Electrical and optical measurements on the first SCUBA-2 prototype 1280 pixel submillimeter superconducting bolometer array.

SCUBA-2 is a submillimeter camera being built for the James Clerk Maxwell Telescope in Hawaii. Bringing CCD style imaging to the submillimeter for the first time, with over 10000 pixels, it will provide a revolutionary improvement in sensitivity and mapping speed. We present results of the first tests on a prototype 1280 pixel SCUBA-2 subarray; the full instrument will be made up of eight such subarrays. The array is made up of transition edge sensor (TES) detectors, with Mo/Cu bilayers as the sensing element. To keep the number of wires reasonable, a multiplexed readout is used. Unlike previous TES arrays, an in-focal plane multiplexer configuration is used, in which the multiplexing elements are located beneath each pixel. To achieve the required performance, the detectors are operated at a temperature of approximately 120 mK. We describe the results of a basic electrical and optical characterization of the array, demonstrating that it is fully operational. Noise measurements were made on several pixels and gave a noise equivalent power below 2.5 x 10(-17) W HZ(-0.5), within the requirements for SCUBA-2. The construction of the testbed used to carry out these measurements is also described.

Predicting the response of a submillimeter bolometer to cosmic rays.

Bolometers designed to detect submillimeter radiation also respond to cosmic, gamma, and x rays. Because detectors cannot be fully shielded from such energy sources, it is necessary to understand the effect of a photon or cosmic-ray particle being absorbed. The resulting signal (known as a glitch) can then be removed from raw data. We present measurements using an Americium-241 gamma radiation source to irradiate a prototype bolometer for the High Frequency Instrument in the Planck Surveyor satellite. Our measurements showed no variation in response depending on where the radiation was absorbed, demonstrating that the bolometer absorber and thermistor thermalize quickly. The bolometer has previously been fully characterized both electrically and optically. We find that using optically measured time constants underestimates the time taken for the detector to recover from a radiation absorption event. However, a full thermal model for the bolometer, with parameters taken from electrical and optical measurements, provides accurate time constants. Slight deviations from the model were seen at high energies; these can be accounted for by use of an extended model.