Important Dates in the Development of Modern Atomic and Radiation Physics

1810 Dalton's Atomic theory

Elements are made up of indivisible tiny particles called atoms that cannot be created or destroyed. The atoms in a given element are identical but are distinctly different from atoms in all other elements.

1859 Bunsen and Kirchoff originate spectroscopy

Study of the depedence of physical quantities on frequency. Many types, including electromagnetic (studies intensity of emitted er and amouunt of absorbed er), acoustic, electric energy loss and Auger electron (study of kinetic energy of electrons), mass (study of mass to charge ratios in molecules and atoms), as well as others.

1869 Mendeleev's periodic system of elements

Grouped elements based on chemical properties (not mass.) Left gaps which allowed for yet undiscovered elements.

1873 Maxwell's theory of electromagnetic radiation

Maxwell's equations predicted waves of oscillating electric and magnetic waves through empty space at a speed close to that of light.

1888 Hertz generates and detects electromagnetic waves

Measured Maxwell's waves and demonstrated that the velocity of the radio waves was equal to that of the speed of light.

1895 Lorentz theory of the electron

Lorentz used Maxwell's equations as well as an expression for the force which a charge particle experiences due to E and B fields to construct a microscopic theory (description of matter in terms of atomic fragments, ions, and electrons.) Went beyond in the hopes to say e- is a complete electromagnetic object by saying mass was mass equivalent of its electromagnetic energy contents and inertias was due to its own electromagnetic field.

1895 Roentgen discovers X rays

Studying Crooke's tubes (vaccum tube in which an electric current can be passed from one electrode to another) when he noticed that a small screen in the room was flourescing. Realized this was a new agent, which he named X rays. X rays are not deflected by magnetic fields. X rays ionize air molecules which essentially neutralizes them.

1896 Becquerel discovers radioactivity

Studying fluorescent and phosphorescent materials to see if they were Roentgen's radiation. When studying uranium realized that it gave off radiation regardless of whether or not it phosphoresced.

1897 Thomson measures charge-to-mass ratio of cathode rays (electrons)

Applied an electric field to cathode rays to determine that they were negatively charged. Found that they had momentum (mass*velocity) and therefore mass. Did several experiments to calculate the ratio of mass/charge and velocity by measuring charge, kinetic energy, and current by manipulating the following occasions:
Ne = Q (charge)
(1/2)Nmv2 = W(kinetic energy)
mv/e = Hr = I where r is the radius of curvature of the path of these rays in a uniform magnetic field H
1/2)(m/e)v2 = W/Q
v = 2W/QI
m/e = I2Q/2W

1898 Curies isolate polonium and radium

Marie was studying uranium ores which she found were much more radioactive than uranium itself. She concluded that this might be due undiscovered radioactive elements, which Pierre would help her to isolate

1899 Rutherford finds two kinds of radiation, which he names "alpha" and "beta," emitted from uranium

Working with uranium radiation he reasoned that if the radiation was absorbed homogeminiousily it would only be made up of one type. However there was one type that was easily absorbed (alpha) and another one that had a greater impact(beta.)

1900 Villard discovers gamma rays, emitted from radium

While investigating a radioactive material, noticed that the rays emitted were not affected by electric or magnetic fields and also that they had a much greater penetration than beta rays (what Villard thought he was studying.)

1900 Thomson's "plum pudding" model of the atom

Theory that atom is composed of electrons surrounded by a sea of positive charge.

1900 Planck's constant, h-6.63to^-34 Js*

Planck realized that energy was quantizied and that the ratio between energy levels was h =6.626 0693(11) times10^{-34} J*s =4.135 66743(35)times10^{-15} eV*s. Reduced Planck's constant/Dirac's constant is equal to h/(2*pi)=1.05457168(18)times10^{-34} J*s = 6.5821191(56) times 10^{-16}eV*s. Also explained black body (absorbs all light and reflects very little of it)radiation at this point. It had been stated that with classical mechanics a person would produce uv, x-rays, so Planck realized the need for quanitzation.

1901 First Nobel prize in physics awarded to Roentgen

For the discovery of x rays.

1902 Curies obtain 0.1 g pure RaCl2 from several tons of pitchblend

For her doctorate Marie isolate a decigram of Radium Chloride from several tons of uranitite. From this she was able to determine the atomic mass of Radium to be 225 grams.

1905 Einstein's special theory of relativity (E=mc^2)

Principle of relativity
No matter an observer's position or velocity in the universe, all physical laws will appear constant. Observer cannot then determine either own absolute velocity or the direction of his travel in space.

