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GCSE · Physics

P4b Radioactivity, Half-Life and Nuclear Uses

AQA 8464 · Calculator allowed · about 105 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Radiation at Work: From Hospital Wards to Power Stations

Original text written for Revision Library.

Radioactive sources are used across the UK every day, often without most people realising it. In hospitals, gamma-emitting tracers such as technetium-99m are injected into patients so that doctors can see inside the body using a gamma camera, without the need for surgery. In industry, beta sources sit inside rollers at paper mills, constantly checking that each sheet is the correct thickness. Even the humble smoke detector on a kitchen ceiling usually contains a tiny alpha source. Meanwhile, nuclear power stations such as Hinkley Point C in Somerset use controlled nuclear fission to generate electricity for millions of homes, releasing no carbon dioxide while doing so. Understanding half-life, radiation hazards and the differences between alpha, beta, gamma and neutron radiation helps scientists and engineers choose the right type of radiation, and the right isotope, for each job, and to use these sources safely.

Alpha, β and γ are three types of nuclear radiation emitted by unstable nuclei.

(a)State what an α particle consists of.(1)

(b)State what a β particle is.(1)

(c)State which of α, β or γ radiation has no mass and no charge.(1)

(d)Put α, β and γ radiation in order of increasing penetrating power (least penetrating first).(1)

(Total for Question 1 is 4 marks)

A technician places a GM tube near an unknown radioactive source. She measures the count rate (with background already subtracted) using different absorbers placed between the source and the tube:

Absorber: none, corrected count rate = 620 counts/min
Absorber: paper, corrected count rate = 618 counts/min
Absorber: 5 mm aluminium, corrected count rate = 15 counts/min
Absorber: 25 mm lead, corrected count rate = 14 counts/min

(a)Identify the type(s) of radiation emitted by the source, using the results above to explain your reasoning.(3)

(b)State one variable that should be kept the same throughout this investigation to make it a fair test.(1)

(c)Explain why the technician subtracts a background count rate (measured with no source present) from each of her readings before recording the results shown above.(2)

(Total for Question 2 is 6 marks)

Nuclides can be written in the form (mass number)/(atomic number) Symbol, for example an α particle is written 4/2 He. Americium-241 (241/95 Am) is an α-emitting source used in household smoke detectors. It decays by α emission to form an isotope of neptunium (Np).

(a)Write the balanced nuclear equation for the α decay of americium-241, giving the mass number and atomic number of the neptunium nuclide formed.(2)

(b)A different isotope, americium-243 (243/95 Am), also decays by α emission. Without writing the full equation, state the mass number and atomic number of the daughter nuclide formed.(2)

(Total for Question 3 is 4 marks)

Cobalt-60 (60/27 Co) is a β-emitting source used to sterilise medical equipment. During β decay, a neutron in the nucleus changes into a proton and an electron, and the electron is emitted from the nucleus as a β particle, written 0/-1 e.

(a)Write the balanced nuclear equation for the β decay of cobalt-60 to form nickel (Ni).(2)

(b)Explain why the mass number stays the same during β decay, but the atomic number increases by 1.(2)

(Total for Question 4 is 4 marks)

After α or β decay, a nucleus is sometimes left in an excited (higher-energy) state. It loses this extra energy by emitting γ radiation. Technetium-99m (99/43 Tc) is an excited nucleus used in medical imaging.

(a)Complete the nuclear equation for technetium-99m emitting γ radiation to form technetium-99 in its stable (ground) state.(1)

(b)Explain why the mass number and atomic number of the nucleus do not change when it emits γ radiation.(1)

(c)Neutrons can also be emitted from unstable nuclei, for example during nuclear fission (covered later in this pack). State the charge of a neutron, and explain why neutron radiation can still be hazardous despite having no charge.(2)

(Total for Question 5 is 4 marks)

Radioactive decay is described as a random process, and the activity of a source is measured in becquerels (Bq).

(a)Explain what is meant by describing radioactive decay as random.(2)

(b)State what is meant by a source having an activity of 1 Bq.(1)

(Total for Question 6 is 3 marks)

A radioactive source has an initial activity of 480 Bq. Its half-life is constant. After 3 hours, the activity of the source has fallen to 60 Bq.

