V" 



654 



TABLE 716.— DEFINITIONS OF SOME TERMS USED IN NUCLEAR PHYSICS 



(concluded) 



Isomer. — As applied to an isotope, it is one of two or more that have the same atomic 

 number and weight but different radioactive properties. 



Isotope. — One of two or more atomic nuclei that differ in weight but have the same 

 atomic number, thus the same chemical characteristics. 



eh 

 Magnetic moment. — Nuclear unit of = -; — rj- = 5.05 X 10 24 erg/oersted where M = 



4rrMc 

 mass of proton. 



eh 

 Magneton (Bohr). — The magnetic moment of the electron = ^F^ — =9.27X10"" 



MtZirc 



erg/oersted. 



Mass-energy ratio. — The relativistic relation between mass and energy, i.e., E = mc 2 . 



Mass, rest. — The mass of a particle M when at rest. See Table 714. 



Mass-velocity ratio. — The variation of mass with velocity. v= velocity, then 



Mo 

 M v = — =^= , c — velocity of light. (See Table 714.) 



c 2 



Meson (Mesotron). — See Table 720. 



Maximum velocity. — The highest velocity for any material substance, i.e., the velocity 

 of light. 



Mev. — A unit of energy ; an electron moving under an emf of 10 6 v. (1.603 X 10~ 6 ergs). 

 See energy units (Table 654). 



Molecule. — An aggregate of two or more atoms of a substance that exists as a unit. 



Momentum, angular of nucleus, measured in units U — fi = h/2ir. 



Negatron. — See negative electron. (Sometimes spelled negaton.) 



Neutrino. — See Table 720. 



Neutron. — A neutral particle with a mass about the same as the proton. See Table 720. 



Nucleon. — General name for protons and neutrons. 



Nucleus. — The central part of an atom, i.e., what is left of an atom after all the outer 

 electrons are stripped off. 



Packing fraction. — Related to the mass lost when the atom was formed = — x —\ 



/I 



where M is the atomic weight of the atom and A the atomic number. 



Photon. — The quantum of radiation = hv. 



Proton. — The nucleus of the smallest unit mass, the smallest isotope of the hydrogen 

 atom. 



Positron. — See electron. (Sometimes written positon.) 



Quantum = hv, a so-called atom of energy, h = Planck constant. See photon. 



Radioactivity. — Natural breakdown of atoms. (See page 672.) 



Range of a particle. — The distance it can move through different media. 



Rest mass. — The mass of any particle at rest. 



Shower. — (Cosmic rays.) See Bursts. Showers may extend a very great distance, i.e., 

 several hundred meters, and have about 10 15 ev energy. 



Spin. — Unit of nuclear spin = H = fi = h/2w. 



Synchrotron. — See Table 718. 



Tritium. — See Triton. 



Triton. — The isotpe of hydrogen that has three times the atomic weight of the proton. 



Ultimate particle. — See Table 720. 



Valence electrons. — The electrons of an atom, in the outer shell that determines its 

 chemical valency. 



Van de Graaff generator. — See Table 718. 



Volt-electron, ve. — A unit of energy equal to that of an electron moving under an 

 e:nf of 1 volt = 1.602 X 10" 12 ergs. 



X-rays. — A radiation of very short wavelengths that results when an electron is stopped 

 (or started) very quickly, as when striking a metal target. (See page 692.) 



SMITHSONIAN PHYSICAL TABLES 



