PRODUCTION OF CONTINUOUS AND CHARACTERISTIC X-RADIATION 



a superimposition of thin target spectra for 

 all electron energies up to the incident 

 energy. The fomi of the thick target distribu- 

 tion can be calculated only if the range- 

 energy relationship is known. The scattering 

 of the electrons causes a further modification 

 of the spectral distribution in a given direc- 

 tion, because the electrons in the beam are 

 no longer travelhng in a defined direction 

 with respect to the point of observation. The 

 problem is not amenable to exact calculation 

 but it is clear that the spectral distributions 

 will contain a greater proportion of soft 

 radiation than is observed from thin targets. 

 Using the spectral distribution ly = const., 

 together with the then accepted Thomson- 

 Whiddington law of energy loss, Kramers 

 obtained a thick target distribution of the 

 form /, = KZ (vo — »') wliich is in fact very 

 closely obeyed in practice, as will be shown 

 below. The intensity per unit frequency in- 

 terval is related to the intensit,y per unit 

 energy interval, and to the somewhat more 

 familiar intensity per unit wavelength in- 

 terval, by the relations 



L = hL 



c 



The continuous spectrum from massive 

 targets of several elements was investigated 

 by Kuhlenkampff (1922), at an angle of ap- 

 proximately 90° to the electron beam, using 

 accelerating voltages between 7 and 12 kV. 

 It was established that the Kramers rela- 

 tionship was closely applicable down to about 

 0.4 j^o . A small additional term, independent 

 of kilovoltage and photon energy but pro- 

 portional to Z^, was found to be necessary. 

 This second term is important only in the 

 immediate vicinity of the high energy limit. 

 These measurements w^ere extended to higher 

 accelerating voltages (20-50 kV) by Kuhlen- 

 kampff and Schmidt (1943), and to lower 

 voltages (1-2 kV) by Neff (1951). Some of 

 the latter data are reproduced in Fig. 5. 



The spectral distribution in the forward 

 direction from electron-opaque targets con- 



Fig. 5. Spectral distribution from a massive 

 target of platinum (Xeff, 1951). 



2 4 6 8 10 



Quantum Energy (kev) 



Fig. 6. Spectral distribution in the forward di- 

 rection from an electron-opaque gold target (Dy- 

 son, 1959a). 



sisting of foils in the region of 1 mg/cm^ in 

 thickness has been investigated recently 

 (Cosslett and Dyson (1957); Dyson (1959a)) 



657 



