A low-noise charge-sensitive preamplifier for an X-ray detector of a Nb-based superconducting tunnel junction

A low-noise charge-sensitive preamplifier with cooled 4-JFETs was developed for an X-ray detector of a Nb-based superconducting tunnel junction with a large capacitance and a low leakage current. The low-noise characteristics for a wide range of shaping times were obtained by using a selected JFET, 2SK 190 in the first stage of the preamplifier. A new formula to simulate the preamplifier noise was developed and applied to the optimization of preamplifier design. A high-energy resolution of 66 eV for 5.9 keV X-ray and 51 eV for pulser signals was obtained by using the optimized preamplifier with an X-ray detector of a 178 μm × 178 μm Nb/Al-AlO2/Nb superconducting tunnel junction with a junction capacitance of 1900pF and a bias current of 1.44nA.