148 BIOLOGICAL EFFECTS OF RADIATION 



Such a couple will yield 800 /xa. current for 1 mw. absorbed radiant power. 

 It will measure as a minimum 0.17 erg/sec. /cm.^ irradiation, which for 

 this size of receiver amounts to a least detectable radiant power 5 X 10~* 

 erg/sec. If a sensitive thermocouple is combined with a resonance 

 amphfier, the least detectable radiant power is 6 X 10"^ erg/sec. With 

 a photovoltaic cell, such as the photronic cell, having a large receiver, 

 11 cm. 2 in area, an irradiation of 10~^ erg/sec. /cm. ^ can be detected. 

 Owing to its large receiver, however, it is absorbing a far greater amount 

 of energy, the radiant power incident upon the surface being 10"^ erg/sec. 



In case of a photoelectric cell with electrometer measurement, we 

 may be dealing with a detector whose internal resistance is 10^^ ohms, its 

 area may be anywhere from 5 to 20 cm. 2, its inherent time of response is 

 extremely small, probably of the order of 10"'' sec. An electrometer 

 which might be used for this purpose might have a period of 10 sec, a 

 voltage sensitivity of 2 X 10~^, a current sensitivity of 10~^'. Such 

 a photocell, depending upon the type selected, may yield a current 

 anywhere from 0.1 to 10.0 /xa. for 1 mw. incident radiant power. With 

 such an arrangement, irradiation may be detected as small as 5 X 10~^ 

 erg/sec. /cm.^ radiant power for a 5-cm.^ area. If one does not require 

 a small time of response, the photoelectric cell may be used with an 

 FP54 amplifier, requiring 100 sec. for time of observation. Such an 

 arrangement may have a least measurable voltage characteristic of 

 3 X 10 "^, corresponding to 3 X 10~'^ amp. Such a device would respond 

 to a least observable irradiation of 3 X 10~^ erg/sec. /cm. 2. This corre- 

 sponds for a 20-cm.2 area to 6 X 10~^ erg/sec. of incident radiant power. 

 It will be noticed that so far only with this extremely sensitive device 

 is the sensitivity of the eye approached, which is able to detect an irradi- 

 ation of 1.3 X 10~^ erg/sec. /cm. 2, but owing to its very small effective 

 area (pupil), the corresponding radiant power is only 7.7 X 10"^° erg/sec. 



The most sensitive of all radiation detectors is the photocell Geiger 

 counter. With this arrangement, irradiation of 9 X 10~" erg/sec. /cm. - 

 has been detected. This corresponds to 4.5 X 10~^° erg/sec. of incident 

 radiant power. This is the only instrument which exceeds the sensitivity 

 of the eye to radiant power. However, for the same period of response, 

 none even approach it. 



In this table, we have presented first the absolute black-body detec- 

 tors, having only a very low sensitivity; second, the secondary black-body 

 instruments, whose deflection when properly constructed is proportional 

 to the incident radiation; third, the selective detectors of the photoelectric 

 type whose sensitivity varies markedly with wave-length. The sensi- 

 tivity as a function of wave-length of these detectors will be taken up 

 in detail in a later section. For most practical radiation measurements, 

 a secondary black-body instrument is used in conjunction with a standard 