Called "special" because applies Galilieo's theory of relativity to flat space time where the effects of gravity can be ignored. So physical laws same in all frames of reference. Also speed of light is constant.

1905 Einstein's explanation of photoelectric effect, introducing light quanta (photons of energy E=hv)

Explained the photoelectric effect mathematically. When er hits a metal surface sometimes electrons bounce off and other times they do not. Einstein realized that this has to do with the frequency of the photons. If the frequency of the photons is below the threshold frequency the electrons will not have enough energy to leave the metal. However, if the frequency of the photons is above the threshold frequency they will have enough energy not only to leave but also some left over kinetic energy to leave with a velocity.

1909 Millikan's oil drop experiment, yielding precise value of electronic charge, e=1.6010^-19C*

Balanced oil droplets between two electrodes by electrical and gravitational fields. Since he knew the value of the electrical field he could determine the charge on the droplets. Repeated this experiment several times to find out that all of his charges where multiples of one number, which he realized was e.

1910 Soddy establishes existence of isotopes

While working with radioactivity he realized that some of the disintegrates to had similair but different atomic masses,but the exact same chemical properties. He thus concluded that they were the same element, but isotopes of one another. Would later win the nobel prize for this work.

1911 Rutherford discovers atomic nucleus

Performed the gold experiment in which he fired alpha particles at gold. Many of them went through with only small deflections. However, a few of them had very large deflections and some of those even fired backwards. Thus disproved the plum pudding model. Rutherford concluded that the atom was made of mostly empty space with a small positively charged nucleus at the center and negatively charged electrons surrounding it.

1911 Wilson cloud chamber

Used to detect ionizing radiation. Consists of an enclused environment with supercooled and supersaturated water vapor. When an alpha or beta particle interacts with this mixture it will ionize and a mist will form around it because it is at the point of condensation. Thus the particles will leave a trail through the enclused area.

1912 von Laue demonstrates interference (wave nature) of X rays

First to suggest that crystals could be used to determine the diffraction of X rays. Then carried out the experiment to realize that x rays have a wavelike nature.

1912 Hess discovers cosmic rays

Prior to Hess assumed that all radiation occurs from the ground or radioactive gases (from the ground.) Took up a balloon and found that radiation increased as you go closer to the atmosphere. Concluded that some type of strong radiation must come from space and hit the atmosphere.

1913 Bohr's theory of the H atom

Orbiting electrons are in orbits with discrete quantized energies.Laws of classical mechanics do not apply when an electron moves from one energy level to another. when an electron moves from one orbital to another the excess energy is carried off by a photon.L(angular momentum)=n*h/(2*pi.) Seen as a semiclassica (mix of classical physics and qm) or solar system like model. Only worked for atoms with one electron, far too simple.

1914 Franck-Hertz experiment demonstrates discrete atomic energy levels in collisions with electrons.

In their experiment electrons were accelerated towards a glass envelope full of mercury vapor. Past this there was a negative voltage that would assist in measuring voltage. They noticed that current would increase and then all of the sudden start to decrease. They realized that when the current was increasing the e-s were participating in elastic collisions (e-s retain most of their energy) and when it was near zero at inelastic collisions (energy of electrons transferred to mercury to get to an excited state.

1915 Einstein's General relativity

Unifies special relativity with Newton's law of universal gravitation by saying that gravity is not a force but a manifestation of curved space and time. Spacetimes is 4D where the four dimensions are mass, energy, and momentum.

1917 Rutherford produces first artificial nuclear transformation

Bombarded nitrogen nuclei with alpha particles. Produced oxygen where there was none before. First successful alchemist. :-V

1922 Compton effect

Thomson Scattering
This cannot explain any shift in the waves wavelength though
When a wave hits a particle, the waves electric and magnetic componenets will accelerate the particle. This will cause the particle to emit radiation and thus scatter the incident wave.

However the wave is found to have a longer wavelength and less energy. If we assume that the wave is made of photons which have matter characteristics, energy and momentum, and wave characteristics, wave length and frequency, then when the photons collide with the electrons they transfer some of their energy causing the electrons to recoil.

1924 de Broglie particle wavelength, lambda=h/momentum

All matter has a wavelength nature (momentum is characteristic of particles and wavelength waves.)

1924 Uhlenbeck and Goudsmit ascribe electron with intrinsic spin h/2

When atoms weres placed in a strong magnetic field it was found that one color of light would cause the spectral lines to split into two. The idea that the atom was spinning was first proposed by Kronig but quashed by Pauli. However, Uhlenbeck and Goudsmit are credited with the discovery.

