Transcript Integrative Studies 410 Our Place in the Universe

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 126C but this is redundant!
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
Becquerel
M. Curie
I. Curie
P. Curie
• 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
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 suns
Were 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
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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:
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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
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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