BICEP Array Detectors and Instrumentation at 30/40 GHz: Design, Performance, and Deployment to the South Pole for Constraining Primordial Gravitational Waves

Citation

Mohamed Soliman, Ahmed Mohamed (2023) BICEP Array Detectors and Instrumentation at 30/40 GHz: Design, Performance, and Deployment to the South Pole for Constraining Primordial Gravitational Waves. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/srdx-w019. https://resolver.caltech.edu/CaltechTHESIS:05232023-174453276

Abstract

The discovery of the Cosmic Microwave Background (CMB) in the 1960s has provided strong observational evidence for the Big Bang cosmological model to describe the origin and evolution of the universe. The theory of cosmic inflation was developed in the 1980s to account for the initial density perturbations by a period of exponential expansion in the early Universe to solve the horizon, flatness and monopole problems. Many inflation models predict potentially detectable primordial gravitational-waves (PGWs) background that imprint a B-mode polarization pattern in the CMB. The amplitude of the inflationary B-mode polarization depends on the energy scale of inflation and is parameterized by the tensor-to-scalar ratio r. The detection of a B-mode pattern would open a new window to probe the energy scale at the beginning of time when the universe was a mere fraction of a second old after the Big Bang.

The BICEP/Keck collaboration is building a series of experiments located at the Amundsen-Scott South Pole Station to map the polarization of the CMB at degree angular scales using small-aperture telescopes. Our latest BICEP/Keck publications use data collected through 2018 and report the strongest constraints r_0.05 < 0.036 at 95% confidence. The current sensitivity on r is limited by the variance from the gravitational lensing. BICEP/Keck is starting a collaboration with the South Pole Telescope (SPT) team to develop delensing techniques to improve future constraints on r. Characterizing Galactic foregrounds, especially synchrotron emission, remains a priority in order to improve constraints as statistical sensitivity continues to improve. The motivation for this thesis is to develop a highly sensitive receiver at 30 and 40 GHz, at frequencies where the synchrotron foreground dominates. BICEP Array represents the latest phase in the BICEP/Keck experiments, and will map the polarization of the CMB at 30/40, 95, 150, and 220/270 GHz. BICEP Array will search for PGWs with unprecedented sensitivity levels on r by characterizing and removing Galactic synchrotron and dust emission from our maps of the CMB.

My PhD thesis focuses on the technology development for high sensitivity detectors and instrumentation to successfully deploy the first BICEP Array receiver at 30 GHz and 40 GHz to the South Pole in order to constrain the Galactic synchrotron foreground. My dissertation presents the receiver design and performance. I will first explain the engineering design principles, the fabrication and a laboratory demonstration of single-color antenna-coupled Transition Edge Sensor (TES) bolometers. Secondly, I will discuss the design and demonstration of dual-color detectors at 30 and 40 GHz that gain receiver sensitivity by increasing the optical throughput and bandwidth of each pixel. I also developed microstrip diplexer circuits that divide the detector bandwidth into two CMB observing channels. I optimized this approach to design the dual-color bowtie-coupled detector at 90/150 GHz. Thirdly, I will introduce a new wide-band corrugated focal plane module design to minimize the beam mismatch systematic at 30 and 40 GHz bands simultaneously. Our receivers map polarization of the CMB by taking the difference between co-located and orthogonally polarized pair of detectors. Polarized beam difference measurements show a differential beam response due to a shift between the polarization beam centers within a pixel due to an electromagnetic interaction with the focal plane frame. The residual beams leak a temperature to polarization (T-P) in the CMB polarization maps and can produce a false B-mode signal that introduces non-negligible systematic errors for BICEP Array measurements to come with improved sensitivity. The wide-band design reduces this effect and associated systematic errors for 30 and 40 GHz receiver. I also developed a new single-band corrugated focal plane module design for 150 GHz receiver. I performed laboratory measurements of these designs at 30, 40, and 150 GHz to verify the modelled response. The corrugation design will also be extended to the 220/270 GHz receiver. Fourthly, I will show my contributions to the receiver deployment, integration and calibration during the first 2020 observing season. The measurements will include the full optical characterization of the detector camera, in-lab and on-sky sensitivity at the South Pole. I will also describe the tests done to diagnose the challenges during the first season and new upgrades during the second 2022 season to improve the overall sensitivity of the receiver. Improved detector modules have been installed during the 2023 season to further boost the mapping speed for measuring the synchrotron foreground.

The technologies developed for BICEP Array feed into capabilities for the upcoming CMB-S4 program. For example, I used similar methods to design a diplexer for a CMB-S4 dual-color feedhorn-coupled detector design at 90/150 GHz. I will also detail my work on the cryogenic implementation and test of an Adiabatic Demagnetization Refrigerator suitable for demonstrating 100 mK CMB-S4 detector arrays in a prototype 95/150 GHz telescope planned to observe on the BICEP Array.

Thesis Files