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.