(a)Define the term half-life.(1)

(b)Show that the half-life of this source is 1 hour.(3)

(Total for Question 7 is 4 marks)

A student investigates the half-life of a radioactive source using a Geiger-Muller (GM) tube connected to a counter (required practical style).

(a)Before the experiment, the student records the background count rate three times: 18, 20 and 16 counts per minute (cpm). Calculate the mean background count rate.(1)

(b)The student then measures the count rate from the source every minute. Her results are shown below (corrected count rate = raw count rate minus the mean background count rate found in part (a)).

Time (min): 0, 1, 2, 3, 4, 5
Raw count rate (cpm): 818, 418, 218, 118, 68, 43
Corrected count rate (cpm): 800, 400, X, 100, Y, 25

Calculate the missing corrected count rate values X (at 2 minutes) and Y (at 4 minutes).(2)

(c)Use the corrected count rate values from part (b) to determine the half-life of the source. Show your working.(3)

(d)The student repeats the whole experiment two more times and calculates a mean half-life from her three sets of results. Explain how this improves the quality of her conclusion.(1)

(e)State one precaution the student should take when handling the radioactive source, to reduce her exposure to radiation.(1)

(f)The source used in this experiment emits β radiation. Suggest why a GM tube is a more suitable method of detecting this radiation than simply observing the source.(2)

(Total for Question 8 is 10 marks)

A radioactive isotope used in industry has a half-life of 8 days. A newly-prepared sample has an activity of 640 Bq.

(a)Calculate the activity of the sample after 24 days.(2)

(b)The sample can only be disposed of as low-level waste once its activity has fallen to 40 Bq or below. Calculate the minimum time that must pass before this is possible.(2)

(Total for Question 9 is 4 marks)

A nurse spills a small amount of a radioactive liquid tracer on her glove while preparing an injection.

(a)State whether this is an example of contamination or irradiation, giving a reason for your answer.(2)

(b)Explain why contamination is generally considered a greater long-term hazard than irradiation, and suggest one way the nurse's contamination in part (a) could have been avoided.(2)

(Total for Question 10 is 4 marks)

Radon gas is a naturally occurring radioactive gas that can build up inside some homes, especially in areas with granite rock, and is an α emitter.

(a)Explain why breathing in radon gas is much more hazardous than being near a sealed α source held at a safe distance outside the body.(2)

(b)Suggest one practical way homeowners in high-radon areas can reduce radon levels inside their homes.(1)

(Total for Question 11 is 3 marks)

The table below shows the approximate contribution of different sources to average UK background radiation exposure.

Radon gas (from rocks and soil): 50%
Medical (X-rays and treatments): 14%
Cosmic rays (from space): 12%
Ground and buildings: 14%
Food and drink: 8%
Nuclear industry, fallout and other man-made sources: 2%

(a)State two sources of background radiation from the table that are natural (not caused by human activity).(1)

(b)Calculate the total percentage of background radiation from man-made sources shown in the table.(2)

(c)A pilot flies at high altitude for much of their working life. Explain why their annual radiation dose from background radiation is likely to be higher than average.(2)

(Total for Question 12 is 5 marks)

The properties of α, β and γ radiation make each type suited to different practical uses. For each use below, explain why the stated type of radiation is suitable.

(a)Smoke detectors contain a small radioactive source that ionises the air between two electrodes, allowing a small current to flow. Explain why an α source (rather than β or γ) is used.(2)

(b)In a paper mill, a radioactive source is placed on one side of the paper as it is produced, with a detector on the other side, to monitor its thickness. Explain why a β source (rather than α or γ) is used.(2)

(c)Gamma radiation is used to sterilise medical equipment and some foods. Explain why γ (rather than α or β) is suitable for this use.(2)

(d)A γ-emitting isotope with a short half-life, such as technetium-99m, is injected into patients as a medical tracer so doctors can image internal organs. Explain why both properties, γ emission and short half-life, make it suitable for this use.(2)

(Total for Question 13 is 8 marks)

Radiation dose is measured in sieverts (Sv) and takes into account the type of radiation and the amount absorbed by the body's tissues.

(a)State two factors that affect the radiation dose a person receives from a source.(2)

(b)Workers at a nuclear power station are only allowed to spend a limited, carefully controlled amount of time in areas of the plant with higher radiation levels, and they wear a radiation dose badge at all times. Explain why limiting their time reduces their radiation dose, and why wearing a dose badge does not, by itself, reduce the dose they receive.(2)

(Total for Question 14 is 4 marks)

Gamma radiation is often used in radiotherapy to treat cancer, aiming a beam of radiation from outside the body at a tumour. Evaluate the use of γ radiation for treating cancer in this way, referring to the properties of γ radiation that make it suitable, and to the risks involved.

