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One of the main neutron sources for the astrophysical s process is the reaction ^{13}C(α,n)^{16}O, taking place in thermally pulsing asymptotic giant branch stars at temperatures around 90 MK. To model the nucleosynthesis during this process the reaction cross section needs to be known in the 150-230 keV energy window (Gamow peak). At these sub-Coulomb energies, cross section direct measurements are severely affected by the low event rate, making us rely on input from indirect methods and extrapolations from higher-energy direct data. This leads to an uncertainty in the cross section at the relevant energies too high to reliably constrain the nuclear physics input to s-process calculations. We present the results from a new deep-underground measurement of ^{13}C(α,n)^{16}O, covering the energy range 230-300 keV, with drastically reduced uncertainties over previous measurements and for the first time providing data directly inside the s-process Gamow peak. Selected stellar models have been computed to estimate the impact of our revised reaction rate. For stars of nearly solar composition, we find sizeable variations of some isotopes, whose production is influenced by the activation of close-by branching points that are sensitive to the neutron density, in particular, the two radioactive nuclei ^{60}Fe and ^{205}Pb, as well as ^{152}Gd.

The fusion reaction $^{12}\mathrm{C}(^{12}\mathrm{C},\mathrm{p})^{23}\mathrm{Na}$ has been studied from $E=2.00$ to 4.00 MeV by particle spectroscopy. The data reveal broad resonances above $E=3.00$ MeV and are compatible with previously reported resonance structure around $E=2.1$ MeV. The data were limited at low energies by low count rates as well as possible background contributions. This experiment extends the previously achieved low-energy measurement by charged particle spectroscopy to 2 MeV, which corresponds to the high-energy side of the astrophysically relevant temperature. Present knowledge of level structures and nonresonant contribution cannot explain the results of the present experiment, which may change the $^{12}\mathrm{C}+^{12}\mathrm{C}$ reaction rate significantly. Despite the progress decreasing the low-energy limit, any extrapolation into the astrophysical energy range remains highly uncertain based on available experimental data.

Background: The origin of fluorine is a widely debated issue. Nevertheless, the $^{15}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{F}$ reaction is a common feature among the various production channels so far proposed. Its reaction rate at relevant temperatures is determined by a number of narrow resonances together with the direct capture and the tails of the two broad resonances at ${E}_{\mathrm{c}.\mathrm{m}.}=1323$ and 1487 keV.Purpose: The broad resonances widths, ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$ and ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\alpha}}$, have to be measured with adequate precision in order to better determine their contribution to the $^{15}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{F}$ stellar reaction rate.Methods: Measurement through the direct detection of the $^{19}\mathrm{F}$ recoil ions with the European Recoil separator for Nuclear Astrophysics (ERNA) were performed. The reaction was initiated by a $^{15}\mathrm{N}$ beam impinging onto a $^{4}\mathrm{He}$ windowless gas target. The observed yield of the resonances at ${E}_{\mathrm{c}.\mathrm{m}.}=1323$ and 1487 keV is used to determine their widths in the $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ channels.Results: We show that a direct measurement of the cross section of the $^{15}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{F}$ reaction can be successfully obtained with the recoil separator ERNA, and the widths ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$ and ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\alpha}}$ of the two broad resonances have been determined. While a fair agreement is found with earlier determination of the widths of the 1487 keV resonance, a significant difference is found for the 1323 keV resonance ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\alpha}}$.Conclusions: The revision of the widths of the two more relevant broad resonances in the $^{15}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{F}$ reaction presented in this work is the first step toward a more firm determination of the reaction rate. At present, the residual uncertainty at the temperatures of the $^{19}\mathrm{F}$ stellar nucleosynthesis is dominated by the uncertainties affecting the direct capture component and the 364 keV narrow resonance, both so far investigated only through indirect experiments.

