Detectors for the Atacama B-mode Search Experiment

Abstract

Inflation is the leading theory for explaining the initial conditions that brought about our

homogeneous and isotropic Universe. It predicts the presence of gravitational waves in the

early Universe, which implant a characteristic B-mode polarization pattern on the Cosmic

Microwave Background (CMB). The Atacama B-mode Search (ABS) experiment is a po-

larimeter observing from Cerro Toco (located in the Atacama desert of Chile at an altitude

of 5190 m), searching for the yet undetected B-mode signal. ABS carries 480 superconduct-

ing Transition Edge Sensor (TES) Bolometers that couple 150 GHz radiation via planar

Ortho-Mode Transducers (OMTs) mounted at the output of corrugated feedhorns. The

feedhorn beam is projected onto the sky through crossed Dragonian reflectors, a set of

reflective and absorptive filters, and a rotating Half Wave Plate (HWP) that modulates

any polarized sky signal at 10.2 Hz. The bolometers are cooled to 300 mK by a He3-He4

adsorption fridge system backed by pulse tubes. The reflectors are located within the 4 K

cavity of the cryostat, while the HWP is mounted on frictionless air bearings above the

cryostat window. This thesis discusses the development and construction of the ABS de-

tector focal plane, and presents results of its performance in the field through August 2012.

The ABS detector array sensitivity of 31 μKs1/2 , together with the experiment's unique

set of systematic controls, and expected multi-year integration time, could detect a B-mode

signal with tensor to scalar ratio r ∼ 0.1.