X-ray detection by Nb STJs above 1.4 K

We have designed and tested a 5×5 mm² chip which includes 24 “island” Josephson tunnel junctions (STJs) with square, circular and diamond geometries, two arrays of 100 “island” junctions and other detection devices. The junctions were based on Nb-AlOx-Nb trilayers, obtained on Si substrate by a process consisting of underlayer Nb deposition, self-aligned insulation and anodization steps. The junction quality was extensively tested by measurements of the I–V characteristics down to T=1.2 K. I_cR_n values up to 2.16 mV have been obtained. Preliminary results concerning the response of these samples to 6 keV X-ray radiation are discussed. Effects of the substrate on the detector response have also been observed and reported. A rough estimation of the energy resolution gives about 700 eV. The quality of the junctions is such that the electronic noise is the predominant limitation to the ultimate resolution.