Lecture 5

Question 1

What element is the least thermodynamically stable

Answer 1

1. Hydrogen

Question 2

What happened directly after the big bang

Answer 2

1. Universe filled mostly with H which condensed and cooled to form stars

Question 3

What were the three nuclear reactions that have been proposed to account for the various types of stars and the observed abundances of the elements

Answer 3

1. Exothermic processes in stellar interiors: these include (successively) hydrogen burning, helium burning, carbon burning
2. Neutron capture processes: these include the s-process (slow neutron capture) and the r-process (rapid neutron capture)
3. Miscellaneous processes: these include the p-process (proton capture) within stars, and the x-process which involves spallation by galactic cosmic rays in interstellar regios.

Question 4

Describe hydrogen burning

Answer 4

1. Fusion
2. At very high temps and pressures (T=10 million K/ 200 billion bar pressure), the first nuclear fusion reactions start
3. H combines with itself to form helium 4
4. Mass loss associated with hydrogen fusion results in hydrogen burning being highly exothermic

Question 5

What are the equations of hydrogen burning

Answer 5

1. 1H+1H-beta+ –>2H
2. 2H+1H –> 3He
3. 3He + 3He –> 4He + 2 1H
4. Overall 4 1H –> 4He + 2beta+ + 26MeV

Question 6

Why is combining of protons unlikely

Answer 6

1. Both have +ve charge
2. Have to be bought into internuclear distance

Question 7

What can catalyse the hydrogen burning/fusion

Answer 7

1. 12C and 13N
2. Starts at 15 million K
3. Contributes to 1% of our sun - centre is 15.7 million K, but majority not hot enough
4. Becomes dominant H burning process > 17 million K

Question 8

Describe the CNO thermonuclear cycle

Answer 8

1. Accounts for 10% in our sun
2. 12C adds proton to form 13N
3. 13N - beta+ –> 13C
4. 13C + 1H –> 14N
5. 14N+1H–> 15O
6. 15O-beta+ –> 15 N
7. 15N is unstable- addition of 1H means it kicks out the He, producing 12C again

Question 9

In heavier stars what happens

Answer 9

1. Additional nuclear reactions (proton additions) occur at >500millionK
2. e processes
3. Elements up to iron are formed- most stable

Question 10

How are elements heavier than iron formed

Answer 10

1. S Process
2. R process

Question 11

Describe s- process

Answer 11

1. Slow neutron addition to elements followed by beta decay
2. 56Fe + 3 1n –> 59Fe –>59Co + beta-
3. Elements up to 209Bi can be formed
4. Stable nuclei are enriched as they are less susceptible to neutron capture

Question 12

Describe helium and carbon burning

Answer 12

1. When the hydrogen is used up in the cored the star collapses further and the temperature rises to 100 million K
2. Helium then starts its fusion reactions
3. Can’t make 8Be- as unstable and endothermic reaction
4. Triple alpha process instead
5. 3 Particles combined in one go- combine to make 12C- Highly exothermic
6. if we combine 3 particles to make 1- release a lot of energy- kicked out by another particle- so not possible as nowhere for energy to go
7. maybe 12C has excited nuclear states that sits exactly at 7.6 MeV- exactly the same amount of energy of energy released in nuclear fusion reaction
8. All the energy sits in the excited state- then lose energy by gamma radiation etc to get to 12C stable state.
9. Then can add He particles to 12C to make multiples of He e.g 16O,20Ne, 24Mg etc up to 48Ti

Question 13

What is R-processes

Answer 13

1. Rapid addition of neutrons
2. When sun mass is fairly large
3. Once all the matieral in the sun is burnt up - equilibrium of all the energy released in fusion process which expand the star and gravitational pull cant be sustained
4. Gravitational collase- supernova
5. Nuclei are squished together to point where they cant be squeezed further- internuclear distance which gives shockwave
6. A core of 1000 billion K in core leads to rapid addition of neutrons until beta instability is too excessive
7. Elements up to 254Cf- typical decay of supernova decays with half life of 55 days - matches half life of 254Cf

Question 14

What are x-processes

Answer 14

1. Interstellar cosmic collisions produce remaining isotopes
2. 6,7Li, 9Be, 10,11B
3. All relatively rare
4. Not made in stars

Question 15

How was nuclear fission first discovered

Answer 15

1. Neutron added to 235U
2. Didn’t make the next element
3. Broke into large chunks and neutrons and binding energy released
4. 142Ba made

Question 16

Show reaction of 235U + 1n to make Ba and what that shows

Answer 16

1. –> 142Ba + 91Kr + 3 n
2. Neutrons produced
3. Shows chain reaction possible

Question 17

What was the first sub-critical reactor

Answer 17

1. 500kg U/paraffin sustained by neutron source in the centre
2. Possible to generate chain reaction but had to have neutron source to keep it going
3. Not sustainable

Question 18

What was first nuclear chain reaction

Answer 18

1. First nuclear chain reaction of 60 tonnes U
2. 400 tonnes graphite
3. Cd rods
4. Kept fission reaction going on its own- sustainable

Question 19

Describe problems of nuclear fission reactions

Answer 19

1. Products themselves are radioactive due to their high neutron/proton ratio
2. All very heavily radiating
3. Generally half life decreases as you get beta decay to small n/p ratio until you get stable element
4. Get a huge amount of different nuclides of different elements
5. Very messy reaction

Question 20

What does 6 fissions produce

Answer 20

1. Produces 15 highly energetic neutrons
2. More than required for chain reaction

Question 21

How can neutrons be lost

Answer 21

1. Leaving into the surroundings
2. Neutron capture by other nuclides - e.g. 238U absorbs neutrons and ends up as plutonium- neutron is lost

Question 22

What are the different types of fission

Answer 22

1. Less than one neutron causing another fission- sub critical and will die out
2. Exactly one neutron- critical
3. More than 1 neutron - super critical, exponential increase in fissions= explosion

Question 23

What does the type of fission depend on

Answer 23

1. Surface/volume ratio (mass,shape)
2. Purity of number of neutrons