To measure extremely faint signals like Cosmic Microwave Background Polarization (a few percent of CMB anisotropy) it is necessary to use very high sensitivity radiometers. This means to adopt low noise cryogenic front-end and long integration times. This is the case of BaR-SPOrt (Balloon borne Radiometer for Sky Polarization Observations), an experiment designed to measure the CMB polarization at sub-degree angular scales. In the millimeter range, where coherent radiometers (polarimeters) are typically employed, usual mechanical coolers can represent a limit to the final sensitivity due to their base temperature instability. As a matter of fact, in correlation polarimeter, temperature fluctuations of the front-end devices, can both mimic a polarized signal and severely limit instrumental sensitivity. Here we discuss in detail the thermal design of the cryostat housing the instrument with particular attention to the closed loop cryocooler adopted, which is able to guarantee 6W at 77K with a stability better than 0.1 K over several hours.
Zannoni, M., Macculi, C., Carretti, E., Cortiglioni, S., Ventura, G., Monari, J., et al. (2004). Thermal design and performance evaluation of the BaR-SPOrt cryostat. In Millimeter and Submillimeter Detectors for Astronomy II (pp.735-743). SPIE--the International Society for Optical Engineering [10.1117/12.551671].
Thermal design and performance evaluation of the BaR-SPOrt cryostat
ZANNONI, MARIO
Primo
;
2004
Abstract
To measure extremely faint signals like Cosmic Microwave Background Polarization (a few percent of CMB anisotropy) it is necessary to use very high sensitivity radiometers. This means to adopt low noise cryogenic front-end and long integration times. This is the case of BaR-SPOrt (Balloon borne Radiometer for Sky Polarization Observations), an experiment designed to measure the CMB polarization at sub-degree angular scales. In the millimeter range, where coherent radiometers (polarimeters) are typically employed, usual mechanical coolers can represent a limit to the final sensitivity due to their base temperature instability. As a matter of fact, in correlation polarimeter, temperature fluctuations of the front-end devices, can both mimic a polarized signal and severely limit instrumental sensitivity. Here we discuss in detail the thermal design of the cryostat housing the instrument with particular attention to the closed loop cryocooler adopted, which is able to guarantee 6W at 77K with a stability better than 0.1 K over several hours.
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