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Superconducting transition edge sensors (TESs) have made dramatic progress since their invention some 65 years ago (Andrews et al 1949 Phys. Rev. 76 154–155; Irwin and Hilton 2005 Topics Appl. Phys. 99 63–149) until now there are major imaging arrays of TESs with as many as 7588 separate sensors. These are extensively used by astronomers for some ground-breaking observations (Hattori et al 2013 Nucl. Instrum. Methods Phys. Res. A 732 299–302). The great success of TES systems has tended to overshadow other superconducting sensor developments. However there are other types (Sobolewski et al 2003 IEEE Trans. Appl. Supercond. 13 1151–7; Hadfield 2009 Nat. Photonics 3 696–705) which are discussed in papers within this special edition of the journal. Here we describe a quite different type of detector, also applicable to single photon detection but possessing possible advantages (higher sensitivity, higher operating temperature) over the conventional TES, at least for single detectors.

We present a review of emerging x-ray and gamma-ray spectrometers based on arrays of superconducting transition-edge sensors (TESs). Special attention will be given to recent progress in TES applications and in understanding TES physics.

Molybdenum is a common material for superconducting thin films used, for example, in transition edge sensors as Mo/metal bilayers. For such applications long term stability of the superconducting parameters is essential, especially the critical temperature and the transition width. Our analysis of the effect of film oxidation on these parameters for Mo/AuPd bilayers shows that even slight oxidation will have a significant effect. A direct comparison shows the potential of sidewall passivation to reduce the degradation and stabilize these parameters.