Hall effect, deep level transient spectroscopy (DLTS) and optical absorption measurements were employed in concert to determine the position of the vanadium acceptor level in vanadium and nitrogen doped 6H and 4H SiC. Hall effect results indicate that the acceptor position in 4H SiC is at 0.80 eV beneath the conduction band edge, and 0.66 eV for the 6H polytype. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 0.80 eV and a capture cross section of 1.8×10−16 cm−2. The optical absorption measurements proved that the levels investigated are related to isolated vanadium, and therefore the vanadium acceptor level. Based on the DLTS measurements and secondary ion mass spectroscopy data, the maximum solubility of vanadium in SiC was determined to be 3.0×1017 cm−3. At these incorporation limits and with the depth of the level, the vanadium acceptor level could be used in the creation of semi-insulating silicon carbide.

In stars, the fusion of $$^{22}$$ 22 Ne and $$^4$$ 4 He may produce either $$^{25}$$ 25 Mg, with the emission of a neutron, or $$^{26}$$ 26 Mg and a $$\gamma $$ γ ray. At high temperature, the ($$\alpha ,n$$ α , n ) channel dominates, while at low temperature, it is energetically hampered. The rate of its competitor, the $$^{22}$$ 22 Ne($$\alpha $$ α ,$$\gamma $$ γ )$$^{26}$$ 26 Mg reaction, and, hence, the minimum temperature for the ($$\alpha ,n$$ α , n ) dominance, are controlled by many nuclear resonances. The strengths of these resonances have hitherto been studied only indirectly. The present work aims to directly measure the total strength of the resonance at $$E_{\text {r}}$$ E r  = 334 keV (corresponding to $$E_{\text {x}}$$ E x  = 10949 keV in $$^{26}$$ 26 Mg). The data reported here have been obtained using high intensity $$^4$$ 4 He$$^+$$ + beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched $$^{22}$$ 22 Ne gas target, and a 4$$\pi $$ π bismuth germanate summing $$\gamma $$ γ -ray detector. The ultra-low background rate of less than 0.5 counts/day was determined using 63 days of no-beam data and 7 days of $$^4$$ 4 He$$^+$$ + beam on an inert argon target. The new high-sensitivity setup allowed to determine the first direct upper limit of 4.0$$\,\times \,$$ × 10$$^{-11}$$ - 11 eV (at 90% confidence level) for the resonance strength. Finally, the sensitivity of this setup paves the way to study further $$^{22}$$ 22 Ne($$\alpha $$ α ,$$\gamma $$ γ )$$^{26}$$ 26 Mg resonances at higher energy.

Studies of charged-particle reactions for low-energy nuclear astrophysics require high sensitivity, which can be achieved by means of detection setups with high efficiency and low backgrounds, to obtain precise measurements in the energy region of interest for stellar scenarios. High-efficiency total absorption spectroscopy is an established and powerful tool for studying radiative capture reactions, particularly if combined with the cosmic background reduction by several orders of magnitude obtained at the Laboratory for Underground Nuclear Astrophysics (LUNA). We present recent improvements in the detection setup with the Bismuth Germanium Oxide (BGO) detector at LUNA, aiming to reduce high-energy backgrounds and to increase the summing detection efficiency. The new design results in enhanced sensitivity of the BGO setup, as we demonstrate and discuss in the context of the first direct measurement of the 65 keV resonance (Ex = 5672 keV) of the 17O(p,gamma)18F reaction. Moreover, we show two applications of the BGO detector, which exploit its segmentation. In case of complex gamma-ray cascades, e.g. the de-excitation of Ex = 5672 keV in 18F, the BGO segmentation allows to identify and suppress the beam-induced background signals that mimic the sum peak of interest. We demonstrate another new application for such a detector in form of in-site activation measurements of a reaction with beta+ unstable product nuclei, e.g., the 14N(p,gamma)15O reaction.