Nuclear and quantum physicsIB Physics SL: Topic test

20 questions, 54 marks

IB Physics SL

Nuclear and quantum physics topic test

Total 54 marks

Name

Class

Date

1
A mercury discharge lamp contains mercury vapour at low pressure. When a current passes through it, mercury atoms are excited to higher energy levels and then fall back down, emitting photons. One transition is from an energy level at −2.7 eV to a lower level at −7.7 eV.
(a)
What is the energy of the photon released in this transition?
[1 mark]
A2.7 eV
B5.0 eV
C7.7 eV
D10.4 eV
(b)
What is the frequency of the emitted photon? (h = 6.63 × 10⁻³⁴ J s, 1 eV = 1.60 × 10⁻¹⁹ J)
[1 mark]
A2.5 × 10¹⁵ Hz
B6.0 × 10¹⁴ Hz
C3.0 × 10⁸ Hz
D1.2 × 10¹⁵ Hz
(c)
Explain how observing a bright line at this photon's frequency in the spectrum of a distant star provides evidence that mercury vapour is present in the star's outer layers.
[2 marks]
Total for question 1: 4 marks
2
A radioactive sample of technetium-99m, used in a hospital scan, has an initial activity of 6.4 × 10⁸ Bq. Its half-life is 6.0 hours.
(a)
What is the activity of the sample 18 hours after it is prepared?
[1 mark]
A8.0 × 10⁷ Bq
B1.6 × 10⁸ Bq
C3.2 × 10⁸ Bq
D2.1 × 10⁸ Bq
(b)
What fraction of the original technetium-99m nuclei remains undecayed after 24 hours?
[1 mark]
A1/4
B1/8
C1/16
D1/32
(c)
State and explain how the count rate recorded by a detector near the patient would be expected to change with time, and explain why radioactive decay is described as a random process.
[2 marks]
Total for question 2: 4 marks
3
When a nucleus of uranium-235 captures a slow neutron, it splits into two smaller daughter nuclei together with two or three fast neutrons, releasing about 200 MeV of energy per fission. The daughter nuclei are typically neutron-rich and highly radioactive, emitting beta and gamma radiation, and must be stored as high-level waste for a very long time before their activity falls to a safe level.
(a)
Explain, in terms of chain reactions, why a moderator is needed in a reactor using uranium-235 as its fuel, even though each fission releases fast neutrons capable of causing further fission.
[3 marks]
(b)
State how the daughter nuclei produced in fission typically decay, naming the additional particle emitted alongside the beta particle, and explain why these products must be managed as radioactive waste for a long period.
[4 marks]
Total for question 3: 7 marks
4
In the core of a certain star, deuterium (²₁H) nuclei fuse with tritium (³₁H) nuclei to form helium-4 (⁴₂He) and a neutron. The nuclear masses are: deuterium 2.014102 u, tritium 3.016049 u, helium-4 4.002602 u and the neutron 1.008665 u. The energy released by this and other fusion reactions supports the star against its own gravity. 1 u is equivalent to 931.5 MeV.
(a)
Determine the energy released by this fusion reaction, in MeV, and explain how this energy supports the star against gravitational collapse.
[6 marks]
(b)
The binding energy per nucleon of helium-4 (7.07 MeV) is higher than that of deuterium (1.11 MeV) or tritium (2.83 MeV). Explain, with reference to binding energy per nucleon, why this reaction releases energy, and discuss why more massive stars are able to fuse progressively heavier elements, up to iron, as they evolve, while lower-mass stars are not.
[6 marks]
Total for question 4: 12 marks
5
A sealed discharge tube contains an unknown gas at low pressure. When a high voltage is applied, the gas emits light only at specific wavelengths. A spectrometer shows bright emission lines at 434 nm, 486 nm and 656 nm. A reference chart shows that hydrogen gas produces bright emission lines at exactly these three wavelengths, while no other common gas produces this combination of lines.
(a)
What can be concluded about the identity of the unknown gas?
[1 mark]
AIt cannot be determined without seeing the gas directly
BIt is a mixture of several gases, since three lines are seen
CIt is most likely hydrogen gas, since the lines match hydrogen's reference spectrum
DIt cannot be hydrogen, because hydrogen absorbs at these wavelengths rather than emitting
(b)
Why does the gas emit light only at these specific wavelengths, rather than a continuous range of wavelengths?
[1 mark]
AThe gas atoms have discrete, fixed energy levels, and each line corresponds to a specific transition between two of them
BThe spectrometer can only detect three wavelengths at a time
CThe gas atoms have continuous energy levels, but only three happen to be excited
DThe lines are caused by reflection of the applied voltage's frequency
(c)
One of the lines, at 656 nm, corresponds to a transition between two energy levels of the hydrogen atom. Calculate the energy difference between these two levels, in eV. (h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹, 1 eV = 1.60 × 10⁻¹⁹ J)
[2 marks]
Total for question 5: 4 marks
6
In a school experiment, a radioactive source of unknown type is placed a fixed distance from a detector. With no absorber present, the detector records a certain count rate. A sheet of paper placed between the source and detector has almost no effect on the count rate. A 5 mm sheet of aluminium reduces the count rate to close to the background level.
(a)
What type of radiation is the source most likely emitting?
[1 mark]
AAlpha particles only
BGamma rays only
CAlpha and gamma together
DBeta particles
(b)
Why is this type of radiation more penetrating than alpha particles but less penetrating than gamma rays?
[1 mark]
AIt has no charge and no mass, so it interacts weakly with matter
BIt has a much smaller mass and charge than an alpha particle, so it ionises matter more gradually as it passes through
CIt travels much faster than gamma rays, so it has less time to interact with atoms
DIt is uncharged but has significant mass, so it only interacts with nuclei
(c)
The source is later found to be strontium-90, which decays by beta-minus emission to yttrium-90. Write a nuclear equation for this decay, and identify what else is emitted.
[2 marks]
Total for question 6: 4 marks
7
A sample of plutonium-239 is shaped into a sphere. If the sphere is too small, most of the neutrons produced by fission escape from its surface before they can cause further fission, so the chain reaction dies out. If the sphere is large enough, on average exactly one neutron from each fission goes on to cause another fission, and the reaction proceeds at a constant, controlled rate. This is called the critical mass.
(a)
Explain, in terms of neutron escape from the surface, why increasing the radius of the plutonium sphere makes it more likely to reach a self-sustaining chain reaction.
[3 marks]
(b)
State what is meant by the critical mass of a fissile sample, and describe, in terms of the number of neutrons per fission that go on to cause further fission, what happens if the sample's mass is (i) below and (ii) above the critical mass.
[4 marks]
Total for question 7: 7 marks
8
A single fission of a uranium-235 nucleus releases about 200 MeV of energy; uranium-235 has 235 nucleons. A single deuterium–tritium fusion reaction, like the one in an earlier question, releases about 17.6 MeV and involves 5 nucleons in total (2 + 3).
(a)
Using the binding energy per nucleon curve, which rises steeply for light nuclei, peaks around nucleon number 60, and falls slowly for heavy nuclei such as uranium-235, explain why splitting a uranium-235 nucleus into two medium-mass fragments releases energy.
[6 marks]
(b)
Compare the energy released per nucleon involved in this single uranium-235 fission with the energy released per nucleon in the deuterium–tritium fusion reaction, and use the shape of the binding energy per nucleon curve to explain why fusion of light nuclei generally releases more energy per nucleon than fission of heavy nuclei.
[6 marks]
Total for question 8: 12 marks

End of questions