IGCSE Physics 0625 — Topic 5: Nuclear Physics — Paper 2 (MCQ) Cambridge Challenge 2

Question 1

In the alpha-particle scattering experiment, three observations were recorded. Observation 1: most alpha particles passed straight through the foil. Observation 2: a few were deflected through small angles. Observation 3: a very small number were deflected through more than 90°.

Which observation, on its own, shows that the nucleus is POSITIVELY charged, and how?

AObservation 1 alone: passing straight through is only possible if the nucleus attracts and then releases each alpha particle in turn BObservation 3 alone: only a negatively charged nucleus could stop a fast-moving positive alpha particle completely and then turn it back along the direction it came from CNone of them: the charge of the nucleus was already known from chemistry before the experiment was carried out DObservations 2 and 3 together: the deflections are consistent with repulsion between the positive alpha particles and a concentration of like charge

Question 2

A radon nuclide is written as ²¹⁹₈₆Rn, meaning nucleon number 219 and proton number 86.

How many neutrons does this nucleus contain?

A86 B305 C219 D133

Question 3

A neutral lithium atom has 3 protons, 4 neutrons and 3 electrons. It loses one electron to become an ion.

Which row describes the ion?

ACharge 0, with 3 protons, 5 neutrons and 2 electrons; a neutron forms to keep the atom neutral BCharge +1, with 3 protons, 4 neutrons and 2 electrons; the nucleus is completely unchanged CCharge −1, with 3 protons, 4 neutrons and 2 electrons; the nucleus is completely unchanged DCharge +1, with 2 protons, 4 neutrons and 2 electrons; a proton leaves with the electron

Question 4

Four nuclides are listed: P has 8 protons and 8 neutrons; Q has 8 protons and 10 neutrons; R has 9 protons and 9 neutrons; S has 10 protons and 10 neutrons.

Which two nuclides are isotopes of the same element?

AP and R, because they both contain the same total number of particles in the nucleus BQ and R, because they both contain exactly the same number of neutrons in the nucleus CP and S, because the ratio of protons to neutrons is exactly one to one in each of them DP and Q, because they have the same proton number but different nucleon numbers

Question 5

Which row correctly compares a proton and an electron?

AThe proton’s charge is roughly 2000 times the electron’s charge, and its mass is also roughly 2000 times larger BTheir charges are equal in size but opposite in sign, and the proton’s mass is roughly 2000 times the electron’s CThe proton has charge but the electron has none, which is why only the proton is found inside the nucleus DTheir charges are equal in size but opposite in sign, and the two particles also have roughly equal masses

Question 6

An alpha particle is emitted by an unstable nucleus.

What is an alpha particle?

AA helium nucleus: two protons and two neutrons bound together, with charge +2 BA fast-moving electron ejected from the nucleus, carrying a charge of −1 CA helium atom: two protons, two neutrons and two orbiting electrons, with no overall charge DA hydrogen nucleus: a single proton carrying a charge of +1 and no neutrons at all

Question 7

A nucleus undergoes a change.

Which change turns the nucleus into a nucleus of a DIFFERENT element?

AA change in the number of neutrons it contains BA change in the number of protons it contains CA change in the number of electrons orbiting it DA change in its energy when it emits a gamma ray

Question 8

Before the scattering experiment, some scientists pictured the atom as a sphere of spread-out positive charge with electrons dotted through it.

If that picture had been correct, what would the alpha-scattering experiment have shown?

AExactly the results that were seen, because spread-out charge and a nucleus deflect alphas identically BNo alpha particles passing through at all, because spread-out charge would fill all the empty space CAll the alpha particles passing through with, at most, very small deflections — none bouncing back DAll the alpha particles bouncing straight back from the surface of every atom in the foil

Question 9

Radioactive decay is described as both random and spontaneous.

Which statement is correct?

ADecay is random for small samples only; in a large sample the nuclei coordinate so that exactly half decay in each half-life BIt is impossible to predict which particular nucleus will decay next, but strong heating always makes every kind of unstable nucleus decay at a noticeably faster rate CIt is impossible to predict which nucleus will decay next, and the decay rate cannot be changed by heating, pressure or chemical reactions DEach nucleus decays after exactly one half-life, so the time of each individual decay can be predicted precisely in advance

Question 10

A polonium-210 nucleus (proton number 84) decays by emitting an alpha particle.

