Elementary Particles

1
Elementary Particles

• A Brief History to 1932
• By Rick Dower
• Roxbury Latin School

2
Democritus(late 5th Century B.C.)

• Supposed that the cosmos consisted of atoms and
  the void , i.e. very small indivisible particle
  and empty space.

3
Aristotle(384 322 B.C.)

• Opposed atomism. Supposed the cosmos consisted of
  a plenum of infinitely divisible particles
  Earth, Water, Air, Fire on Earth and Aether in
  the heavens.

4
Rene Descartes(1596 - 1650)

• Followed Aristotle in believing the Cosmos is
  full of invisible particles. Supposed those
  particles whirling in vortices are responsible
  for gravity of Earth and planetary motion.

5
Isaac Newton(1642 - 1729)

• Argued from the orbital motion of planets and
  comets that space contains no aetherial medium.
  It seems probable to me that God in the
  Beginning formd Matter in solid, massy, hard,
  impenetrable, moveable Particles (Opticks,
  1730).

6
John Dalton(1766 - 1844)

• Proposed the idea of the chemical atom in New
  System of Chemical Philosophy (1808-1827) and
  gave first set of relative atomic weights based
  on combining proportions of elements in compounds.

7
Kinetic Theory of Gases

• First proposed by Daniel Bernouli (1738).
  Rediscovered by John Herapath (1820) but rejected
  (opposed to Newtons static model). Revived and
  refined by Rudolph Clausius (1856) James Clerk
  Maxwell (1859) and Ludwig Boltzmann. Atomic size
  and mass calculated from measurements.

8
Joseph John Thomson(1856 - 1940)

• Experimentally determined the mass to charge
  ratio of cathode rays (electrons) and inferred
  that they are constituents of all chemical atoms.
  From an estimate of electron charge he determined
  an approximate mass of electron (about 1/2000 of
  H atom).

9
Pierre and Marie Curie(1859 - 1906) and (1867
1934)

• Investigated radioactive elements, discovered
  polonium and radium, showed exponential decay of
  radioactive substance, measured atomic mass of
  radium, showed vast energy release of radioactive
  process.

10
Ernest Rutherford(1871 1937)

• Named a and b radiation based on absorption of
  rays, explored radioactive decay series, proposed
  atomic transmutation in radioactive elements
  (1902), showed a particles are He nuclei,
  developed nuclear model of atom (1910) based on a
  particle scattering, demonstrated artificial
  transmutation (proton ejected when a collided
  with N nucleus, measured nuclear size.

11
Albert Einstein(1879 1955)

• His 1905 paper on the photoelectric effect
  proposed that light energy comes in quantum
  units (later called photons). The energy of a
  photon is E hf, where f frequency of the
  light and h Plancks constant.

12
Photons

• Robert Millikans experiments (1914-1916)
  verified Einsteins photoelectric theory.
• Arthur Holly Comptons measurements of x-ray
  scattering by electrons in metals (1922-23)
  verified existence of photons.

13
James Chadwick(1891 1974)

• In 1932 Chadwick showed that the penetrating
  radiation produced when beryllium was bombarded
  by fast a particles was an electrically neutral
  particle with mass about that of a proton, i.e. a
  neutron.

14
Neutrino

• In 1914 Chadwick observed that electrons emitted
  in b decay have a continuous energy spectrum up
  to some maximum rather than a discreet energy. In
  1930 Wolfgang Pauli proposed the emission of
  another particle (called a neutrino by Fermi)
  which shares energy with the b particle in the
  decay.

15
Cosmic Rays

• In 1910 Father Theodor Wulf measured more
  radiation at the top of the Eiffel Tower than
  expected. In 1911-1912 Victor Hess made
  observations in balloons up to 5350 m above sea
  level. He showed that radiation increased with
  altitude. Robert Millikan verified Hesss
  observations in the 1920s and called them cosmic
  rays.

16
Positrons

• In 1932 Carl Anderson showed an observation of a
  cosmic ray particle in a cloud chamber that had
  characteristics of an electron but curved the
  opposite way in a magnetic field. He discovered
  the positrons predicted by Diracs theory of
  electrons.

17
Accelerators

• John Cockcroft and Ernest Walton developed a
  voltage multiplying circuit to accelerate
  protons. In 1932 they disintegrated Li atoms into
  two a-particles as a result of bombardment by
  770 kV protons.
• Ernest Lawrence built his first successful
  cyclotron to provide acceleration in several
  small increments in 1930.