A. D. Turner

Titanium nitride films for ultrasensitive microresonator detectors

Titanium nitride (TiNx) films are ideal for use in superconducting microresonator detectors for the following reasons: (a) the critical temperature varies with composition (0 107) and have noise properties similar to resonators made using other materials, while the quasiparticle lifetimes are reasonably long, 10–200u2002μs. TiN microresonators should therefore reach sensitivities well below 10−19u2002Wu2009Hz−1/2.

Silicon nitride micromesh bolometer array for submillimeter astrophysics

We present the design and performance of a feedhorn-coupled bolometer array intended for a sensitive 350-mum photometer camera. Silicon nitride micromesh absorbers minimize the suspended mass and heat capacity of the bolometers. The temperature transducers, neutron-transmutation-doped Ge thermistors, are attached to the absorber with In bump bonds. Vapor-deposited electrical leads address the thermistors and determine the thermal conductance of the bolometers. The bolometer array demonstrates a dark noise-equivalent power of 2.9 x 10(-17) W/ radicalHz and a mean heat capacity of 1.3 pJ/K at 390 mK. We measure the optical efficiency of the bolometer and feedhorn to be 0.45-0.65 by comparing the response to blackbody calibration sources. The bolometer array demonstrates theoretical noise performance arising from the photon and the phonon and Johnson noise, with photon noise dominant under the design background conditions. We measure the ratio of total noise to photon noise to be 1.21 under an absorbed optical power of 2.4 pW. Excess noise is negligible for audio frequencies as low as 30 mHz. We summarize the trade-offs between bare and feedhorn-coupled detectors and discuss the estimated performance limits of micromesh bolometers. The bolometer array demonstrates the sensitivity required for photon noise-limited performance from a spaceborne, passively cooled telescope.

SPIDER: Probing the Early Universe with a Suborbital Polarimeter

We evaluate the ability of SPIDER, a balloon-borne polarimeter, to detect a divergence-free polarization pattern (B-modes) in the Cosmic Microwave Background (CMB). In the inflationary scenario, the amplitude of this signal is proportional to that of the primordial scalar perturbations through the tensor-to-scalar ratio r. We show that the expected level of systematic error in the SPIDER instrument is significantly below the amplitude of an interesting cosmological signal with r=0.03. We present a scanning strategy that enables us to minimize uncertainty in the reconstruction of the Stokes parameters used to characterize the CMB, while accessing a relatively wide range of angular scales. Evaluating the amplitude of the polarized Galactic emission in the SPIDER field, we conclude that the polarized emission from interstellar dust is as bright or brighter than the cosmological signal at all SPIDER frequencies (90 GHz, 150 GHz, and 280 GHz), a situation similar to that found in the Southern Hole. We show that two ~20-day flights of the SPIDER instrument can constrain the amplitude of the B-mode signal to r<0.03 (99% CL) even when foreground contamination is taken into account. In the absence of foregrounds, the same limit can be reached after one 20-day flight.

SPIDER: A balloon-borne CMB polarimeter for large angular scales

We describe SPIDER, a balloon-borne instrument to map the polarization of the millimeter-wave sky with degree angular resolution. Spider consists of six monochromatic refracting telescopes, each illuminating a focal plane of large-format antenna-coupled bolometer arrays. A total of 2,624 superconducting transition-edge sensors are distributed among three observing bands centered at 90, 150, and 280 GHz. A cold half-wave plate at the aperture of each telescope modulates the polarization of incoming light to control systematics. SPIDERs first flight will be a 20-30-day Antarctic balloon campaign in December 2011. This flight will map ~8% of the sky to achieve unprecedented sensitivity to the polarization signature of the gravitational wave background predicted by inflationary cosmology. The SPIDER mission will also serve as a proving ground for these detector technologies in preparation for a future satellite mission.

ACBAR: the Arcminute Cosmology Bolometer Array Receiver

We describe the Arcminute Cosmology Bolometer Array Receiver (ACBAR); a multifrequency millimeter-wave receiver designed for observations of the cosmic microwave background and the Sunyaev-Zeldovich effect in clusters of galaxies. The ACBAR focal plane consists of a 16 pixel, background-limited, 240 mK bolometer array that can be configured to observe simultaneously at 150, 220, 280, and 350 GHz. With 4-5 FWHM beams and a 3° azimuth chop, ACBAR is sensitive to a wide range of angular scales. ACBAR was installed on the 2 m Viper telescope at the South Pole in 2001 January. We describe the design of the instrument and its performance during the 2001 and 2002 observing seasons.

