Preamplifier development for superconducting tunnel junction array X-ray detector electronics

Abstract

We have developed a very low noise preamplifier that meets the low-cost, high-density requirements for use with superconducting tunnel junction (STJ) arrays. The design allows the STJs' bias points to be digitally set at the ȝV level. We then trace out noise versus bias curves under computer control and, by showing the correlation between this noise and detector energy resolution, establish the ability to automatically select optimum STJ operating points. Working with a 75 ȝm x 75 ȝm STJ with a Ta x-ray absorber, we achieved better than 7 eV FWHM at the 525 eV O-K line with about 2 eV coming from the preamplifier.

112-Pixel Arrays of High-Efficiency STJ X-Ray Detectors

“…2, center). A BF862 n-channel JFETs provides an input voltage noise e n ≈ 0.8 nV/√Hz, and is combined with a OP1 op-amp for a dc open-loop gain A OL > 10 6 [8]. A secondary feedback loop adjusts the JFET bias current so that its gate provides the desired STJ bias voltage without using a blocking capacitor [8].…”

New Developments in Superconducting Tunnel Junction X-Ray Spectrometers for Synchrotron Science

“…1) [4]. For low noise we use a composite preamplifier with two BF862 JFETs in parallel at the input with a total transconductance g m = 45 mS and a voltage noise of 0.8 nV/ √ Hz.…”

“…The total parts cost is $13 per channel, so that the 32-channel boards currently being developed at XIA can be priced to fit into typical research budgets. Details of the circuitry and a discussion of noise and performance have been published [4,6]. Figure 2 shows characteristics of a 67 µm × 67 µm STJ as the DAC setting is stepped across the voltage range in the STJ subgap region.…”

“…For details, see Ref. [4] STJ detectors should have higher quantum efficiency and effective area than the 36-pixel Nb-based STJs currently in use, while still providing an energy resolution of ∼10 eV FWHM and count rate capabilities of ∼10,000 counts/s per pixel.…”