Characterization, 1064 nm photon signals and background events of a tungsten TES detector for the ALPS experiment

The high efficiency, low-background, and single-photon detection with transition-edge sensors (TES) is making this type of detector attractive in widely different types of application. In this paper, we present first characterizations of a TES to be used in the Any Light Particle Search (ALPS) experiment searching for new fundamental ultra-light particles. Firstly, we describe the setup and the main components of the ALPS TES detector (TES, millikelvin-cryostat and SQUID read-out) and their performances. Secondly, we explain a dedicated analysis method for single-photon spectroscopy and rejection of non-photon background. Finally, we report on results from extensive background measurements. Considering an event-selection, optimized for a wavelength of $1064\mathrm{n}\mathrm{m}$, we achieved a background suppression of $\sim {10}^{-3}$ with a $\sim 50\mathrm{\%}$ efficiency for photons passing the selection. The resulting overall efficiency was $23\mathrm{\%}$ with a dark count rate of $8.6\cdot {10}^{-3}{\mathrm{s}}^{-1}$. We observed that pile-up events of thermal photons are the main background component.