Section 12 - Nuclear Physics

Question 1

How has our model of the atom changed over time?

Answer 1

1. Democritus proposed that all matter was made up of little, identical lumps called atoms
2. In 1804 John Dalton proposed a hypothesis that each element was made up of a different type of atom which were tiny spheres
3. J.J. Thomson discovered that electrons could be removed ffrom atoms
4. Thomson suggested that atoms were spheres of positive charge with tiny negative electrons in them - plum pudding
5. Rutherford suggested that atoms didn’t have uniformly distributed charge and density

Question 2

What is the Rutherford scattering?

Answer 2

1. Fired a beam of alpha particles at thin gold foil
2. A circular detector screen surrounding the gold foil and the alpha source was used to detect alpha particles deflected by any angle
3. Most of the alpha particles went straight through the foil, while a few were deflected by a large angle
4. Some were deflected back by 90°
5. Occurs in a vacuum so that no collisions between air particles and alpha particles can occur

Question 3

What were the conclusions of the Rutherford scattering?

Answer 3

1. Most of the atom must be empty space because most of the alpha particles passed straight through the foil
2. The nucleus must have a large positive charge, as some positively charged alpha particles were repelled and deflected by a large angle
3. The nucleus must be small as very few alpha particles were deflected back
4. Most of the mass must be in the nucleus, since the fast alpha particles (with high momentum) are deflected by the nucleus

Question 4

What is the equation for electron capture?

Answer 4

X + e- –> z-1Y + Ve

Question 5

Why is ionising radiation dangerous?

Answer 5

It can kill or mutate cells, which could lead to mutations and lead to things such as cancer

Question 6

What is an application of beta decay, and why is beta used?

Answer 6

• Used to measure the thickness of paper or alumnium foil
• Alpha isn’t used as it’s 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

Question 7

What is the inverse sqaure law and what type of radiation does it apply to?

Answer 7

• Gamma
• The intensity is inversely proportional to the square of the distance from the source (W/m^2 - watts per square meter)

Question 8

Why might experimental evidence not support the inverse square law?

Answer 8

1. The random nature of the radiation count
2. Dead-time in the G-M detector
3. d is not the real distance between source and detector
4. Source is not a point source
5. The source may not be a pure gamma emitter
6. Assumes no absorption between source and detector

Question 9

What investigation can be used to show the inverse sqaure law?

Answer 9

1. Measure the background radiation using a Geiger Muller tube, without the source in the room
2. Put the gamma source at a set distance (1m) from the GM tube and measure the count rate per minute - record 3 measurements for each distance and take an average
3. Do this for at least 8 distances going up in 10cm intervals
4. Take away the background radiation from each reading
5. Square each of the distances
6. Plot a graph of the count rate per minute against 1/distance sqaured
7. If it is a straight line through the origin, then it confirms 1/d^2 and count rate per minute are directly proportional

Question 10

What is background radiation?

Answer 10

Radiation that is constantly in the surrounding from sources such as rocks and food

Question 11

What is the decay constant?

Answer 11

The probability of a nucleus decaying per second (s^-1)

Question 12

What is half life?

Answer 12

The time it takes for half of the unstable nuclei in a substance to decay
Or
The time it takes for the activity of a nucleus to half (Bq - decays per second)

Question 13

Why is Technetium 99m useful in medicine?

Answer 13

• It releases gamma radiation
• It has a short half life therefore it doesn’t stay radioactive for long
• Half life of 6 hours: long enough for it to be detected
• It can be made near to the hospital
• Easy to detect outside the patient

Question 14

What does the graph of n against Z show?

Answer 14

• The relationship between proton number and neutron number
• The graph shows a stability curve which starts at N=Z until N value of 20
• After that the graph curves upwards and becomes steeper

Question 15

Where on the stability curve does β- decay occur and why?

Answer 15

• Above the stability line, because the nuclei found there contains too many neutrons
• When beta minus decay occurs the neutron turns into a proton and it becomes more stable

Question 16

Where on the curve does β+ decay occur and why?

Answer 16

• Below the stability line, because the isotopes found there have too many protons
• When beta plus decay occurs the proton turns into neutrons

Question 17

How do the heavier nuclei often decay?

Answer 17

• Through alpha decay because alpha decay emits a helium nucleus
• This causes the nuclei to become less heavy and more stable

Question 18

How can you use energy conservation to find the closest approach of a particle?

Answer 18

• We know the kinetic energy a particle will have initially and the electrostatic potential energy that repels it
• At some point these are equal to each other and this is the closest approach

Question 19

What is the equation used in the closest approach equation?

Answer 19

Ep = Qq/ 4πε0r
* Q is the charge of an alpha particle (2e)
* q is the charge of the target nucleus q=Ze
* ε0 = permittivity of free space
* r = the distance of closest approach (m)

Question 20

How is electron diffraction used to determine the diameter of a radius?

Answer 20

• 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

Question 21

When using electron diffraction to estimate nuclear radius, how does the intensity vary with the diffraction angle?

Answer 21

1. The diffraction pattern is very similar to that of a light source shining through a circular aperture - a central bright maxima containing the majority of the incident electrons, surrounded by other dimmer rings (maxima)
2. The intensity of the maxima decreases as the angle of diffraction increases - the intensity never hits zero, it just gets very close

Question 22

What is the difference in size between the nuclear and atomic radius?

