GM tube) and compare with no absorber;
(almost) no decrease with paper shows no alpha is emitted / a large decrease would show alpha;
Allow
repeat with the GM tube moved more than about 5 cm from the source for MP1
Do not accept
increase the thickness of aluminium
(e)
no exposure to ionising radiation / safer / no need for real sources / quick to repeat;
37IGCSE-PHY-7-036Comparing fission and fusion for future power stationsPaper 2 onlyGrade 8–96 marks
All nuclear power stations operating today use nuclear fission of uranium-235. Scientists are trying to build power stations that use nuclear fusion of hydrogen isotopes (deuterium, which can be extracted from seawater, and tritium).
(a)
Compare nuclear fission and nuclear fusion as ways of generating electricity in the future. Your answer should include how energy is released in each process and the advantages and disadvantages of each.
(6)
Show mark scheme
(a)
Indicative content:
MP1. fission: (large) U-235 nucleus absorbs a neutron and splits into two smaller nuclei (and neutrons);
MP2. fusion: two small / light (hydrogen) nuclei join to form a larger nucleus;
MP3. in fusion there is a loss of mass which is released as energy / fission products have kinetic energy;
MP4. fission is a controlled chain reaction (control rods / moderator) / proven, reliable technology already in use;
MP5. fission daughter nuclei are radioactive / produce long-lived radioactive waste that must be stored safely for a very long time;
MP6. fusion product (helium) is not radioactive / much less long-lived radioactive waste;
MP7. fusion fuel (deuterium from seawater) is plentiful whereas uranium is limited / must be mined;
MP8. fusion needs very high temperature and pressure because nuclei are positive and repel (electrostatic repulsion);
MP9. fusion is hard to contain / no fusion power station yet generates electricity commercially;
MP10. fission accidents can release radioactive material (contamination) / fusion cannot run away as a chain reaction;
Allow
award 1 mark per MP to a maximum of 6, then use the levels to confirm the mark
both processes release energy without producing carbon dioxide can be credited once in place of any one MP
Do not accept
fusion splits nuclei
fusion produces no radiation at all
fission waste becomes safe after one half-life
Level 0 (0)
No rewardable content.
Level 1 (1–2)
Isolated statements about fission or fusion, for example a simple description of one process or one advantage, with little comparison.
Level 2 (3–4)
Describes both processes and gives some advantages or disadvantages, with some linked comparison; reasons may be incomplete (e.g. high temperature stated without the repulsion of nuclei).
Level 3 (5–6)
Clear, balanced comparison of both processes covering how energy is released and at least two linked advantages and disadvantages of each, with physics reasons (e.g. radioactive daughter nuclei, electrostatic repulsion requiring high temperature and pressure).
38IGCSE-PHY-7-037Measuring blood volume with a sodium-24 tracerPaper 2 onlyGrade 8–99 marks
Doctors can measure the total volume of blood in a patient's body using a radioactive tracer, sodium-24. The graph shows how the activity of a small reference sample of sodium-24 changes with time. A solution containing sodium-24 with a total activity of 32000 Bq is injected into the patient's blood. A few minutes later, when the tracer has mixed with the blood, a 10.0 cm³ sample of blood is taken and sent to a laboratory. Because of a delay, the activity of the blood sample is measured 30 hours after the injection. It is 16 Bq.
(a)
Use the graph to determine the half-life of sodium-24.
(2)
(b)
Determine the total volume of blood in the patient's body. Assume that the decay of the sodium-24 during the few minutes before the sample is taken is negligible. Give your answer in cm³.
(4)
(c)
State one other assumption you made in (b).
(1)
(d)
A doctor says that measuring the activity of the sample 30 hours after the injection gives a less reliable result than measuring it straight away. Suggest why.
(2)
Show mark scheme
(a)
time for activity to fall to half / from 1200 to 600 (Bq) shown;
15 h (accept 14 h to 16 h);
Allow
any other pair of values a factor of 2 apart, e.g. 600 → 300 Bq between 15 h and 30 h
39IGCSE-PHY-7-038Steam bubbles in water-moderated and graphite-moderated reactorsPaper 2 onlyGrade 8–911 marks
In a nuclear reactor, water is pumped through the core to cool the fuel rods. Reactor A uses the cooling water as its moderator. Reactor B uses a large block of graphite as its moderator. Cooling water flows through channels in the graphite, as shown in the diagram. Water slows down neutrons and also absorbs some neutrons. Graphite slows down neutrons but absorbs very few. Steam is much less dense than liquid water.
