When deriving resonance strengths using the thick-target yield approximation, for very narrow resonances it may be necessary to take beam energy straggling into account. This applies to gas targets of a few keV width, especially if there is some additional structure in target stoichiometry or detection efficiency. The correction for this effect is shown and tested on recent studies of narrow resonances in the Ne-22(p, gamma)Na-23 and N-14(p, gamma)O-15 reactions. Copyright (C) EPLA, 2018

L. Wagner,1, 2 S. Akhmadaliev,1 M. Anders,1, 2 D. Bemmerer,1, ∗ A. Caciolli,3, 4 St. Gohl,1, 2 M. Grieger,1, 2 A. Junghans,1 M. Marta,5 F. Munnik,1 T. P. Reinhardt,2 S. Reinicke,1, 2 M. Röder,1, 2 K. Schmidt,1, 2 R. Schwengner,1 M. Serfling,1, 2 M. P. Takács,1, 2 T. Szücs,1 A. Vomiero,6 A. Wagner,1 and K. Zuber2 Helmholtz-Zentrum Dresden-Rossendorf (HZDR), 01328 Dresden, Germany Technische Universität Dresden, 01069 Dresden, Germany Dipartimento di Fisica e Astronomia, Università degli studi di Padova, 35131 Padova, Italy Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Padova, 35131 Padova, Italy GSI Helmholtzzentrum für Schwerionenforschung, D-64291 Darmstadt, Germany Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Luleå, Sweden (Dated: April 23, 2018)

The N-14(p, gamma) O-15 reaction is the slowest reaction of the carbon-nitrogen cycle of hydrogen burning and thus determines its rate. The precise knowledge of its rate is required to correctly model hydrogen burning in asymptotic giant branch stars. In addition, it is a necessary ingredient for a possible solution of the solar abundance problem by using the solar N-13 and O-15 neutrino fluxes as probes of the carbon and nitrogen abundances in the solar core. After the downward revision of its cross section due to a much lower contribution by one particular transition, capture to the ground state in O-15, the evaluated total uncertainty is still 8%, in part due to an unsatisfactory knowledge of the excitation function over a wide energy range. The present work reports precise S factor data at twelve energies between 0.357 and 1.292 MeV for the strongest transition, capture to the 6.79-MeV excited state in O-15, and at ten energies between 0.479 and 1.202 MeV for the second strongest transition, capture to the ground state in O-15. An R-matrix fit is performed to estimate the impact of the new data on astrophysical energies. The recently suggested slight enhancement of the 6.79-MeV transition at low energy could not be confirmed. The present extrapolated zero-energy S factors are S-6.79(0) = 1.24 +/- 0.11 keV b and S-GS(0) = 0.19 +/- 0.05 keV b.

This corrects the article DOI: 10.1103/PhysRevLett.115.252501.

The Big Bang Nucleosynthesis (BBN) theory allows to calculate the abundance of isotopes createdin the early universe. Interestingly, the primordial abundance of deuterium deduced from observations of pristine gas at high redshifts is more accurate with respect to the computed value because the BBN calculation is affected by the paucity of data for the deuterium-burning reaction D(p,g)3He. In fact, only a single dataset is currently available in the BBN energy range, in which the authors state systematic error of 9%. The concern for the D(p; g)3He cross section error is made worse by the fact that the theoretical and experimental values do not agree at the level of 20%. A new measurement is presently in progress at the LUNA (Laboratory for Underground Nuclear astrophysics) accelerator, operating deep underground at the Gran Sasso Laboratory, Italy. The main goal is the study of the D(p; g)3He cross section in the BBN energy range with accuracy. The LUNA measurement is described and preliminary results are discussed and compared with ab-initio calculations. The impact of this measurement in cosmology and particle physics is also highlighted. In particular, a precision measurement allows to derive the universal baryon density Wb with accuracy comparable to the one obtained by the PLANCK experiment. Finally, the accurate knowledge of the D(p; g)3He cross section increases the sensitivity to probe the existence of relativistic particles (e.g. sterile neutrinos, hot axions etc.) not foreseen in the standard model.