What are the nucleon number and proton number of the daughter nucleus?

ANucleon number 206, proton number 84 BNucleon number 208, proton number 83 CNucleon number 214, proton number 86 DNucleon number 206, proton number 82

Question 11

A phosphorus-32 nucleus (proton number 15) decays by emitting a beta particle.

What is the daughter nucleus?

ANucleon number 31, proton number 15 BNucleon number 32, proton number 16 CNucleon number 28, proton number 13 DNucleon number 32, proton number 14

Question 12

A nucleus in an excited state emits a gamma ray.

What happens to the nucleus?

AIt gains energy from the emission and becomes more unstable than it was before BIt loses energy, but its proton number and nucleon number both stay exactly the same CIts proton number rises by one, because the gamma ray carries away a negative charge DIts nucleon number falls by one, because the gamma ray is a particle emitted from the nucleus

Question 13

Alpha and beta particles travel at right angles to a uniform electric field between two charged plates. The beta particles are deflected far more than the alpha particles, and in the opposite direction.

Why are the beta particles deflected more?

AA beta particle has a far smaller mass than an alpha particle, and this outweighs its smaller charge BA beta particle carries a far larger charge than an alpha particle, so it feels a much larger electric force CBeta particles travel more slowly than alpha particles, so the field has more time to act on each one DAlpha particles are deflected only by magnetic fields, so an electric field leaves them almost unaffected

Question 14

A detector gives a count rate of 620 with no absorber between it and a source. Inserting a sheet of paper leaves the count rate at 618. Inserting 5 mm of aluminium instead reduces it to 210. Inserting several centimetres of lead reduces it to 25, the background level.

Which radiations does the source emit?

AAlpha, beta and gamma all together BBeta and gamma, but not alpha CGamma only, because it is the only radiation that reaches the lead DAlpha and beta, but not gamma

Question 15

Alpha radiation is the most strongly ionising of the three radiations, yet it has the shortest range in air.

Which statement explains the connection?

AAir absorbs alpha particles chemically, by reacting with the helium as it forms, before much ionisation can happen BThe connection is a coincidence: how strongly a radiation ionises the air it passes through has no effect at all on how far that radiation is able to travel before stopping CEach ionising collision transfers energy from the particle, so the radiation that ionises most heavily uses up its energy over the shortest distance DAlpha particles are so small and fast that they slip between air molecules, ionising them from a distance as they pass

Question 16

A beta particle is an electron, yet it is emitted from the nucleus, which contains no electrons.

Where does the beta particle come from?

AA neutron in the nucleus changes into a proton, and the electron created in that change is ejected at high speed BOne of the atom’s orbiting electrons falls into the nucleus and is then thrown back out at a much higher speed CA proton in the nucleus changes into a neutron, releasing the positive charge as a fast-moving electron DThe nucleus stores a small number of electrons between its protons, and one of these escapes during the decay

Question 17

Which list gives properties of gamma radiation?

AElectromagnetic wave; travels at the speed of light; uncharged; the least ionising of the three radiations BParticle of matter; travels at about a tenth of the speed of light; uncharged; stopped completely by thin aluminium CElectromagnetic wave; travels faster than light in a vacuum; uncharged; unable to pass through the human body DElectromagnetic wave; travels at the speed of light; negatively charged; the most ionising of the three radiations

Question 18

A radium-226 nucleus contains 88 protons and 138 neutrons. It decays by emitting an alpha particle.

How many neutrons does the daughter nucleus contain?

A137 B136 C138 D134

Question 19

A source has an activity of 960 Bq. Its half-life is 5.0 hours.

What is its activity after 20 hours?

A240 Bq B48 Bq C60 Bq D120 Bq

Question 20

In a laboratory the background count rate is 20 counts/min. With a source in place, a detector reads 260 counts/min. The source’s half-life is 45 minutes.

What will the DETECTOR read 90 minutes later?

A60 counts/min B80 counts/min C140 counts/min D65 counts/min

Question 21

A sample of a radioactive nuclide is left for exactly three half-lives.

What fraction of the original nuclei has DECAYED?

