664 

 TABLE 720.— SOME FUNDAMENTAL PARTICLES OF MODERN PHYSICS* 



Electron. — A negatively charged stable particle. The negative charge surrounding the 

 nuclei in all neutral atoms consists entirely of electrons. 



Positron. — A particle of the same mass, Me, as an ordinary electron. It has a positive 

 electrical charge of exactly the same amount as that of an ordinary electron (which is 

 sometimes called negatron). Positrons are created either by the radioactive decay of cer- 

 tain unstable nuclei or, together with a negatron, in a collision between an energetic (more 

 than one Mev) photon and an electrically charged particle (or another photon). A positron 

 does not decay spontaneously but on passing through matter it sooner or later collides 

 with an ordinary electron and in this collision the positron-negatron pair is annihilated. 

 The rest energy of the two particles, which is given by Einstein's relation E = mc ! 

 and amounts to 1.0216 Mev altogether, is converted into electromagnetic radiation in the 

 form of one or more photons. 



Proton. — This is the nucleus of an ordinary hydrogen atom. It has a positive charge 

 of exactly the same amount as that of an electron and a mass Mp which is 1837 times 

 larger than M e and is a stable particle. No experimental evidence of negative protons has 

 been found as yet. 



Neutron. — An electrically neutral particle of mass only very slightly greater (by a 

 factor of 1.0013) than that of the proton. Neutrons are produced in various nuclear re- 

 actions. In the free state a neutron is unstable, decaying spontaneously with a half-life of 

 about 10 minutes into a proton, and electron and (presumably) a neutrino. When passing 

 through matter a neutron can also be captured by atomic nuclei. 

 Deuteron.t — Nucleus of H 2 . 

 a-particle.t — Nucleus of He 4 . 



Meson. — Two types of particles of mass intermediate between that of the electron and 

 proton have been discovered in cosmic radiation and in the laboratory. The one particle 

 with mass about 215 m e is called M-meson, the other with about 280 m, 7r-meson. Mesons 

 of both positive and negative charge have been found and there is now reasonably good 

 evidence for neutral mesons. Both types of mesons decay spontaneously. Some evidence 

 exists for a meson of mass about 1000 m«. 



Neutrino. — An electrically neutral particle of mass very much smaller than that of the 

 electron and possibly zero. There exists as yet no direct experimental evidence for the 

 existence of neutrinos since they interact extremely weakly with matter (e.g., only a small 

 fraction of neutrinos passing through a body of solar mass would be absorbed). There 

 exist, however, extensive measurements on the momentum and energy of the parent and 

 daughter nucleus and of the emitted )3-particle in a /3-decay process. These measurements 

 show that energy and momentum (as well as spin and charge) in such a process can be 

 conserved if, and only if, a light neutral particle such as the neutrino is emitted together 

 with the /3-particle. 



Photon. — A photon (or 7-ray) is a quantum of electromagnetic radiation which has zero 

 rest mass and an energy of h (Planck's constant) times the frequency of the radiation. 

 Photons are generated in collisions between nuclei or electrons and in any other process in 

 which an electrically charged particle changes its momentum. Conversely photons can be 

 absorbed (i.e., annihilated) by any charged particle. 



There have been some reports of other particles than those listed above. 



• Prepared by E. E. Salpeter and W. K. H. Wolfgang. 

 quantum), hv: y; value (\ — .6n) = 3.310X10" 12 ergs. 



SMITHSONIAN PHYSICAL TABLES 



t Not fundamental. t The photon (radiation 



