TopmetalSe, an open-source low-noise CMOS charge imager for Selena.

The Selena neutrino experiment couples an amorphous selenium (aSe) ionization target to a complimentary metal-oxide semiconductor (CMOS) pixel array as an imaging detector for next generation neutrino physics. The high Q-value of 82Se and the excellent image-based event classification allows for a neutrinoless double beta decay search free from environmental backgrounds. We present the R&D status of the Selena experiment, as well as the design and characterization of the first prototype pixel array designed for Selena, the TopmetalSe. Fabricated in the Skywater 130nm open-source process, the prototype consists of a 100x100 array of pixels with a 15um pitch. Each pixel measures charge through an exposed topmetal layer, which is directly connected to an in-pixel charge sensitive amplifier. We demonstrate the low noise and high charge collection of the first TopmetalSe devices coupled to aSe.