Most of the R&D has been focused on designing and fabricating both SELENDIS detectors and a dedicated laser-based calibration set-up. When envisioning detectors capable of detecting single electron/hole pair equivalent energies, even calibration devices need to be thought of and adapted to experimental constraints. In our case, detectors are placed at 20 mK (that is -273,13 °C) in a cryostat, under vacuum, separated from room temperature by screens at various temperature stages following a ‘Russian doll’ architecture. We have developed a table-top laser-based system using a pulsed 1550nm laser-diode equipped with an integrated photodiode to allow for a continuous monitoring of optical power during calibration measurements. Remotely controlled variable attenuators confer a dynamic range in optical power allowing for the simultaneous calibration of 4 different detectors at energies ranging from 10’s of keV down to a single electron hole pair. A custom made flange permits the optical fibers feedthrough to the inner part of the cryostat where unjacked fibers of only 125 um diameter travel across the temperature stages from 300 K to 20 mK to reach the detectors. The acquisition electronics and software have been adapted to record timing and optical power information of laser-diode induced events and disentangle them from background events. This whole setup was successfully tested and validated down to energies as low as the detector trigger thresholds.
The detector design phase extended over more than a year. The optimal and final detector configurations resulted from studies based on : 1) finite element simulations of electric field lines in various detector geometries 2) Comparative study of the performance of prototypes and existing detectors with different electrode designs. From an initially envisioned array of two 30-g Si and Ge crystals, we finally opted for an array of six 3-g Ge crystals. We built, assembled, and instrumented four detectors with NTD thermal sensors. Two of these detectors were operated as pure calorimeters in a first step towards validating the phonon resolution objective of 15 eV (rms), which while long-delayed due to cryogenic issues was eventually exceeded with 13 eV. Two other crystals were equipped with aluminum electrodes lithographed in a grid scheme, known to withstand a high voltage without leaking. The two remaining crystals were optically polished in preparation of the next generation of SELENDIS detectors with electrodes - made of Sapphire wafers with a thin film of Al - separated from the crystal by a vacuum gap, allowing for voltages even greater than 100V to be applied.
Although the design, fabrication and first performance tests were all successful, the detectors equipped with electrodes were only ready to be operated at the very end of the fellowship. This delay, mostly due to both cryogenic issues and the Covid pandemic, postponed the detector applications initially planned to occur during the fellowship.
