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The Nucleus
• Let’s go further down… what’s in a a nucleus?
• Protons– Mass about 2000 times that of an electron
– Positive electric charge
– The nucleus of hydrogen
• Neutrons– Mass a tiny bit higher than that of the proton (0.1% difference)
– No electric charge – neither attracted nor repelled electrically
– Discovered by Chadwick at Cambridge (where else?) in 1932
• Discovered by breaking nuclei apart in scattering experiments
Isotopes
• Protons and neutrons clump together in many different combinations
• Number of protons determines the chemical element– Equal number of electrons in a neutral atom
• Nuclei with a given number of protons can have different numbers of neutrons – different “isotopes” of the elements– Ex.: Carbon nucleus (6 protons) can have 6, 7, or 8 neutrons
• Notation: 12C indicates that form of carbon with 12 total protons and neutrons– C has 6 protons always, so 12C has 6 neutrons, 13C has 7, etc.
– Sometimes written as but this is redundant!C126
Number of protons
Isotopes
• Different isotopes are chemically identical (essentially)– Same number of electrons, pattern of electron waves
– Nuclear mass varies from isotope to isotope, but heavy in any case!
• Many isotopes are “radioactive” – the nuclei can break apart spontaneously– “Half life” is the time it takes for half the nuclei in a sample to
decay
– Alternatively, the time at which a given nucleus as a 50/50 chance of decaying
• Occur with different abundances in nature– E.g., 98.89% of naturally occurring C is 12C
A Puzzle
• The protons repel each other electrically
• So what holds the nucleus together??
• There must be a new kind of force that operates on the nuclear scale and is much stronger than electrical forces
• Today we call this force the “strong force” • It acts between (any combination of) protons
and neutrons
• Short-ranged – doesn’t reach too far outside the nucleus– Think velcro
Radioactivity
• Discovered in the late 1800’s by Becquerel– Found that Uranium and other
substances would expose photographic film
• Studied by the Curies: Marie, Pierre and their daughter Irene– Marie, Pierre and Becquerel shared the
1903 Nobel prize in physics
– Marie won another in 1911 (the first person to win two!)
– Irene won the 1935 Nobel in chemistry
Becquerel M. Curie
I. Curie P. Curie
Radiation• Radioactivity happens when nuclei break apart
– Usually spontaneously
– Stuff typically comes flying out
• Many substances are radioactive– All isotopes heavier than Bismuth (atomic number 83)
– Many isotopes of lighter elements
• Radioactive materials emit three distinct kinds of “rays”
• Alpha rays – just Rutherford’s old alpha particles– Actually a helium nucleus: 2 protons and 2 neutrons stuck together
– 4He, in other words
• Beta rays (actually electrons)
• Gamma rays (actually high energy light)
Nuclear Energy and Weapons
• Since the strong nuclear force is so much stronger than electrical forces, processes involving re-arrangements of protons and neutrons (“nuclear reactions”) involve much more energy than do chemical reactions– Basic QM reason: protons and neutrons are confined to a very
small space – they thus have very high speeds!
– About a million times more energy is involved in a typical nuclear reaction than in a typical chemical reaction
• This is why atomic weapons are more powerful than chemical explosives
• Also what makes nuclear energy attractive
• Two basic processes: fission and fusion
Nuclear Fission• Splitting apart of unstable heavy nuclei, with release of
large amounts of energy
• Relies on a “chain reaction” to create sustained energy– Neutrons released go on to create more fissions
The First Reactor December 1942
Enrico Fermi
Atomic Weapons in World War II• Physics of the nucleus studied intensely in the 1930s
• In October 1939, Einstein wrote to President Roosevelt suggesting that extremely powerful bombs could perhaps be made based on nuclear fission, and encouraging the USA to continue research in this area– AE had been a pacifist, but he was afraid the Nazis would develop the
bomb first
– Heisenberg was in charge of the Germans’ atomic bomb project during WWII – didn’t make much progress, as it turned out
• “Manhattan Project” begun August 1942– Many top physicists worked on it
• First atomic weapon tested July 16, 1945, Alamogordo, NM
Trinity
J. Robert Oppenheimer
If the radiance of a thousand sunsWere to burst at once into the sky,That would be like the splendor of the Mighty One...I am become Death,The shatterer of Worlds.
