Design and characterization of new 90 GHz detectors for the Cosmology Large Angular Scale Surveyor (CLASS)

TL;DR

This paper presents the design, fabrication, and laboratory characterization of new 90 GHz detectors for the CLASS telescope, aiming to improve stability and efficiency in measuring CMB polarization patterns.

Contribution

It introduces design modifications to TES bolometers and provides in-lab performance data, including noise and bandpass measurements, for the upgraded detectors.

Findings

Median NEP of 12.3 aW/√s indicating photon noise limit

Successful assembly and installation of four new detector wafers

Detectors are expected to be photon noise limited in field

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) is a polarization-sensitive telescope array located at an altitude of 5,200 m in the Chilean Atacama Desert. CLASS is designed to measure "E-mode" (even parity) and "B-mode" (odd parity) polarization patterns in the Cosmic Microwave Background (CMB) over large angular scales with the aim of improving our understanding of inflation, reionization, and dark matter. CLASS is currently observing with three telescopes covering four frequency bands: one at 40 GHz (Q); one at 90 GHz (W1); and one dichroic system at 150/220 GHz (G). In these proceedings, we discuss the updated design and in-lab characterization of new 90 GHz detectors. The new detectors include design changes to the transition-edge sensor (TES) bolometer architecture, which aim to improve stability and optical efficiency. We assembled and tested four new detector wafers, to replace four modules of the W1 focal plane. These detectors were installed into the W1 telescope, and will achieve first light in the austral winter of 2022. We present electrothermal parameters and bandpass measurements from in-lab dark and optical testing. From in-lab dark tests, we also measure a median NEP of 12.3 $\mathrm{aW\sqrt{s}}$ across all four wafers about the CLASS signal band, which is below the expected photon NEP of 32 $\mathrm{aW\sqrt{s}}$ from the field. We therefore expect the new detectors to be photon noise limited.