Design and Validation of a Cold Load for Characterization of CMB-S4 Detectors

TL;DR

This paper introduces a novel cryogenic blackbody source, or cold load, designed for characterizing CMB-S4 detectors, featuring operation at 1 K, a 3He heat switch, and external optical coupling, validated with a Spider experiment detector array.

Contribution

The paper presents a new cold load design with three innovative features for CMB detector characterization, validated through thermal and optical tests.

Findings

Cold load operates effectively at 1 K without disturbing other cryogenic stages.

External optical signals can be coupled without adding significant load.

Design successfully validated with a 150 GHz detector array.

Abstract

We present the design and validation of a variable temperature cryogenic blackbody source, hereinafter called a cold load, that will be used to characterize detectors to be deployed by CMB-S4, the next-generation ground-based cosmic microwave background (CMB) experiment. Although cold loads have been used for detector characterization by previous CMB experiments, this cold load has three novel design features: (1) the ability to operate from the 1 K stage of a dilution refrigerator (DR), (2) a 3He gas-gap heat switch to reduce cooling time, and (3) the ability to couple small external optical signals to measure detector optical time constants under low optical loading. The efficacy of this design was validated using a 150 GHz detector array previously deployed by the Spider experiment. Thermal tests showed that the cold load can be heated to temperatures required for characterizing CMB-S4's detectors without significantly impacting the temperatures of other cryogenic stages when mounted to the DR's 1 K stage. Additionally, optical tests demonstrated that external signals can be coupled to a detector array through the cold load without imparting a significant optical load on the detectors, which will enable measurements of the CMB-S4 detectors' optical time constants.