MEDICO-BIOLOGIC RESEARCH 



mann glass is a little greater than by beryl- 

 lium. A scheme of the AF.G-50A Machlett 

 tube with the vacuum camera is shown in 

 Fig. 3. The similar camera was made for 

 our tube by R. H. Archer (Ottawa, Civic 

 Hospital). 



Until now we have discussed the continu- 

 ous radiation produced by the tungsten 

 target. Yet there is another way to obtain 

 soft and ultra soft x-rays by using higher 

 kilovoltages. These x-rays, so-called "char- 

 acteristic", are emitted at a definite tension 

 by a target made of a specific material. 

 The above mentioned Duane-Hunt law (S) 

 is not applicable to characteristic rays. 

 Their wavelength depends upon other laws 

 discussed in works on the subject where 

 one can find both the target material and 

 the necessary tension for the emission 

 of this radiation. It is possible to make these 

 characteristic rays monochromatic using 

 special filters. Targets of chromium (X 2.287 

 A at 6 kv), iron (X 1.935 A at 7 kv) and 



o 



copper (X 1.539 A at 9 kv) are producers of 

 characteristic radiation most frecjuently 

 used (38). This characteristic monochromatic 

 radiation is especially important for precise 

 quantitative and qualitative analyses. These, 

 however, meet with many practical difficul- 

 ties in their application to biological speci- 

 mens. The detailed description of methods 

 may be found elsewhere (Clark) (38). 



The size of the focal spot is very important 

 for obtaining sharp microradiographs. This 

 size is 1.5 mm square in projection in the 

 Machlett AEG-50A tube, and is 40 m in the 

 Hilger & Watt microfocus tube. It may be 

 reduced to still smaller size through the ap- 

 plication of the condenser and objective 

 lenses as is done in the Cosslett-Nixon tube 

 for x-ray projection microscopy or in the 

 electron microscope. 



The smaller the focal spot the more is the 

 geometrical resolution of x-rays, i.e. the less 

 is the penumbra formation. These depend- 

 encies are discussed in detail in special pub- 

 lications on the subject (11)(23)(38)(90)(99). 



VILCUUM CAMERA ADAPTER - 



VKUUM CAMCU ^ 



PHOTDOW>MIC PlJffC 



- GASKCTED JOMT 



SKOMCN I S TO 10 HOIONS THICK I 



Fig. 3. Scheme of Machlett AEG-50A tube with 

 vacuum camera, specimen and plate in place as 

 given by Lurie (86). 



The scattering in air of both kinds of 

 secondary rays, modified and unmodified by 

 the Compton effect, is quite considerable for 

 soft rays used in microradiography (see 

 equation 3). However, the influence of scat- 

 tered rays on image formation may be re- 

 duced to a minimum by using several dia- 

 phragms on the path between tube target 

 and object. Lead pipes covering the entire 

 distance between the tube target and the 

 cassette cover may be used with the same 

 effect. Such a pipe is shown in Fig. 23. 



Object Factor. With a rough approx- 

 imation the x-ray absorption (m) in biologi- 

 cal tissues may be expressed in such an ec^ua- 

 tion: 



7n = k\^- Z* (approximately) -|- 0.2 (3) 



where X is the wave length, Z (AN) number 

 of element in atomic chart, k the constant 

 and 0.2 the coefficient of scattering. 



Hydrogen (Z 1), carbon (Z 6), nitrogen 

 (Z 7), oxygen (Z 8) and calcium (Z 20) are 

 the main elements present in animal (hu- 

 man) body in considerable quantities; sulfur 

 (Z 16), iron (Z 26) and phosphorus (Z 15) 

 comprise only a small percentage of body 

 components. Zuppinger (116) gives for ma- 

 croradiology the following data on linear 

 coefficients of attenuation (Table 2). From 



595 