Answer 22

1. The radius of an atom is about 0.05nm
2. The radius of the smallest nucleus is about 1fm (1x10^-15m)
3. Nuclei are really tiny compared to the size of the whole atom

Question 23

What is the relationship between the nuclear radius and the nucleon number?

Answer 23

1. When data from nuclear radii experiments is plotted on a graph of nuclear radius R against the cube root of the nucleon number A^1/3, the line of best fit gives a straight line
2. This shows the linear relationship between R and A^1/3
3. R∝A^1/3
4. R=R0A^1/3

Question 24

What is nuclear density?

Answer 24

1. The volume each nucleon takes up in a nucleus is about the same
2. As protons and neutrons haven nearly the same mass, all nuclei have a similar density
3. The nuclear density is around 1.45x10^17 kgm^-3
4. Nuclear matter’s density is enormous - much larger than atomic density, suggesting that an atom contains lots of empty space with most of its mass in the nucleus

Question 25

What are the 4 types of nuclear radiation?

Answer 25

• Alpha - a helium nucleus (2 protons and 2 neutrons), relative charge of +2 and mass (u) 4
• Beta-minus - electron, relative charge of -1 and mass (u) negligible
• Beta-plus - positron, relative charge of +1 and mass (u) negligible
• Gamma - short wavelength, high frequency electromagnetic wave, relative charge 0 and mass (u) 0

Question 26

What are the penetrating capabilities of nuclear radiation?

Answer 26

• Alpha - strongly ionising, slow, absorbed by paper or a few cm of air, affected by the magnetic field
• Beta-minus - weakly ionising, fast, absorbed by ~3mm of aluminium, affected by the magnetic field

Question 27

What is the mass defect and why is there one?

Answer 27

• The difference between the total mass of all the nucleons separately compared to the mass of the nucleus
• As energy is needed to bring the constituent parts of the nucleus together, the mass equivalent of energy is lost and the total mass decreases

Question 28

What is binding energy

Answer 28

The energy required to separate a nucleus into its consituent parts

Question 29

What is the most stable nuclei and why?

Answer 29

Iron has the perfect number of protons and neutrons, making it energetically favorable and less likely to decay or undergo fission compared to other elements

Question 30

What is nuclear fission?

Answer 30

• Where a unstable nucleus splits into 2 smaller nuclei
• Often occurs with the larger nuceli
• The binding energy per nucleon increases when fission occurs, therefore the overall process releases energy

Question 31

What is fusion?

Answer 31

• When 2 smaller nuclei fuse together to create a larger nuclei
• The new nucleus has a larger binding energy per nucleon than the old nucei therefore energy is released in the process

Question 32

Why is it difficult to make fusion occur on Earth?

Answer 32

• There is a large repulsion between the 2 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

Question 33

How is fission used in nuclear reactors?

Answer 33

• Rods of Uranium-235 absorb the thermal neutrons and become unstable and then split into 2 daughter nuclei
• It also releases 2/3 more neutrons that then go on to be reabsorbed by another Uranium-235 in a chain reaction

Question 34

What is the purpose of a moderator?

Answer 34

• To slow down the neutrons so they become thermal neutrons and therefore will travel slow enough to be absorbed by the uranium
• This is done through elastic collisions between the moderator and neutrons
• Water can be used

Question 35

Why are control rods essential for a nuclear power station?

Answer 35

• They stop the chain reaction from being out of control
• They absorb the neutrons so that only 1 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
• Boron is often used

Question 36

What is the purpose of using water as a coolant?

Answer 36

• Allows heat from the nuclear reactor to escape, which stops the reactor from overheating

Question 37

What is the critical mass?

Answer 37

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

Question 38

Which waste products from nuclear reactors cause the highest risk?

Answer 38

Spent fuel rods

Question 39

How is high level waste disposed of?

Answer 39

• They are first stored in cooling ponds
• Then placed in steel containers which are buried deep underground

Question 40

What is low level waste contained in?

Answer 40

They are sealed in containers and put underground until it’s safe again

Question 41

What is the mass unit?

Answer 41

The mass relative to 1/12th the mass of Carbon-12

Question 42

A thermal nuclear reactor produces radioactive waste.
State the source of this waste and discuss some of the problems faced in dealing
with the waste at various stages of its treatment.
Your answer should include:
* the main source of the most dangerous waste
* a brief outline of how waste is treated
* problems faced in dealing with the waste, with suggestions for overcoming
these problems.

Answer 42

1. The (highly radioactive/ most dangerous) waste are the fission fragments
   from the fission of uranium-235 or from (spent) fuel rods.
2. The waste is initially placed in cooling ponds/water
3. Plutonium/uranium is separated to be recycled
4. High level waste is vitrified/made solid into (pyrex) glass
5. Then placed in (stainless) steel/lead/concrete cylinders/containers/bunkers to be stored deep underground
6. Transporting waste presents a potential danger to the public so waste is
   transported enclosed in impact/crash resistant/extra thick and strong
   casings Or processed onsite or nearby
7. In liquid form the (high level) waste may leak hence the need to vitrify
   (and barrel in steel)
8. The waste will be radioactive for hundreds/thousands of years so long
   term storage needs to be in geologically stable areas (deep
   underground).