PicoWatt infrared power measurement using an absolute cryogenic radiometer

We report on initial measurements of the low-temperature thermal properties of a device that is similar to the experimental apparatus used for absolute cryogenic radiometry (ACR) within the Low Background InfraRed (LBIR) facility at NIST. The device consists of a receiver cavity mechanically and thermally connected to a temperature-controlled stage through a thin-walled polyimide tube which serves as a weak thermal link. In order to evaluate the functionality of the device for use in a cryogenic radiometer, we measured the thermal resistance and thermal time constant of the system within the temperature range of 1.8 4.4 K. The measured thermal resistance and thermal time constant at 1.883 K were 2400 ± 500 (K/mW) and 24 ± 6 (s). This value for the thermal resistance should result in about an order-of-magnitude improvement in radiometer sensitivity compared with the present ACR within LBIR. Although the sensitivity should improve by about an order-of-magnitude, the measured time constant is nearly unchanged with respect to previous ACRs within LBIR, due to the reduced dimensions of the receiver cavity. Finally, the thermal conductivity inferred from the measured thermal resistance and geometrical parameters was computed, with an average value of 0.015 (W/m-K), and compared with other measurements of polyimide from the literature.