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 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 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

How do radioactive isotopes decay?

Answer 13

Exponentially

Question 14

Why is Technetium 99m useful in medicine?

Answer 14

• 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 15

What does the graph of n against Z show?

Answer 15

• 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 16

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

Answer 16

• 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 17

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

Answer 17

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

Question 18

How do the heavier nuclei often decay?

Answer 18

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

Question 19

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

Answer 19

• 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 20

What is the equation used in the closest approach equation?

Answer 20

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 21

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

Answer 21

• 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
• Use the de Broglie wavelengt equation if an equation is required

Question 22

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

Answer 22

1. 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 23

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

Answer 23

1. The radius of an atom is about 0.05nm
2. The radius of the smallest nucleus is about 1fm (1x10^-15m)

Question 24

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

Answer 24

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 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 an 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 - in liquid form the high level waste might leak hence the need to vitrify 5. 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 6. The waste will be radioactive for hundreds/thousands of years so long term storage needs to be in geologically stable areas - deep underground in steel containers

Question 43

How do you find how many photons are incident on an area per second?

Answer 43

I/I0=A/A0 which essentially means that the intensity of the photons you have over the total number of photons in a second = the area you have/ total area of the sphere 4pir^2

Question 44

How do you find the power output of a nuclear reactor?

Answer 44

E=mc^2 P=mc^2/t

Question 45

Explain why there is an imbalance in the number of protons and neutrons, referring to the fundamental forces

Answer 45

1. Strong nuclear force - 0-0.5fm repulsive, 0.5-3fm attractive, 3fm+ negligible 2. Electromagnetic force acts between protons - long range 3. Fewer protons are needed to reduce the repulsive forces

Question 46

What is the difference between U-235 and U-238?

Answer 46

U-235 undergoes fission U-238 doesn't, it scatters the released neutrons

Question 47

Why do neutrons need to be thermal neutrons?

Answer 47

To increase the probablility of them being absorbed by the U-235 or To allow the neutron to be absorbed by the fuel/U-235

Question 48

What are the benefits of using nuclear power?

Answer 48

1. Less climate change as little greenhouse gas emitted 2. Small amounts of fuel give the same amount of energy 3. Some can quickly adjust their output

Question 49

How do you improve the measurement taken for the background radiation in an experiment?

Answer 49

1. Remove all radioactive sources from the room 2. Zero the counter 3. Measure on the same day the experiment is carried out

Question 50

What's the difference between becoming irradiated and experiencing radiation?

Answer 50

To be irradiated the nucleus has to be affected

Question 51

Why does the number of collisions for the kinetic energy of a neutron to be reduced deoend on the nucleon number of the moderator atoms?

Answer 51

* More (kinetic) energy is lost when the mass of the moderator atom/nucleus is closer to the mass of the neutron * So the number of collisions needed increases with nucleon number