Experimental radio cosmology

Abstract:

The Q/U Imaging ExperimenT (QUIET) employs coherent receivers at 43GHz and 95GHz, operating on the Chajnantor plateau in the Atacama Desert in Chile, to measure the anisotropy in the polarization of the CMB. QUIET primarily targets the B modes from primordial gravitational waves. The combination of these frequencies gives sensitivity to foreground contributions from diffuse Galactic synchrotron radiation. Between 2008 October and 2010 December, >10,000hours of data were collected, first with the 19-element 43GHz array (3458hours) and then with the 90-element 95GHz array. Each array observes the same four fields, selected for low foregrounds, together covering ~1000deg^2. This paper reports initial results from the 43GHz receiver which has an array sensitivity to CMB fluctuations of 69uK sqrt(s). The data were extensively studied with a large suite of null tests before the power spectra, determined with two independent pipelines, were examined. Analysis choices, including data selection, were modified until the null tests passed. Cross correlating maps with different telescope pointings is used to eliminate a bias. This paper reports the EE, BB and EB power spectra in the multipole range ell=25-475. With the exception of the lowest multipole bin for one of the fields, where a polarized foreground, consistent with Galactic synchrotron radiation, is detected with 3sigma significance, the E-mode spectrum is consistent with the LCDM model, confirming the only previous detection of the first acoustic peak. The B-mode spectrum is consistent with zero, leading to a measurement of the tensor-to-scalar ratio of r=0.35+1.06-0.87. The combination of a new time-stream double-demodulation technique, Mizuguchi-Dragone optics, natural sky rotation, and frequent boresight rotation leads to the lowest level of systematic contamination in the B-mode power so far reported, below the level of r=0.1

Abstract:

In this paper, we will present the design and the simulation of a 230GHz finline Ultra-wide IF Bandwidth SIS mixer. This mixer will be used in a novel millimeter-wave heterodyne interferometer: GUBBINS. GUBBINS is designed to demonstrate high surface brightness mm-wave interferometry at modest spatial and spectrum resolution. Its observational targets are the spectrum of the Sunyaev-Zel'dovich effect in the brightness galaxy cluster[3]. The archetype of the mixer design described here is an antipodal finline SIS mixer designed by Paul Grimes in Oxford Experimental Cosmology group in 2008[1]. Here several improvement and modification are made to simplify the design and fabrication, and also enhance the IF bandwidth. An unilateral finline replaces the complicated antipodal finline. No RF bandpass filter is needed after finline. The tuning circuit design presented here aims to achieve wider RF coupling bandwidth, even though only a single junction is used. A multi-stage IF transformer follows the IF bonding pad matching the IF output of the mixer to the input of the IF amplifier, as well as reducing the impact of the parasitical capacitance introduced by the RF finline and RF radial stub.

Abstract:

We present the first, complete 230 GHz feedhorn array manufactured by direct drilling of smooth-walled horns into a single plate of aluminium. The horn design process, based on a genetic algorithm, is described and the fabrication process, via direct drilling using shaped drill bits, is presented. We present cross coupling and beam pattern measurements of a close-packed pair of the smooth-walled horns fabricated in a single block of aluminium. We also present a prototype 37 horn array, again fabricated by drilling into a single block. Our measurements show that our designs and fabrication techniques will be robust when applied to large focal arrays of horns consisting of hundreds or thousands of feedhorns. We expect our smooth-walled horn designs and novel manufacturing techniques will offer an attractive, low-cost alternate to traditional horn arrays consisting of electroformed corrugated horns.