(Total for Question 15 is 6 marks)

A radioactive source used in an industrial thickness gauge has a half-life of 4 hours. At the start of a shift its activity is 2000 Bq.

(a)Calculate the activity of the source 12 hours later.(2)

(b)Health and safety rules state that the source must be replaced once its activity falls below 100 Bq. By finding the activity after each successive half-life, determine the minimum number of complete half-lives, and hence the minimum time, needed for the activity to fall below 100 Bq.(3)

(Total for Question 16 is 5 marks)

Nuclear fission is the process used to release energy in nuclear power stations. This content applies to separate/triple Physics only.

(a)Describe what happens during the nuclear fission of a uranium-235 nucleus, starting from a slow-moving (thermal) neutron being absorbed.(3)

(b)Explain what is meant by a chain reaction, and state the condition needed for a chain reaction to be self-sustaining.(2)

(c)In a nuclear reactor, control rods (for example, made of boron) are raised or lowered in the reactor core. Explain how control rods are used to control the rate of fission reactions.(2)

(Total for Question 17 is 7 marks)

When a uranium-235 nucleus (235/92 U) absorbs a slow-moving neutron (1/0 n), it can undergo fission to form xenon-140 (140/54 Xe), an isotope of strontium (Sr), and 2 further neutrons. This content applies to separate/triple Physics only.

(a)Use conservation of mass number and atomic number to determine the mass number and atomic number of the strontium nuclide formed.(3)

(b)State the type of nuclear reaction taking place, and describe, in terms of mass, how energy is released during the reaction.(2)

(Total for Question 18 is 5 marks)

Nuclear fusion is the process that releases energy in stars, including the Sun. This content applies to separate/triple Physics only.

(a)Describe what happens during nuclear fusion, using hydrogen nuclei fusing to form helium as an example.(2)

(b)Explain why extremely high temperature and pressure are needed for nuclear fusion to take place.(2)

(c)State one similarity and one difference between the energy released in nuclear fusion and in nuclear fission.(2)

(Total for Question 19 is 6 marks)

A patient at a hospital is injected with a radioactive tracer, technetium-99m, which has a half-life of 6 hours and an initial activity of 400 MBq. Hospital policy states that a patient cannot be discharged home until the activity of the tracer remaining in their body has fallen to 25 MBq or below.

(a)Calculate the number of half-lives needed for the activity to fall from 400 MBq to 25 MBq.(2)

(b)The patient is injected at 09:00. Calculate the earliest time the patient can be discharged.(2)

(c)Technetium-99m is chosen for this scan partly because it is a γ emitter with a relatively short half-life. Evaluate why these two properties make it a good choice for a diagnostic tracer used inside the body, compared with using a longer half-life α-emitting isotope instead.(2)

(Total for Question 20 is 6 marks)

Mark scheme · P4b Radioactivity, Half-Life and Nuclear Uses

Question 1

(a) B1 2 protons and 2 neutrons (a helium nucleus), oe
(a) Answer: 2 protons and 2 neutrons (a helium nucleus).
(b) B1 a fast-moving/high-energy electron, emitted from the nucleus, oe
(b) Answer: A fast-moving electron emitted from the nucleus.
(c) B1 γ (radiation), cao
(c) Answer: Gamma radiation.
(d) B1 α, then β, then γ, cao
(d) Answer: Alpha, then β, then γ.

Question 2

(a) B1 the count rate barely changes when paper is added (620 to 618), so no significant α radiation is present, since α would be almost completely absorbed by paper, oe
(a) B1 the count rate drops sharply (from about 618 to about 15 counts/min) when aluminium is added, showing that β radiation is present, since β is absorbed by a few mm of aluminium, oe
(a) B1 the count rate changes very little between aluminium and lead (15 to 14), showing the remaining radiation is γ, since γ is only slightly reduced even by thick lead; conclusion: the source emits β and γ radiation, oe
(a) Answer: The source emits β and γ radiation (no significant α radiation is present).
(b) B1 any one of: the distance between the source and the GM tube; the counting time for each reading; the GM tube/detector used, oe
(b) Answer: The distance between the source and the GM tube (or the counting time for each reading).
(c) M1 there is always some radiation present from natural background sources (e.g. cosmic rays, rocks) that the GM tube would detect even with no source present, oe
(c) A1 subtracting this background gives a corrected count rate due only to the source under investigation, making the results accurate/valid, oe
(c) Answer: So the recorded count rate is due only to the source being investigated, not background radiation, giving accurate, valid results.

