First End-to-End Testing of Prototype CMB-S4 Cold Detector Modules and Readout Chains

Abstract

The cosmic microwave background (CMB), relic light emitted after the Big Bang, is a key probe of the early universe and inflationary theory. The CMB stage four (CMB-S4) project was intended as a ground-based telescope survey to map the temperature and polarization of the CMB with over 500 000 detectors. We report measurements from the first end-to-end test of prototype CMB-S4 detector and readout modules. The detector wafers consisted of 90- and 150-GHz transition-edge sensor (TES) detectors coupled to conical feedhorns via orthomode transducers and operated at 100 mK inside a dilution refrigerator. Signals were read out using a superconducting quantum interference device-based time-division multiplexing system. To support CMB-S4’s plans for eight different types of detector wafers, the cryogenic readout electronics were modular. We report on the prototype design and performance, including characterization of TES response, readout performance, and optical response measured with a cryogenic cold load (readout here meaning everything that moves signals from detectors to external computers). We also report on the performance of a convolutional neural network to identify valid loadcurves for TES parameter characterization as part of TES characterization software; this software was used to characterize TESs to test the readout chain and identify problems in the test bed. We discuss identifying heating in and heatsinking a large detector module assembly. These tests show that the prototype CMB-S4 detector wafer and readout chain function as intended. While CMB-S4 will not move forward as planned, its design, prototyping, and test beds will benefit the development of future experiments.