224 RADIATION BIOLOGY 



vacuum on a permanent backing of nitrocellulose or other plastic, receiver 

 elements of less than O.Ol-yu thickness can be obtained. Platinum with 

 a temperature coefficient of resistance of 0.3 per cent per degree centi- 

 grade and nickel with a value of 0.6 per cent per degree centigrade have 

 been used most extensively. The metal bolometers usually have a rela- 

 tively low resistance in the range 10-100 ohms. The evaporated-nickel 

 bolometer described by Bilhngs, Barr, and Hyde (1947) had an area of 

 6 mm^, a time constant of 4 msec, and a minimum detectable power of 

 3.3 X 10~^ w, with a modulation frequency of 30 cps and an amplifier 

 with a 100-cps band width. The minimum detectable irradiance is about 

 0.5 nw cm~^ under these conditions. Reduction of the band width to 

 1 cps or the use of a slow meter can reduce the minimum detectable 

 power by a factor of 10. 



2. Thermistor. Thin, blackened semiconductor flakes composed of a 

 mixture of oxides, including those of nickel, manganese, and cobalt, have 

 a negative temperature coefficient of about 4 per cent per degree centi- 

 grade and a very high specific resistance. The temperature coefficient 

 of such semiconductor flakes, commonly called "thermistors," is about 

 ten times that of the common metals. Becker and Moore (1946) devel- 

 oped the Sett element, or thermistor bolometer, for infrared use, and 

 Wormser (1953) gave extensive data on its performance. A thermistor 

 bolometer with a receiver 0.2 mm wide, 2.5 mm long (0.5 mm-), and 

 10 iJL thick has a resistance of the order of 3 megohms; therefore it can be 

 coupled directly to the grid circuit of a vacuum-tube amplifier. Since it 

 is not necessary to evacuate these instruments, they are usually mounted 

 directly on quartz or glass or supported in air. The sensitivity and time 

 constant, as given in Table 3-16, vary with the method of mounting. 

 The shortest time constant and lowest sensitivity are obtained with the 

 quartz-backed element, whereas the slowest bolometers with the maxi- 

 mum sensitivity are mounted in air. As with metal bolometers, two ele- 

 ments are mounted in th.e same housing so that only one element receives 

 the incident flux. 



3. Superconductor. Andrews et al. (1946) have made use of the very 

 large temperature coefficient of resistance of certain types of conductors 

 near their superconducting range of temperature, where the resistance is 

 approaching zero. The superconducting bolometer (Fuson, 1948) has 

 the highest sensitivity of all thermal detectors, but the inconvenience of 

 operating at the extremely low temperature of about 15°K required for 

 the niobium nitride conductor eliminates it at the present time as a 

 practical detector for general use. The minimum detectable power is 

 about 6 X 10-1" w (Bell et al, 1946). 



4. Bolometer circuits. A bolometer detector may be coupled to the 

 measuring device through (1) a d-c Wheatstone bridge and galvanometer, 

 (2) a simple Wheatstone bridge supplied with alternating current, the 