R. Depalo, 2 F. Cavanna, M. Aliotta, M. Anders, 6 D. Bemmerer, ∗ A. Best, A. Boeltzig, C. Broggini, C.G. Bruno, A. Caciolli, 2 G. F. Ciani, P. Corvisiero, T. Davinson, A. Di Leva, Z. Elekes, F. Ferraro, A. Formicola, Zs. Fülöp, G. Gervino, A. Guglielmetti, C. Gustavino, Gy. Gyürky, G. Imbriani, M. Junker, R. Menegazzo, V. Mossa, F. R. Pantaleo, D. Piatti, 2 P. Prati, O. Straniero, 7 T. Szücs, M. P. Takács, 6 and D. Trezzi

Big Bang nucleosynthesis (BBN) describes the production of light nuclei in the early phases of the Universe. For this, precise knowledge of the cosmological parameters, such as the baryon density, as well as the cross section of the fusion reactions involved are needed. In general, the energies of interest for BBN are so low (E < 1MeV) that nuclear cross section measurements are practically unfeasible at the Earth’s surface. As of today, LUNA (Laboratory for Underground Nuclear Astrophysics) has been the only facility in the world available to perform direct measurements of small cross section in a very low background radiation. Owing to the background suppression provided by about 1400 meters of rock at the Laboratori Nazionali del Gran Sasso (LNGS), Italy, and to the high current offered by the LUNA accelerator, it has been possible to investigate cross sections at energies of interest for Big Bang nucleosynthesis using protons, 3He and alpha particles as projectiles. The main reaction studied in the past at LUNA is the 2H(4He,\( \gamma\))6Li . Its cross section was measured directly, for the first time, in the BBN energy range. Other processes like 2H(p,\( \gamma\))3He , 3He(2H, p)4He and 3He(4He,\( \gamma\))7Be were also studied at LUNA, thus enabling to reduce the uncertainty on the overall reaction rate and consequently on the determination of primordial abundances. The improvements on BBN due to the LUNA experimental data will be discussed and a perspective of future measurements will be outlined.

The dipole strength distribution of Se-80 was studied in a photon-scattering experiment by using bremsstrahlung produced with an electron beam of energy 11.5 MeV at the linear accelerator ELBE. We identified 180 gamma transitions up to an energy of 9.6 MeV, and analyzed the strength in the quasicontinuum of the spectrum. Simulations of statistical gamma-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section below the neutron-separation energy derived in this way was combined with the photoabsorption cross section obtained from an earlier (gamma,n) experiment and used as an input for the calculation of Se-79(n,gamma) reaction rates on the basis of the statistical reaction model.

Background: The Ne-22(p,gamma)Na-23 reaction is the most uncertain process in the neon-sodium cycle of hydrogen burning. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars and classical novae, its uncertainty is mainly due to a large number of predicted but hitherto unobserved resonances at low energy.Purpose: A new direct study of low-energy Ne-22(p,gamma)Na-23 resonances has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), in the Gran Sasso National Laboratory, Italy.Method: The proton capture on Ne-22 was investigated in direct kinematics, delivering an intense proton beam to a Ne-22 gas target. gamma rays were detected with two high-purity germanium detectors enclosed in a copper and lead shield suppressing environmental radioactivity.Results: Three resonances at 156.2 keV [omega gamma = (1.48 +/- 0.10) x 10(-7) eV], 189.5 keV [omega gamma = (1.87 +/- 0.06) x 10(-6) eV] and 259.7 keV [omega gamma = (6.89 +/- 0.16) x 10(-6) eV] proton beam energy, respectively, have been observed for the first time. For the levels at E-x = 8943.5, 8975.3, and 9042.4 keV excitation energy corresponding to the new resonances, the gamma-decay branching ratios have been precisely measured. Three additional, tentative resonances at 71, 105, and 215 keV proton beam energy, respectively, were not observed here. For the strengths of these resonances, experimental upper limits have been derived that are significantly more stringent than the upper limits reported in the literature.Conclusions: Based on the present experimental data and also previous literature data, an updated thermonuclear reaction rate is provided in tabular and parametric form. The new reaction rate is significantly higher than previous evaluations at temperatures of 0.08-0.3 GK.