Background: The $^{17}O(p,\ensuremath{\gamma})^{18}F$ reaction affects the production of key isotopes (e.g., $^{18}F$ and $^{18}O$) in the explosive hydrogen burning that powers classical novae. Under these explosive conditions, the reaction rate is dominated by contributions from a narrow resonance at ${E}_{\mathrm{c}.\mathrm{m}.}=183\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ and by the combined contributions of direct capture and low-energy tails of broad resonances. At present, the astrophysical reaction rate is not well constrained because of the lack of data in the energy region appropriate to classical novae.Purpose: This study aims at the measurement of the $^{17}O$$(p,\ensuremath{\gamma})^{18}F$ reaction cross section in order to determine its reaction rate in the temperature region appropriate to explosive hydrogen burning in novae.Method: The $^{17}O$$(p,\ensuremath{\gamma})^{18}F$ reaction cross section was measured using both the prompt detection of the emitted $\ensuremath{\gamma}$ rays and an activation technique. Measurements were carried out at the Laboratory for Underground Nuclear Astrophysics (Gran Sasso, Italy) where the strongly reduced cosmic-ray-induced background allows for improved sensitivity compared to previous studies.Results: The $^{17}O$$(p,\ensuremath{\gamma})^{18}F$ reaction cross section was measured in the range ${E}_{\mathrm{c}.\mathrm{m}.}=160$ to $370\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$. The strength of the ${E}_{\mathrm{c}.\mathrm{m}.}=183\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ resonance, $\ensuremath{\omega}\ensuremath{\gamma}=1.67\ifmmode\pm\else\textpm\fi{}0.12\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{eV}$, was determined with unprecedented precision. The total $S$ factor was obtained through a combined fit of prompt $\ensuremath{\gamma}$-ray and activation results. An overall global fit including other existing data sets was also carried out and a recommended astrophysical reaction rate is presented.Conclusions: The reaction rate uncertainty attained in this work is now below the required precision for nova models. We verified, following a full set of hydrodynamic nova models, that the abundances of oxygen and fluorine isotopes obtained with the present reaction rate are determined with 10% precision and put firmer constraints on observational signatures of novae events.

We give the LUNA results on the $\ensuremath{\sigma}(E)$ cross section measurement of a key reaction of the proton-proton chain strongly affecting the calculated neutrino luminosity from the Sun: ${}^{3}\mathrm{He}{(}^{3}\mathrm{He},2p{)}^{4}\mathrm{He}$. Because of the cosmic ray suppression provided by the Gran Sasso underground laboratory, it has been possible to measure $\ensuremath{\sigma}(E)$ throughout the energy range in which this reaction occurs in the Sun, i.e., down to $16.5\mathrm{keV}$ center of mass energy. The data clearly show the cross section increase due to the electron screening effect but they do not exhibit any evidence for a narrow resonance suggested to explain the ${}^{8}\mathrm{B}$ and ${}^{7}\mathrm{Be}$ solar neutrino flux reduction.

The behavior of ion-implanted As in (100) silicon wafers, following thermal oxidation, has been investigated by Rutherford backscattering spectroscopy, atomic force microscopy, transmission electron microscopy, and extended x-ray-absorption fine structure. The adopted fluences ($3\ifmmode\times\else\texttimes\fi{}{10}^{15}$ and $3\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) and oxidation conditions (wet 920 \ifmmode^\circ\else\textdegree\fi{}C, dry 1100 \ifmmode^\circ\else\textdegree\fi{}C) span quite a broad range of phenomena, giving rise to As diffusion in the bulk, and/or segregation and precipitation at the ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}$ interface. The surface roughness is correlated to that measured at the interface, although the oxide presence strongly reduces the value with respect to that present at the interface. Rough interfaces and surfaces are formed when the arsenic concentration exceeds the solid solubility and precipitation occurs. The SiAs precipitates are characterized by a monoclinic structure with low surface energy for the (100) facet, as determined by the Wulff plot. Residual roughness is left at the oxide surface even if precipitates initially formed dissolve during subsequent oxidation. The depth profile of the dopant has been quantitatively computed by the analytical solution of the diffusion equation, taking into account the interface movement, the As redistribution at the interface between oxide and bulk silicon, the formation, growth, and dissolution of precipitates, and, of course, the drive-in process. The dependence of the diffusion coefficient on the dopant concentration has been also considered and determined as a converging parameter, considering iteratively the differential equation solution. In all the investigated cases the agreement between experimental data and calculations has been found to be good.

