Planck-LFI: Design and Performance of the 4 Kelvin Reference Load Unit

TL;DR

This paper details the design, materials, and performance of a cryogenic reference load system used in the Planck-LFI radiometers, crucial for stable signal comparison in cosmic microwave background measurements.

Contribution

It presents the innovative design and optimization of a 4.5 K reference load system tailored for multiple frequency bands in the Planck-LFI instrument.

Findings

Achieved stable reference signals at 4.5 K for all bands

Optimized horn and target design for each frequency band

Validated thermal and RF properties of materials used

Abstract

The LFI radiometers use a pseudo-correlation design where the signal from the sky is continuously compared with a stable reference signal, provided by a cryogenic reference load system. The reference unit is composed by small pyramidal horns, one for each radiometer, 22 in total, facing small absorbing targets, made of a commercial resin ECCOSORB CR (TM), cooled to approximately 4.5 K. Horns and targets are separated by a small gap to allow thermal decoupling. Target and horn design is optimized for each of the LFI bands, centered at 70, 44 and 30 GHz. Pyramidal horns are either machined inside the radiometer 20K module or connected via external electro-formed bended waveguides. The requirement of high stability of the reference signal imposed a careful design for the radiometric and thermal properties of the loads. Materials used for the manufacturing have been characterized for thermal, RF and mechanical properties. We describe in this paper the design and the performance of the reference system.