METHOD AND APPARATUS FOR EVALUATING SUPERCONDUCTING TUNNEL JUNCTION DETECTOR NOISE VERSUS BIAS VOLTAGE

A technique for characterizing the noise behavior of a superconducting tunnel junction (STJ) detector as a function of its applied bias voltage Vb by stepping the STJ's bias voltage across a predetermined range and, at each applied bias, making multiple measurements of the detector's current, calculating their mean and their standard deviation from their mean, and using this standard deviation as a measure of the STJ detector's noise at that applied bias. Because the method is readily executed under computer control, it is particularly useful when large numbers of STJ detectors require biasing, as in STJ detector arrays In a preferred implementation, the STJ is measured under computer control by attaching it to a digital spectrometer comprising a digital x-ray processor (DXP) coupled to a preamplifier that can set the STJ's bias voltage Vb using a digital-to-analog converter (DAC) controlled by the DXP. An on-board digital-signal-processor in the DXP is then programmed to implement the technique by stepping through a sequence of bias voltages using the DAC and, for each voltage, capturing 1000 values of the detector current Id from the DXP's baseline filter and then computing their mean and standard deviation Id. Minima in the plot of Id. vs Vb are shown to correspond to minima in the plot of detector energy resolution vs Vb, allowing a plot or table of Id. vs Vb values to be used to locate an optimum value of Vb for the detector operating point.