BICEP2 II: Experiment and Three-Year Data Set

TL;DR

BICEP2 targets the inflationary B-mode signature in the CMB polarization on degree angular scales by deploying a high-density array of antenna-coupled TES bolometers in a compact, on-axis refractor at the South Pole. The instrument combines low optical loading, tight control of systematics, and a sophisticated data pipeline to achieve photon-noise-limited sensitivity; three years of observations yielded deep polarization maps (≈87 nK-arcmin) over ~384 deg^2 and an array NET of ≈15.8 μK√s. The study thoroughly characterizes the instrument, optics, detectors, cryogenics, shielding, and data reduction, and demonstrates that the measurements are dominated by sky signals rather than systematics, enabling the first detection of B-mode power at degree scales in companion analyses. The work lays the groundwork for future Keck/Spider follow-ups and multi-frequency foreground cleaning, significantly advancing constraints on the inflationary tensor-to-scalar ratio and the energy scale of inflation.

Abstract

We report on the design and performance of the BICEP2 instrument and on its three-year data set. BICEP2 was designed to measure the polarization of the cosmic microwave background (CMB) on angular scales of 1 to 5 degrees ($\ell$=40-200), near the expected peak of the B-mode polarization signature of primordial gravitational waves from cosmic inflation. Measuring B-modes requires dramatic improvements in sensitivity combined with exquisite control of systematics. The BICEP2 telescope observed from the South Pole with a 26~cm aperture and cold, on-axis, refractive optics. BICEP2 also adopted a new detector design in which beam-defining slot antenna arrays couple to transition-edge sensor (TES) bolometers, all fabricated on a common substrate. The antenna-coupled TES detectors supported scalable fabrication and multiplexed readout that allowed BICEP2 to achieve a high detector count of 500 bolometers at 150 GHz, giving unprecedented sensitivity to B-modes at degree angular scales. After optimization of detector and readout parameters, BICEP2 achieved an instrument noise-equivalent temperature of 15.8 $μ$K sqrt(s). The full data set reached Stokes Q and U map depths of 87.2 nK in square-degree pixels (5.2 $μ$K arcmin) over an effective area of 384 square degrees within a 1000 square degree field. These are the deepest CMB polarization maps at degree angular scales to date. The power spectrum analysis presented in a companion paper has resulted in a significant detection of B-mode polarization at degree scales.

Figures (24)

• Figure 1: The Bicep2 telescope in the mount, looking out through the roof of the Dark Sector Laboratory (DSL) located 800 m from the geographic South Pole. The three-axis mount allows for motion in azimuth, elevation, and boresight rotation (also called "deck rotation"). An absorbing forebaffle and reflective ground screen prevent sidelobes from coupling to nearby objects on the ground. A flexible environmental seal or "boot" maintains a room temperature environment around the cryostat and mount. The telescope forms an insert within the liquid helium cryostat. The focal plane with polarization-sensitive TES bolometers is cooled to 270 mK by a $^4$He/$^3$He/$^3$He sorption refrigerator. The housekeeping electronics (§\ref{['sec:housekeeping']}) and Multi-Channel Electronics (MCE, §\ref{['sec:mce']}) attach to the lower bulkhead of the cryostat.
• Figure 2: Bicep2 absorbing forebaffle, flexible environmental seal (the "boot"), and ground shield. The telescope and mount sat below the boot inside the Dark Sector Laboratory.
• Figure 3: The telescope optical system. All components (except the window) were anti-reflection coated to provide minimal reflection at 150 GHz. All optics below the 40 K nylon filter were cooled to 4 K, providing low and stable optical loading. Due in large part to the radially symmetric design, simulations predict well-matched beams for two idealized orthogonally polarized detectors at the focal plane.
• Figure 4: Cross-sectional view of the telescope insert. The entire telescope insert assembly is cooled to $4~\mathrm{K}$ by a thermal link to a liquid helium bath. The optics tube provides rigid structural support for the optical chain, including the lenses, filters, and aperture stop. The camera tube assembly houses the sub-kelvin sorption refrigerator and the cryogenic readout electronics in a radiatively and thermally protected enclosure. The sub-kelvin focal plane assembly sits within a superconducting Nb magnetic shield. The focal plane is thermally connected to the fridge via a passive thermal filter.
• Figure 5: Cross-sectional view of the sub-kelvin hardware. The superconducting Nb magnetic shield is heat-sunk to $350~\mathrm{mK}$. Within, the focal plane is isolated from thermal fluctuations by eight carbon fiber legs. A thin aluminized Mylar shroud extends from the top of the focal plane assembly to the bottom of the Nb magnetic shield to minimize radio frequency pickup. Temperature stability is maintained through the combined use of active and passive filtering. The passive thermal filter, on the bottom of the focal plane, serves to roll off thermal fluctuations at frequencies relevant to science observations, while active temperature control modules maintain sub-millikelvin stability over typical observation cycles.