The dipole strength distribution of $^{80}\mathrm{Se}$ was studied in a photon-scattering experiment by using bremsstrahlung produced with an electron beam of energy 11.5 MeV at the linear accelerator ELBE. We identified $180\phantom{\rule{0.28em}{0ex}}\ensuremath{\gamma}$ transitions up to an energy of 9.6 MeV, and analyzed the strength in the quasicontinuum of the spectrum. Simulations of statistical $\ensuremath{\gamma}$-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section below the neutron-separation energy derived in this way was combined with the photoabsorption cross section obtained from an earlier $(\ensuremath{\gamma},n)$ experiment and used as an input for the calculation of $^{79}\mathrm{Se}(n,\ensuremath{\gamma})$ reaction rates on the basis of the statistical reaction model.

The scientific program and curent status of the planned accelerator laboratory in the Felsenkeller shallow-underground facility in Dresden, Germany, are reviewed.

To precisely determine BBN 6Li production, the cross-section of the nuclear reaction 2H(α, γ)6Li must be directly measured within the astrophysical energy range of 30–400keV. This measure requires an ultra-low γ-ray background in the experimental set-up. We have realized the conditions matching these very strict requirements at LUNA, the deep underground accelerator laboratory active in the INFN Gran Sasso National Laboratory (LNGS), Italy: the γ-ray spectrometer background has been reduced down to reach unmatched low levels, comparable to the good ones experienced in dedicated off-line underground ultra low γ counting rate. We present and discuss the γ-ray background reduction reached in the HpGe spectrometer, where most of the remaining γ-ray background seen in the spectra are coming from the energetic deuterons scattered in the gas target by the α beam. Thanks to the low neutron environmental background at LUNA, the effect of this weak flux of 2–3MeV neutrons on HpGe detectors has been studied in details and the results are presented and discussed.

We report on measurements of resonance strengths and energies for the \( E_{p} = 151\) and 193 keV resonances in the 18O(p,\(\alpha\))15N and 17O(p,\(\alpha\))14N reactions, respectively, obtained during commissioning of a new setup for alpha-particle detection studies at the LUNA underground laboratory. Our values, \(\omega\gamma (151)=164.2\pm 0.9_{stat} {}^{+12.1}_{-11.7} {}_{syst}\) meV and \(\omega\gamma (193)=1.68\pm 0.03_{stat} \pm 0.12_{syst}\) meV, are in excellent agreement with those reported in the literature. New values of resonance energies are \( E_{p}=151.2 \pm 0.3\) keV and \( E_{p}=194.8 \pm 0.3\) keV, respectively, this latter with the highest precision to date. Comparative background measurements in silicon detectors overground and underground were also carried out, yielding up to a factor of 15 in background suppression at LUNA at energies around 200keV. This clearly demonstrates the usefulness of underground measurements in charged-particles experiments, especially at low detection energies.

We report on measurements of resonance strengths and energies for the and 193 keV resonances in the O-18(p,)N-15 and O-17(p,)N-14 reactions, respectively, obtained during commissioning of a new setup for alpha-particle detection studies at the LUNA underground laboratory. Our values, meV and meV, are in excellent agreement with those reported in the literature. New values of resonance energies are keV and keV, respectively, this latter with the highest precision to date. Comparative background measurements in silicon detectors overground and underground were also carried out, yielding up to a factor of 15 in background suppression at LUNA at energies around 200keV. This clearly demonstrates the usefulness of underground measurements in charged-particles experiments, especially at low detection energies.

The dipole strength distribution of Ge-74 was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.0 and 12.1 MeV at the linear accelerator ELBE. We identified 94 levels with spin J = 1 up to an excitation energy of 8.9 MeV and analyzed the strength in the quasicontinuum of states. Simulations of statistical gamma-ray cascades were performed to estimate intensities of inelastic transitions and to correct the intensities of the ground-state transitions for their branching ratios. The photoabsorption cross section below the neutron-separation energy derived in this way is combined with the photoabsorption cross section obtained from an earlier (gamma,n) experiment and compared with phenomenological approximations.

