The Magnetic Microbolometer Detection Chain: A Proposed Detection System to Observe the B Modes of the Cosmic Microwave Background

Abstract

Several experiments are currently carried out to measure the magnitude of the B mode polarization of the cosmic microwave background (CMB). It is a strong indicator of the presence of gravitational waves from the early universe inflationary epoch. As the average variations of the B mode components of the CMB are expected to be of the order of a few tens of nK or below, the detection of these polarized signals requires an ultrasensitive system. This article is focused on CMB detection at frequencies around the 150 GHz band of the electromagnetic spectrum, near the peak of the CMB 2.7K blackbody band of the EM spectrum. We propose a readout system for CMB cryogenic detection based on a software-defined radio (SDR) that uses frequency division multiplexing (FDM), a Goertzel channelizer and a radio frequency microwave SQUID multiplexer (uMUX) working at the cryogenic temperatures of ~320mK. These interfaces can be used to read an array of 1024 magnetic microbolometers (MMBs) as detectors that are photon-limited for CMB detection in the band of interest. As part of the requirements for these measurements, we introduce a design of the detection and read out chain and show its expected performance and potential implementation. The proposed system can read the desired number of detectors from an array in a modular way, which allows future expansions, and its frequency division multiplexing system improves the cooling capacity of the cryostat by minimizing the amount of active cryogenic electronics. In this article, we first describe this proposed FDM readout chain and then present noise measurements of a test implementation.