Question 3

(a) M1 α particle shown correctly as 4/2 He and equation attempted using conservation of mass number and atomic number
(a) A1 241/95 Am -> 237/93 Np + 4/2 He, cao
(a) Answer: 241/95 Am -> 237/93 Np + 4/2 He
(b) M1 mass number = 243 - 4 = 239
(b) A1 atomic number = 95 - 2 = 93 (neptunium-239), cao
(b) Answer: Mass number 239, atomic number 93 (neptunium-239).

Question 4

(a) M1 nickel nuclide correctly identified as 60/28 Ni, oe
(a) A1 60/27 Co -> 60/28 Ni + 0/-1 e, cao
(a) Answer: 60/27 Co -> 60/28 Ni + 0/-1 e
(b) B1 a neutron changes into a proton (and an emitted electron), so the total number of nucleons (protons + neutrons) in the nucleus stays the same, oe
(b) B1 the number of protons increases by 1, as a neutron becomes a proton, which increases the atomic number by 1, oe
(b) Answer: A neutron converts into a proton, so the nucleon (mass) number is unchanged, but the proton (atomic) number increases by 1.

Question 5

(a) B1 99/43 Tc -> 99/43 Tc + γ radiation (no change in mass number or atomic number), cao
(a) Answer: 99/43 Tc -> 99/43 Tc + γ radiation
(b) B1 γ radiation is electromagnetic radiation/energy, with no mass and no charge, so no particles are lost from the nucleus, oe
(b) Answer: Gamma radiation has no mass and no charge, so its emission does not change the number of protons or neutrons in the nucleus.
(c) B1 the charge of a neutron is 0 (neutral), cao
(c) B1 neutrons are highly penetrating and can be absorbed by nuclei in the body, which can make previously stable nuclei unstable/radioactive, or can transfer energy through collisions, causing ionisation indirectly, oe
(c) Answer: Charge = 0. Although uncharged, neutrons penetrate deeply and can be absorbed by nuclei (making them unstable) or cause ionisation indirectly through collisions.

Question 6

(a) B1 it is impossible to predict which particular nucleus in a sample will decay next, or exactly when any individual nucleus will decay, oe
(a) B1 each nucleus has the same constant probability of decaying in a given time interval, regardless of external conditions (e.g. temperature, pressure) or how long it has already existed, oe
(a) Answer: It is impossible to predict when any individual nucleus will decay; each nucleus has an equal, constant probability of decaying in a given time, unaffected by external conditions.
(b) B1 one decay (nuclear disintegration) per second, cao
(b) Answer: One decay per second.

Question 7

(a) B1 the time taken for the activity (or count rate, or number of undecayed nuclei) of a radioactive sample to halve, oe
(a) Answer: The time taken for the activity of a radioactive source to fall to half its original value.
(b) M1 480 -> 240 after 1 half-life
(b) M1 240 -> 120 -> 60 after 2 further half-lives (3 half-lives in total to reach 60 Bq)
(b) A1 3 half-lives occur in 3 hours, so half-life = 1 hour, cao
(b) Answer: 1 hour

Question 8

(a) B1 18 cpm, cao
(a) Answer: 18 counts per minute
(b) B1 X = 200 (218 - 18), cao
(b) B1 Y = 50 (68 - 18), cao
(b) Answer: X = 200 cpm, Y = 50 cpm
(c) M1 recognises the corrected count rate halves at each successive time shown: 800, 400, 200, 100, 50, 25
(c) M1 identifies that this halving occurs at regular 1-minute intervals throughout the data
(c) A1 half-life = 1 minute, cao
(c) Answer: 1 minute
(d) B1 repeating and taking a mean reduces the effect of random error/anomalous readings, giving a more precise and reliable half-life value, oe
(d) Answer: Repeating and averaging reduces the effect of random error, giving a more precise and reliable half-life value.
(e) B1 any one of: use tongs/forceps (never handle directly); keep the source at arm's length/maximise distance; store it in a lead-lined container when not in use; minimise the time the source is out of its container; do not point the source at herself or others, oe
(e) Answer: Use tongs to handle the source (or another valid precaution, e.g. maximise distance, minimise handling time, or use shielding).
(f) M1 β radiation cannot be seen, heard or felt, so it cannot be detected using human senses, oe
(f) A1 a GM tube detects ionising radiation directly, producing an electrical pulse each time a particle enters it, which can be counted to give an objective, quantitative measurement, oe
(f) Answer: Beta radiation cannot be detected by human senses; a GM tube detects each ionising particle electronically, giving an accurate, quantitative count rate.