Intermediate mass fragments (IMF) and light particles emitted from the Ni-58 + Ni-58 reaction at a beam energy of 375 MeV have been studied. The fragments and light particles were measured in coincidence with 4 pi gamma-ray spectrometer. The Z=6 (C) kinetic energy spectra and the distribution of the final nuclei in coincidence with the emitted C are well described by Hauser-Feshbach calculations extended to many channels. A detailed study of C-Psi and 3 alpha-gamma correlations indicate a strong selectivity of the IMF decay. Our results indicate that the IMF can populate nuclei that are not accessible via multiple light particle (Z

Angle-resolved photoemissions studies were conducted on single-crystal (TaSe4)(2)I samples above and below the charge-density-wave transition temperature (T-CDW) of 260 K. We observed a shift of the leading photoemission edge between 300 and 60 K consistent with resistivity measurements performed on the same sample: a band gap opens throughout the Brillouin zone below T-CDW. However, several aspects of the data are difficult to reconcile with any standard model.

Classical novae are important contributors to the abundances of key isotopes, such as the radioactive $^{18}\mathrm{F}$, whose observation by satellite missions could provide constraints on nucleosynthesis models in novae. The $^{17}\mathrm{O}(p,\ensuremath{\gamma})^{18}\mathrm{F}$ reaction plays a critical role in the synthesis of both oxygen and fluorine isotopes, but its reaction rate is not well determined because of the lack of experimental data at energies relevant to novae explosions. In this study, the reaction cross section has been measured directly for the first time in a wide energy range ${E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\simeq}200--370\text{ }\text{ }\mathrm{keV}$ appropriate to hydrogen burning in classical novae. In addition, the ${E}_{\mathrm{c}.\mathrm{m}.}=183\text{ }\text{ }\mathrm{keV}$ resonance strength, $\ensuremath{\omega}\ensuremath{\gamma}=1.67\ifmmode\pm\else\textpm\fi{}0.12\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$, has been measured with the highest precision to date. The uncertainty on the $^{17}\mathrm{O}(p,\ensuremath{\gamma})^{18}\mathrm{F}$ reaction rate has been reduced by a factor of 4, thus leading to firmer constraints on accurate models of novae nucleosynthesis.

Keywords: BAND DISCONTINUITIES ; INTERFACE ; HETEROSTRUCTURES ; HOMOJUNCTION ; SPECTROSCOPY ; STATES Note: Cnr,ist struttura mat,i-0004 frascati,italy. Dellorto, t, ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland.ISI Document Delivery No.: QB022 Reference LSE-ARTICLE-1994-012View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

An in-beam \ensuremath{\gamma}-ray study performed with the $^{121}\mathrm{Sb}$${(}^{19}$F,4n) reaction has established the high spin level structure of the N=78 nucleus $^{138}\mathrm{Nd}$. States up to I=21\ensuremath{\Elzxh} and 8.5 MeV excitation energy have been indentified. High spin states are described in the framework of the cranking model and of the interacting boson model with broken pairs. The calculations reproduce levels up to I=18\ensuremath{\Elzxh} including the two ${10}^{+}$ states which, from the feeding cascades, are identified as the \ensuremath{\nu}${\mathit{h}}_{11/2}^{\mathrm{\ensuremath{-}}1}$ and \ensuremath{\pi}${\mathit{h}}_{11/2}^{2}$ excitations. Cranked Strutinsky type calculations predict opposite shapes for the two different types of excitations.

