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.
The 12C(p,γ) and 13C(p,γ) reaction cross sections are currently under investigation in the low-background environment of the Laboratory for Underground Nuclear Astrophysics. Both reactions are being studied using different types of solid targets, and employing complementary detection techniques (HPGe spectroscopy, total absorption spectroscopy and activation counting). To reduce systematic uncertainties, targets must be accurately characterized and their degradation monitored under the intense (~ 400 µA) beam of the LUNA400 accelerator. We present the experimental techniques employed, and the analyses developed for the study of these reactions.
Nuclear reaction cross sections are essential ingredients to predict the evolution of AGB stars and understand their impact on the chemical evolution of our Galaxy. Unfortunately, the cross sections of the reactions involved are often very small and challenging to measure in laboratories on Earth. In this context, major steps forward were made with the advent of underground nuclear astrophysics, pioneered by the Laboratory for Underground Nuclear Astrophysics (LUNA). The present paper reviews the contribution of LUNA to our understanding of the evolution of AGB stars and related nucleosynthesis.
The next years will see the completion of the radioactive ion beam facility SPES (Selective Production of Exotic Species) and the upgrade of the accelerators complex at Istituto Nazionale di Fisica Nucleare – Legnaro National Laboratories (LNL) opening up new possibilities in the fields of nuclear structure, nuclear dynamics, nuclear astrophysics, and applications. The nuclear physics community has organised a workshop to discuss the new physics opportunities that will be possible in the near future by employing state-of-the-art detection systems. A detailed discussion of the outcome from the workshop is presented in this report.
The 12C(p, γ)13N reaction cross section is currently under investigation in the low-background environment of the Laboratory for Underground Nuclear Astrophysics (LUNA). It is being studied using different types of solid targets, and employing two complementary detection techniques: HPGe spectroscopy and activation counting. To reduce systematic uncertainties, targets have been accurately characterized and their degradation under the intense beam of the LUNA-400 accelerator monitored. We present the experimental techniques and the corresponding analyses used to extract the reaction cross section.
The preliminary results of the GALTRACE (GALILEO TRacking Array for Charged Ejectiles) demonstrator are reported. GALTRACE is an array of Silicon PAD detectors for particle spectroscopy and discrimination to be employed in low-energy nuclear physics experiments with stable and radioactive beams at the Legnaro National Laboratories (LNL, Italy). The readout is perfomed with multi-channel, VLSI preamplifiers realized in AMS 350 nm technology, directly wire-bonded on the PCB. These preamplifiers have a resolution of 125 electrons rms and a risetime of 10 ns with a 4 pF capacitance referred to the input. The preamplifiers have a spectroscopic dynamic energy range of 40 MeV. This value is boosted by more than one order of magnitude by an innovative fast-reset device that allows for 40-700 MeV spectroscopy with a resolution of less than 0.3% FWHM. After preamplifier test-bench characterization, a full validation of a TRACE demonstrator including detector, front-end electronics, single-ended to differential converters and digitalization system has been performed. The resolution of the 60 active channels, evaluated at the 5486 keV 241 Am alpha peak, is 35±5 keV
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