Exploring BiSn Sphere Attachment of Absorbers for Gamma-Ray TES

A Super-Conducting ENergetic x-ray Telescope (ASCENT) is a concept for a future balloon-borne high-energy X-ray telescope in the energy range 60–85 keV to study gamma-ray emissions of 67.87 keV and 78.32 keV from the radioactive isotope ⁴⁴Ti. For the focal plane instrumentation, ASCENT will use Mo-Cu/Mo-Au bilayer transition edge sensor (TES) microcalorimeter gamma-ray detectors with tin (Sn) absorbers. ‘Spectrometer to Leverage Extensive Development of Gamma-ray TESs for Huge Arrays using Microwave Multiplexed Enabled Readout’ (SLEDGEHAMMER), a detector development project at the National Institute of Standards and Technology, acts as the basis for the detector arrays for ASCENT. SLEDGEHAMMER has tin (Sn) absorbers attached to the SU-8 epoxy posts, lithographically placed on the detectors, but we are also considering other geometries for the chips where the absorbers are attached to the chips separated from the TESs, which could help to avoid parallel path for a current flow around the detectors with these BiSn sphere attachments. In this work, we are reporting on developing a method to attach Tin (Sn) absorbers to the transition edge sensors (TESs) with 0.2 mm diameter BiSn solder spheres replacing epoxy. The goal is to improve the thermal conductivity between the absorbers and the TESs compared to what was achieved using epoxy, potentially reducing the presence of an athermal component in the tails of signal pulses. We describe our efforts toward finding optimal temperature and pressure conditions for making this contact and the progress toward contact resistance measurements of these joints.