Development of X-ray detection system by using an on-chip coil inetgrated superconducting tunnel junction

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An on-chip coil integrated superconducting tunnel junction (OC^2-STJ) was developed for high performance X-ray detector. The OC^2-STJ consists of a Nb-based junction and a superconducting microstrip coil deposited on the insulating layer deposited on the top electrode of the STJ. The microstrip coil generates the magnetic field in the insulating layer of the junction for suppression of Josephson current. The fabricated OC^2-STJ chip was mounted in a cooling capsule with the ^<55>Fe X-ray source. The cooling capsule was attached to the mixing chamber of a compact ^3He-^4He dilution refrigerator. The OC^2-STJ chip was held at a constant temperature in the capsule against the Joule heat generation in the current feed line of the strip coil during the detector operation. Electric charges produced by tunneling of excited quasiparticles by incident X-rays were collected by a charge sensitive preamplifier. To reduce noise components in the output signals, a field effect transistor (FET) of the signal input was placed on the 1 K pot of the refrigerator through a weak thermal link. The temperature of the input FET was regulated at 100 K by a heater to generate the output voltage pulses with the minimum noise equivalent charge. The temperature of the OC^2-STJ chip was kept around 600 mK with regulating the FET temperature at 1 K. In the operation of the OC^2-STJ for X-ray detection, the current from 10 to 30 mA was supplied to the microstrip coil. Response characteristics were obtained for a diamond-shaped Nb-based tunnel junction with a sensitive area of 100 × 100 μm^2 in the OC^2-STJ chip. A stable operation of the OC^2-STJ was demonstrated without using usual external electromagnets.