1925 Pauli exclusion principle

No two electrons in at atom can occupy the same quantum state. Therefore they can not have the same four quantum numbers.

1925 Heisenberg's first paper on quantum mechanics

Said that the failure of Bohr's model was that it was based on quantities that could not be observed. He wanted to replace this model with one in which the variables were observed using the variables of frequency and intensity. His first paper introduced matrix mechanics, a necessary fix because xp was not equal to px. The results from matrix mechanics agreed with Bohr's hydrogen model and later experiments.

1926 Schroedinger's wave mechanics

Began with de Brolgie's hypothesis and introduced the wave function, psi. A linear differential equation, second ordiner in spatial coordinates and first order in regards to time. The linearity allows for the superposition of solutions to account for interfence effects and also the construction of wave packets to represent particles. Changed the atomic model from Bohr's to the idea of an electron being an oscillating cloud that oscillates in such a way to form a standing wave around the nucleus.

1927 Heisenberg Uncertainity Principle

In classical mechanics if one knows an objects position and velocity as well as the force being applied to it, everything is known about the molecule. However, in the atomic world with very short wavelength and high momentums it is impossible to know both the particle's positiion and momentum at the same time.

1927 Mueller discovers that ionizing radiation produces genetic mutations

Exposed fruit flies to radiation and found that it increases mutations in the flies.

1927 Birth of quantum elecrodynamics, Dirac's paper on "The Quantum Theory of the Emission and Absorption of Radiation

The Quantum Mechanics of Heisenberg and Schrodinger did not work with the theory of principle of relativity. It became apparent that quantum mechanics was self contradictory, however, this could be fixed by quantum field theory. Quantum field should be regarded as a collection of photons. Emission and absorption of radiation cooresponds to the creation and destruction of photons.

1927 Davisson and Germer confirm particles exhibit wavelike properties.

Observed the diffraction of a stream of e- similair to that of a diffraction of a beam of light.

1928 Dirac's relativistic wave equation of the electron

Proposed a wave equation that was linear in both space and time variables. Showed new equation contained intrinsic angular momentum for electron. Fine structure of hydrogen atom came out in the model. Implied existence of positive electron. Quantum field theory can be used to describe all fundamental particles. When a quantum field becomes excited this cooresponds to particle-antiparticle pairs being created from radiation, as the amount of pairs decrease the excited field will also decrease.

1930 Bethe quantum-mechanic stopping-power theory

Stopping power of collisions can be determined by calculating average energy ransferred to the randomly distributed electrons. Bethe came up iwth a formula that gies the collisional stopping power for relativistic electrons.

1930 Lawrence invents cyclotron

Accelerates charged particles using high frequncy and alternating voltage. The magnetic field (almost perpendicular) causes the particles to go around in a circle. As more energy is applied the particles spiral outward. Lots of advantages over a linear accelerator, more compact so particles can reach higher speeds.

1932 Anderson discovers positron

The anti-particle of electrons. Have a charge of +1, spin 1/2 and the same mass as an electron. Created through radioactive decay. First evidence of anitmatter.Discovered by passing cosmic rays through gas chamber and magnet surrounded lead plate which bends particles according to charge.

1932 Chadwick discovers neutron

Electrically neutral particles with about the same mass as protons. Orginally observed as a type of high penetrating radiation when alpha particles emitted from polonium hit some of the lighter elements. Free neutrons are unstable and thus only exist from nuclear reaction, high energy reactions, and nuclear disintegrations.

1932 Splitting of the atom by Cockcroft and Walton

1933 Szilard nuclear chain reactions

Suggested that if neutron driven process releases more neutrons than number required to start it a nuclear chain reaction will begin. Then experimented to realized that each rcollision on average releases two neutrons.
Bombarded lithium atom with high energy protons and caused it to split into helium and other chemical elements.

1934 Joliot-Curie and Joliot produce artificial radioisotopes

Changed aluminum into a previousily unknown form of silicon (an isotope.)

1935 Yukawa predicts the existences of mesons, responsible for short-range nuclear force

Believed that electromagnetic force was infinite because the exchange particles were massless. Though short range strong force was dueo to the exchange of a heavy particle he named a meson.

1936 Gray's formalization of Bragg-Gray principle

The ionization produced from a gas-filled cavity medium is proprotional to the energy abosrbed in the medium.When the cavity is small enough that it doesn't change number or distribution of the electrons in the medium without the caivty Energy absorbed= (Ionization per unit volume produced in gas)(Average energy lost by secondary electrons per pair of ions formed in the gas)(ration of stopping power of medium and gas for secondary electrons)

1937 Muons found in cosmic radiation

Mass closed ot the one predicted by Yukawa found but turns out not to interact in the strong interaction.

