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تسجيل مستخدم جديدCollapse of the N=28 shell closure in $^{42}$Si
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The energies of the excited states in very neutron-rich $^{42}$Si and $^{41,43}$P have been measured using in-beam $gamma$-ray spectroscopy from the fragmentation of secondary beams of $^{42,44}$S at 39 A.MeV. The low 2$^+$ energy of $^{42}$Si, 770(19) keV, together with the level schemes of $^{41,43}$P provide evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that $^{42}$Si is best described as a well deformed oblate rotor.
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The single-particle structure of the $N=27$ isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly-magic $^{48}$Ca toward the drip line. $^{43}$S was studied employing the one-neutron knockout reaction from a radio
active $^{44}$S beam. Using a combination of prompt and delayed $gamma$-ray spectroscopy the level structure of $^{43}$S was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the $^{44}$S ground-state configuration has a strong intruder component. The results were confronted with shell model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the $2p_{3/2}$ orbital in the experiment indicates that the breakdown of the $N=28$ magic number is more rapid than the theoretical calculations suggest.
Measurements of the N=28 isotones 42Si, 43P and 44S using one- and two-proton knockout reactions from the radioactive beam nuclei 44S and 46Ar are reported. The knockout reaction cross sections for populating 42Si and 43P and a 184 keV gamma-ray obse
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Excited states of the neutron-rich isotopes $^{42,44}$S and $^{41,43}$P have been studied via inverse-kinematics proton scattering from a liquid hydrogen target, using the GRETINA $gamma$-ray tracking array to extract inelastic scattering cross secti
ons. Deformation lengths of the $2^+_1$ excitations in $^{42,44}$S have been determined and, when combined with deformation lengths determined with electromagnetic probes, yield the ratio of neutron-to-proton matrix elements $M_n/M_p$ for the $2^+_1$ excitations in these nuclei. The present results for $^{41,43}$P$(p,p)$ are used to compare two shell model interactions, SDPF-U and SDPF-MU. As in a recent study of $^{42}$Si, the present results on $^{41,43}$P favor the SDPF-MU interaction.
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. E
xperimental 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.
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