Development of silicon interposer: towards an ultralow radioactivity background photodetector system

It is of great importance to develop a photodetector system with an ultralow radioactivity background in rare event searches. Silicon photomultipliers (SiPMs) and application-specific integrated circuits (ASICs) are two ideal candidates for low background photosensors and readout electronics, respectively, because they are mainly composed of silicon, which can achieve good radio-purity without considerable extra effort. However, interposers, used to provide mechanical support and signal routes between the photosensor and the electronics, are a bottleneck in building ultralow background photodetectors. Silicon and quartz are two candidates to construct the low background interposer because of their good radio-purity; nevertheless, it is non-trivial to produce through silicon vias (TSV) or through quartz vias (TQV) on the large area silicon or quartz wafer. In this work, based on double-sided TSV interconnect technology, we developed the first prototype of a silicon interposer with a size of 10~cm$\times$10~cm and a thickness of 320~$\mu$m. The electrical properties of the interposer are carefully evaluated at room temperature, and its performance is also examined at -110~$^\circ$C with an integrated SiPM on the interposer. The testing results reveal quite promising performance of the prototype, and the single photoelectron signals can be clearly observed from the SiPM. The features of the observed signals are comparable with those from the SiPM mounted on a normal FR4-based PCB. Based on the success of the silicon interposer prototype, we started the follow-up studies that aimed to further improve the performance and yield of the silicon interposer, and eventually to provide a solution for building an ultralow background photodetector system

Experimental investigation of Ca isotopic fractionation during abiotic gypsum precipitation

Experiments investigating Ca isotopic fractionation during gypsum precipitation were undertaken in order to elucidate the mechanisms and conditions that govern isotopic fractionation during mineral precipitation. Both stirred and unstirred free drift gypsum precipitation experiments were conducted at constant initial ionic strength (0.6 M) and variable initial saturation states (4.8–1.5) and Ca2+:SO42− ratios (3 and 0.33). Experimental durations varied between 0.5 and 190 h, while temperature (25.9–24.0 °C), pH (5.8–5.4) and ionic strength (0.6–0.5) were relatively constant. In all experiments, 20–80% of the initial dissolved Ca reservoir was precipitated. Isotopically light Ca preferentially partitioned into the precipitated gypsum; the effective isotopic fractionation factor (Δ44/40Cas–f = δ44/40Casolid − δ44/40Cafluid) of the experimental gypsum ranged from −2.25‰ to −0.82‰. The log weight-averaged, surface area normalized precipitation rates correlated with saturation state and varied between 4.6 and 2.0 μmol/m2/h. The crystal size and aspect ratios, determined by SEM images, BET surface area, and particle size measurements, co-varied with precipitation rate, such that fast growth produced small (10–20 μm), tabular crystals and slow growth produced larger (>1000 μm), needle shaped crystals. Mass balance derived δ44Cas and Δ44Cas–f, calculated using the initial fluid δ44Ca and the mass fraction of Ca removed during precipitation (fCa) as constraints, suggest that the precipitate was not always sampled homogeneously due to the need to preserve the sample for SEM, surface area, and particle size analyses. The fractionation factor (αs–f), derived from Rayleigh model fits to the fluid and calculated bulk solid, ranged from 0.9985 to 0.9988 in stirred experiments and 0.9987 to 0.9992 in unstirred experiments. The αs–f demonstrated no clear dependence on either precipitation rate or initial saturation state in stirred reactors, but exhibited a positive dependence on rate in unstirred experiments. The differences in αs–f between stirred and unstirred reactors, as well as a general correlation between αs–f and crystal morphology, led us to hypothesize that growth on different crystal faces controls the isotopic composition of gypsum. We also explore the idea that speciation in solution explains the difference between experiments in which the only major difference was the Ca2+ to SO42− ratio in solution. The importance of understanding the environmental controls on the fractionation factor during mineral precipitation is highlighted in this study. The fractionation factor of gypsum precipitation near chemical equilibrium was found to be ∼0.9995, rather than 1, indicating that even at near equilibrium conditions, the δ44Ca of minerals are not likely to record the δ44Ca of the solution directly. However, the measurable isotopic fractionation associated with gypsum formation does suggest that a gypsum-based proxy may be useful in constraining Ca cycling in marginal environments over geologic time scales. Model examples are provided that demonstrate how such a proxy would operate