XIV. 



X RAYS AND X IRRADIATION 453 



or powder X-ray patterns. To the biologist this information has been 

 most important in the study of viruses or biologies, as penicillin, 

 when crystals may be formed, or in the analysis of elastic fibers. 



Summary volumes covering some of these points are listed in the 

 general bibliography (Z-^)- 



A. SOURCES OF X-RAY RADIATION 



It is presumed that the readers of this volume will be interested 

 primarily in investigating the biological effects of X irradiation rather 

 than in the construction and maintenance of X-ray apparatus. 

 Many will depend on hospitals or physical laboratories, where regulai- 

 X-ray equipment is being maintained, for their source of radiation. 

 Consequently, only a minimum acquaintance with X-ray tubes will 

 be needed. Where possible, the irradiations had best be made in a 

 ph^^sical laboratory. The biological material will then have the 

 benefit of trained specialists in both the biological and irradiation 

 fields. The necessary conditions of tube operation will be assured 

 and proper measurement will be made of the dose of X rays applied 

 to the tissues. But, since investigators are not equally fortunate, it 

 is well for all to be aware of some of the necessities of X-ray exposure 

 and protection. A few major points may be mentioned. 



The early X-ray tubes were gas-filled types operating at rela- 

 tively low voltages. These tubes delivered radiant energy of rela- 

 tively long wavelengths and, therefore, had low penetrating power. 

 The biological effects were largely surface effects. With further de- 

 velopment and water cooling these gas-filled tubes became highly ef- 

 fective generators of X rays in the wavelength region from 8 A. (an 

 angstrom unit = 10"^ cm.) to 0.3 A. Owmg to their long wave- 

 length, the X-radiation dosage rate in ionization units (roentgen or 

 r. unit, defined later) delivered by these tubes can reach very high 

 values. 



The constructional features of a gas X-ray tube are reproduced 

 in Figure 1. For contrast, Figure 2 shows a vacuum type tube of a 

 closely similar design. The cathode (C) focuses the high speed elec- 

 trons on the target or anode, thus generating the X rays, which are 

 emitted from the window at K (47). The low gas pressure in this 

 tube, about 10 ~* mm. of mercury, is controlled by vacuum pumps 

 and an adjustable air leak. The water-cooled anode may be com- 

 posed of any suitable metal desired. An anode metal is often 



