432 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



when radiation is incident on a detector. Usage varies considerably, so that it is well 

 to make sure which factors (window transmission, sensitive volume, energy depend- 

 ence, etc.) are included in a given case. 



electrometer: An instrument for measuring charge, usually by mechanical forces 

 exerted on a charged electrode in an electric field. 



electron : An elementary particle of rest mass m, equal to 9.107 X IQ-^s g and charge 

 equal to 4.802 X 10"'" statcoul. Its charge may be either positive or negative. The 

 positive electron is usually called a positron; the negative electron is sometimes called 

 the negatron. Most frequently the term electron means negatron. The negative elec- 

 tron is a constituent of all atoms. In a neutral atom the number of electrons is equal 

 to the atomic number Z. 



electron volt (ev) : A unit of energy equal to the work done in transferring one elec- 

 tron against a potential difference of 1 volt. 



electroscope: An instrument for detecting an electric charge by means of the 

 mechanical forces exerted between electrically charged bodies. 



electrostatic generator : A high-voltage generator in which the potential is produced 

 by the work done in the mechanical transport of electrical charges. See Van de Graaff 

 generator. 



element: (a) A substance all of whose atoms have the same atomic number {h) 

 A naturally occurring mixture of isotopes, (c) A class of atom having a particular 

 atomic number as the distinguishing characteristic. 



elementary particle: A term loosely applied today to the electron; proton; neutron; 

 positron ; positive and negative mu mesons ; positive, negative, and neutral pi mesons ; 

 positive, negative, and neutral V particles; neutrino; and some similar entities, the 

 existence of which is speculative or which have not yet been thoroughly characterized 

 by experiment. 



^ emulsion, nuclear : A photographic emulsion specially designed to permit observa- 

 tion of the individual tracks of ionizing particles. 



endosteum : The ti.ssue lining the internal cavity of the bone. 



endothelium : The layer of simple squamous cells lining the inner surface of the cir- 

 culatory organs and certain other closed body cavities. 



enriched material : Material in which the amount of one or more isotopes of a con- 

 stituent has been increased. 



epilation (depilation) : The temporary or permanent removal of hair. 



epiphysis : A part or process of a bone which ossifies separately and subsequently 

 becomes joined to the main part of the bone. Growth persists until the epiphyses are 

 closed. 



epithelium : The purely cellular, nonvascular layer covering all the free surfaces of 

 the Ixxly, cutaneous, mucous, and serous, including the glands and other structures 

 derived therefrom. 



erythema : An abnormal redness of the skin, due to distention of the capillaries with 

 blood. It can be caused by many different agents, e.g., heat, certain drugs, ultravio- 

 let rays, and ionizing radiations. 



erythrocyte : A red blood corpuscle. 



exchange reaction : See isotopic exchange. 



exit dose : Dose of radiation at surface of body opposite to that on which the beam 

 is incident. 



extrapolation ionization chamber: An ionization chamber with electrodes whose 

 spacing can be adjusted and accurately determined to permit extrapolation of its read- 

 ings to zero chamber volume. 



film badge : An appropriately packaged photographic film for detecting radiation 



