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We are developing a device for measuring the heat capacity of candidate absorber materials for gamma‐ray microcalorimeters with the goal of finding materials with low heat capacity and high stopping power to improve detector efficiency. To date, only Sn has been effective as an absorber, and speculation is that other materials suffer from anomalously high heat capacities at low temperatures. The key component of the measurement device is a 17 mm×17 mm low heat capacity silicon platform suspended by Kevlar fibers designed for accepting 1 g to 2 g samples, and whose heat capacity can be characterized prior to attaching a sample. The platform has a thin film Pd/Au heater deposited directly on the silicon, and a semiconducting thermometer bonded to the surface. The heat capacity is determined from C = Gτ, where G is the in‐situ measured conductance and x is the measured temperature decay time from a step change in applied heat. For a platform without samples, decay periods on the order of 0.3 to 0.05 seconds ...

Normal‐metal/insulator/superconductor (NIS) tunnel junctions can be integrated with transition edge sensor (TES) bolometers to provide additional cooling to the sensor. For example, a TES array cooled to 300 mK by a 3He refrigerator could be enhanced with additional on‐chip NIS cooling to decrease the effective bath temperature of each TES to 100 mK. The noise of the array with on‐chip cooling would be ∼40% lower than that of a similar array without on‐chip cooling. Additionally, the lower bath temperature allows a more physically robust design with the same saturation power per pixel. However, on‐chip NIS cooling does not come for free. While the NIS coolers reduce the bath temperature seen by each pixel, they also dissipate about 100 times as much power as the pixel alone and increase the fabrication complexity. We describe a potential thermal design of a 256 pixel TES bolometer array with on‐chip cooling. We show that such an array is feasible despite the increase in power dissipation.