Elementary Particles - Boston University: Physics Department

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Elementary Particles
A Brief History to 1932
By Rick Dower
Roxbury Latin School
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.
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.
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.
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 form’d Matter in solid, massy, hard,
impenetrable, moveable Particles” (Opticks, 1730).
John Dalton
(1766 - 1844)
• Proposed the idea of the “chemical atom” in
New System of Chemical Philosophy (18081827) and gave first set of relative atomic
weights based on combining proportions of
elements in compounds.
Kinetic Theory of Gases
• First proposed by Daniel Bernouli (1738).
Rediscovered by John Herapath (1820) but
rejected (opposed to Newton’s static
model). Revived and refined by Rudolph
Clausius (1856) James Clerk Maxwell
(1859) and Ludwig Boltzmann. Atomic size
and mass calculated from measurements.
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).
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.
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.
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 = Planck’s constant.
Photons
• Robert Millikan’s experiments (1914-1916)
verified Einstein’s photoelectric theory.
• Arthur Holly Compton’s measurements of
x-ray scattering by electrons in metals
(1922-23) verified existence of photons.
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.
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.
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 Hess’s observations in the 1920s
and called them cosmic rays.
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 Dirac’s theory of electrons.
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.

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