The EBEX Balloon-borne Experiment - Detectors and Readout

EBEX was a long-duration balloon-borne experiment to measure the polarization of the cosmic microwave background. The experiment had three frequency bands centered at 150, 250, and 410 GHz and was the first to use a kilopixel array of transition edge sensor bolometers aboard a balloon platform. We describe the design and characterization of the array and the readout system. From the lowest to highest frequency, the median measured detectors' average thermal conductances were 39, 53, and 63 pW/K, the medians of transition temperatures were 0.45, 0.48, and 0.47 K, and the medians of normal resistances were 1.9, 1.5, and 1.4 Ω; we also give the measured distributions. With the exception of the thermal conductance at 150 GHz, all measured values are within 30% of their design. We measure median low-loop-gain time constants τ ₀ = 88, 46, and 57 ms. Two measurements of bolometer absorption efficiency gave results consistent within 10% and showing high (∼0.9) efficiency at 150 GHz and medium (∼0.35 and ∼0.25) efficiency at the two higher bands. We measure a median total optical power absorbed of 3.6, 5.3, and 5.0 pW. EBEX pioneered the use of the digital version of the frequency domain multiplexing system. We multiplexed the bias and readout of 16 bolometers onto two wires. The median per-detector noise-equivalent temperatures are 400, 920, and 14,500 . We compare these values to our preflight predictions and to a previous balloon payload. We discuss the sources of excess noise and the path for a future payload to make full use of the balloon environment.