132 UADIATION IJIOLOGY 



of low Ileal capacity ininuT.scd in an atmosphere of xenon in a .small cham- 

 ber. The rear wall of the chamber is a thin collodion film, .silvered on the 

 outside surface. When exposed to radiation, the absorbed heat is 

 rapidly transferred to the gas, which expands, deforming the collodion 

 wall. This slight deformation is readily detected by an optical system 

 whereby an image of a grid is cau.sed to move across another grid, vary- 

 ing the light received on the face of a photocell, as the membrane is 

 deformed. The detector is intended to be used with chopped radiation. 

 With .such a cell, energies of 5 X 10~" watt may be detected (Golay, 

 1949). 



Ultimate Sensitivity of Thermal Detectors. The sensitivity of any ther- 

 mal detector of radiation is ultimately limited by the random fluctuations 

 to be expected on thermodynamic grounds, in the temperature of any 

 body in equilibrium with its environment (Myers, 1946; Jones, 1947). It 

 would be impractical to attempt to measure a temperature change due to 

 incident radiation, which is small compared to these random fluctuations. 

 Alternatively, from a different but equivalent point of view, one may 

 regard the sensitivity as limited by the inevitable statistical fluctuations 

 in the heat radiation emitted and received at all times by any body. 

 Since a thermal detector is sensitive to radiation of all wave lengths, it is 

 sensitive to the thermal radiation emitted by its surroundings. Any 

 attempt to detect a radiation beam of energy less than the fluctuations to 

 be expected in the thermal radiation energy received from (and emitted 

 to) the surroundings (Fellgett, 1949) would be impractical. 



Such considerations set a lower limit to the sensitivity of thermal 

 detectors expo.sed to a surround at ordinary temperatures, at about 

 3 X 10~'"^ watt for a detector of area 1 mm- and response time of 1 sec 

 (Jones, 1947). To achieve higher sen.sitivity in any radiation detector, 

 it is necessary to limit the wave-length region to which it is sensitive, in 

 order thereby to reduce the fluctuation in the detector output, due to the 

 fluctuation in incident thermal radiation energy. Thus, for instance, for 

 objects at ordinary laboratory temperatures, the intensity of emitted 

 thermal radiation of wave length less than 6000 A is negligible. There- 

 fore, a photoelectric detector which is .sensitive only to radiations of wave 

 length less than 6000 A is entirely insensitive to the fluctuations in ther- 

 mal radiation. 



PHOTOCHEMIC.\L DETECTORS 



If monochromatic radiation is employed or if the spectral energy dis- 

 tribution of the radiation concerned is known, photochemical processes 

 may be conveniently used as a mea.sure of radiation intensity. For 

 absolute determinations, the quantum yield of the photochemical reaction 

 must fir.st have been determined at all wave lengths of interest by calibra- 

 tion again.st a standard thermal detector. The use of a photochemical 