Question 9

(a) M1 24 / 8 = 3 half-lives
(a) A1 640 -> 320 -> 160 -> 80; activity = 80 Bq, cao
(a) Answer: 80 Bq
(b) M1 640 / 40 = 16 = 2⁴, so 4 half-lives are needed (or: continuing from 80 Bq, one further half-life gives 40 Bq, 4 half-lives in total)
(b) A1 time = 4 x 8 = 32 days, cao
(b) Answer: 32 days

Question 10

(a) B1 contamination, cao
(a) B1 because radioactive material is now on her (on the glove/skin), and will continue to irradiate her until it is removed, rather than her simply being near a source, oe
(a) Answer: Contamination, because radioactive material is now on her, continuing to expose her until removed, rather than her just being near a source.
(b) B1 contamination keeps radioactive material in contact with (or inside) the body, giving continued exposure that could last a long time, until it is removed or decays, unlike irradiation, which stops as soon as the person moves away from the source, oe
(b) B1 any sensible precaution, e.g. wearing gloves/protective clothing while handling the liquid tracer, working over a tray to contain spills, oe
(b) Answer: Contamination keeps the source in continued contact with the body, giving a prolonged dose, unlike a brief external exposure. Precaution: wearing gloves/protective clothing while handling the tracer.

Question 11

(a) B1 outside the body, α particles have a very short range and are absorbed by a few cm of air (or the outer dead layer of skin), so they cannot reach living cells, oe
(a) B1 if inhaled, the gas is inside the lungs in direct contact with living tissue, and α radiation is strongly ionising, so it deposits all its energy over a very short distance, causing intense localised damage to lung cells/DNA, oe
(a) Answer: Outside the body α radiation is stopped before it reaches living cells; inhaled radon is in direct contact with lung tissue, where strongly ionising α radiation causes serious localised cell/DNA damage.
(b) B1 any sensible answer, e.g. improve ventilation (fit an extractor fan or a radon sump beneath the floor to vent the gas outside); seal cracks in floors/walls to stop gas entering the building, oe
(b) Answer: Improve ventilation, for example by fitting a radon sump or extractor fan beneath the floor to vent the gas safely outside.

Question 12

(a) B1 any two of: radon gas; cosmic rays; ground and buildings; food and drink (both needed for the mark), oe
(a) Answer: Radon gas and cosmic rays (or any two natural sources from the table).
(b) M1 identifies medical (14%) and nuclear industry/fallout (2%) as the man-made sources
(b) A1 14 + 2 = 16%, cao
(b) Answer: 16%
(c) B1 at high altitude there is less atmosphere above to absorb cosmic rays, so exposure to cosmic radiation is greater than at ground level, oe
(c) B1 because the pilot spends a large amount of time flying, their cumulative dose over a year builds up to be higher than someone mostly at ground level, oe
(c) Answer: Less atmosphere at altitude absorbs less cosmic radiation, and the pilot's large amount of flying time increases their cumulative annual dose.

