Tables 492-193. -jgy 



RONTGEN (X-RAYS) RAYS. ^ ' 



TABLE 492. — X-rays, General Properties. 



X-rays are produced whenever and wherever a cathode ray hits matter. They are invisible, 

 of the same nature as, and travel with the velocity of light, affect photographic plates, excite 

 phosphorescence, ionize gases and suffer deviation neither by magnetic nor electric fields as do 

 cathode rays. In an ordinary X-ray tube (vacuum order o.ooi to o.oi_ mm Hg) the cathode 

 (concave for focusing, generally of aluminum) rays are focused on an anticathode of high atomic 

 weight (W, Pt, high atomic weight, high melting point, low vapor pressure, to avoid sputtering, 

 high thermal conductivity to avoid heating). Depth to which cathode rays penetrate, order 

 of 0.2 X io~^ cm in Pb, 90,000 volts (Ham, 1910), 24 x io~* cm in Al, 22,000 volts (Warburg, 

 1915). Note: High speed H and He molecules (2 x lo^ cm/sec.) can penetrate o.ooi to 0.006 

 mm mica; He a particles (2 x 10^ cm/sec), 0.04 mm glass. 



The X-rays from an ordinary bulb consist of two main classes: 



Heterogeneous ("general," "independent") radiation, which depends solely on the speed of 

 the parent cathode rays. It is always present and its range of hardness (wave-lengths) depends 

 on the range of speeds of the cathode rays. Its energy is proportional to the 4th power of these 

 speeds. 



Homogeneous ("characteristic," "monochromatic") radiation {K, L, M, etc. radiations, 

 see Table 498 for wave-lengths), characteristic of the metal of the anticathode. Generated only 

 when cathode rays are sufficiently fast. There is a critical velocity for each characteristic 

 radiation from each material, proportional to the atomic weight of the anticathode. The critical 

 velocity for the K radiation is Vj^ = A-^ 10^, when A is the atomic weight of the radiator (e.g. 

 anticathode); V^ = i/2{A — 48)10*. 



The following relation has been found to hold experimentally between the voltage V through 

 which the cathode particles fall and the maximmn frequency v of the X-rays produced: eV 

 = hv, where e is the electronic charge and h, Planck's constant. Blake and Duane (Phys. Rev. 

 10, 624, 191 7) found for h, 6.555 X lo"^^ erg second. 



As the speed of the cathode rays is increased, shorter and shorter wave-lengthed "independent" 

 X-rays are produced until the critical speed is reached for the "characteristic" rays; with faster 

 speeds, the cathode rays become at first increasingly effective for the characteristic radiation, 

 then less so as the independent radiation again predominates. 



When cathode rays hit the anticathode some 75 per cent are reflected, the more the heavier 

 its atomic weight. The chances of the remainder hitting an atom so as to generate an X-ray 

 are slight; only i /looo or 1/2000 of the original energy goes into X-rays. If Ex and Ee are the 

 energies of the X and the parent cathode rays, A the atomic weight of the anticathode, /3 the 

 velocity of the cathode rays as fraction of the light value (3 x lo^" cm/sec), Beatty showed 

 (Pr. R. S. 1913) that Ex = Ec (.51 X lo^^jS^); (j^jg refers only to the independent radiations; 

 when characteristic radiations are excited their energy must be added and the tube becomes 

 considerably more efficient. No quantitative expression for the latter has been developed. 



When an X-ray strikes a substance three types of radiation result : scattered (sometimes called 

 secondary) X-rays, characteristic X-rays and corpuscular rays (negatively charged particles). 

 The proportions of the rays depend on the substance and the quality of the primary rays. When 

 the substance is of low atomic weight, by far the greater portion of the X-rays, if of a penetrating 

 type, are scattered. With elements of the Cr-Zn group most of the resulting radiation is "charac- 

 teristic." With the Cu group the scattered radiation (1/200) is negligible. Heavier elements, 

 both scattered and characteristic X-rays. Corpuscular radiation greater, mass for mass, for 

 elements of high atomic weight and may mask and swamp the characteristic radiation. Hence 

 an X-ray tube beam, heterogeneous in quality, allowed to fall on different metals, — Cu, Ag, Fe, 

 Pt, etc., — excites characteristic X-rays of wide range of qualities. Exciting ray must be harder 

 than the characteristic radiation wished. The higher the atomic weight of the material struck 

 (radiator), the more penetrating the quality of the resulting radiation as shown by the following 

 table, which gives X, the reciprocal of the distance in cm in Al, through which the rays must pass 

 in order that their intensity will be reduced to 1/2.7 of their original intensity. 

 TABLE 493. — Rontgen Secondary Rays. 



With the radiator at 45 ° to the primary X-rays at most only about 50 per cent of the energy 

 goes to characteristic rays and only about i/io of the latter escape the surface of the radiator. 

 The /3 radiations of radioactive elements may possibly be regarded (Rutherford) as a characteristic 

 radiation produced by the expulsion of the a particles. The hardness of some corresponds to the 

 K and L radiations. 



For more complete data on X-rays, see X-rays, G. W. C. Kaye, Longmans, 1917, upon which 

 these X-ray tables are greatly based. 

 Smithsonian Tables. 



