First Tests of Prototype SCUBA-2 Superconducting Bolometer Array

Abstract

Abstract. We present results of the first tests on a 1280 pixel superconducting bolometer array, a prototype for SCUBA-2, a sub-mm camera being built for the James Clerk Maxwell Telescope in Hawaii. The bolometers are TES (transition edge sensor) detectors; these take advantage of the large variation of resistance with temperature through the superconducting transition. To keep the number of wires reasonable, a multiplexed read-out is used. Each pixel is read out through an individual DC SQUID; room temperature electronics switch between rows in the array by biasing the appropriate SQUIDs in turn. Arrays of 100 SQUIDs in series for each column then amplify the output. Unlike previous TES arrays, the multiplexing elements are located beneath each pixel, making large arrays possible, but construction more challenging. The detectors are constructed from Mo/Cu bi-layers; this technique enables the transition temperature to be tuned using the proximity effect by choosing the thickness of the normal and superconducting materials. To achieve the required performance, the detectors are operated at a temperature of approximately 120 mK. We describe the results of a basic characterisation of the array, demonstrating that it is fully operational, and give the results of signal to noise measurements. Astronomy at sub-mm wavelengths (hundreds of microns) is relatively new. In the last decade, state of the art systems have improved from single pixel detectors to arrays of hundreds of pixels. There are huge advantages to be gained from increasing the number of pixels still further, if this can be done without compromising performance. This paper describes tests on a prototype 1280 pixel detector sub-array for a new instrument, SCUBA-2 [1]. With over 10 000 pixels in total, this will give a revolutionary improvement over existing instruments in sensitivity and mapping speed, and bring CCD-style imaging to the sub-mm for the first time. To appear in the proceedings of the 24th International Conference on LowBolometers are the most sensitive detectors for broadband measurements at sub-mm wavelengths. Most existing arrays use semiconductors as the temperature sensing element. However, these have reached the sensitivity limits possible for this technology. Moreover, it is difficult to scale such arrays to more than a few hundred pixels since they involve manual steps in the construction of the pixels, and require separate wiring to each detector.SCUBA-2 uses TES (transition edge sensor) detectors. These obtain high sensitivity from the large change in resistance through the superconducting transition. A constant bias voltage results in negative feedback to temperature changes due to changes in incident power, keeping the detector in the superconducting transition without the need for active temperature control. The transition temperature is tuned via the proximity effect by choosing the thicknesses in a molybdenum-copper bi-layer. Thermal isolation of the sensing element is achieved using a silicon nitride membrane [2]. This choice...