Question 13

(a) B1 α radiation has a short range in air and is strongly ionising, so it can ionise the air in the small gap between the electrodes, oe
(a) B1 it is easily absorbed by smoke particles, cutting the ionisation/current and triggering the alarm; γ is too weakly ionising and would pass through smoke without being absorbed, and β has a longer range than needed, oe
(a) Answer: Alpha is strongly ionising over a short range, so it ionises the air gap but is readily absorbed by smoke, cutting the current and triggering the alarm.
(b) B1 β radiation is partially absorbed by the paper, so the amount reaching the detector changes as the thickness changes, allowing the thickness to be monitored, oe
(b) B1 α would be completely absorbed even by thin paper, giving no useful variation in signal; γ would pass through with very little absorption regardless of thickness changes, oe
(b) Answer: Beta is partially absorbed by the paper, so the detector reading varies usefully with thickness; α would be fully absorbed and γ barely absorbed at all.
(c) B1 γ radiation is highly penetrating, so it can pass through packaging and equipment, killing bacteria/microorganisms throughout the item, oe
(c) B1 it does not need direct contact and does not make the equipment radioactive, and does not heat or melt the equipment unlike heat sterilisation; α and β would not penetrate the packaging, oe
(c) Answer: Gamma penetrates packaging/equipment fully, killing microorganisms throughout, without leaving the item radioactive or damaging it with heat.
(d) B1 γ radiation can pass out of the body to be detected externally by a γ camera without being significantly absorbed by tissue, unlike α or β, which would be absorbed inside the body, oe
(d) B1 a short half-life means the isotope decays away quickly, reducing the total radiation dose received and the length of time the patient remains radioactive, oe
(d) Answer: Gamma escapes the body to be detected externally with minimal tissue damage; the short half-life limits the patient's total radiation dose and exposure time.

Question 14

(a) B1 any one of: type of radiation; distance from the source; time exposed to the source; amount of shielding used; activity of the source (1 mark per correct factor, max 2), oe
(a) Answer: Any two of: type of radiation, distance from the source, time of exposure, shielding used, activity of the source.
(b) B1 reducing the time spent exposed to a radiation source reduces the total dose received, since dose builds up the longer a person is exposed, oe
(b) B1 a dose badge only measures/records the radiation dose a worker has received; it does not block, absorb or reduce the radiation reaching them, oe
(b) Answer: Less exposure time means a lower total dose; the badge only measures the dose received, it does not shield the worker or reduce the radiation itself.

Question 15

Level 1 (1-2): Basic, isolated statement(s) are made about gamma radiation and/or its use in treating cancer, with little or no linking between ideas. The answer may only state that gamma is used to treat cancer or that it is penetrating.
Level 2 (3-4): Relevant points are made about how the properties of gamma radiation make it suitable for treating tumours, and at least one associated risk is identified, with some linking of ideas. Reasoning is mostly logical but the evaluation is limited.
Level 3 (5-6): A detailed, coherent explanation links the properties of gamma radiation (penetration, ionising ability) to its suitability for treating internal tumours, alongside a clear discussion of risks to healthy tissue, and a reasoned judgement weighing benefit against risk. The answer is logically structured throughout and uses correct scientific terminology.
Indicative content:
- Gamma radiation is highly penetrating, so a beam can be aimed from outside the body at a tumour deep inside without invasive surgery.
- Ionising radiation damages the DNA of the cells it passes through, including cancer cells, preventing them from dividing or killing them.
- The beam can be aimed from several different angles, or rotated around the patient, so the doses overlap and concentrate on the tumour, while any one healthy area only receives a low dose from a single direction.
- Healthy cells near the tumour and along the beam's path can still be damaged or killed, causing side effects such as nausea, fatigue or skin damage.
- Radiation can damage the DNA of healthy cells without killing them, which can cause mutations and a small additional risk of the patient developing further cancers later.
- Treatment is often given in small fractionated doses over several sessions, which helps healthy tissue recover between doses while still damaging the tumour cumulatively.
- Radiotherapy can treat tumours that would be difficult, risky or impossible to remove by surgery, or can be used alongside surgery to destroy any remaining cancer cells.
- An overall judgement should weigh the benefit (destroying the tumour without invasive surgery) against the risks (damage to healthy tissue, side effects, small increased future cancer risk).

Question 16

(a) M1 12 / 4 = 3 half-lives
(a) A1 2000 -> 1000 -> 500 -> 250; activity = 250 Bq, cao
(a) Answer: 250 Bq
(b) M1 continues halving from 250 Bq: after 4 half-lives = 125 Bq (still at or above 100 Bq, not yet below); after 5 half-lives = 62.5 Bq
(b) M1 identifies that 5 half-lives is the minimum whole number of half-lives needed for the activity to fall below 100 Bq
(b) A1 minimum time = 5 x 4 = 20 hours, cao
(b) Answer: 20 hours (5 half-lives)

