653 



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



Alpha-particle. — A helium atom, stripped of its outer electrons, that is expelled from 

 a radioactive material. 



Artificial disintegration. — Breaking down of an atom by a controlled experiment. 



Atom. — The smallest particle of any material substance that can exist as such. 



Atomic bomb. — A bomb depending upon atomic energy. (U or Pu fission.) 



Atomic energy. — Energy due to some breaking down of an atom. 



Atomic mass unit, amu.-(l) The mass of a unit atomic weight (see Dalton). (2) An 

 energy unit equal to the mass energy (mc 2 ) of a unit atomic mass (1/16 mass O 10 ) = 

 1.4921 X.IO" 4 ergs = 931.3 Mev. 



Atomic number. — The value of the positive charge of the atom. This determines the 

 chemical properties. 



Atomic weight. — Chemical : The relative weight of an atom taking the oxygen atom, 

 found in nature, as having a weight of 16. Physical : The relative weight of an atom taking 

 the oxygen isotope 16 as having a weight of 16. This makes the ratio of physical to chemi- 

 cal scale = 1.000272 ± .000005. 



Barn. — Unit area cross section of nucleus =r 10~ 24 cm\ 



Baryton. — See Table 720. See meson. 



Beta-ray. — An electron expelled from a radioactive material. 



Betatron.— See Table 718. 



Binding energy. — The energy due to the packing of an element assuming that the ele- 

 ment is made up of protons, electrons, and neutrons. 



Bursts (cosmic ray). — A very great output of particles due to a cosmic-ray encounter 

 with an atom. 



Cathode rays. — Electrons that are driven from the negative electrode (the cathode) of 

 a discharge tube. (See Table 758.) 



Chain reaction. — A reaction in which one or more of the products of the reaction 

 keeps it going, i.e., such as the fission of 92 U 235 . 



Compton effect. — The change in wavelength due to the scattering of radiation by a 

 material substance. 



Cosmic rays. — A radiation that falls upon the outer atmosphere, generally thought to 

 come from outer space. (See page 710.) 



Cosmos. — The entire universe. 



Cross section, a. — The proportionality constant between the beam intensity and the 

 number of particles, considered, that strike a target. It has the dimension of an area. See 

 Barn. 



Cyclotron. — See Table 718. 



De Broglie wavelength. — For a particle of mass m and velocity v, the De Broglie 

 wavelength X = h/mv. 



Delta-rays. — Electrons that are emitted from certain materials due to a-ray bombard- 

 ment. 



Deuterium. — See deuteron. 



Deuteron. — This isotope of hydrogen that has twice the atomic weight of the proton. 



Electron ±. — The smallest particle of electricity that can exist. 

 Positron, + electron. (Charge + 4.8025 X 10- lo esu.) 

 Negatron, — electron. (Charge — 4.8025 X 10' n esu.) 



Electron shell. — The shell that is used to describe the location of the outer electrons 

 of an atom. These are K,L,M,N,0. (See Table 658.) 



Energy units. — See Table 654. Erg : 



ev — The energy equal to that of an electron moving under an emf of 1 volt = 1.602 X 



lO" 12 ergs. 

 Mev — The energy equal to that of an electron moving under an emf of 10° volts, 

 amu — The mass-energy of a unit mass of atomic weight = 1.492 X 10" 3 ergs. 

 Mass unit— Energy value of one gram = 8.987 X 10 20 ergs. 



Fission. — The breaking down of a heavy atom into two parts of about equal mass. 

 (See page 706.) 



Gamma-rays. — Radiation of very short wavelength that results from some radioactive 

 breakdown. (See Tables 747-752.) 



H-rays. — Hydrogen atoms that are emitted from certain materials due to a-ray bom- 

 bardment. 



h. — Planck constant. See quantum. 



h or h = h/2TT. 



Isobar. — One of two or more nuclei that have the same weight but different atomic 

 numbers. 



(continued) 



SMITHSONIAN PHYSICAL TABLES 