(a)
During a fault, bubbles of steam form in the cooling water in reactor A. The rate of fission in reactor A decreases. Explain why.
(3)
(b)
The same fault occurs in reactor B. Explain why the rate of fission in reactor B could increase.
(3)
(c)
Explain why the effect in reactor B could make the reactor increasingly difficult to control.
(2)
(d)
The core of each reactor is surrounded by thick concrete. Name two types of radiation that the concrete must absorb, and explain why it does not need to be designed to stop alpha particles.
(3)
Show mark scheme
(a)
less (liquid) water / fewer water molecules so neutrons are slowed down less / less moderation;
fast neutrons are less likely to be absorbed by U-235 nuclei / to cause fission;
fewer neutrons go on to cause further fissions / chain reaction slows down;
Allow
MP3: fewer than one neutron per fission causes another fission
ignore 'fewer neutrons absorbed by the water' unless used to argue the rate increases
Do not accept
steam absorbs more neutrons
(b)
graphite still slows neutrons / moderation unchanged;
less water so fewer neutrons absorbed (by water);
more neutrons cause fission / chain reaction speeds up / more than one neutron per fission causes another fission;
Do not accept
steam is a better moderator than water
(c)
more fissions so more (kinetic) energy is transferred to the fuel / water, so the temperature rises;
more water turns to steam, so even fewer neutrons are absorbed and the rate of fission rises further (runaway / positive feedback);
Allow
MP2: the increase causes a further increase
Do not accept
steam slows neutrons more
(d)
neutrons;
gamma (rays);
alpha particles (have very short range) absorbed by the fuel rods / steel vessel / air before reaching the concrete;
Allow
MP3: alpha cannot penetrate even a sheet of paper / thin metal
Do not accept
beta particles (not credited as one of the two: they are absorbed by the fuel cladding and steel vessel)
40IGCSE-PHY-7-040Helium-3 fusion and carbon fusion in starsPaper 2 onlyGrade 8–910 marks
Stars release energy by nuclear fusion. In the core of the Sun, the temperature is about 15 million °C and the pressure is extremely high. One of the fusion reactions in the core is: ³₂He + ³₂He → ⁴₂He + ¹₁H + ¹₁H In the core of stars much more massive than the Sun, the temperature is even higher. In these stars, hydrogen nuclei also fuse with carbon nuclei: ¹²₆C + ¹₁H → ¹³₇N
(a)
Show that the first reaction is balanced in terms of mass number and charge.
(2)
(b)
The total mass of the products of the first reaction is less than the total mass of the two helium-3 nuclei. Explain what this tells you about the reaction.
(2)
(c)
Explain why the fusion of a hydrogen nucleus with a carbon nucleus needs a higher temperature than the fusion of two hydrogen nuclei.
(3)
(d)
A student says: "The second reaction is really fission, because a carbon nucleus is hit by a small particle, just as a uranium-235 nucleus is hit by a neutron in fission." Explain why the student is wrong.
a correct statement that the totals are equal on both sides with the numbers shown
Do not accept
totals stated as equal without any numbers
adding mass number and atomic number together
(b)
(some) mass is lost / converted;
energy is released;
Allow
MP2: energy released as kinetic energy of products / radiation
Do not accept
mass is destroyed and energy is absorbed
(c)
carbon nucleus has more protons / larger positive charge (6 vs 1);
greater electrostatic repulsion (between the nuclei);
nuclei need more kinetic energy / higher speed to get close enough to fuse (so higher temperature);
Allow
MP2: stronger repulsion between like charges
Do not accept
carbon is heavier so needs more energy to move (as the reason for MP2)
gravitational attraction is weaker
(d)
in this reaction two (light) nuclei join to form one larger nucleus / nitrogen-13 is larger than carbon-12;
in fission a large / heavy nucleus splits into two smaller (daughter) nuclei (and neutrons);
the hydrogen nucleus / proton is positively charged and must overcome electrostatic repulsion (very high temperature), but a neutron is uncharged so is not repelled;
Allow
MP3: fission can happen at ordinary temperatures because the neutron has no charge
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