A3/4 B1/8 C7/8 D1/3

Question 22

The mass of a radioactive isotope in a sample falls from 80 mg to 10 mg in 36 days.

What is the half-life of the isotope?

A36 days B9.0 days C4.5 days D12 days

Question 23

A decay graph for a source shows the corrected count rate falling from 1000 counts/min at time zero to 500 counts/min at 250 s, and to 250 counts/min at 500 s.

What is the half-life, and what feature of the graph gives it?

A250 s — the time at which the graph first begins to curve instead of falling in a straight line B125 s — half of the time taken for the count rate to fall from 1000 to 250 counts/min C500 s — the time taken for the count rate to fall by 750 counts/min from its starting value D250 s — the time taken for the count rate to fall to half of any starting value

Question 24

Two gamma-emitting isotopes are available for use as a medical tracer injected into a patient: isotope X with a half-life of 6 hours, and isotope Y with a half-life of 3 years.

Why is X the better choice?

AIts activity lasts long enough for the scan but then dies away quickly, so the patient’s radiation dose after the test is small BIsotope Y would decay completely before the scan could begin, because long half-lives mean rapid disappearance CIts shorter half-life means the gamma radiation it emits is more penetrating, so the detectors placed outside the patient’s body receive a much clearer and sharper signal DA short half-life means the isotope is cheaper to produce, and cost is the main consideration in medical physics

Question 25

A detector records the corrected count rate from a source: 600 at 0 min, 424 at 1 min, 300 at 2 min, 212 at 3 min, 150 at 4 min.

What is the half-life of the source?

A2 minutes B1 minute C3 minutes D4 minutes

Question 26

Living wood gives a corrected count rate of 60 counts/min per sample due to carbon-14, which has a half-life of 5700 years. A wooden axe handle found in a peat bog gives 15 counts/min for an identical sample.

What is the approximate age of the axe handle?

A17 100 years B22 800 years C5700 years D11 400 years

Question 27

A source must be stored until its activity has fallen below 1% of its initial value.

What is the minimum whole number of half-lives that must pass?

A7 B100 C5 D10

Question 28

A student says: "After one half-life, half of the sample’s nuclei have decayed. After two half-lives, all of them have decayed."

Which correction should the student make?

ANo correction is needed: two halves make a whole, so the sample is exhausted after exactly two half-lives BThe second half-life takes twice as long as the first, which is why some nuclei survive beyond two half-lives CAfter two half-lives three-quarters of the original nuclei still remain undecayed, because each half-life only ever removes a quarter of whatever sample was there originally DIn each half-life, half of the nuclei REMAINING at its start decay — so a quarter of the original nuclei are still undecayed after two half-lives

Question 29

A student measures a source’s count rate over several hours but forgets to subtract the background count rate before finding the half-life from a graph.

What effect does this mistake have on the calculated half-life?

AIt comes out too long, because the constant background makes the count rate appear to fall by a smaller proportion than it really does BIt has no effect on the final result, because the background is present in every single reading and therefore cancels itself out automatically when the graph is drawn CThe graph becomes a rising curve instead of a falling one, so no half-life can be read from it at all DIt comes out too short, because background radiation adds random spikes that make the curve fall away more steeply

Question 30

A source containing a single isotope of half-life 15 hours has an activity of 4800 Bq.

How long will it take for the activity to fall to 300 Bq?

A240 hours B45 hours C60 hours D16 hours

Question 31

A detector in an empty laboratory, with no sources present, still records a steady count rate.

What is the largest contributor to this background radiation in most locations?

ALeakage from nearby nuclear power stations operating normally BThe residue of historical nuclear weapons testing in the atmosphere CSignals from mobile phone masts and other transmitters nearby DRadon gas seeping from rocks and soil into the air

Question 32

The radiation from a small gamma source spreads out in all directions. A detector 10 cm from the source is moved to 20 cm away.

What happens to the count rate and why?

AIt stays the same, because gamma radiation is hardly absorbed at all by 10 cm of air BIt falls to zero, because gamma radiation from small sources has a range of about 15 cm in air CIt falls to roughly a quarter, because the radiation has spread over a sphere with four times the area DIt falls to roughly a half, because the detector is twice as far away from the source

Question 33

An engineer must choose a radioactive source for detecting the position of a leak in an underground water pipe, by adding the source to the water and scanning at ground level.

