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