Advanced ACTPol Cryogenic Detector Arrays and Readout

Abstract: Advanced ACTPol is a polarization-sensitive upgrade for the 6 m aperture Atacama Cosmology Telescope (ACT), adding new frequencies and increasing sensitivity over the previous ACTPol receiver. In 2016, Advanced ACTPol will begin to map approximately half the sky in five frequency bands (28-230 GHz). Its maps of primary and secondary cosmic microwave background (CMB) anisotropies -- imaged in intensity and polarization at few arcminute-scale resolution -- will enable precision cosmological constraints and also a wide array of cross-correlation science that probes the expansion history of the universe and the growth of structure via gravitational collapse. To accomplish these scientific goals, the Advanced ACTPol receiver will be a significant upgrade to the ACTPol receiver, including four new multichroic arrays of cryogenic, feedhorn-coupled AlMn transition edge sensor (TES) polarimeters (fabricated on 150 mm diameter wafers); a system of continuously rotating meta-material silicon half-wave plates; and a new multiplexing readout architecture which uses superconducting quantum interference devices (SQUIDs) and time division to achieve a 64-row multiplexing factor. Here we present the status and scientific goals of the Advanced ACTPol instrument, emphasizing the design and implementation of the Advanced ACTPol cryogenic detector arrays.

• The paper presents the AdvACT upgrade which improves CMB mapping through advanced multichroic detector arrays and refined readout systems.
• It utilizes continuously rotating metamaterial silicon half-wave plates and a high-density SQUID multiplexing architecture to mitigate noise and systematic errors.
• The project’s enhanced sensitivity is expected to nearly halve uncertainties in key cosmological parameters and improve cross-correlation studies with other surveys.

Advanced ACTPol Cryogenic Detector Arrays and Readout

The article presents a comprehensive exploration of the Advanced ACTPol (AdvACT) upgrade, an enhanced version of the original ACTPol receiver for the Atacama Cosmology Telescope. This upgrade focuses on increasing the sensitivity and bandwidth of the receiver to achieve more accurate mapping of the Cosmic Microwave Background (CMB) in polarization and intensity across a wide range of frequencies.

Scientific Objectives and Methodology

AdvACT aims to map nearly half of the sky in five frequency bands ranging from 28 GHz to 230 GHz, replacing the existing ACTPol receiver arrays. This enhancement is achieved by deploying four new multichroic arrays with a significantly improved detector count. The principal scientific objectives include providing more precise constraints on cosmological models and key parameters like the baryon and matter densities as well as extending the exploration into the epoch of cosmic inflation and the growth of structure in the universe.

The authors detail the instrumentation upgrades designed to support these ambitious scientific goals. For instance, AdvACT employs continuously rotating metamaterial silicon half-wave plates to manage polarization systematics, effectively modulating large angular scale signals for improved detection capabilities. The substantial increase in detector count will achieve map noise levels that enable detailed observations of various cosmological phenomena.

Instrument Design and Implementation

The AdvACT upgrade involves substantial updates to the existing hardware. It reuses the ACTPol's existing cryostat infrastructure, combined with new multichroic detector arrays. These arrays leverage transition-edge sensor (TES) bolometers fabricated on large 150 mm wafers. Each detector within an array is read out using an advanced multiplexing architecture that utilizes superconducting quantum interference devices (SQUIDs). This system permits a 64-row multiplexing factor—significantly higher than previous implementations—therefore facilitating high-density data collection efficiently.

Optical upgrades include new silicon lenses with broadband metamaterial anti-reflective coatings and feedhorn arrays optimized for the new frequency bands. These configurations serve to maintain high levels of polarization efficiency and reduce optical loss.

Numerical and Projected Outcomes

AdvACT's heightened sensitivity is expected to nearly halve the uncertainty of several cosmological parameters, translating to a significant improvement in understanding the lambda cold dark matter (ΛCDM) model. Moreover, the upgrade anticipates the detection of a multitude of secondary CMB anisotropies, such as the Sunyaev-Zel'dovich effects and gravitational lensing signals, that can be cross-correlated with other surveys for improved cosmological insights.

These advancements highlight how AdvACT will enhance capabilities for robust cross-correlation studies, leveraging overlaps with various other astrophysical and cosmological surveys, including the LSST and eROSITA, among others. This collaborative approach is projected to yield detections of a high number of galaxy clusters and millimeter sources, with pivotal implications for dark matter distribution and neutrino mass constraints.

Concluding Remarks and Future Prospects

The deployment of AdvACT represents a significant step forward in ground-based CMB research. The phased implementation of upgraded arrays between 2016 and 2018 marks a decisive period of development and integration. This work is supported by various national and international funding bodies, underscoring the wide-reaching collaboration and commitment to advancing understanding of the cosmos. Looking further ahead, the innovative technologies and methodologies refined through AdvACT could set a precedent for future astronomical instrumentation and research endeavors.