The rate of the carbon-nitrogen-oxygen (CNO) cycle of hydrogen burning is controlled by the {sup 14}N(p,{gamma}){sup 15}O reaction. The reaction proceeds by capture to the ground states and several excited states in {sup 15}O. In order to obtain a reliable extrapolation of the excitation curve to astrophysical energy, fits in the R-matrix framework are needed. In an energy range that sensitively tests such fits, new cross-section data are reported here for the four major transitions in the {sup 14}N(p,{gamma}){sup 15}O reaction. The experiment has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) 400-kV accelerator placed deep underground in the Gran Sasso facility in Italy. Using a composite germanium detector, summing corrections have been considerably reduced with respect to previous studies. The cross sections for capture to the ground state and to the 5181, 6172, and 6792 keV excited states in {sup 15}O have been determined at 359, 380, and 399 keV beam energy. In addition, the branching ratios for the decay of the 278-keV resonance have been remeasured.

The {sup 15}N(p,{gamma}){sup 16}O reaction represents a breakout reaction linking the first and second cycles of the CNO cycles redistributing the carbon and nitrogen abundances into the oxygen range. The reaction is dominated by two broad resonances, at E{sub p}=338 and 1028 keV, and a direct capture contribution to the ground state of {sup 16}O. Interference effects between these contributions both in the low-energy region (E{sub p}

Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar $H$-shell burning. In particular, the strengths of low-energy resonances with $El200$ keV in $^{25}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{26}\mathrm{Al}$ determine the production of $^{26}\mathrm{Al}$ and a precise knowledge of these nuclear data is highly desirable. Absolute measurements at such low energies are often very difficult and hampered by $\ensuremath{\gamma}$-ray background as well as changing target stoichiometry during the measurements. The latter problem can be partly avoided using higher-energy resonances of the same reaction as a normalization reference. Hence the parameters of suitable resonances have to be studied with adequate precision. In the present work we report on new measurements of the resonance strengths $\ensuremath{\omega}\ensuremath{\gamma}$ of the $E=214$, $304$, and $326$ keV resonances in the reactions $^{24}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{25}\mathrm{Al}$, $^{25}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{26}\mathrm{Al}$, and $^{26}\mathrm{Mg}$($p,\ensuremath{\gamma}$)$^{27}\mathrm{Al}$, respectively. These studies were performed at the LUNA facility in the Gran Sasso underground laboratory using multiple experimental techniques and provided results with a higher accuracy than previously achieved.

A detailed comparison between measured energy spectra and cross sections of {sup 1,2,3}H and {sup 4}He, evaporated from the composite system {sup 96}Ru ({ital E}{sup *}{congruent}113 MeV), and the predictions of the statistical model, has been carried out. Results obtained with transmission coefficients derived from optical model (OM), ingoing-wave boundary-condition model (IWBCM) and fusion systematics (FS) are presented. The best overall description of the data is obtained using IWBCM transmission coefficients, with reduced {ital s}-wave barriers, including, in the level density, deformations slightly larger than those predicted by the rotating liquid drop model (RLDM). {sup 2,3}H yields are well reproduced by IWBCM transmission coefficients, while they are largely overestimated by the OM ones; the failure is found to be related to the imaginary part of the optical potential which accounts for the absorption of nonfusion reactions, in peripheral collisions. No combination of phase space and transmission coefficients is able to reproduce, simultaneously, the low emission barrier and the cross section observed for protons.

The 3He(alpha,gamma)7Be reaction presently represents the largest nuclear uncertainty in the predicted solar neutrino flux and has important implications on the big bang nucleosynthesis, i.e., the production of primordial 7Li. We present here the results of an experiment using the recoil separator ERNA (European Recoil separator for Nuclear Astrophysics) to detect directly the 7Be ejectiles. In addition, off-beam activation and coincidence gamma-ray measurements were performed at selected energies. At energies above 1 MeV a large discrepancy compared to previous results is observed both in the absolute value and in the energy dependence of the cross section. Based on the available data and models, a robust estimate of the cross section at the astrophysical relevant energies is proposed.