The ^{22}Ne(p,γ)^{23}Na reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle affects the synthesis of the elements between ^{20}Ne and ^{27}Al in asymptotic giant branch stars and novae. The ^{22}Ne(p,γ)^{23}Na reaction rate is very uncertain because of a large number of unobserved resonances lying in the Gamow window. At proton energies below 400 keV, only upper limits exist in the literature for the resonance strengths. Previous reaction rate evaluations differ by large factors. In the present work, the first direct observations of the ^{22}Ne(p,γ)^{23}Na resonances at 156.2, 189.5, and 259.7 keV are reported. Their resonance strengths are derived with 2%-7% uncertainty. In addition, upper limits for three other resonances are greatly reduced. Data are taken using a windowless ^{22}Ne gas target and high-purity germanium detectors at the Laboratory for Underground Nuclear Astrophysics in the Gran Sasso laboratory of the National Institute for Nuclear Physics, Italy, taking advantage of the ultralow background observed deep underground. The new reaction rate is a factor of 20 higher than the recent evaluation at a temperature of 0.1 GK, relevant to nucleosynthesis in asymptotic giant branch stars.

The 22Ne(p,γ)23Na reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle is active in asymptotic giant branch stars as well as in novae and contributes to the nucleosythesis of neon and sodium isotopes. In order to reduce the uncertainties in the predicted nucleosynthesis yields, new experimental efforts to measure the 22Ne(p,γ)23Na cross section directly at the astrophysically relevant energies are needed. In the present work, a feasibility study for a 22Ne(p,γ)23Na experiment at the Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV accelerator deep underground in the Gran Sasso laboratory, Italy, is reported. The ion-beam-induced γ-ray background has been studied. The feasibility study led to the first observation of the E p=186 keV resonance in a direct experiment. An experimental lower limit of 0.12 × 10−6 eV has been obtained for the resonance strength. Informed by the feasibility study, a dedicated experimental setup for the 22Ne(p,γ)23Na experiment has been developed. The new setup has been characterized by a study of the temperature and pressure profiles. The beam heating effect that reduces the effective neon gas density due to the heating by the incident proton beam has been studied using the resonance scan technique, and the size of this effect has been determined for a neon gas target.

The strength of the ${E}_{\mathrm{p}}=1.842$ MeV resonance in the ${}^{40}$Ca(p,$\ensuremath{\gamma}$)${}^{41}$Sc reaction is determined with two different methods: First, by an absolute strength measurement using calcium hydroxide targets, and second, relative to the well-determined strength of the resonance triplet at ${E}_{\ensuremath{\alpha}}$ = 4.5 MeV in the ${}^{40}$Ca($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)${}^{44}$Ti reaction. The present new value of $\ensuremath{\omega}\ensuremath{\gamma}=(0.192\ifmmode\pm\else\textpm\fi{}0.017)$ eV is 37% (equivalent to $3.5\ensuremath{\sigma}$) higher than the evaluated literature value. In addition, the ratio of the strengths of the 1.842 MeV ${}^{40}$Ca(p,$\ensuremath{\gamma}$)${}^{41}$Sc and 4.5 MeV ${}^{40}$Ca($\ensuremath{\alpha}$,$\ensuremath{\gamma}$)${}^{44}$Ti resonances has been determined to be $0.0229\ifmmode\pm\else\textpm\fi{}0.0018$. The newly corrected strength of the 1.842-MeV resonance can be used in the future as a normalization point for experiments with calcium targets.

The photoabsorption cross section of Ta-181 up to the neutron-separation energy is deduced using bremsstrahlung produced with an electron beam of 9.6 MeV energy. The analysis of the measured gamma-ray spectra includes the quasicontinuum of levels at high energy. Simulations of gamma-ray cascades are performed to estimate intensities of inelastic transitions and branching ratios of the ground-state transitions. The resulting photoabsorption cross section shows enhanced dipole strength in the energy range from 5 to 8 MeV, which may be related to a pygmy dipole resonance. The results of the present experiment are compared with predictions of a quasiparticle-random-phase approximation in a deformed basis. A combination of the present experimental data and (gamma, n) data is used as an input to the statistical code TALYS applied to calculate cross sections and reaction rates of photonuclear reactions that are important for the nucleosynthesis of Ta-180.