– The Bhagavad-Gita
Atomic Weapons
• Two weapons used against Japan in August 1945
• Japan surrendered quickly after the second attack
• In the late 1940s new and more powerful bombs based on nuclear fusion were developed in the USA (the “Hydrogen bomb”)– Also development of rocket
technology
• The Soviet Union caught up quickly, aided by spies
“Now we're all sons of bitches.”– Ken Bainbridge,Trinity Test Director,speaking to Oppenheimer
More Details• The difficulty with building such weapons is in isolating
enough 235U (“enrichment”) or making 239Pu– Only about 1% of naturally occurring Uranium is 235U
– Needs to be enriched to about 90% for use in weapons
– Very challenging (fortunately!)
– Much lower enrichment level needed for reactor use
• “Breeder” reactors can make 239Pu from 235U– Could help expand uranium resources
– Part of why reactors are worrisome, though
• WW2 bombs had yields of 15-20 “kilotons”– Means same energy released as 15-20 thousand tons of TNT
• Later “H bombs” typically in the 1-10 megaton range– I.e., 1-10 thousand kilotons or 1-10 million tons of TNT
Fission Reactors• Use 235U or 239Pu in a controlled
reaction
• Energy is used to heat water, which drives a turbine
• Problems:– limited fuel supply
– possibility of accidents
– dangerous byproducts
– expensive technology
– limited lifetime of power plant due to radiation
Accidents• Three Mile Island (1979, Pennsylvania, USA)
– Partial core meltdown
– No radiation released or (identifiable) injuries
– Occurred a few days after the release of The China Syndrome
– Contributed to negative public opinion of nuclear energy
• Chernobyl (1986, Ukraine)– Explosion, fire, core meltdown
– Both reactor design and operator error were responsible
– Heroic efforts by firefighters and other workers, many of whom died
– Radiation spread over large parts of Europe, including Scandinavia, the UK, and northern France
Chernobyl
Chernobyl• 203 people hospitalized immediately, of whom 31 died
– Mostly firefighters and other rescue workers– They were not told of the high radiation levels!
• Many others exposed to high levels of radiation– Expect increase in cancer rate– Appears to have been an increase in the rate of thyroid cancer
among children in the area– Estimating how much is due to the accident is very difficult– According to one study, expect about 18,000 cancer deaths,
eventually• For comparison, about 50,000 people are killed each year in auto
accidents in the US
• WHO: 56 direct deaths• Evacuation and resettlement of over 360,000 people
Nuclear Fusion
• A process in which light nuclei combine (fuse) to form a heavier nucleus, with a large energy release
• The physics mechanism behind the sun’s burning, as well as thermonuclear weapons (the “H bomb”)
• The “holy grail” of energy sources– Cheap fuel (hydrogen!)
– Lots of energy
– Safe
– No harmful by-products, unlike current fission reactors
Basic Reactions
Why is it so hard?• The nuclei don’t want to be close together
– Like charges repel each other
• To overcome this repulsion, you need very high temperature and pressure
Powering the Sun
• The sun is a cloud of (mostly) hydrogen (90%) and helium (9%)
• Held together by gravity, which tries to make it collapse
• As it collapsed, heat and pressure built up near the center
• Eventually nuclear fusion begins – the source of the sun’s power!
• Total energy output equivalent to 100 billion 1 megaton bombs going off every second!
Heavier Elements
• As He is produced in a star’s core, it can fuse to make even heavier nuclei
• If the star is very large (lots of gravitational pressure), elements up to iron (Fe) can be produced
• When the star runs out of fuel, it explodes – a “supernova”– The heaver the star, the more violent the death
• The heavy elements (C, N, O, …) are thrown out into space, and can collapse into new stars and planets
• Essentially all of the elements heavier than Li are made in stars and supernovae!
Supernova 1987A
