Nuclear Physics

Question 1

15. Describe an experiment to verify that a radioactive source is (4 marks):
    a. A β–particle emitter
    b. A α-particle emitter

Answer 1

• Place radiation detector close to the source and determine the count rate
• Use a thin sheet of paper
• If it is α radiation, then the count rate will drop dramatically to almost background rate
• If it is β radiation, then aluminium at least a couple of mm thick will substantially reduce the count rate

Question 2

16. Describe what conclusions can be drawn about the structure of the atom from Rutherford’s experiment in which α-particles are scattered by gold nuclei (4 marks).

Answer 2

• Alpha-scattering provided evidence for the nucleus of the atom
• The nucleus contained most of the mass of the atom
• The nucleus has a very small size (10-14m) because very few α-particles were scattered through large angles
• The nucleus was (positively) charged because it repelled the (positive) α-particles [note: the sign of the charge cannot be deduced from the experiment, this is because they did not know about the fusion so they would not have realised that if the nucleus was negative fusion would have occurred]

Question 3

17. Discuss two advantages and two disadvantages of producing electrical power by nuclear fission. (6 marks)

Answer 3

• Problems with the reaction getting out of control
• Maintaining the reaction so that it proceeds continuously
• Does not produce acid rain or waste gases that could cause pollution
• Risks from radiation: emissions due to an accident(1); emissions from radioactive waste(1); long half-life of some of the waste products(1)

Question 4

18. It is suggested that some radioactive waste could be stored underground in simple metal containers and would not cause damage to the environment. Comment on this suggestion (4 marks)

Answer 4

• Ground acts as a good shield
• Satisfactory for short time
• But half-lives likely to be long
• Corrosion, leakage etc.

Question 5

19. Explain the technique of radioactive carbon dating (4 marks)

Answer 5

• Living plants / animals absorb C-14
• Once dead, the plant does not take in any more C-14
• The fraction of C-14 to C-12 nuclei / number of C-14 nuclei / activity of C-14 nuclei measured in dead and living sample
• X = x0e-(lamda)t used with data above to estimate the age.

Question 6

20. Suggest why the method of carbon dating is likely to be unreliable if a sample is: less than 200 years old (1 mark), more than 60000 years old (1 mark).

Answer 6

• A/A0 is too small to measure accurately (must answer in terms of change in number of C-14)
• Activity is very small / difficult to measure / comparable to background radiation (not that there are too few C-14 atoms  there are still a large number!)

Question 7

21. Describe the process of induced nuclear fission (2 marks)

Answer 7

• A neutron is absorbed by a massive nucleus

* The nucleus is split into two daughter nuclei and one or more neutrons

Question 8

22. Explain why the total mass of the individual nucleons of a deuterium nucleus is different from the mass of the deuterium nucleus (3 marks)

Answer 8

• Mass of nucleus < total mass of nucleons
• Energy must be supplied to the nucleus to separate the nucleons
• E = mc2 and E is the binding energy and m is the mass defect

Question 9

23. Explain how nuclear fission can provide energy (2 marks)

Answer 9

• The total binding energy of the products is greater than the binding energy of the original nucleus
• The difference in the binding energies is released as energy

Question 10

24. Suggest a suitable material which can be used as a moderator in a fission reactor and explain its role. QWC: In your answer you should make clear how a moderator works at the microscopic level (4 marks)

Answer 10

• Moderator: water / graphite / carbon
• It slows down the fast moving neutrons
• Slow-moving neutrons have a greater chance of causing fission
• QWC: The neutrons make collisions with the moderator nuclei and transfer some of their kinetic energy

Question 11

25. Describe the nature and the range of the three forces acting on the protons and neutrons in the nucleus (5 marks)

Answer 11

• Any five from:
• Gravitational force
• This force is attractive
• And is long ranged / obeys 1/r2 relationship
• Strong nuclear force
• This force is attractive at larger distances or repulsive at shorter distances
• Is short ranged / 10-14
• Electrostatic / electrical force
• This force is repulsive between protons / zero between neutrons / zero between protons and neutrons
• Is long ranged / obeys 1/r2 relationship