Kern- und Teilchenphysik I Lecture 7: Nuclear fusion
Prof. Nico Serra
Dr. Patrick Owen, Dr. Silva Coutinho
http://www.physik.uzh.ch/de/lehre/PHY211/HS2016.html
Nuclear Fission and Fusion
We have seen that fission is the fragmentation of a heavy nucleus into 2
more stable components Usually requires to be triggered by a slow neutron
Nuclear Fusion
In this case to need to bring to 2 nuclei close to each other for the strong force to come into action
Need to overcome the Coulomb barrier due to nuclei repulsion Need to give some energy for the reaction to occur
The difference in binding energy is released
Some interesting processes
Most interacting nuclei are isotopes of hydrogen (Z=1) for 2 reasons:
Fusion
In order for nuclear fusion to take place the energy must be high enough to tunnel into the Coulomb barrier
The reaction rate per unit volume is given by
Gamow Factor
cross section
Reaction Rate
Rate of nuclear fusion,
proportional to the fusion probability.
Fusion takes place in a narrow energy.
Big Bang Nucleosynthesis
Big Bang model:
The Universe was initially very dense and very hot The Universe started to expand and cooled down
When temperature dropped sufficiently hadrons became stable
When the temperature dropped below 2.22 MeV (about 200s after the Big Bang) the deuteron becomes stable
Other processes follow
There are no stable nuclei with A=5 and A=8 so 7Li is the heaviest nucleus produced during the big bang
Stars
In stars nuclear fusion happens since the gravitational pressure is sufficient to bring two positively charged nuclei close enough
In stars like the sun (with core temperature lower than 15M Kelvin) the proton-proton cycle take place
Deuterium formation
Stars
In stars nuclear fusion happens since the gravitational pressure is sufficient to bring two positively charged nuclei close enough
In stars like the sun (with core temperature lower than 15M Kelvin) the proton-proton cycle take place
The pp fusion is followed by a beta decay, where a position and an
electron neutrino are emitted
The flux of this neutrinos can be
computed accurately (solar neutrino problem)
Stars
In stars nuclear fusion happens since the gravitational pressure is sufficient to bring two positively charged nuclei close enough
In stars like the sun (with core temperature lower than 15M Kelvin) the proton-proton cycle take place
Deuterium-proton fusion
Stars
In stars nuclear fusion happens since the gravitational pressure is sufficient to bring two positively charged nuclei close enough
In stars like the sun (with core temperature lower than 15M Kelvin) the proton-proton cycle take place
Helium-3 fusion
Stars
In stars nuclear fusion happens since the gravitational pressure is sufficient to bring two positively charged nuclei close enough
In stars like the sun (with core temperature lower than 15M Kelvin) the proton-proton cycle take place
Alpha particle formation
Problem
The Sun weights 2x1030Kg, the chemical composition is 71%H, 27% He and 2% heavier elements. The luminosity is 4x1026W
- Compute how much H is converted into He every second
- How much did the Sun loose in till now (5x109 years)
Problem
The Sun weights 2x1030Kg, the chemical composition is 71%H, 27% He and 2% heavier elements. The luminosity is 4x1026W
- Compute how much H is converted into He every second
- How much did the Sun loose in till now (5x109 years)
Burning Helium
If the star is heavier enough the star stats burning 4He with the chain
With no hydrogen to contest the gravitational pressure the star collapses
If we the star is much smaller than the Sun, has no enough mass to ignite other nuclear reactions (the Coulomb barrier is larger) it will become a white dwarf
When this process starts at a large rate there is still some hydrogen in the exterior of the star and it is heat up and the outer mantel
Burning Iron
Heavy star have enough gravitational pressure to ignite reaction of heavier nuclides
Product of star
Nuclear Products
- Onion structure of stars
- Heavy element in the internal shells
- Shell burning at interfaces
