The Low Background Infrared (LBIR) facility has developed and tested the components of a new detector for calibration
of infrared greater than 1 pW, with 0.1 % uncertainty. Calibration of such low powers could be valuable for the
quantitative study of weak astronomical sources in the infrared. The pW-ACR is an absolute cryogenic radiometer
(ACR) employing a high resolution transition edge sensor (TES) thermometer, ultra-weak thermal link and miniaturized
receiver to achieve a noise level of around 1 fW at a temperature of 2 K. The novel thermometer employs the
superconducting transition of a tin (Sn) core and has demonstrated a temperature noise floor less than 3 nK/Hz<sup>1/2</sup>. Using
an applied magnetic field from an integrated solenoid to suppress the Sn transition temperature, the operating
temperature of the thermometer can be tuned to any temperature below 3.6 K. The conical receiver is coated on the
inside with infrared-absorbing paint and has a demonstrated absorptivity of 99.94 % at 10.6 μm. The thermal link is
made from a thin-walled polyimide tube and has exhibited very low thermal conductance near 2x10<sup>-7</sup> W/K. In tests with
a heater mounted on the receiver, the receiver/thermal-link assembly demonstrated a thermal time constant of about 15 s.
Based on these experimental results, it is estimated that an ACR containing these components can achieve noise levels
below 1 fW, and the design of a radiometer merging the new thermometer, receiver and thermal link will be discussed.
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 Radiometry
(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 increase in radiometer sensitivity compared
with the present ACR within LBIR. Although the sensitivity should increase 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.