Fission: Heavy Elements can reduceenergy (i.e. increase bindingenergy per nucleon) bysplitting roughly in half.
U 235 fission fragment distribution
Excess Neutrons235
92U + n → 8638Sr + 150
54Xe
23592U + n → 86
38Sr + 13254Xe +
Chain Reaction
Reproduction Factor / Criticality
What affects k?– Number of Nuclei / Shape of Mass
critical mass– Number of Released Neutrons per
Fission– Cross Section– Is it Fissile?
What do I want k to be?
Neutron Cross Section / Thermalization
Problem – How to make k~1?
Maximum k for natural U is 1.32– Not every neutron absorbed by U-235 causes
fission.– U-238 absorbs slow neutrons without
undergoing fission Any additional loss of neutrons reduces k Solutions
– Enriched uranium (from 0.7 to ~3 percent)– Heavy water
Control– Control Rods --- Neutron Absorbers
SCRAMSafety Control Rod Axe Man
Breeder Reactors
Then, after ~ 24 min…
Then, after ~ 2.3 days…
U❑238 +𝑛→ U∗
❑239
U❑❑
239 → Np∗+𝑒−❑239
Np❑❑
239 → Pu∗+𝑒−❑239
Nuclear Power Pro/Con Clean energy – no greenhouse
gasses Cheap Can be safe if properly regulated Radioactivity Disasters (but that’s a problem
whenever you have concentrated energy!)
Nuclear proliferation
Inform yourself!
It’s inevitable It’s CLEAN and CAN be done safely and environmentally
responsibly It accounts for about 20% of our electricity today … and 80% of France’s electricity Get past “not in my back yard”
I burned down a field too! There's no such thing as a "safety candle." Grasshopper eyes turn red before they
pop. If you let things get out of control,
sometimes you won't be able to get them back into control.
There are people who can help (it's less embarrassing not to need them, but if you do, you'd better call them).
It's not going to stay a secret forever. You can get through this.
Fusion--Two nuclei combine to form a heavier nucleus
First reaction in early stars
11H + 1
1H → 21H + e+ + νe
mass before - mass after = energy released/c 2
(2MH1 – 2Me) – (MH2 –Me + Me) = (2x1.0078250u - 0.0005486u) - 2.0141018 u = 0.0009996 u (1uc2=931 MeV)
Q = 0.93 MeV
Fusion--Two nuclei combine to form a heavier nucleus
More reactions in stars
11H + 1
1H → 21H + e+ + νe
11H + 2
1H → 32He
11H + 3
2He → 42He + e+ + νe
32He + 3
2He → 42He + 1
1H + 11H
Fusion--Two nuclei combine to form a heavier nucleus
One reaction in a hydrogen bomb
21H + 3
1H → 42He + n
Total mass before -Total mass after = energy released5.0302 u - 5.0113 u = .0189 u ≈ 17.6 MeV
Fusion Reactors Much less radioactivity
– Mostly produces helium-4, but materials near the reaction can absorb neutrons, etc.
Hydrogen is everywhere! It doesn’t work . . . Yet
– Protons repel each other until you get them really close
– Really high temperatures needed
Plasma Fusion
TFTR
Inertial Confinement Fusion
Other practical uses of nuclear physics…
Nuclear medicine– Nuclear treatment (destroy tumors)– Tracer imaging– PET scan– NMR (MRI) ?
Power for space probes Dating materials Sterilization of equipment or food Smoke detectors
Irradiated with
neutrons
Detected electrons emitted
Van Dyck’s
“Saint Rosalie Interceding for the
Plague-Stricken of Palermo”
After a few hours- manganese
(found in umber,A dark pigment)
After 4 days- phosphorous
(found in charcoal)
Jackson Pollock – Galaxy
Irradiated with neutrons
Jackson Pollock – Galaxy
Detected secondaryemission of truth
And beyond Particle physics, QCD, the standard
model Unification theories / theory of
everything String theory
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