Search

The 14N(p,γ)15O reaction is the slowest process of the CN cycle, and thus it is of high astrophysical interest since it regulates the total rate of energy and neutrinos production through the cycle. The 14N+p ground state capture is strongly influenced by a sub-threshold resonance corresponding to the 6.79 MeV state in 15O. The width of this resonance is a major source of uncertainty in the extrapolation of the reaction cross section in the Gamow energy window. Preliminary results of a new Doppler Shift Attenuation measurement of the lifetime of the 6.79 MeV state in 15O are discussed. The level of interest was populated via the 2H(14N,n)15O reaction in inverse kinematics at 32 MeV beam energy. The gamma-rays emitted in the decay of the 6.79 MeV level to the ground state were detected with the AGATA Demonstrator array of high-purity germanium detectors. The sensitivity of the shape of the peak in the gamma-ray energy spectrum to the level lifetime is investigated comparing the experimental peaks with detailed Monte Carlo simulations of the reaction mechanisms and the gamma-ray emission and detection. Nuclear levels in 15N (also populated in the 14N+2H reaction) for which the lifetimes are known in the literature provided a test of the analysis technique. [ABSTRACT FROM AUTHOR]

The preliminary results of a new direct measurement of the lifetime of the first excited 3/2+ state in 15O are discussed. An accurate evaluation of this lifetime is of paramount importance for the determination of the cross section of the 14N(p,γ)15O reaction, the slowest one in the CNO cycle, at the energies of the solar Gamow peak. The 2H(14N,15O)n reaction in inverse kinematics at 32MeV beam energy (XTU Tandem, LNL) was used to populate the level of interest, which decays via a 6.79 MeV E1 gamma-ray transition to the ground state. Gamma rays were detected with 4 triple clusters of HPGe detectors of the AGATA Demonstrator array. The energy resolution and position sensitivity of this state-of-the-art gamma-ray spectrometer have been exploited to investigate the Doppler Shift Attenuation effect on the lineshape of the gamma-ray peak in the energy spectrum. The deconvolution of the lifetime effects from those due to the kinematics of the emitting nuclei has been performed using detailed Monte Carlo simulations of the gamma emission and detection. CDCC-CRC calculations for the nucleon transfer process have been used for this purpose and preliminary results are shown. [ABSTRACT FROM AUTHOR]

The large solid angle magnetic spectrometer for heavy ions PRISMA was operated at LNL in conjunction with the highly efficient CLARA set-up up to the end of March 2008. The AGATA Demonstrator is being mounted around the target area of PRISMA replacing the CLARA set-up. It will allow to carry out nuclear structure and reaction mechanism studies with better resolution and higher statistics. New detectors for light ions and slow moving heavy ions have been designed for the focal plane of the spectrometer. [ABSTRACT FROM AUTHOR]

The lifetimes of excited states of the 122Ba ground—state band, populated via the 108Cd(16O,2n)122Ba and the 112Sn(13C,2n)122Ba reactions, have been measured using the Recoil Distance Doppler—Shift method. The level scheme of 122Ba has also been revised. [ABSTRACT FROM AUTHOR]

By comparing the excitation energies of analogue states in isobaric multiplets, several nuclear structure properties can be studied as a function of the angular momentum up to high spin states. In particular, the mirror nuclei 35Ar and 35Cl show large differences between the excitation energies of analogue negative-parity states at high spin, confirming the important contribution of the relativistic electromagnetic spin-orbit interaction to the Coulomb energy. The single-particle character of the configuration of these states is reproduced with very good accuracy by shell model calculations in the sd and pf shells valence space. In addition, evidence of isospin mixing is deduced from the El transitions linking positive and negative parity states. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The excited states in 146Gd have been re-investigated with the 144Sm(α,2n) reaction using a modern Ge γ-ray array including a polarimeter. Amongst the non-yrast states populated in this reaction we have identified the aligned 6+ member of the two-phonon octupole quartet from the observation of the E3 branching to the one phonon 3- state. Our results represent the first observation of a 6+→3-→0+ E3 cascade in an even-even nucleus. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The energy spectra and relative transition rates of 176,178,180Os were found to be very similar to the values calculated in the framework of the critical point symmetry X(5). In order to enable more stringent tests, including also absolute transition probabilities, two coincidence RDDS lifetime measurements were performed at the Laboratori Nazionali di Legnaro with the GASP spectrometer using the 152,154Sm(29Si,5n)176,178Os reactions. The deduced transition quadrupole moments in 178Os agree well with the X(5) predictions. For 176Os only preliminary results were obtained so far. These preliminary data support an X(5) like structure also in the case of 176Os. The experimental data of both nuclei are compared to calculations in the framework of the interacting boson model (IBM) and of the general collective model (GCM). © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