1938 Hahn and Strassman observe nuclear fission

Proved that an isotope of barium was produced after Urnaium was bombared by a neutron.

1942 First nuclear chain reaction

Chicago Pile-1, Dec 2, first nuclear reactor (CP-1), Fermi had previousily realized that when a nucleus might split neutrons might become free, this could thus produce more fission reactions.

1942-1946 Manahatted Project

Formally known as Manhattan Engineering District (MED)
Under U.S. Army Corpes of Engineers under General Groves with scientific research by Oppenheimer
Three successful nuclear weapons: Trinity, Little Boy, and Fat Man

1945 Trinity Test

July 16, New Mexico, plutonium.

1945 First atomic bomb

August 6, Little Boy, Uranium, Hiroshima.

1945 First plutonium bomb

August 9, Fat Man, Plutonium, Nagaski.

1947 Discovery of meson

Lattes, Muirhead, Occhialini, and Powell while conducting a high altitude experiment discover the pion, which turned out to be a meson.

1948 Transistor invented by Shockley, Bardeen, and Brattain

Allows variable current from an external source to flow from one terminal to another by either which one is smaller voltage or an external source. A semiconductor that can be used for amplification, switching, voltage stabilitzation, signal modulation, as well as other things.

1952 Explosion of first fusion device (hydrogen bomb)

Fission bomb is exploded next to fusion chamber. The radiation from the fission reaction (X and gamma rays) compress and heat the materials needed to start a reaction. Interestingly the neutrons emitted from fusion can incude a final fission stage with depleted uranium.

1953 First maser

Produces coherent electromagnetic waves. "microwave amplification by stimulateed emission of radation."

1956 Discovery of nonconservation of parity by Lee and Yang

Before this parity was a well established principle of physics. Parity involves transforming the algebraic signs of the coordinate system (x becomes -x, y becomes - y, and z becomes -z.) So a system goes from being right handed to left handed or vice versa. Two applications of parity return the system back to its original state.Electromagnetic and strong interactions are invariant under parity transformation. Lee and Yang predicted the nonconservation of parity in the weak intereaction.

1957 Wu proves nonconservation of parity in weak interactions

Studied beta decay of Cobalt-60 by lowering the temperature to about .01 K. He polarized the nuclear spins along the direction of the applied magnetic field. Then then direction of the electrons emitted were studied, if parity was conserved equal numbers should be emitted in both directions. However, it was determines that more electrons were emitted in the direction opposite to that of the magnetic field and opposite to the nuclear spin. This experiment and subsequent ones demonstrated that a neutroni has an intrinisic angular momentum pointed in a direction opposite of its velocity (thus making it a left handed particle.)

1958 Discovery of Van Allen Radiation Belts

Torus of energetic charged particles (plasma) around the Earth. Trapped by Earth's magnetic field. When belts become to full particles strike the upper atmosphere and fluoresce which causes the polar aurora. Confirmed under Explorer III missions. Think of as two belts, inner radiation belt (mostly protons) and outer radiation belt (mostly electrons.)

1960 First successful laser

"Light amplification by Stimulated Emission of Radiation." Optical source that emits photons in a coherent beam.

1964 Gell-Mann and Zweig independently introduce quark model

Classification scheme of hadrons in terms of their valence elecrons (quarks and anti-quarks) because they give rise tot he quantum numbers of the hadrons. Quantum numbers are of two kinds, one Poincare symmetry and the other isospin. Quark model uses standard assignment of quantum numbers to quarks spin 1/2, baryon number 1/3, electric charge 2/3 for u quark and --1/3 for d and s. Antiquarks have opposite quantumnumbers.

1967 Salam, Weinberg, Glashow indepedently propose theories that unify weak and electromagnetic interactions

In particle physics the electromagnetic and weak interactions are modeled as two different aspects of the same force, the electroweak force.

1972 First beam of 200-GeV protons at Fermilab

1973 First step in proving existence of electroweak force

Discovery of neutral currents in neutrino scattering by Gargamelle collaboration.

1978 Penzias and Wilson awarded Nobel Prize for 1965 discovery of 2.7 K microwave radiation permeating space, presumably remnant of "big bang" some 10-20 billion years ago

1981 270 GeV proton-antiproton colliding-beam experiment at European laboratory for Particle Physics (CERN); 540 GeV center-of-mass energy equivalent to laboratory energy fo 150,000 GeV