449 research outputs found
Evolution of the N=28 shell closure: a test bench for nuclear forces
The evolution of the N=28 shell closure is investigated far from stability.
Using the latest results obtained from various experimental techniques, we
discuss the main properties of the N=28 isotones, as well as those of the N=27
and N=29 isotones. Experimental results are confronted to various theoretical
predictions. These studies pinpoint the effects of several terms of the
nucleon-nucleon interaction, such as the central, the spin-orbit, the tensor
and the three-body force components, to account for the modification of the
N=28 shell gap and spin-orbit splittings. Analogies between the evolution of
the N=28 shell closure and other magic numbers originating from the spin-orbit
interaction are proposed (N=14,50, 82 and 90). More generally, questions
related to the evolution of nuclear forces towards the drip-line, in bubble
nuclei, and for nuclei involved in the r-process nucleosynthesis are proposed
and discussed.Comment: 40 pages,15 figures, Procceedings Nobel Symposium 2012, accepted for
publication in Physica Script
Lifetimes of 26Al and 34Cl in an astrophysical plasma
We study here the onset of thermal equilibrium affecting the lifetimes of
26Al and 34Cl nuclei within a hot astrophysical photon gas. The 26Al isotope is
of prime interest for gamma ray astronomy with the observation of its delayed
(t_1\2=0.74 My) 1.809MeV gamma-ray line. Its nucleosynthesis is complicated by
the presence of a short lived (t_1\2=6.34s) spin isomer. A similar
configuration is found in 34Cl where the decay of its isomer (34mCl, t_1\2=32m)
is followed by delayed gamma-ray emission with characteristic energies. The
lifetimes of such nuclei are reduced at high temperature by the thermal
population of shorter lived levels. However, thermal equilibrium within 26Al
and 34Cl levels is delayed by the presence of the isomer. We study here the
transition to thermal equilibrium where branching ratios for radiative
transitions are needed in order to calculate lifetimes. Since some of these
very small branching ratios are not known experimentally, we use results of
shell model calculations.Comment: 11 pages, 5 figures, Latex, accepted for publication in Phys. Rev.
Evolution of the N=50 gap from Z=30 to Z=38 and extrapolation towards 78Ni
The evolution of the N=50 gap is analyzed as a function of the occupation of
the proton f5/2 and p3/2 orbits. It is based on experimental atomic masses,
using three different methods of one or two-neutron separation energies of
ground or isomeric states. We show that the effect of correlations, which is
maximized at Z=32 could be misleading with respect to the determination of the
size of the shell gap, especially when using the method with two-neutron
separation energies. From the methods that are the least perturbed by
correlations, we estimate the N=50 spherical shell gap in 78Ni. Whether 78Ni
would be a rigid spherical or deformed nucleus is discussed in comparison with
other nuclei in which similar nucleon-nucleon forces are at play.Comment: 7 pages, 8 figures, accepted for publication PRC (22 december 2011
High-spin states in the five-valence-particle nucleus 213Po
Excited states in 213Po have been populated using the 18O + 208Pb reaction at 85 MeV beam energy and studied with the Euroball IV -multidetector array. The level scheme has been built up to 2.0 MeV excitation energy and spin I 25/2 ~ from the triple -coincidence data. Spin and parity values of several yrast states have been assigned from the -angular properties. The configurations of the yrast states are discussed using results of empirical shell-model calculations and by analogy with the neighbouring nuclei. The spin and parity values of several low-spin states of 213Po previously identified from the -decay of 213Bi are revised
First observation of high-spin states in Po: Probing the valence space beyond ^{208}Pb
Excited states in Po-214 have been populated using the O-18 + Pb-208 reaction at 85-MeV beam energy and studied with the Euroball IV gamma-multidetector array. The level scheme has been built up to similar to 2.7-MeV excitation energy and spin I = 12 (h) over bar from the triple-gamma coincidence data. Spin and parity values of most of the observed states have been assigned from the gamma-angular properties. The configurations of the yrast states are discussed using results of empirical shell-model calculations and by analogy with the neighboring nuclei. The Po-214 level scheme established in this work constitutes an important step for the determination of the effective nucleon-nucleon interactions beyond N = 126
