Astrophysics, And Diffuse Emission Research Articles

Design and calibration of a cryogenic blackbody calibrator at centimeter wavelengths

We describe the design and calibration of an external cryogenic blackbody calibrator used for the first two flights of the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) instrument. The calibrator consists of a microwave absorber weakly coupled to a superfluid liquid helium bath. Half-wave corrugations viewed 30° off axis reduce the return loss below −35 dB within a compact footprint. Ruthenium oxide resistive thermometers embedded within the absorber monitor the temperature across the face of the calibrator. The thermal calibration transfers the calibration of a reference thermometer to the flight thermometers using the flight thermometer readout system. The calibrator thermometry is stable in time over four years, with statistical uncertainty in the temperature calibration of order 2 mK near 2.7 K, limited primarily by thermal fluctuations in the liquid helium bath. Observations of the superfluid transition demonstrate that the absolute temperature scale is accurate within 0.3 mK.

An Instrument to Measure the Temperature of the Cosmic Microwave Background Radiation at Centimeter Wavelengths

The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) is a balloon-borne instrument to measure the temperature of the cosmic microwave background at centimeter wavelengths. ARCADE uses narrow-band cryogenic radiometers to compare the sky to an external full-aperture calibrator. To minimize potential sources of systematic error, ARCADE uses a novel open-aperture design which maintains the antennas and calibrator at temperatures near 3 K at the mouth of an open bucket Dewar, without windows or other warm objects between the antennas and the sky. We discuss the design and performance of the ARCADE instrument from its 2001 and 2003 flights.

The Temperature of the Cosmic Microwave Background at 10 GHz

We report the results of an effort to measure the low-frequency portion of the spectrum of the cosmic microwave background (CMB) radiation, using a balloon-borne instrument called the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE). These measurements are to search for deviations from a thermal spectrum that are expected to exist in the CMB as a result of various processes in the early universe. The radiometric temperature was measured at 10 and 30 GHz using a cryogenic open-aperture instrument with no emissive windows. An external blackbody calibrator provides an in situ reference. Systematic errors were greatly reduced by using differential radiometers and cooling all critical components to physical temperatures approximating the antenna temperature of the sky. A linear model is used to compare the radiometer output to a set of thermometers on the instrument. The unmodeled residuals are less than 50 mK peak to peak with a weighted rms of 6 mK. Small corrections are made for the residual emission from the flight train, atmosphere, and foreground Galactic emission. The measured radiometric temperature of the CMB is 2.721 ± 0.010 K at 10 GHz and 2.694 ± 0.032 K at 30 GHz.

Reionization and structure formation with ARCADE

Structure formation in the early universe is expected to produce a cosmological background of free–free emission. The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) will measure the spectrum of the cosmic microwave background at centimeter wavelengths to detect this signature of structure formation. I describe the ARCADE instrument and the cryogenic engineering required to achieve millikelvin accuracy at such long wavelengths.

A low noise thermometer readout for ruthenium oxide resistors

The thermometer readout and thermal control system for the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) experiment is described, including the design, testing, and results from the first flight of ARCADE. The noise is equivalent to about 1 Ω or 0.15 mK in a second for the RuO2 resistive thermometers at 2.7 K. The average power dissipation in each thermometer is 1 nW. The control system can take full advantage of the thermometers to maintain stable temperatures. Systematic effects are still under investigation, but the measured precision and accuracy are sufficient to allow measurement of the cosmic background spectrum.