XV. ELECTRONS, NEUTRONS, AND ALPHA I'AUTH'LES 545 



the agar is contained in a metal dish, the thickness of agar should be 

 comparable with the maximum |8-ray range so that there is no am- 

 biguity as to the nature of the scattering medium. 



The dose rate with this arrangement is simply J„ r.e.p. per minute 

 where J» is the ionization per unit volume at 0°C. and 760 mm. of 

 mercury. Since it is difficult to know the volume of a paper chamber 

 accurately it is generally more convenient to calibrate the chamber 

 by covering it with a 4 mm. thick graphite cap and to measure the 

 7-ray ionization produced by a radium source of known strength. It 

 may be assumed that the ionization produced under such circum- 

 stances due to the 7 rays of radium filtered through 0.5 mm. platinum 

 is at the rate 8.3 electrostatic units per cubic centimeter per hour at 

 1 cm. from a point source of 1 mg. For further details, several recent 

 papers should be consulted (56,89,100). 



External Proton and Alpha-Ray Sources. Scattering is negli- 

 gible in the case of protons and a particles but a technical problem 

 arises in connection with the collection of the ions, particularly in the 

 case of a-ray ionization. A field of 500 v. per centimeter fails to 

 collect about 20% of the ions formed in air by a rays traveling in the 

 direction of the electric field (84,90). The same field collects all but 

 about 2% of the ions when the particles are traveling perpendicular 

 t o the field. It is therefore advisable always to define the beam to be 

 used by a stop, as in the first method described for /3 rays, but to use 

 a transverse collecting field between parallel plates, which define the 

 depth of the volume from which ions are collected, as, for example, in 

 the manner shown in Figure 16c. The dose rate to specimens at the 

 level of the upper surface of the stop will be given by the same formula 

 as for the jS-ray dose rate but with the inclusion of a factor / to allow 

 for the different value of T^air for protons and a particles. Thus : 



(lose rate = 



fk'I 



(12) 



or, when R/d is small : 



fk'I/irRH (13) 



where/ = 1.11 for protons and 1.09 for a particles; and A;' is the 

 mass stopping power ratio of tissue and air for either protons or a 

 particles if the dose is expressed in r.e.p. per minute or the electronic 



