XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 521 



stand atmospheric pressure without imploding, if mounted on a grid 

 so that the area of unsupported film is very small (48). The t\v(j de- 

 signs of apparatus referred to deliver electron current densities of 

 0.2-0.5 na,. per square centimeter to exit slits 0.35 X 6.3 mm.^ The 

 resultant maximum dose rates to the superficial micron of exposed 

 tissue varies with kilovoltage from about 5 X 10'' r. per second at I 

 kv. to 5 X 10* r. per second at 15 kv. with the smaller apparatus, 

 from 10^ r. per second at 10 kv. to 2 X 10* r. per second at 100 kv. with 

 the larger apparatus. The dose rate is related to the current per unii 

 area by the following general formula, which is applicable to l)()tli 

 electrons and protons: 



dose rate = 3.88 X Wii/A) A r.e.p./sec. 



= 3.49 X 10^ ('i/A)A energy units/sec, 



where (i/A) is the current density in milliamperes per square centi- 

 meter and A is the ion density of the radiation in ions per micron of 

 tissue. 



No essential difference in principle is involved in accelerating 

 electrons to voltages between 100 kv. and 2,000,000 volts. A number 

 of forms of high tension supply are commercially available up to 200 

 kv. Apparatus to supply higher voltages at constant potentials may 

 be bought but is to be regarded as a special reciuirement (49, p. 1391). 

 The electrostatic generator is a convenient and flexible apparatus for 

 generating a variety of radiations, including constant potential elec- 

 tron beams at 1-2 m.e.v. Currents of 300 /xa. may be obtained with- 

 out great difficulty at 2 m.e.v. and focused to a window 5 cm.^ in 

 area, giving a current density of 60 jua. per square centimeter and an 

 average dose rate of over 10,000,000 r. per second throughout a depth 

 of about 9 mm. of tissue. The extent to which the dose rate varies 

 throughout this depth of tissue has been measured by Trump, Van 

 de Graaff, and Cloud (50) and is shown in Figure 12. 



Apparatus generating electron beams at high voltage should (jm 

 no account be approached except under conditions in which everj' 

 part of the body surface is protected by sufficient absorbing material 

 to stop the /3 rays completely. It is not alwaj^s appreciated liow 

 easily /3 rays are s(;attered even by air; and in at least one instance 

 several observers were seriously burned by looking for less than hali' 

 a minute and at a distance of several feet at the electron beam as it 

 emerged into air from a 1.2 m.e.\-. generator (51). 



