5,120 Superconducting Bolometers for the PIPER Balloon-Borne CMB Polarization Experiment

We are constructing the Primordial Inflation Polarization Explorer (PIPER) to measure the polarization of the cosmic microwave background (CMB) and search for the imprint of gravity waves produced during an inflationary epoch in the early universe. The signal is faint and lies behind confusing foregrounds, both astrophysical and cosmological, and so many detectors are required to complete the measurement in a limited time. We will use four of our matured 1,280 pixel, high-filling-factor backshort-under-grid bolometer arrays for efficient operation at the PIPER CMB wavelengths. All four arrays observe at a common wavelength set by passband filters in the optical path. PIPER will fly four times to observe at wavelengths of 1500, 1100, 850, and 500 μm in order to separate CMB from foreground emission. The arrays employ leg-isolated superconducting transition edge sensor bolometers operated at 128 mK; tuned resonant backshorts for efficient optical coupling; and a second-generation superconducting quantum interference device (SQUID) multiplexer readout. We describe the design, development, and performance of PIPER bolometer array technology to achieve background-limited sensitivity for a cryogenic balloon-borne telescope.