[en] A radiation detector is housed in a chamber in a heat conductor which is positioned in a dewar flask. A thermo-electric cooler is joined to one end of the heat conductor and a fin heat dissipation device is joined to the cooler. A thermistor senses the temperature of the heat conductor. Control means respond to the sensed temperature to operate the cooler to remove heat to stabilize the temperature of the chamber. The assembly extends into a conduit through which flows material, radiation from which is to be detected, the fin assembly being exposed to the atmosphere. The detector, e.g. a scintillation crystal and a photomultiplier perform better at the stabilized temperature, which may be less than that of the flowing material such as warm oil. (author)