A LIQUID-HELIUM-COOLED ABSOLUTE REFERENCE COLD LOAD FOR LONG-WAVELENGTH RADIOMETRIC CALIBRATION

We describe a large (78 cm) diameter liquid-helium-cooled blackbody absolute reference cold load for the calibration of microwave radiometers. The load provides an absolute calibration near the liquid-helium (LHe) boiling point, with total uncertainty in the radiometric temperature of less than 30 mK over the 2.5-23-cm wavelength (12-1.3 GHz) operating range. Emission from those parts of the cold load not immersed in LHe is less-than-or-equal-to 25 mK and the reflection coefficient is less-than-or-equal-to 3.5X10(-4). This cold load has been used at several wavelengths at the South Pole, Antarctica and at the White Mountain Research Station, California to calibrate spectral measurements of the cosmic microwave background radiation. For the instruments operated at 20-, 12-, 7.9-, and 4.0-cm wavelength at the South Pole, the total corrections to the LHe boiling-point temperature (approximately 3.8 K) were 48+/-23, 18+/-10, 10+/-18, and 15+/-16 mK, respectively. In operation, the average LHe loss rate was less-than-or-equal-to 4.4 l/h, allowing day-long periods of operation without a LHe fill. The boiloff rate is not strongly dependent on the radiative load at the aperture, yielding very stable operation and radiometric performance. Design considerations, radiometric and thermal performance, and operational aspects are discussed. A comparison with other LHe-cooled reference loads including the predecessor of this cold load is given.