Gamma thermometers have been in routine use in nuclear power plants for power distribution monitoring for more than 35 years. These instruments depend upon the heating of a metal block by gamma rays. The generated heat is proportional to the specific power of nearby fuel rods. Usually heat is allowed to escape to a sink through a controlled heat path and the temperature drop is measured by thermocouples. If the heat sink is at a known temperature, a single thermocouple may be employed to measure the temperature of the metal block.
Fredrik B. H. Jensen,
"Measurement of nuclear reactor local heat rates by optical fiber infrared emission," Optical Engineering 36(8), (1 August 1997). https://doi.org/10.1117/1.601424