184 Applied Biophysics 



larly suitable for direct use in body cavities during therapeutic 

 treatment. Very compact vmits have been developed, with small 

 ionized volume and large electrical capacity, so that large doses 

 can be measured. Chambers are now being used inside needle- 

 like sheaths which can actually be inserted into the tissues, 

 during treatment. Condenser chambers have the advantage that 

 several may be used simultaneously, so that an extended field 

 of radiation may be rapidly surveyed. Another particularly suit- 

 able application is the so-called ''protection chamber" for record- 

 ing the dose received by workers owing to small amounts of 

 stray radiation. 



The Meastirement of Gamma Rays in Rontgens 



To turn again to the more theoretical side of radiation 

 measurement, the desire to measure gamma radiation in rontgens 

 has resulted in great advances in the understanding of the 

 ionization chamber and of the energy exchange between radia- 

 tion and matter generally. Special interest attached to the prob- 

 lem of the gamma radiation from radium, in particular the dose 

 rate produced by 1 milligram of radium at 1 cubic centimeter, 

 when filtered by 0.5 millimeter of platinum (to cut out the 

 primary beta radiation) — the so-called specific gamma-ray dose 

 rate of radium. 



As early as 1931, Mayneord ^^ estimated this quantity from 

 the known energy output of the radium gamma radiation (ob- 

 tained by calorimetric measurements by Ellis and Wooster), 

 and from the known absorption coefficient of air, to be 8.7 r 

 per hour, and a measurement with a "thimble" chamber cali- 

 brated by comparison with an X-ray dosemeter gave 9.2 r per 

 hour, in reasonable agreement. Mayneord, in 1933,^^ further 

 estimated this quantity from Eve's constant (the number of ion 

 pairs per second per unit volume produced in air at 1 cubic 

 centimeter from the quantity of radium C in equilibrium with 

 1 gram of radium) as 8.9 r per hour. But at the same time, 

 attempts to measure the specific gamma-ray dose rate directly 

 with "free-air" chambers led to values of only about one-third 



