Superconducting NbN microstrip detectors

Superconducting NbN strip transmission line counters and coupling circuits were processed on silicon wafers using thin-film techniques, and they were characterized with several methods to verify the design principles. The stripline circuits, designed using microwave design rules, were simulated using a circuit design tool enhanced to include modelling of the superconducting lines. The strips, etched out of the 282 nm thick top NbN film with resistivity 284 μΩ cm at 20 K, have critical temperatures in the range 12–13 K and a critical current density approximately Jc(0)=3.3×105 A/cm2. The linearized heat transfer coefficient between the strip and the substrate is approximately 1.1×105 W/m2 K and the healing length is about 1.6 μm between 3 and 5 K temperatures. Traversing 5 MeV α-particles caused the strips to quench. No events due to electrons could be detected in agreement with the predicted signal amplitude which is below the noise threshold of our wideband circuitry. The strip bias current and hence the signal amplitude were limited due to a microbridge at the isolator step of the impedance transformer.