Question 17

(a) B1 a slow-moving neutron is absorbed by a U-235 nucleus, forming an unstable (excited) nucleus, oe
(a) B1 this unstable nucleus splits into two smaller daughter nuclei of roughly similar size, oe
(a) B1 the fission releases 2 or 3 fast-moving neutrons and a large amount of energy, mainly as the kinetic energy of the fragments and neutrons, oe
(a) Answer: A neutron is absorbed, forming an unstable nucleus, which splits into two smaller daughter nuclei, releasing 2-3 neutrons and a large amount of energy.
(b) B1 the neutrons released by one fission go on to be absorbed by further U-235 nuclei, causing more fissions, which release more neutrons, so the number of fissions rapidly multiplies, oe
(b) B1 a sufficient mass of fissile material (a critical mass) must be present so that, on average, at least one neutron from each fission goes on to cause another fission, oe
(b) Answer: Neutrons from one fission trigger further fissions, multiplying rapidly; this requires at least a critical mass of fissile material to be present.
(c) B1 control rods absorb some of the free neutrons produced by fission, reducing the number available to cause further fissions, oe
(c) B1 lowering the rods further into the core absorbs more neutrons and slows the reaction; raising them absorbs fewer neutrons, allowing the rate (and power output) to increase, keeping the chain reaction controlled, oe
(c) Answer: Control rods absorb neutrons; lowering them slows the reaction and raising them speeds it up, keeping the chain reaction steady and controlled.

Question 18

(a) M1 atomic number: 92 + 0 = 54 + Z(Sr) + 0, so Z(Sr) = 38
(a) M1 mass number: 235 + 1 = 140 + A(Sr) + 2, so A(Sr) = 94
(a) A1 strontium-94 (atomic number 38, mass number 94), cao
(a) Answer: Strontium-94 (atomic number 38, mass number 94).
(b) B1 nuclear fission, cao
(b) B1 the total mass of the products (fragments and neutrons) is very slightly less than the total mass before the reaction; this 'missing' mass is converted into a large amount of energy, released mainly as kinetic energy of the fragments and neutrons, oe
(b) Answer: Nuclear fission. A small amount of mass is converted into a large amount of energy, released mainly as kinetic energy of the fission products.

Question 19

(a) B1 two light nuclei (e.g. hydrogen nuclei) join together to form a single heavier nucleus (e.g. helium), oe
(a) B1 this releases a large amount of energy (and, depending on the reaction, other particles), oe
(a) Answer: Two hydrogen nuclei fuse together to form a heavier helium nucleus, releasing a large amount of energy.
(b) B1 all nuclei are positively charged, so they strongly repel each other (electrostatic repulsion) as they approach, oe
(b) B1 extremely high temperature gives nuclei enough kinetic energy, and extremely high pressure pushes them close enough together, to overcome this repulsion so the strong nuclear force can bind them, oe
(b) Answer: Positively charged nuclei repel each other; extreme temperature and pressure give them enough energy and closeness to overcome this repulsion and fuse.
(c) B1 similarity: both release a very large amount of energy compared with chemical reactions, because mass is converted into energy in both, and both involve changes to the nucleus rather than just electrons, oe
(c) B1 difference: fusion of light nuclei releases more energy per kilogram of fuel than fission of heavy nuclei, oe
(c) Answer: Similarity: both convert mass into a large amount of energy through changes to the nucleus. Difference: fusion releases more energy per kilogram of fuel than fission.

Question 20

(a) M1 400 / 25 = 16 = 2⁴
(a) A1 4 half-lives, cao
(a) Answer: 4 half-lives
(b) M1 4 x 6 = 24 hours
(b) A1 09:00 the following day, cao
(b) Answer: 09:00 the next day
(c) B1 γ radiation is weakly ionising and highly penetrating, so it can pass out of the patient's body to be detected by an external γ camera without being significantly absorbed by (or causing much damage to) tissue; an α emitter would not penetrate out of the body to be detected, while causing much greater ionising damage to surrounding tissue from within, oe
(c) B1 the short half-life means the isotope decays away relatively quickly, reducing the total radiation dose received and how long the patient remains radioactive, whereas a longer half-life isotope would keep irradiating the patient (and require precautions around others) for much longer, oe
(c) Answer: Gamma escapes the body for external detection with little tissue damage, unlike α; the short half-life limits the patient's total dose and radioactive time, unlike a longer-lived source.

Radioactivity, Half-Life and Nuclear Uses - Worksheets, Questions and Revision