Some Members of the Atomic Family
 
    The numbers in brackets after the name denote first, the electric charge and second, the mass. The charge on an electron is counted as -1 unit and the electron mass as +1 unit. Thus (+1, 207) means the particle has a positive charge of 1 unit and a mass 207 times that of the electron.
    The mass energy of an electron is 0.51 MeV, hence conversion to mass energies can be made by multiplying the given mass by 0.51. Thus the muon has a mass energy of 106 MeV. Note the letter M is used to denote quantities of millions and the letter G to denote quantities of thousand of million.
 
Photon (0, 0)
  		A quantum of electromagnetic radiation, e.g. light, X-Rays, gamma rays. The concept was introduced by M. Planck in 1900 when he described the emission of light as taking place in "packets" rather than in a steady stream. The energy of a Photon is proportional to the frequency of the radiation and inversely proportional to the wavelength.
 
Leptons
Electron (-1, 1)
  		Discovered by J. J. Thomson in 1897. The number of orbital electrons in an atom determines its chemical properties. Actual rest mass = 9.1 x 10-28g. Emitted as Beta rays by some radioactive nuclei. A stable particle.
 
Positron (+1, 1)
  		Positive counterpart or, "anti-particle", to the electron. Predicted theoretically by P. A. Dirac in 1928 and first discovered in cosmic rays by C. D. Anderson in 1932. Emitted as beta rays by some radioactive nuclei. When positrons and electrons collide they usually annihilate each other and turn into gamma rays: consequentially, positrons only last about 10-10 seconds within ordinary matter, but are stable in isolation.
 
Neutrino (0, 0) &
Anti-neutrino (0, 0)
  		These particles travel at the speed of light and are distinguished from one another by the relation of their spin to their direction of motion. A neutrino is emitted with the positron during positive Beta-decay; and an anti-neutrino with the electron during negative beta-decay. Their interaction with matter is extremely slight. First postulated by Pault in 1933 and detected in 1936. pi-meson decay also produces neutrinos and anti-neutrinos but in 1962 it was proved experimentally that these are a different species. Thus there are two kinds of neutrino each with an anti-neutrino. All these particles are distinguished from the photons by having different spin.
 
Muon (+1, 207)
  		Similar to, but heavier than, the electron and positron; disintegrates into electron (or positron if positive) + neutrino + anti-neutrino.
 
Mesons
Pion (+1, 273)
or (0, 264)
  		The pi-meson. Charged pions decay either into muons and neutrinos or into electrons and neutrinos. Neutral pions decay into gamma rays, into "positron-electron pairs", or both. Pions are intimately connected with nuclear forces, i.e., with the "strong" interaction.
 
Kaon (+1, 966)
or (0, 974)
  		The K-mesons. These decay in many different ways producing other mesons, electrons and neutrinos.
 
Baryons
Proton (+1, 1836.1)
  		The positively-charged constituent of nuclei; the hydrogen nucleus is one proton. Fast moving protons occur in cosmic rays. Does not spontaneously disintergrate.
 
Anti-Proton
(-1, 1836.1)
  		Negative anti-particle of the proton. Its existence was long suspected. Artificially produced and detected for the first time in 1955. Will react with the proton to produce pions and kaons.
 
Neutron (0, 1838.6)
  		Discovered by J. Chadwick in 1932. The neutral constituent of nuclei. When free it spontaneously disintegrates into a proton, an electron and an anti-neutrino, after an average lifetime of about 18 minutes. Passes through matter much more easily than charged particles.
 
Anti-neutron
(0, 1838.6) 		The anti-particle of the neutron from which it is distinguished by properties connected with its magnetic moment and spin. Will react with neutron to produce pions and kaons.
 
Lambda Particle
(0, 2183)
  		Discovered in 1947. Decays into proton plus pion.
 
Sigma Particle
(0, about 2330)
or (+1, about 2330)
  		Various modes of disintegration, producing neutrons, protons, mesons and lambda particles.
 
Omega Particle
(+1, 3272)
  		Predicted by scientists and duly discovered at Brookhaven, New York in 1964.
 
Psi Particle
(0, about 6100)
  		Discovered independently by two laboratories in the USA in 1974. Still under intense study.
 

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