Nuclear Transformations

Question 1

Process occurring in a cyclotron

Answer 1

1. Charged particles are accelerated to high velocities in an EM field
2. This generates a high energy particle beam
3. Particle beam hits target - at sufficiently high energy, this collision leads to nuclear reactions
   (But this is an ‘atom-by-atom’ reaction so is unsuitable for bulk production of isotopes)

Question 2

Neutron bombardment

Answer 2

Used to prepare heavy, trans-uranium elements

Question 3

Neutron bombardment can produce trans-uranium elements up to…

Answer 3

Z = 100 (Fermium, Fm)

Question 4

238U + 22Ne –>

Answer 4

256Md + 1H + 3n

t1/2 = 77 min

Question 5

248Cm + 15N –>

Answer 5

260Lr + 3n

t1/2 = 3 min

Question 6

209Bi + 58Fe –>

Answer 6

266Mt + n

t1/2 = 5 microsecs

Question 7

249Cf + 48Ca –>

Answer 7

294Og + 3n

t1/2 = 890 microsecs

Question 8

What determines the size of a star?

Answer 8

1. Gravity (decreases size)

2. Energy release from fusion (increases size)

Question 9

What happens to the size of a star when all the H in the core is used up?

Answer 9

No more energy is released from H fusion so gravity takes over and the star ‘collapses’ (decreases in size).

Question 10

s process

Answer 10

Slow neutron capture followed by beta decay
- slow in the sense that there is time for beta decay to occur before another neutron is captured
Elements up to 209Bi can be formed

Question 11

r process

Answer 11

Occurs in supernovae! (T > 1 trillion K in the core)
Rapid addition of (100-200) neutrons within seconds, until the beta-instability is too great
- rapid in the sense that the nuclei do not have time to undergo beta decay before another neutron is captured
Elements up to 254Cf are formed
Non-selective process - similar amounts of all heavy nuclei are produced

Question 12

e process

Answer 12

= “proton addition” (slow addition of protons to elements)
Occurs at T > 500 million K
Elements up to Fe are formed
Basically ‘fills in the gaps’ from He/C/O burning to give elements that are not multiples of 4

Question 13

What are the names of the processes that produce elements heavier than iron?

Answer 13

s process and r process

Question 14

x process

Answer 14

Interstellar cosmic collisions to produce elements heavier than H but lighter than C e.g. 6,7Li, 9Be, 10,11Be

Question 15

First successful conversion of an element

Answer 15

14N + 4He —> 17O + 1H

Question 16

Proving the existence of the neutron

Answer 16

9Be + 4He —> 12C + 1n

Question 17

Conditions required for H burning

Answer 17

T > 10 million K

d = 160 gcm-3

Question 18

Conditions required for H burning catalysed by 12C

Answer 18

T > 15 million K

Question 19

Conditions required for He fusion

Answer 19

All H in core used up

T > 100 million K

Question 20

Hoyle resonance

Answer 20

There is an energy difference of only 0.3 MeV between the reaction energy of the triple alpha process and the energy of the 12C* excited state
This energy difference is bridgeable by the thermal energy of the stellar interior
Therefore the triple alpha process is feasible