228 



RADIATION BIOLOGY 



The principal advantage of the thermocouple or thermopile is that it 

 requires no external source of power and its sensitivity is relatively con- 

 stant. Evacuation produces essentially the same gain of 10- to 20-fold 

 as with the bolometer and a decrease in speed. The ultimate sensitivi- 

 ties in terms of signal/noise ratios obtainable with thermocouples cover 

 the same range as those for the bolometers. In general, however, the 

 thermocouple is a low-resistance element, and a high-turns-ratio coupling 

 transformer is required for modulated flux. Transformers with turns 

 ratios of 100-750 are available for couples with resistances in the range 

 10-50 ohms (Robinson, 1952). The thermocouple is coupled directly to 

 the transformer primary, and the secondary is direct- or capacity-coupled 

 to the grid of the first stage, as shown in Fig. 3-26. 



Pneumatic Detector. The pneumatic detector was first described by 

 Hayes and developed by Golay (1947a,b) into an instrument of great 

 reliability and sensitivity. It is one of the most sensitive and fastest 



5 s- in 



INFRARED 

 WINDOW 



PHOTOCELL 



i 1 



8 in 



Fig. 3-27. Diagram of the Golay pneumatic detector. {From Golay, 19476.) 



of the thermal detectors. The Golay pneumatic cell consists of a small 

 chamber containing a receiver of a very thin evaporated film of metal 

 in the center, a window at one end, and a very thin flexible mirror at 

 the other. The metal film, which is about 3 mm in diameter, absorbs 

 the radiant flux entering the cell through the window, heats the surround- 

 ing gas, and causes pressure changes that are detected optically by the 

 distortion of the flexible mirror, as shown in Fig. 3-27. 



The flexible mirror is placed at the focus of an optical system so that it 

 reflects an image of a line grid back onto another portion of the same grid 

 in such a manner that small changes in curvature of the mirror produce 

 large movements in the grid image and large changes in transmitted flux. 

 A photoelectric cell continuously measures the intensity of the flux pass- 

 ing through the line grid. The radiant flux to be measured is modulated 



