Superconducting cold-electron bolometers with JFET readout for OLIMPO balloon telescope

The OLIMPO experiment is a 2.6 m balloon-borne telescope, aimed at measuring the Sunyaev- Zeldovich effect in clusters of Galaxies. OLIMPO will carry out surveys in four frequency bands centered at 140, 220, 410 and 540 GHz. The detector system consists of four bolometer arrays and incorporates new detector technologies that are potential candidates for future space missions. One of these technologies is the Capacitively Coupled Cold-Electron Bolometer (CEB) with JFET readout. The JFET readout coupled to semiconductor-based high-impedence bolometers has been developed already for the BOOMERanG and Planck-HFI experiments. The CEB is a planar antenna-coupled superconductong detector with high sensitivity and high dynamic range. Here, we discuss a scheme to match the relatively moderate dynamic resistance of CEB (∼1kOhm) to the high noise equivalent resistance of JFET (1 MΩ). To achieve noise matching with JFET, a Cold-Electron Bolometer with a weak Superconducting Absorber (SCEB) has been proposed. In voltage-biased mode with voltage higher than (Delta 1-Delta 2) the IV of SIS' junctions has considerably increased dynamic resistance up to the level of Rj1000*Rn. Electron cooling will be still very effective for the incoming power. Simulations show that photon noise level can be achieved at 300 mK for a structure with Ti absorber and Al/Ti tunnel junctions for all frequency ranges with the estimated in-flight optical power load for OLIMPO.