Stephan S. Meyer

Monolithic silicon bolometers

A new type of bolometer detector for the millimeter and submillimeter spectral range is described. The bolometer is constructed of silicon using integrated circuit fabrication techniques. Ion implantation is used to give controlled resistance vs temperature properties as well as extremely low 1/f noise contacts. The devices have been tested between 4.2 and 0.3 K. The best electrical NEP measured is 4 × 10 - 1 6 W/Hz at 0.35 K between 1- and 10-Hz modulation frequency. This device had a detecting area of 0.25 cm2 and a time constant of 20 msec at a bath temperature of 0.35 K.

A measurement of the medium-scale anisotropy in the cosmic microwave background radiation

Observations from the first flight of the Medium Scale Anisotropy Measurement (MSAM) are analyzed to place limits on Gaussian fluctuations in the cosmic microwave background radiation (CMBR). This instrument chops a 30 min beam in a three-position pattern with a throw of +/- 40 min; the resulting data is analyzed in statistically independent single- and double-difference sets. We observe in four spectral channels at 5.6, 9.0, 16.5, and 22.5/cm, allowing the separation of interstellar dust emission from CMBR fluctuations. The dust component is correlated with the IRAS 100 micron map. The CMBR component has two regions where the signature of an unresolved source is seen. Rejecting these two source regions, we obtain a detection of fluctuations which match CMBR in our spectral bands of 0.6 x 10(exp -5) is less than Delta (T)/T is less than 2.2 x 10(exp -5) (90% CL interval) for total rms Gaussian fluctuations with correlation angle 0.5 deg, using the single-difference demodulation. Fore the double difference demodulation, the result is 1.1 x 10(exp -5) is less than Delta(T)/T is less than 3.1 x 10(exp -5) (90% CL interval) at a correlation angle of 0.3 deg.

A large-scale cosmic microwave background anisotropy measurement at millimeter and submillimeter wavelengths

A balloon-borne experiment to measure the anisotropy of the cosmic microwave background radiation at angular scales of 4 deg or greater is reported. The instrument simultaneously measures in four spectral bands centered on 5.6, 8.7, 15.8, and 22.5/cm. Three results are presented: (1) the 95-percent confidence limit for monochromatic anisotropies is 0.0001 or less on angular scales of 10 deg; (2) the Galactic plane dust emission at l = 42 deg is consistent with a nu-squared emissivity law at frequencies above 15/cm, with excess emission below 15/cm; and (3) atmospheric ozone at an altitude of 35 km may form clumps as large as Delta emissivity/emissivity = 0.002. 21 refs.

Millimeter–submillimeter wavelength filter system

We describe the design, fabrication, measurement, and performance of a set of cryogenic millimetersubmillimeter wavelength filters used in a balloonborne bolometric radiometer. The set contains single resonant mesh grids used as dichroic beam splitters, resonant meshes in a double quarter-wave configuration, a commercial inductive grid filter, and high-frequency blocking filters. The resultant system has passbands at λ = 1.73, 1.05, 0.61, 0.44 mm with δλ/λ = 0.23, 0.23, 0.12, 0.06. Limits on high-frequency leakage are deduced from laboratory measurements and from the analysis of flight data. The filter set response to three different sources of radiation is presented to show the method and limitations of our characterization. The key element of the filter system is a resonant periodic array of cross-shaped holes etched in thin aluminum. We give an empirical scaling law for the resonant wavelength as a function of structure parameters for aluminum on 25-µm-thick Mylar. Plots of the transmittance for normally incident radiation and the transmittance and reflectance for a 45° incident radiation are presented.

Early results from the Cosmic Background Explorer (COBE)

The Cosmic Background Explorer, launched November 18, 1989, has nearly completed its first full mapping of the sky with all three of its instruments: a Far Infrared Absolute Spectrophotometer (FIRAS) covering 0.1 to 10 mm, a set of Differential Microwave Radiometers (DMR) operating at 3.3, 5.7, and 9.6 mm, and a Diffuse Infrared Background Experiment (DIRBE) spanning 1 to 300 μm in ten bands. A preliminary map of the sky derived from DIRBE data is presented. Initial cosmological implications include: a limit on the Comptonization y parameter of 10−3, on the chemical potential μ parameter of 10-2, a strong limit on the existence of a hot smooth intergalactic medium, and a confirmation that the dipole anisotropy has the spectrum expected from a Doppler shift of a blackbody. There are no significant anisotropies in the microwave sky detected, other than from our own galaxy and a cosθ dipole anisotropy whose amplitude and direction agree with previous data. At shorter wavelengths, the sky spectrum and anisotropies are dominated by emission from ‘local’ sources of emission within our Galaxy and Solar System. Preliminary comparison of IRAS and DIRBE sky brightnesses toward the ecliptic poles shows the IRAS values to be significantly higher than found by DIRBE at 100 μm. We suggest the presence of gain and zero-point errors in the IRAS total brightness data. The spacecraft, instrument designs, and data reduction methods are described.