High-spin states with seniority v=4,4,6 in 119-126Sn
The 119-126Sn nuclei have been produced as fission fragments in two reactions
induced by heavy ions: 12C+238U at 90 MeV bombarding energy, 18O+208Pb at 85
MeV. Their level schemes have been built from gamma rays detected using the
Euroball array. High-spin states located above the long-lived isomeric states
of the even- and odd-A 120-126Sn nuclei have been identified. Moreover isomeric
states lying around 4.5 MeV have been established in 120,122,124,126Sn from the
delayed coincidences between the fission fragment detector SAPhIR and the
Euroball array. The states located above 3-MeV excitation energy are ascribed
to several broken pairs of neutrons occupying the nu h11/2 orbit. The maximum
value of angular momentum available in such a high-j shell, i.e. for
mid-occupation and the breaking of the three neutron pairs, has been
identified. This process is observed for the first time in spherical nuclei.Comment: 20 pages, 22 figures, 12 tables, accepted for publication in Physical
Review
The X-ray warm absorber and nuclear obscuration in the Seyfert 1.8 galaxy ESO 113-G010
We present the first analysis of the X-ray warm absorber and nuclear
obscuration in the Seyfert 1.8 galaxy ESO 113-G010. We used archival data from
a 100 ks XMM-Newton observation made in 2005. From high resolution spectroscopy
analysis of the RGS data, we detect absorption lines originating from a warm
absorber consisting of two distinct phases of ionisation, with log xi ~ 3.2 and
2.3 respectively. The higher-ionised component has a larger column density and
outflow velocity (N_H ~ 1.6 x 10^22 cm^-2, v ~ -1100 km/s) than the
lower-ionised component (N_H ~ 0.5 x 10^22 cm^-2, v ~ -700 km/s). The shape of
the optical-UV continuum and the large Balmer decrement (H_alpha/H_beta ~ 8)
indicate significant amount of reddening is taking place in our line of sight
in the host galaxy of the AGN; however, the X-ray spectrum is not absorbed by
cold neutral gas intrinsic to the source. We discuss different explanations for
this discrepancy between the reddening and the X-ray absorption, and suggest
that the most likely solution is a dusty warm absorber. We show that dust can
exist in the lower-ionised phase of the warm absorber, which causes the
observed reddening of the optical-UV emission, whereas the X-rays remain
unabsorbed due to lack of cold neutral gas in the ionised warm absorber.
Furthermore, we have investigated the uncertainties in the construction of the
Spectral Energy Distribution (SED) of this object due to obscuration of the
nuclear source and the effects this has on the photoionisation modelling of the
warm absorber. We show how the assumed SEDs influence the thermal stability of
each phase and whether or not the two absorber phases in ESO 113-G010 can
co-exist in pressure equilibrium.Comment: 11 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
High-spin structures of 88Kr and 89Rb: Evolution from collective to single-particle behaviors
The high-spin states of the two neutron-rich nuclei, 88Kr and 89R have been
studied from the 18O + 208Pb fusion-fission reaction. Their level schemes were
built from triple gamma-ray coincidence data and gamma-gamma angular
correlations were analyzed in order to assign spin and parity values to most of
the observed states. The two levels schemes evolve from collective structures
to single-particle excitations as a function of the excitation energy.
Comparison with results of shell-model calculations gives the specific proton
and neutron configurations which are involved to generate the angular momentum
along the yrast lines.Comment: 12 pages, 9 figures, Physical Review C (2013) in pres
High-spin structures of 136Cs
Odd-odd 136Cs nuclei have been produced in the 18O + 208Pb and 12C + 238U
fusion-fission reactions and their gamma rays studied with the Euroball array.
The high-spin level scheme has been built up to ~ 4.7 MeV excitation energy and
spin I ~ 16 hbar from the triple gamma-ray coincidence data. The configurations
of the three structures observed above ~ 2 MeV excitation energy are first
discussed by analogy with the proton excitations identified in the semi-magic
137Cs nucleus, which involve the three high-j orbits lying above the Z=50 gap,
pi g_{7/2}, pi d_{5/2} and pi h_{11/2}. This is confirmed by the results of
shell-model calculations performed in this work.Comment: 6 pages, 4 figures, 3 table
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