Small thermal calorimeters operated at temperatures near 100 mK are sufficiently sensitive to detect single photons, measuring the deposited energy to accuracies approaching 1 eV. A test device has demonstrated 17‐eV FWHM for 6‐keV x‐rays—almost an order of magnitude better than a conventional Si(Li) solid state detector. Further improvement requires the development of monolithic fabrication techniques to reduce the parasitic heat capacities introduced in assembling a discrete device. Current technology should permit the construction of a detector about 1/2 mm2 with good efficiency for 35‐keV x‐rays and a resolution near 25 eV. The ultimate resolution achievable in practice and the tradeoff of collecting energy for resolution depend primarily on the low‐temperature specific heat of the material used to absorb the x‐rays and efficiently thermalize their energy. Finding the optimum absorber is particularly important at higher photon energies. We are studying absorbers that may offer considerably improved performance.

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