THE NUCLEUS OF THE ATOM

• Components of the Atomic Nucleus– Protons– Neutrons and Isotopes– Radioactivity

• Development of Nuclear Weapons• Nuclear Energy• Particle Physics– Standard Model– Four Fundamental Froces

Potential Energy of any Body of Mass

The Proton

• Rest Mass =1.007276466812awu

• Average Lifetime years• Discovered, described,

and named by Ernest Rutherford over a three-year period (1917-1920)

James Chadwick

• Student of Rutherford• Designed an experiment

with Polonium and Beryllium target. Detected an uncharged form of radiation that had a mass approximately equal to the proton.

• He called it the neutron.1891-1974; Britain

The Neutron

• Rest Mass =1.0086649160043awu• Average Lifetime =881.515 sec

Chadwick’s experimental design

Isotopes• Following the discovery of neutrons, the

disagreement between atomic number and atomic mass for the elements became clear.

• Also, the disagreement between different forms of the same element could be explained.

Antoine Henri Becquerel• Discovered penetrating

radiation by uranium salts that exposed photographic plates in the absence of visible light.

• Reported in 1896.

1852-1908; France

Marie & Pierre Curie

Following report of invisible radiation, Pierre and Marie worked on characterizing the radiation and finding other radioactive elements (e.g. Radium). Maria (Marie) Salomea Sklodowska-Curie;

1867-1934; Poland and FrancePierre Curie; 1859-1909; France

Types of Radiation

Paper

Aluminum

LeadM

Alpha Radiation (α-decay)

• Discovered and named by Rutherford• Reduces the atomic number by 2 and atomic

mass by 4• Equivalent to a helium nucleus

Beta Radiation (β-decay)

• Discovered by Becquerel and named by Rutherford

• Neutron decays to a proton, an electron, an electron neutrino

• Initiated in the nucleus by neutron spontaneously changing to proton (mediated by the weak nuclear force)

Gamma Radiation (γ-decay)

• Discovered by Villard and named by Rutherford

• Type of photon – high energy x-ray with frequency >1019 Hz

• Potassium-40 good source

• γ-decay in association with α and/or β decay

Half-Life

• Probabilistic nature• Exponential decay• Rutherford suggested it

as a way to date minerals.

Radiometric Dating

Based on two decay sequences:238U to 206Pb (half-life 700 million years)235U to 207Pb (half-life 4.5 billion years)Usually taken from very stable zircon crystals

Decay Chains

Controlled Nuclear Fission

Otto Hahn1879-1968; Germany

Friedrich Wilhelm ‘Fritz’ Strassman1902-1980; Germany

Lise Meitner1878-1968; Austria, GermanySweden, UK

Leo Szilard

Tuesday, September 12, 1933. The stoplight changed to green. Szilárd stepped off the curb. As he crossed the street time cracked open before him and he saw a way to the future, death into the world and all our woes, the shape of things to come. (Rhodes 1986)

1898-1964; Hungary and USA

Enrico Fermi

• Began to bombard elements with neutrons and transmutated them into different elements

• In USA built first reactor to create a sustained nuclear reaction

• Developed theory of β-decay

1901-1954; Italy and USA

Pile-1; University of Chicago 2 December 1942

Letter to FDR

The Manhattan Project

Little Boy, The Uranium Bomb

Fat Man, The Plutonium Bomb

At Trinity Test SiteLeft: J. Robert Oppenheimer, scientific leader of Los AlamosRight: Gen. Leslie Groves

Plutonium bred from U-238 in reactors at Hanford, WA

The Only Uses of Atomic Devices in War

Little Boy at Hiroshima, 6 August 1945Yield: 16 ktCasualties: >90,000 dead

Fat Man at Nagasaki, 9 August 1945Yield: 21 ktCasualties: >60,000 dead

The Cold War and Destruction Unlimited

Edward Teller (1908-2003)Hungary and USA

10.4mtMIKE1952

W-88WarheadFor Trident II missiles475kt

Nuclear Energy

Accidents

TMI unit 2 28 March 1979 Chernobyl 26 April 1986

Fukushima-1 11 March 2011

Accelerators

Tevatron at Fermilab, near Chicago

Large Hadron Collider, CERN

Appearance of the spray of subatomic particles from a high energy collision

The Standard Model of Particle Physics

The Four Fundamental Forces of Nature

• Strong Nuclear Force: short range but very strong in attracting quarks (exchange of gluons changes color) within hadrons (e.g. protons and neutrons)

• Weak Nuclear Force: short range and weaker than all forces except gravity. Exchange of W and Z bosons between quarks changes their flavor (e.g. U or D).

• Electromagnetic Force: long distance and obeys inverse square law. Photons carry force which is exchanged between leptons.

• Gravitational Force: long distance and obeys inverse square law. Gravitons attract all particles that have mass.

Strong Nuclear Force

• ~100X stronger than EM force

• Binds hadrons (protons & neutrons) in nucleus

• Binds quarks to form hadrons

u = +2/3d = -1/3

Gauge particle is gluon

Weak Nuclear Force

W & Z bosons ~100 times as massive as a proton

Reprise β-decay with quarks and W- bosonResults of weak nuclear interactions

• Radioactive decay• Beta decay• Burning of sun• Initiating the process of hydrogen fusion in

stars.• Production of deuterium• Formation of other heavy nuclei• Radiocarbon dating

Antimatter

• Every particle has an anti-particle (e.g. electron vs. positron)

• Electron-positron annihilation yields gamma radiation