X-ray and optical photon counting detector using superconducting tunnel junctions

The properties of Superconducting Tunnel Junctions (STJ) make them very suitable for low light level astronomical observations. STJ detectors could be used to improve the sensitivity of such a system to be implemented on ground based telescopes. We have developed a modified version of the “SNEP” fabrication process for making Nb/Al-AlO_x-Al/Nb and Ta/Al-AlO_x-Al/Ta/Nb STJ´s. These junctions show a very low sub-gap leakage current at 0.1 K. They use a double thin aluminum trapping layer and a Nb or Ta absorber grown epitaxially on a R-plane sapphire substrate. A new experimental set-up has been completed to achieve photon counting performances for the X-ray and for optical-near infrared wavelengths. An original, room temperature, charge sensitive preamplifier has been built for this purpose and tested with a ⁵⁵Fe X-ray source. The 6 keV peak of this source can be observed in the measured spectrum. Photon counting ability in the near infrared at 0.78 μm is also demonstrated