Design and Performance of a Prototype Magnetic Calorimeter Array for the Lynx X-Ray Microcalorimeter

Abstract

We describe a small-scale demonstration of an X-ray microcalorimeter using metallic magnetic calorimeters (MMCs) that consist of superconducting meander-shaped sensor coils with very fine pitches. These devices are being developed for potential future astrophysics missions, such as Lynx. Because Lynx requires more than 100 000 pixels on a pitch of around 50 μm, the pixels and array designs are challenging to fabricate. We have fabricated prototype superconducting Nb meander coils with pitches of 0.8, 1.2, 1.6, and 5.0 μm to determine the most optimal geometry for coupling the MMC magnetization signal to the SQUIDs used to read them out. The Nb sensor coils and microstrip wiring are buried beneath the surface of a planarized silicon wafer. There are four buried Nb layers, and these are connected through superconducting Nb vias. We have observed a large critical current of ~ 30 mA for pixels with the finest pitch meander, which have a Nb width of 0.4 μm. We have characterized each pixel geometry from the magnetization, heat capacity, and thermal conductance measurements. The preliminary measured energy resolution from pixels in these geometries is also discussed.