Claudio Santonastaso

Context. Carbon burning determines the destiny of stars with masses higher than ≈8 M _⊙ and mass-accreting white dwarfs (WDs) in close binary systems. For this reason, in order to understand several astrophysical phenomena it is necessary to understand this key evolutionary phase; examples include determining the mass limits of CO and ONe WDs, the rates of e-capture, core-collapse, and Type Ia supernovae, and the contribution of massive stars to the chemical evolution. Despite its fundamental importance in astrophysics, the rate of the leading fusion process – the ¹² C+ ¹² C reaction – remains largely uncertain at relevant stellar temperatures. Aims. Revision of the ¹² C+ ¹² C reaction rate of both the α and the p channels and evaluation of the consequent impact on star models of mass ranging between 8 and 20 M _⊙ . Methods. Based on the latest direct measurements of the ¹² C+ ¹² C cross section in the 2.8–4 MeV energy range, we calculated two new lower bounds for the rate of the ¹² C+ ¹² C process. These two estimates are obtained using different assumptions for the low-energy extrapolation of the modified astrophysical S ^* ( E ) factor: one replacing the method adopted by Caughlan & Fowler (1988, At. Data Nucl. Data Tables, 40, 283, CF88), and another based on a hindrance model. Results. The new rates are substantially lower than that commonly used in current stellar models of massive stars and accreting WDs. Specifically, the ratio between the new rate calculated in this work and the rate adopted in CF88 is, at T = 1 GK, 0.2 for the p channel and 0.085 for the α channel. Consequently, the characteristic carbon-burning temperatures and densities shift to higher values than those previously reported and the corresponding lifetime is significantly reduced. The maximum mass for progenitors’ pure CO WDs is higher as well as the maximum CO WD mass. Moreover, in a 20 M _⊙ model close to central Si depletion, the physical structure of the core is less dense in the region between 2 and 4 M _⊙ from the center. This fact should have important consequences for the resulting explosive properties.

This work explores the possibility of using 4H-SiC PIN diodes to provide a high electric field able to induce the Stark effect in ⁷Be atoms implanted in the diode space charge region, modifying the ⁷Be radioactive decay time. A set of PIN diodes of area ranging between 2.12 × 10⁻³ cm² and 9.88 × 10⁻³ cm² was designed and fabricated to reach breakdown voltages up to 1000 V. Be ions were implanted in the epitaxial layer, and then the devices were reverse biased at about 75% of the theoretical breakdown voltage for durations exceeding 100 days, long enough for a precise measurement of the ⁷Be radioactive decay time. Electrical characterization in the pristine state, after Be ion implantation, and after long reverse bias allowed us to verify the suitability of 4H-SiC PIN diodes by assessing both the agreement between simulated and measured performance and the stability of the electric field. Be ion implantation-related defects induced both an increase in the reverse current generation and a decrease in the junction capacitance, though not affecting the breakdown voltage. Comparison with test devices implanted with the stable isotope ⁹Be indicates that any defects introduced by the ⁷Be radioactive decay are below the detection limit of the employed characterization techniques and have a negligible impact on the reverse-blocking characteristics of the diodes. Device simulations allowed us to conclude that the electric field remains close to its theoretical value throughout the experiment duration, confirming the suitability of 4H-SiC diodes for both induction and measurement of ⁷Be lifetime variations.

This study presents the design, fabrication, and electrical characterization of 4H-SiC PIN diodes employed to provide a high electric field able to induce Stark effect in ⁷ Be atoms implanted in the space charge region. Indeed, a variation in the half-life of the ⁷ Be radioactive decay is expected to be achieved by applying an electric field of the order of 10 ⁶ V/cm, which can be produced by reverse-biasing 4H-SiC diodes close to the breakdown voltage. A set of diodes of area ranging between 2.12×10 ^-3 cm ² and 9.88×10 ³ cm ² was designed and fabricated to reach breakdown voltages up to 1000 V. When tested under reverse current limitation set equal to 300 nA, over 50% out of 24 devices could withstand reverse bias exceeding 800 V. This work reports on the characteristics of one diode of 9.88×10 ³ cm ² area, implanted with ⁷ Be and subject to continuous reverse-bias at 750 V for 107 days. Electrical characterization conducted before, during, and after long-term polarization highlighted an increase in the reverse current generation due to implantation-related defects, which however does not affect the breakdown voltage. These considerations lead to the conclusion that the electric field acting on the implanted ⁷Be remains stable over time, confirming the suitability of 4H-SiC diodes for both induction and measurement of ⁷ Be lifetime variations.

In this work we present to our knowledge the most precise measurement of the ⁷Be electron capture decay half-life in a host material. A silicon carbide sample with ~8.62 × 10^{9 7}Be atoms was measured for 83.5 d on an ultra-low background high purity Ge detector located deep underground in the Laboratori Nazionali del Gran Sasso, Italy. The result obtained for the decay half-life is T _1/2 = 53.284 ± 0.016 d, which corresponds to an uncertainty of 0.3‰. Thanks to the high sensitivity achieved, this measurement is paving the way to further investigations on this process aiming to understand how environmental conditions may affect the decay half-life.

The KM3NeT Collaboration is building an underwater neutrino observatory at the bottom of the Mediterranean Sea, consisting of two neutrino telescopes, both composed of a three-dimensional array of light detectors, known as digital optical modules. Each digital optical module contains a set of 31 three-inch photomultiplier tubes distributed over the surface of a 0.44 m diameter pressure-resistant glass sphere. The module also includes calibration instruments and electronics for power, readout, and data acquisition. The power board was developed to supply power to all the elements of the digital optical module. The design of the power board began in 2013, and ten prototypes were produced and tested. After an exhaustive validation process in various laboratories within the KM3NeT Collaboration, a mass production batch began, resulting in the construction of over 1200 power boards so far. These boards were integrated in the digital optical modules that have already been produced and deployed, which total 828 as of October 2023. In 2017, an upgrade of the power board, to increase reliability and efficiency, was initiated. The validation of a pre-production series has been completed, and a production batch of 800 upgraded boards is currently underway. This paper describes the design, architecture, upgrade, validation, and production of the power board, including the reliability studies and tests conducted to ensure safe operation at the bottom of the Mediterranean Sea throughout the observatory’s lifespan.