First results of the COBE satellite measurement of the anisotropy of the cosmic microwave background radiation

Abstract We review the concept and operation of the Differential Microwave Radiometers (DMR) instrument aboard NASAs Cosmic Background Explorer (COBE) satellite, with emphasis on the software identification and subtraction of potential systematic effects. We present preliminary results obtained from the first six months of DMR data and discuss implications for cosmology.

Preliminary DMR measurements of the CMB isotropy

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full-sky maps at frequencies 31.5, 53, and 90 GHz. The redundant channels and matched beams at three frequencies distinguish the DMR from previous large-scale surveys. Galactic emission is seen unambiguously at all three frequencies. The only large-scale anisotropy detected in the cosmic microwave background is the dipole anisotropy. There is no clear evidence for any other large-angular-scale feature in the maps. Without correcting for any systematic effects, we are able to place limits {Delta}T/T{sub 0}{lt}3{times}10{sup {minus}5} for the rms quadrupole amplitude, {Delta}T/T{sub 0}{lt}4{times}10{sup {minus}5} for monochromatic fluctuations, and {Delta}T/T{sub 0}{lt}4{times}10{sup {minus}5} for Gaussian fluctuations (all limits are 95% C.L. with T{sub 0}=2.735 K). The data limit {Delta}T/T{sub 0}{lt}10{sup {minus}4} for any feature larger than 7{degree}. We briefly review the DMR and discuss some implications of these results for cosmology.

Early results from the far infrared absolute spectrophotometer (FIRAS)

The Far Infrared Absolute spectrophotometer (FIRAS) on the Cosmic Background Explorer (COBE) mapped 98% of the sky, 60% of it twice, before the liquid helium coolant was exhausted. The FIRAS covers the frequency region from 1 to 100 cm−1 (10 mm to 0.1 mm wavelength) with a 7° angular resolution. The spectal resolution is 0.2 cm−1 for frequencies less than 20 cm−1 and 0.8 cm−1 for higher frequencies. Preliminary results include: a limit on the deviations from a Planck curve of 1% of the peak brightness from 1 to 20 cm−1, a temperature of 2.735±0.06 K, a limit on the Comptonization parameter y of 10−3, on the chemical potential parameter μ of 10−2, a strong limit on the existence of a hot smooth intergalactic medium, and a confirmation that the dipole anisotropy spectrum is that of a Doppler shifted blackbody. Although there are many unresolved issues about the data processing and analysis of systematic errors, some of which are described here, the instruments is expected to achieve its goal of an accuracy ...

Resonant cryogenic chopper

We describe the design, construction, and performance of a resonant cryogenic chopper that operates at 4.2 K. The chopper is mechanically and thermally robust; it can occult a 2.54-cm aperture at 4.5 Hz while dissipating ~1 mW. Both the stator and rotor magnetic fields are controllable to allow for performance optimization and to help in measuring any possible interference effects. Data on long-term amplitude stability are presented.

Early results from the MIT millimeter and sub‐millimeter balloon‐borne anisotropy measurement

The MIT balloon‐borne bolometric search for Cosmic Microwave Background Radiation (CMBR) anisotropies places the most stringent constraints to date on the fluctuations in the CMBR. Four maps of half of the Northern Hemisphere at 1.8, 1.1, 0.63 and 0.44 mm wavelength, have a beam size of 3.8° with a 1 sigma sensitivity of less than 0.1 mK (thermodynamic) per FOV in each of the first two channels. Analysis of the sky map at 1.8 mm wavelength using a likelihood ratio test for galactic latitudes ‖ b ‖≥15° yields a 95% confidence level (CL) upper limit on fluctuations of the CMBR at ΔT/T≤1.6×10−5 with a statistical power of 92% for Gaussian fluctuations at a correlation angle of 13°. Between 3° and 22°, the upper limit of fluctuations is ΔT/T≤4.0×10−5 (95% CL). An anisotropy is detected in the map but it cannot yet be attributed to primordial sources. The ultimate sensitivity for this experiment is ΔT/T∼7×10−6 (95% CL) over this angular range for Gaussian fluctuations.