In stars, four hydrogen nuclei are converted into a helium nucleus by two competing nuclear fusion processes: the proton - proton chain (p-p) and the carbon - nitrogen - oxygen (CNO) cycle. At temperatures higher than 2×107 K, the CNO cycle dominates the energy production. In particular, its rate is determined by the slowest reaction: 14N(p,γ)15O. Direct measurement in a laboratory at the surface of the Earth is hampered by the background due to the cosmic rays. Here we report on an experiment performed with the LUNA (Laboratory for Underground Nuclear Astrophysics) accelerator placed deep underground in the INFN Gran Sasso laboratory (Italy). Thanks to the cosmic ray suppression provided by the mountain shield, we could measure the 14N(p,γ)15O cross section for the first time directly at energies corresponding to stellar temperatures and with unpreceedent accuracy. In particular we used a large solid angle BGO detector with a differentially pumped windowless gas target. The lowest measured energy is 70.1 keV. Our results have important consequences for the formation of carbon stars, for an independent lower limit on the age of the universe, and for solar neutrino flux. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Coexisting states of prolate and oblate shapes were studied via Coulomb excitation of radioactive 74Kr and 76Kr beams at safe energies at GANIL. The results for the transitional E2 matrix elements for several yrast transitions in both isotopes are in contradiction with the B(E2) values extracted from the lifetimes that are found in the literature. This motivated a new lifetime measurement which showed strong deviations from the previously published values, but is consistent with the results from Coulomb excitation. Having resolved these discrepancies and knowing the B(E2) values with high precision allows to extract the diagonal matrix elements from the Coulomb excitation data. The results confirm the proposed shape coexistence scenario. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The excited states in 146Gd have been re-investigated with the 144Sm(α,2n) reaction using a modern Ge γ-ray array including a polarimeter. Amongst the non-yrast states populated in this reaction we have identified the aligned 6+ member of the two-phonon octupole quartet from the observation of the E3 branching to the one phonon 3- state. Our results represent the first observation of a 6+→3-→0+ E3 cascade in an even-even nucleus. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Lifetime measurements in 134Pr were performed by means of the Recoil distance Doppler-shift and Doppler-shift attenuation methods using the multidetector array EUROBALL, in conjunction with the inner BGO ball. The derived B(E2) transition strengths within the two bands candidates for chiral partners behave differently with increasing spin while the corresponding B(M1) values have a similar behaviour within the experimental uncertainties. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Excited states have been studied in sd-shell nuclei following the 16O (70 MeV) + 24Mg (400 μg/cm2) fusion-evaporation reaction. The GASP spectrometer in conjunction with the charged-particle detector ISIS and the Neutron ring allowed the detection of the γ-rays in coincidence with evaporated light particles. New data on the mirror pairs A=31 and A=35 have been obtained. In particular, the comparison between the level schemes of 35Ar and 35Cl has confirmed the importance of the electromagnetic spin-orbit term, which explains the large Mirror Energy Difference values. Evidence of isospin mixing can be deduced from the E1 transitions. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The feasibility of the entire process of γ-ray tracking is demonstrated experimentally for the first time. The accurracy of the results is verified by the capability to carry out Doppler correction of γ-rays emitted in flight. The resolution of the 847.8 keV transition detected with the MARS detector after Coulomb excitation of a [sup 56]Fe beam could be improved from 15 keV to below 5 keV (FWHM). [ABSTRACT FROM AUTHOR]

Lifetimes of high-spin states in 69As have been measured using Doppler shift attenuation technique with the GASP and RFD setup. The determined transition probabilities indicate large deformation associated with some rotational bands in this nucleus. [ABSTRACT FROM AUTHOR]

The transition strenghts for the low—lying levels of 122,124Ba have been determined analysing the Recoil Distance Doppler Shift data by the Differential Decay Curve Method. The trend of normalised B(E2) reveals a drop in correspondance of the 81+ state. This feature is also present in 122Ba: entering the backbending region the interplay between collective and quasiparticle degrees of freedom have to be carefully taken into account. [ABSTRACT FROM AUTHOR]