8.1 - Nuclear Physics

• Describe the Rutherford Scattering Experiment

  ● A beam of alpha particles was directed at a thin gold foil.

  ● Occurs in a vacuum so that no collisions between air particles and alpha particles can occur.

  ● The experiment was done in order to determine structure of an atom.
• In the Rutherford scattering experiment, it was observed that most alpha particles passed straight through. What can we infer from that?

  Most of the atom is made up of empty space.
• What evidence was there that suggested that the nucleus had a positive charge?


  The nucleus repels the alpha particles and caused them to deflect from their original path, some even bouncing back
• Name 3 types of radiation

  Alpha

  Beta (Plus and minus)

  Gamma
• Order alpha, gamma and beta ionisation from most to least ionising

  Alpha

  Beta

  Gamma
• Order alpha, beta and gamma radiation from most to least penetrating

  Gamma

  Beta

  Alpha
• A sheet of paper can block which type of radiation?

  Alpha
• Electron capture is described as:

  And an electron neutrino

  
• Why is ionising radiation seen as dangerous:


  Because it can kill or mutate cells, which could lead to mutations and lead to things such as cancer
• Which radiation is more harmful inside the human body: alpha, beta or gamma?

  Alpha radiation - it has a high ionising power so can damage more cells. It is also very poorly penetrating, therefore it is not able to leave the body, whereas gamma is highly penetrating.
• Give an example of a real life use of Beta decay and explain why beta is chosen for this

  Beta radiation can be used to measure the thickness of paper or aluminium foil. Alpha isn't used as it is less penetrative and wouldn't reach the detector on the other side of the sheet. Gamma radiation is too penetrative and would pass through everything.
• What type of radiation follows the inverse square law?

  Gamma radiation
• What is intensity measured in?

  Watts per square metre (Wm^-2)
• Describe an experiment which can be used to show the inverse square law and gamma rays

  - Measure the background radiation using a Geiger-Muller Tube, when the gamma radiation is not in the room

  - Put the gamma source at a set distance from the GM tube and measure the count rate per minute. Record 3 measurements for each distance and take an average.

  - Do this for many distances, going up in set increments each time (eg 5cm, 10cm etc)

  - Take away the background radiation for each reading

  - Square each of the distances

  - Plot a graph of the count rate per minute against 1 over distance squared (1/d^2)

  - If it is a straight line through the origin then it confirms that they are directly proportional
• What is background radiation?

  Radiation that is constantly in the surroundings from sources such as rocks and food
• What is the decay constant (lambda)

  The probability of a nucleus decaying per second
• What are the units for the decay constant?

  s^-1
• What is half life?

  The time it takes for half the unstable nuclei in a substance to decay
• What is activity measured in?

  Becquerels (Bq) - Decays per second
• True or False: Radioactivity isotopes decay exponentially


  True

  
• Why is technetium-99m useful in medicine?

  - Because it releases gamma radiation

  - It has a short half life therefore it doesn't stay highly radioactive for long

  - Half life of 6 hours: long enough for it to be detected

  - It can also be made near to the hospital

  - Easy to detect outside the patient

  - 'Clears away' after a few days
• What does the graph of N against Z show?

  It shows the relationship between proton number and neutron number. The graph shows a stability curve which starts as N=Z until N value of 20. After that the graph curves upward and becomes steeper.
• Where on the curve does Beta-Minus decay occur and why?

  Above the stability line, because the nuclei found there contains too many neutrons. Therefore, when beta-minus decay occurs the neutron turns into a proton and it becomes more stable.
• What type of decay occurs below the stability line and why?

  Beta plus decay

  As the isotopes found here often have too many protons. Therefore, when beta plus decay occurs the proton turns into a neutron.
• How does the heavier nuclei often decay?

  Through alpha decay.

  This is because alpha decay emits a helium nucleus (2 protons and two neutrons) therefore causing the nuclei to become less heavy and more stable.
• How is electron diffraction used to determine the diameter of a nucleus?

  - An electron beam is fired at a thin sheet of the desired atom

  - A diffraction pattern is produced on a screen behind

  - Using the angle of a minimum we can use equations to calculate the diameter
• True or False: The density of a nucleus is independent to its radius

  True
• What is the mass defect?


  The difference between the total mass of all the nucleons separately compared to the mass of the nucleus.
• Why is there a mass defect?

  Because energy is needed to bring the constituent parts of a nucleus together, therefore the mass equivalent of the energy is lost and the total mass decreases.
• What is binding energy?

  The energy required to separate a nucleus into its constituent parts

  
• What is nuclear fission?

  - Where an unstable nucleus splits into 2 smaller nucleus

  - Often occurs with the larger nuclei

  - The binding energy per nucleon increases when fission occurs therefore the overall processes releases energy
• What is fusion?


  When 2 small nuclei fuse together to create a larger nuclei. The new nucleus has a larger binding energy per nucleon than the old nuclei therefore energy is released in the process
• Why is it difficult to make fusion occur on earth?

  There is a large repulsion between the two positively charged nuclei, therefore a lot of energy is required to overcome the repulsion and fuse them together.

  It is hard to get a material that can withstand the heat and be cost effective.
• How is fission used in nuclear reactors?

  Rods of uranium-235 absorb neutrons and become unstable and then split into two daughter nuclei. It also releases 2/3 more neutrons. These then go on to be reabsorbed by another uranium-235.
• What is the purpose of a moderator?

  To slow down the neutrons so they travel slow enough to be absorbed by the uranium.

  They do this through elastic collisions between the moderator and the nucleus.
• Why are control rods essential for a nuclear power station?

  They stop the chain reaction from being out of control.

  They absorb neutrons so that only of the neutrons released in each reaction can go on to be absorbed by another uranium

  If not then the nuclear reactor would overheat as too many reactions would happen at once
• Is boron used as a control rod or a moderator?

  Control Rod
• Give an example of a material that can be used as a moderator

  Water
• What is the purpose of using water as a coolant?

  It allows heat from the nuclear reactor to escape, which stops the reactor from overheating.
• What is the critical mass?

  The minimum mass of fuel needed for a chain reaction to occur