An optical fiber-based gamma-ray calorimeter (OFBGC) sensor array which uses a thermal mass with a low thermal conductivity is provided. Advantages of the OFBGC sensor array include: 1) the number of sensors in the OFBGC sensor array is adjustable and limited only by the spatial resolution of the OFBGC sensors, within the OFBGC sensor array, and 2) the OFBGC sensor design is simpler to build than a conventional optical fiber-based gamma thermometer (OFBGT) sensor array. One purpose of the OFBGC is to determine the power distribution in nuclear reactors.

An absorber for electromagnetic calorimeters that comprises a front portion and a rear portion that are made of different materials. The front portion may comprise a first material that is resistant to blow off (which can result in spalling) and vaporization from electromagnetic radiation (e.g., X-rays) exposure. The rear portion may comprise a second material that has a higher atomic number than the first material. For example, the example, the front portion may comprise graphite and the rear portion may comprise aluminum. The front portion may comprise 1 millimeter (mm) (0.039 inches (in)) thick AXM-5Q graphite, and the rear portion may comprise 1 mm (0.039 in) thick 6061-T6 aluminum. Thus, the absorber may fit into existing calorimeter housings and absorb high energy electromagnetic radiation at a sufficient rate, without undesirable levels of blow off or vaporization.