XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 525 



of 32.5, the energy required to produce a pair of ions, and tlie full 

 length of track, in microns, of a particle of energy E^. 



Radon contained in a glass or metal l)ulb of just sufficient thick- 

 ness to stop all the RaC a rays provides a very convenient source 

 of /3 rays, which come from the RaB and RaC, which attains c(iuilil)- 

 rium with the radon about four hours after the bulb is filled. When 

 specimens about 1 mm. in thickness are irradiated, the 7 rays from 

 RaB and RaC may contribute a few per cent of the total dose. 



Lea has given useful particulars about the energy distribution 

 among the /S rays from such a source. The dose rate at 1 cm. from 

 a point source due to the /S rays emitted by RaB and RaC in equilib- 

 rium with 1 millicurie of radon may be taken as about 20 r. per min- 

 ute. The dose rates from other radioactive substances will be about 

 half that from radon. They may be calculated from the following 

 general formula for the dose rate at 1 cm. from a point source of 1 

 millicurie : 



dose rate = 1 . 10 A r.e.p./min. 



= 0.99 A energy units/min. 



where A is the mean linear ion density in tissue of the particles 

 emitted by the source. 



Radium is sometimes mounted on flat plaques of small area cov- 

 ered by very thin Monel metal as a source of jS radiation. Though pri- 

 marily intended for medical applications, such plaques may be found 

 useful for biological research. The energy distribution among the 

 /3 rays falling on a thin biological specimen irradiated by such a source 

 will be intermediate between the distributions given by Lea for the 

 irradiation of thin and thick specimens. The /3-ray dose rate from 

 such plaques and the absorption of the radiation in tissues has been 

 measured by Neary {56) under a variety of experimental conditions. 

 Typically the dose rate was 100 r. per minute at the surface of a 

 plaque, 1.25 cm. in diameter, containing 6.2 mg. radium covered by a 

 0.05 mm. Monel metal foil. When the plaque was covered by very 

 thin rubber and placed in contact with an extended surface of water 

 the dose rate at depths of 0.5, 1, 2, and 4 mm. was found to be 70, 

 49, 25, and 8.5%, respectively, of the surface dose. Variation of dose 

 with depth depends critically on the geometrical arrangement em- 

 ployed since )3-ray scattering rather than energy loss is the controlling 

 factor. 



