We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible misinterpretation of the data. We consider the relationship between ballistic and diffusive transport, the effect of different transport regimes on spin polarization measurements, and the importance of unambiguous identification of the type of transport regime. We find that in a realistic parameter range, the analysis of PCAR spectra of purely diffusive character by a ballistic model yield approximately the same (within ~3%) values of the spin polarization and the barrier strength Z larger by ~ 0.5-0.6. We also consider the dependence of polarization values on Z, and have shown by simple modeling that letting the superconducting gap vary as an adjustable parameter can result in a spurious dependence of the spinpolarization P c on Z. At the same time we analyzed the effects of finite Z on the apparent value of P c measured by the PCAR technique, using a large number of examples from both our own measurements and from the literature. We conclude that there is a systemdependent variation in P c (Z), presumably due to spin-flip scattering at the interface.However, the exact type of this dependence is hard to determine with any statistical certainty.

An international Advisory Group on Uncertainties has published guidelines for the statistical analysis of a simple key comparison carried out by the Consultative Committees of the International Committee of Weights and Measures (CIPM) where a travelling standard of a stable value is circulated among the participants. We discuss several concerns regarding these guidelines. Then, we describe a statistical model based on the Guide to the Expression of Uncertainty in Measurement to establish the key comparison reference value, the degrees of equivalence, and their associated standard uncertainties on the basis of the data submitted by the participants. The proposed statistical model applies to all those CIPM key comparisons where the submitted results are mutually comparable and appropriate for determining the key comparison reference value and the submitted uncertainties are sufficiently reliable.

To implement the detector-based radiometric scale in the new Medium Background Infrared (MBIR) facility at the National Institute of Standards and Technology (NIST), we have developed an electrical substitution cavity radiometer that can be operated just above liquid nitrogen temperature. This MBIR Active Cavity Radiometer (ACR) utilizes a temperature-controlled receiver cone and an independently temperature-controlled heat sink. Being a thermal-type detector, low noise and drift of the radiometer signal depends mainly on low-noise temperature control of the receiver and heat sink. Using high critical-temperature (T c) superconducting thin film temperature sensors in the active control loops, we have achieved closed-loop temperature controllability of better than 10 µK at 89 K for a receiver having an open-loop thermal time constant of about 75 seconds. For a flux level of 1 µW to 10 µW, the rms noise floor over a measurement cycle time is below 20 nW. This is the lowest noise level yet reported for a liquid nitrogen cooled electrical-substitution radiometer, and it is the first demonstration of the use of high-T c superconductors in such a radiometer. Potential uses for this ACR in the MBIR facility include absolute measurement of the broadband radiance of large-area 300 K cryogenic blackbody sources, and absolute measurement of the spectral radiance of laser-illuminated integrating spheres for improved relative spectral responsivity measurements of infrared transfer standard radiometers.

The Low Background Infrared Calibration Facility (LBIR) at the National Institute of Standards and Technology has been in operation for calibration measurements of ihe radiant power emitted fn>rn infrared radiation (IR) sources, such as cryogenic blaclcbodies, for more than 2 years. The IR sources arc sent to NIST by Customers from industiy, government, and university laborato^ii^s. An absolute ciyngenic radiometer is used as the standard detector to measure the total radiant power at its aperture. The low background is provided by a closed cycle helium refrigeration system that maintains the inner parts of Ihe calibration chamber at 20 K. The radiance temper* ature of the blackbody is deduced from the measured power and cottipared with the htackbody temperature sensor data. The calibration procedures and data analysts arc illustrated using the measurements of a typical blackbody.

The Low Background Infrared (LBIR) facility at the National Institute of Standards and Technology (NIST) has performed ten radiance temperature calibrations of low-background blackbodies since 2001, when both the calibration facility and method of calibrating blackbodies were significantly improved. Data from nine of these blackbody calibrations are presented, showing a surprisingly large spread in blackbody performance. While some blackbodies performed relatively well, in no case did the measured radiance temperature agree with the temperature sensors in the blackbody core to within 0.3 K over the entire operating temperature range of the blackbody. Of the nine blackbodies reported, five showed temperature errors greater than 1 K at some point in their operating temperature range. The various sources of uncertainty, such as optical geometry and detector standard uncertainty, are presented with examples to support the stated calibration accuracy. Generic blackbody cavity design features, such as cavity thermal mass, cavity volume and defining aperture placement are discussed and correlated with blackbody performance. Data are also presented on the performance of the absolute cryogenic radiometers (ACRs) that are used as detector standards in the calibration of blackbodies. Recent intercomparisons of all the LBIR ACRs with a trap detector calibrated against the NIST primary optical power measurement standard show that ACRs used to calibrate blackbodies are suitable detector standards and contribute less than 0.02% uncertainty (k = 1) to radiance temperature measurements of the blackbody cavities.