Which source is most suitable?

AA gamma emitter with a half-life of a few hours BA beta emitter with a half-life of a few minutes CAn alpha emitter with a half-life of a few hours DA gamma emitter with a half-life of several hundred years

Question 34

A worker inhales dust containing an alpha-emitting isotope. A colleague standing nearby is exposed to the same isotope only from outside the body.

Why is the situation far more serious for the worker who inhaled the dust?

AOutside the body alpha is stopped harmlessly by air and skin, but inside the lungs every particle’s energy is absorbed by living tissue at very close range BAlpha radiation becomes more penetrating in the warm, moist conditions found inside the human body CThe situation is equally dangerous for both of the workers, because the same isotope always emits exactly the same kind of radiation with the same energy wherever it happens to be DInhaled isotopes decay faster than external ones, because the body’s chemistry shortens their half-life

Question 35

A GM tube connected to a counter is used to examine a weak source. Two 30-second counts taken one after the other give 152 and 174.

What is the best explanation of the difference?

AThe GM tube must be faulty, because a working detector gives identical counts over identical time intervals BRadioactive decay is random, so repeated counts over equal times naturally fluctuate around an average value CThe background radiation switched off during the first count and came back on during the second count DThe source’s half-life elapsed between the readings, so the second count should have been half the size of the first

Question 36

A quality-control system checks the fill level of closed metal drink cans on a production line, using a radioactive source on one side and a detector on the other, level with the required fill line.

Which source should be used, and what does a HIGH count rate at the detector mean?

AA beta source; a beta particle count is unaffected by the liquid, so the count rate never changes BA gamma source; a high count rate means the liquid is above the fill line, so the can is over-filled CA gamma source; a high count rate means the liquid is below the fill line, so the can is under-filled DAn alpha source; a high count rate means the can on the line is completely empty of liquid

Question 37

A school laboratory keeps a set of sealed radioactive sources.

Which set of precautions is correct for handling them?

AWear thin rubber gloves, which absorb all three types of radiation completely, and carry the sources quickly by hand so that the whole job is finished sooner BHandle sources only in a darkened room, because radioactive emissions are strongest in bright light CWarm each source gently before use, so that its half-life lengthens and its activity becomes safer to work with DLift sources with long-handled tongs, point them away from the body, and return them to their shielded store as soon as possible

Question 38

When not in use, a gamma source is kept in a thick-walled lead box rather than a steel cupboard.

Why is lead used?

ALead stops the source from decaying while it is stored, preserving its activity for later use BLead is airtight, and gamma radiation cannot pass through any container that excludes air CLead is dense enough to absorb most of the gamma radiation, reducing the dose to people nearby to a low level DLead reflects all of the gamma radiation back into the inside of the box, so that none of it can ever escape and reach the people who are working outside

Question 39

In a nuclear reactor, a uranium-235 nucleus absorbs a neutron and splits into two smaller nuclei, releasing energy and two or three fast neutrons.

Why are the released neutrons essential to the reactor’s operation?

AThey generate the electric current directly, because moving neutrons constitute a flow of charge BThey combine with the fission fragments inside the core to rebuild fresh uranium fuel, so that the reactor never needs to be refuelled while it is running CThey can go on to cause fission in further uranium nuclei, sustaining a chain reaction that keeps releasing energy DThey carry away the waste products of each fission, keeping the fuel rods clean during operation

Question 40

Fusion of hydrogen nuclei powers the Sun, but requires temperatures of millions of degrees.

Why is such an extreme temperature necessary?

AThe hydrogen must first be boiled into a gas, because fusion is impossible in liquids and solids BThe positively charged nuclei repel each other, and only at enormous speeds can they approach closely enough to fuse CHigh temperature is needed to split each hydrogen atom’s nucleus apart into fragments before those fragments are able to recombine as helium and release energy DThe heat is needed to ignite the hydrogen, which then burns with oxygen to release the Sun’s energy

Paper 2 — Multiple Choice

Section A: The Nuclear Atom

Section B: Radioactive Decay

Section C: Half-Life

Section D: Detection and Uses

Section E: Safety, Fission and Fusion