SPIDER: a balloon-borne large-scale CMB polarimeter

Spider is a balloon-borne experiment that will measure the polarization of the Cosmic Microwave Background over a large fraction of a sky at ~ 1° resolution. Six monochromatic refracting millimeter-wave telescopes with large arrays of antenna-coupled transition-edge superconducting bolometers will provide system sensitivities of 4.2 and 3.1 μKcmb√s at 100 and 150 GHz, respectively. A rotating half-wave plate will modulate the polarization sensitivity of each telescope, controlling systematics. Bolometer arrays operating at 225 GHz and 275 GHz will allow removal of polarized galactic foregrounds. In a 2-6 day first flight from Alice Springs, Australia in 2010, Spider will map 50% of the sky to a depth necessary to improve our knowledge of the reionization optical depth by a large factor.

The Keck Array: a pulse tube cooled CMB polarimeter

The Keck Array is a cosmic microwave background (CMB) polarimeter that will begin observing from the South Pole in late 2010. The initial deployment will consist of three telescopes similar to BICEP2 housed in ultracompact, pulse tube cooled cryostats. Two more receivers will be added the following year. In these proceedings we report on the design and performance of the Keck cryostat. We also report some initial results on the performance of antenna-coupled TES detectors operating in the presence of a pulse tube. We find that the performance of the detectors is not seriously impacted by the replacement of BICEP2s liquid helium cryostat with a pulse tube cooled cryostat.

Silicon nitride micromesh bolometer arrays for SPIRE

We are developing arrays of bolometers based on silicon nitride micromesh absorbers for the Spectral & Photometric Imaging Receiver (SPIRE) on the Far Infra-Red and Submillimeter Space Telescope (FIRST). The bolometers are coupled to a close-packed array of 1 f(lambda) feedhorns which views the primary mirror through a cooled aperture stop. Feedhorn-coupled bolometers minimize the detector area and throughput and have good optical efficiency. A 1 f(lambda) feedhorn array provides, higher mapping speed than a 2 f(lambda) feedhorn array and reduces the number of jitters required to produce a fully sampled map, but at the cost of more detectors. Individual silicon nitride micromesh bolometers are already able to meet the performance requirements of SPIRE. In parallel we are developing transition-edge detectors read out by SQUID current amplifier. The relatively large cooling power available at 300 mK enables the array to be coupled to a cold SQUID multiplexer, creating a monolithic fully multiplexed array and making large format arrays possible for SPIRE.

Antenna-Coupled TES Bolometer Arrays for CMB Polarimetry

We describe the design and performance of polarization selective antenna-coupled TES arrays that will be used in several upcoming Cosmic Microwave Background (CMB) experiments: SPIDER, BICEP-2/SPUD. The fully lithographic polarimeter arrays utilize planar phased-antennas for collimation (F/4 beam) and microstrip filters for band definition (25% bandwidth). These devices demonstrate high optical efficiency, excellent beam shapes, and well-defined spectral bands. The dual-polarization antennas provide well-matched beams and low cross polarization response, both important for high-fidelity polarization measurements. These devices have so far been developed for the 100 GHz and 150 GHz bands, two premier millimeter-wave atmospheric windows for CMB observations. In the near future, the flexible microstrip-coupled architecture can provide photon noise-limited detection for the entire frequency range of the CMBPOL mission. This paper is a summary of the progress we have made since the 2006 SPIE meeting in Orlando, FL.

Spider optimization: Probing the systematics of a large-scale B-mode experiment

Spider is a long-duration, balloon-borne polarimeter designed to measure large-scale cosmic microwave background (CMB) polarization with very high sensitivity and control of systematics. The instrument will map over half the sky with degree angular resolution in the I, Q, and U Stokes parameters in four frequency bands from 96 to 275 GHz. Spiders ultimate goal is to detect the primordial gravity-wave signal imprinted on the CMB B-mode polarization. One of the challenges in achieving this goal is the minimization of the contamination of B-modes by systematic effects. This paper explores a number of instrument systematics and observing strategies in order to optimize B-mode sensitivity. This is done by injecting realistic-amplitude, time-varying systematics into a set of simulated time streams. Tests of the impact of detector noise characteristics, pointing jitter, payload pendulations, polarization angle offsets, beam systematics, and receiver gain drifts are shown. Spiders default observing strategy is to spin continuously in azimuth, with polarization modulation achieved by either a rapidly spinning half-wave plate or a rapidly spinning gondola and a slowly stepped half-wave plate. Although the latter is more susceptible to systematics, the results shown here indicate that either mode of operation can be used by Spider.