Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 7 de 7
Filtrar
Más filtros










Base de datos
Intervalo de año de publicación
1.
Phys Rev Lett ; 110(3): 032501, 2013 Jan 18.
Artículo en Inglés | MEDLINE | ID: mdl-23373914

RESUMEN

The decay of (19)O(ß(-)) and (19)Ne(ß(+)) implanted in niobium in its superconducting and metallic phases was measured using purified radioactive beams produced by the SPIRAL GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within a 1σ error bar. This measurement casts strong doubts on the predicted strong electron screening in a superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for (19)Ne and 400(320) eV for (19)O. Precise determinations of the half-lives were obtained for (19)O, 26.476(9) s, and for (19)Ne, 17.254(5) s.

2.
Phys Rev Lett ; 102(16): 162503, 2009 Apr 24.
Artículo en Inglés | MEDLINE | ID: mdl-19518704

RESUMEN

Six proton-emitting states in 19Ne were studied through the inelastic scattering reaction H(19Ne,p);{19}Ne; (p)18F. Their energies and widths were derived from the protons detected at zero degree, while proton-proton angular correlations between the detector at zero degree and a segmented annular detector were used to determine their spin value. In addition to the known states, a new broad J=1/2 resonance has been evidenced at E_{x} approximately 7.9 MeV, approximately 1.45 MeV above the proton emission threshold. By introducing this resonance, the 18F(p,alpha)15O destruction rate in novae is significantly enhanced. This reduces the chance to observe the cosmic gamma-ray emission of 18F from novae in space telescopes.

3.
Phys Rev Lett ; 101(15): 152501, 2008 Oct 10.
Artículo en Inglés | MEDLINE | ID: mdl-18999593

RESUMEN

The 18O(p,alpha)15N reaction is of primary importance in several astrophysical scenarios, including fluorine nucleosynthesis inside asymptotic giant branch stars as well as oxygen and nitrogen isotopic ratios in meteorite grains. Thus the indirect measurement of the low energy region of the 18O(p,alpha)15N reaction has been performed to reduce the nuclear uncertainty on theoretical predictions. In particular the strength of the 20 and 90 keV resonances has been deduced and the change in the reaction rate evaluated.

4.
Phys Rev Lett ; 95(3): 031101, 2005 Jul 15.
Artículo en Inglés | MEDLINE | ID: mdl-16090732

RESUMEN

We report on the observation of a previously unknown resonance at E(lab)(R)=194.1+/-0.6 keV in the 17O(p,alpha)14N reaction, with a measured resonance strength omegagamma(palpha)=1.6 +/- 0.2 meV. We studied in the same experiment the 17O(p,gamma)18F reaction by an activation method and the resonance-strength ratio was found to be omegagamma(palpha)/omegagamma(pgamma) = 470 +/- 50. The corresponding excitation energy in the 18F compound nucleus was determined to be 5789.8 +/- 0.3 keV by gamma-ray measurements using the 14N(alpha, gamma)18F reaction. These new resonance properties have important consequences for 17O nucleosynthesis and gamma-ray astronomy of classical novae.

5.
Phys Rev Lett ; 93(8): 081102, 2004 Aug 20.
Artículo en Inglés | MEDLINE | ID: mdl-15447168

RESUMEN

We report on the observation of a new resonance at E(lab)(R)=190 keV in the 17O(p,gamma)18F reaction. The measured resonance strength amounts to omegagamma(pgamma)=(1.2+/-0.2)x10(-6) eV. With this new value, the uncertainties in the 17O(p,gamma)18F and 17O(p,alpha)14N thermonuclear reaction rates are reduced by orders of magnitude at nova temperatures. Our significantly improved reaction rates have major implications for the galactic synthesis of 17O, the stellar production of the radioisotope 18F, and the predicted oxygen isotopic ratios in nova ejecta.

6.
Phys Rev Lett ; 86(18): 3985-8, 2001 Apr 30.
Artículo en Inglés | MEDLINE | ID: mdl-11328076

RESUMEN

We have measured the cross section of the 7Be(p,gamma)8B reaction for E(c.m.) = 185.8, 134.7, and 111.7 keV using a radioactive 7Be target (132 mCi). Single and coincidence spectra of beta+ and alpha particles from 8B and 8Be* decay, respectively, were measured using a large acceptance spectrometer. The zero energy S factor inferred from these data is 18.5+/-2.4 eV b and a weighted mean value of 18.8+/-1.7 eV b (theoretical uncertainty included) is deduced when combining this value with our previous results at higher energies.

7.
Astrophys J ; 526(2): L97-L100, 1999 Dec 01.
Artículo en Inglés | MEDLINE | ID: mdl-10550287

RESUMEN

Classical novae emit gamma-ray radiation at 511 keV and below (with a cutoff at around 20-30 keV), related to positron annihilation and its Comptonization in the expanding envelope. This emission has been elusive up to now because it occurs at epochs well before the maximum in optical luminosity, but it could be detected by some sensitive instrument on board a satellite, provided that the nova is close enough and that it is observed at the right moment. The detection of this emission, which is a challenge for current and future gamma-ray instruments, would shed light into the physical processes that occur in the early phases of the explosion, which are invisible in other lower energy ranges. A good prediction of the emitted fluxes and of the corresponding detectability distances with different instruments relies critically on a good knowledge of reaction rates relevant to 18F destruction, which have been subject to strong revision after recent nuclear spectroscopy measurements. With respect to previous results, smaller ejected masses of 18F are predicted, leading to smaller emitted fluxes in the 20-511 keV range and shorter detectability distances.

SELECCIÓN DE REFERENCIAS
DETALLE DE LA BÚSQUEDA