The rate of the hydrogen-burning carbon-nitrogen-oxygen (CNO) cycle is controlled by the slowest process, 14N(p,γ )15O, which proceeds by capture to the ground and several excited states in 15O. Previous extrapolations for the ground state contribution di

Solar neutrino fluxes depend both on astrophysical and on nuclear physics inputs, namely on the cross sections of the reactions responsible for neutrino production inside the Solar core. While the flux of solar $^{8}\mathrm{B}$ neutrinos has been recently measured at Superkamiokande with a 3.5% uncertainty and a precise measurement of $^{7}\mathrm{Be}$ neutrino flux is foreseen in the next future, the predicted fluxes are still affected by larger errors. The largest nuclear physics uncertainty to determine the fluxes of $^{8}\mathrm{B}$ and $^{7}\mathrm{Be}$ neutrinos comes from the $^{3}\mathrm{He}$($\ensuremath{\alpha},\ensuremath{\gamma}$)$^{7}\mathrm{Be}$ reaction. The uncertainty on its S-factor is due to an average discrepancy in results obtained using two different experimental approaches: the detection of the delayed \ensuremath{\gamma} rays from $^{7}\mathrm{Be}$ decay and the measurement of the prompt \ensuremath{\gamma} emission. Here we report on a new high precision experiment performed with both techniques at the same time. Thanks to the low background conditions of the Gran Sasso LUNA accelerator facility, the cross section has been measured at ${E}_{\mathrm{c}.\mathrm{m}.}=170$, 106, and 93 keV, the latter being the lowest interaction energy ever reached. The S-factors from the two methods do not show any discrepancy within the experimental errors. An extrapolated $S(0)=0.560\ifmmode\pm\else\textpm\fi{}0.017$ keV barn is obtained. Moreover, branching ratios between the two prompt \ensuremath{\gamma}-transitions have been measured with 5--8% accuracy.

The flux of {sup 7}Be and {sup 8}B neutrinos from the Sun and the production of {sup 7}Li via primordial nucleosynthesis depend on the rate of the {sup 3}He({alpha},{gamma}){sup 7}Be reaction. In an extension of a previous study showing cross section data at 127-167 keV center-of-mass energy, the present work reports on a measurement of the {sup 3}He({alpha},{gamma}){sup 7}Be cross section at 106 keV performed at Italy's Gran Sasso underground laboratory by the activation method. This energy is closer to the solar Gamow energy than ever reached before. The result is {sigma}=0.567{+-}0.029{sub stat}{+-}0.016{sub syst} nb. The data are compared with previous activation studies at high energy, and a recommended S(0) value for all {sup 3}He({alpha},{gamma}){sup 7}Be activation studies, including the present work, is given.

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-gamma technique and with inferior precision. Using a windowless gas target, high beam intensity, and low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined at 127, 148, and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S factor to solar energies.

New photoemission data are presented for a series of Ce-doped or undoped, reduced or nonreduced, Nd cuprates. The data, taken with high signal-to-noise level in the constant-initial-state mode, demonstrate the presence of Cu+, in contrast with previous photoemission and electron-energy-loss studies.

The ${}^{143}\mathrm{Tb}$ nucleus has been studied with the ${}^{92}\mathrm{Mo}$ (${}^{54}\mathrm{Fe},$ $3p\ensuremath{\gamma}$) reaction at 240-MeV incident energy, extending the systematics of odd-$A,$ $N=78$ isotones to higher $Z.$ For the yrast band, excited states up to a spin of ${59/2}^{\ensuremath{-}}$ have been observed. The first allowed backbend occurs at a rotational frequency of $\ensuremath{\Elzxh}\ensuremath{\omega}=0.38\mathrm{MeV}.$ Three- and five-quasiparticle structures were identified, similar to those seen in lower mass $N=78$ isotones. Two of these structures have characteristics typical of mixed proton-neutron oblate configurations with one pair of aligned ${h}_{11/2}$ neutrons. In addition, two bands consisting of stretched $E2$ transitions have been observed with no known analogous structures in neighboring nuclei. The results are discussed in terms of the cranking model.