130 RADIATION BIOLOGY 



niul spectral variation of sensitivity of various phosphors, in conjunction 

 with the spectral response of phototubes, to make wave-length-selective 

 detectors (e.g., Luckiesh and Taylor, 1940; Kerr, 1947j. 



THERMAL DKTECTORS 



The measurement of radiant energy in absolute units is most com- 

 monly accomplished by the total absorption of such energy in an appro- 

 priate substance, accompanied by a measurement of the increase in tem- 

 perature of the absorber. If the heat capacity of the absorber is known, 

 the energy content of the radiation may then be readily calculated. In 

 practice, the increase in temperature of the absorber may be measured as a 

 resultant change in electrical resistance (bolometer), as an electromotive 

 force (thermocouple), or as a mechanical deformation induced by gas 

 expansion (Golay cell). Radiation detectors based on a measurement of 

 the change in temperature consequent to radiation absorption are called 

 thermal detectors. 



To absorb totally the radiant energy, the detector must be "black" to 

 all wave lengths represented in the radiation to be measured. It is pos- 

 sible to prepare such "black" surfaces by vacuum deposition, under 

 appropriate circumstances, of such substances as bismuth, zinc, platinum, 

 or gold (Pfund, 1930, 1933, 1937a,b; Harris and McGinnies, 1948). Such 

 surfaces are known to absorb all incident radiation from 0.2 m to beyond 



15 ^l. 



These detectors may be calibrated by the use of a radiation beam of 

 known energy content, thereby avoiding the necessity of a direct measure- 

 ment of their heat capacity. Such a defined beam may be obtained from 

 standard lamps, available from the U.S. Bureau of Standards (Goblentz 

 and Stair, 1933), operated under precisely defined conditions. 



Bolometer. Measurements of the change in electrical resistance of a 

 detector, consequent to the absorption of radiant energy, are conveniently 

 carried out by using the detecting bolometer as one arm of an initially 

 balanced resistance bridge. The change in resistance leads to unbalance 

 of the bridge with a resultant unbalance voltage which may be amplified 

 to a readily meterable magnitude. A similar bolometer, shielded from 

 the radiation, may be placed in an appropriate arm of the bridge to com- 

 pensate for variations in ambient temperature. If the thermal capacity 

 of the detector is low, it may be used with chopped radiation, with a 

 resultant oscillatory unbalance voltage, which may be amplified by an 

 alternating-current amplifier; in such cases the amplifier may well be 

 .sharply tuned to the chopping frequency to improve the signal-noise 



ratio. 



To produce a bolometer detector of high sensitivity, it is desirable to 

 deposit the absorbing coating upon a substance with a high temperature 

 coefficient of resistance. Among the metallic substances, nickel and 



