Tables 655 and 656 

 RONTGEN RAYS (X RAYS) 



TABLE 655. — Constants for Cathode-Ray Speeds in Matter 



537 



Material 



Beryllium 12. 0.75 X io 9 



Aluminum 17. I.! " 



Copper 56. 3^6 " 



Silver 66. 



Gold 138. 



Moist air, 76 cm, 18 C 0.0062 0.44 X 10 



4-2 

 9.0 



TABLE 656.— X-Ray Emission 



X Rays are generated whenever and wherever swiftly moving electrons (cathode rays) 

 strike matter. This process occurs in gas discharge tubes at moderately low pressures 

 (about 0.001 to 0.01 mm Hg) ; the gas-filled X-ray tube is based on this principle. The 

 Coolidge tube, in which the gas pressure is so low (less than io" 4 mm Hg) as not to play 

 a part, is superior for most purposes : the electrons, supplied by a hot filament incorporated 

 in the cathode, are given a high velocity by the application of a high potential (as high as 

 300,000 v, in certain types) ; these cathode rays are directed against an area (" focal spot ") 

 on the anode ("target," " anticathode ") where the X rays are generated. 



These X rays are of two types: continuous spectrum rays ("heterogeneous," "general," 

 or "white" radiation) and characteristic rays (line spectra). 



Continuous spectrum X rays are a direct result of the acceleration of the cathode rays 

 due to their close contacts with the atoms of the anticathode. The spectrum energy dis- 

 tribution of this radiation, from a tube whose electrodes are maintained at a constant 

 potential difference (V), is described very roughly by the formula (for a more accurate 

 type of formula, see the I.C.T. vol. 6) 



J v dp=C(v — »)^/^dv v<v (1) 



for an energy-frequency graph, or by 



J x d\= (A7X 2 ) -I 1/X0 - i/X \e~WdX X>\„ (2) 



for an energy-wave length graph. In these two formulae, J v or J\ is the energy between 

 frequencies, v and v + dp, or wave lengths X and X + d\, respectively, e, the base of 

 natural logarithms, and v» and X the highest frequency and shortest wave length, respec- 

 tively of the spectrum ("high frequency limit," "short wave-length limit," "spectrum 

 limit"). For X rays generated inside the anticathode c and k are zero; this simplifies the 

 formulae, the exponential term becoming unity. For the X ray obtained outside the tube, 

 c and k have values, estimates of which are tabulated in Table 660. The factor, c or k, 

 determines the energy of the X rays ; the convenient way to evaluate this energy is, instead 

 of assigning numerical values to c or k, to evaluate Ei and h (Table 660). v and X 

 depend only on the voltage (F), the relations being: 



X =i2.336/F (3) 



v = 243.0 X io ls V (4) 



(X and X are expressed throughout this section in Angstrom units, io -8 cm, and v and v<> 

 are in sec." 1 , and V is in kilovolts absolute.) 



The energy of the continuous spectrum X rays, Ei, produced in the anticathode 

 ordinarily comprises a major fraction of the total X-ray energy generated; the energy 

 of the characteristic rays, E 2 , comprises the minor fraction. (Ei + Ei) is only an exceed- 

 ingly small fraction of the electrical energy, JSs, supplied to the tube. E1/E1 is called the 

 efficiency of production of continuous spectrum X rays, and is closely represented by the 

 formula 



£,/£, = ZV X 13 X io- T . 



where Z is the atomic number of the material of the anticathode, and V is expressed in 

 kilovolts. On account of losses by absorption in the anticathode and in the walls_ of the 

 tube only a small part of this energy generated inside the anticathode gets outside the 

 tube. Table 660 supplies some numerical values of this " usable " energy, for tubes similar 

 to the standard commercial types. 



Characteristic X rays result from the ionization of atoms, either (1) by direct cathode 

 ray impact, or (2) by absorption of X rays. In the anticathode of an X-ray tube both 

 these processes occur. With a silver anticathode, for example, at any voltage between 35 

 and 80 kv, process (1) accounts for about 65% of the energy of the characteristic rays. 



Smithsonian Tables 



