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 39A MeV. The low 2(+) energy of (42)Si, 770(19) keV, together with the level schemes of (41,43)P, provides 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 transfer of neutrons onto 24 Ne has been measured using a reaccelerated radioactive beam of 24 Ne to study the ðd; pÞ reaction in inverse kinematics. The unusual raising of the first 3=2 þ level in 25 Ne and its significance in terms of the migration of the neutron magic number from N ¼ 20 to N ¼ 16 is put on a firm footing by confirmation of this state's identity. The raised 3=2 þ level is observed simultaneously with the intruder negative parity 7=2 À and 3=2 À levels, providing evidence for the reduction in the N ¼ 20 gap. The coincident gamma-ray decays allowed the assignment of spins as well as the transferred orbital angular momentum. The excitation energy of the 3=2 þ state shows that the established USD shell model breaks down well within the sd model space and requires a revised treatment of the proton-neutron monopole interaction. DOI: 10.1103/PhysRevLett.104.192501 PACS numbers: 21.10.Hw, 21.10.Jx, 23.20.Lv, 25.60.Je The monopole part of the nucleon-nucleon interaction is now recognized as having a major effect on nuclear shell structure far from stability [1,2]. The interaction between valence protons and neutrons is sufficient to alter the energies of single-particle levels so that different magic numbers (or shell gaps) appear, and this can substantially affect the collective [3] and magnetic [4] properties and basic quantities such as the lifetime [5]. Nucleon transfer reactions induced by light ions are an established experimental tool for studying single-particle structure [6]. Here we employ the ðd; pÞ reaction in inverse kinematics to explore the disappearance of the N ¼ 20 magic number (and its replacement by N ¼ 16) in the neutron-rich neon isotones. As will be shown, the measurement of the differential cross sections of the light ejectiles plus the coincident gamma decays of the residual nucleus brings a new power to this type of study.Recent work using other techniques has provided evidence for the emergence of N ¼ 16 as a magic number in this region, but has not identified the single-particle structure in an unambiguous manner through measurements of the spectroscopic factors and spins. In a study of the decay of 25 F [7] the increased energy of the 0d 3=2 neutron orbital was inferred. This made use of a preliminary analysis of the present work [8] and concluded that the energy shift was consistent with the monopole effect [7]. In a study of 27 Ne using the ðd; pÞ reaction but without detecting the protons [9], a reduced gap between the 0d 3=2 and higher negative parity orbitals was deduced. This agreed with nucleon removal studies [10]. Finally, in recent studies of 23 O by transfer [11] and 25 O by proton removal [12] the 0d 3=2 state was found to have an increased excitation energy, but the required modifications to the shell-model interaction were not mutually consistent [11,12]. While an extensive review including the emergence of the N ¼ 16 magic number has recently been published [2], further quantitative data are needed in order to understand this monopole effect properly....

A compact, quasi-4π position sensitive silicon array, TIARA, designed to study direct reactions induced by radioactive beams in inverse kinematics is described here. The Transfer and Inelastic All-angle Reaction Array (TIARA) consists of 8 resistive charge division detectors forming an octagonal barrel around the target and a set of double-sided silicon-strip annular detectors positioned at each end of the barrel. The detector was coupled to the γ-ray array EXOGAM and the spectrometer VAMOS at the GANIL Laboratory to demonstrate the potential of such an apparatus with radioactive beams. The reaction, well known in direct kinematics, has been carried out in inverse kinematics for that purpose. The observation of the ground state and excited states at 7.16 and 7.86 MeV is presented here as well as the comparison of the measured proton angular distributions with DWBA calculations. Transferred l-values are in very good agreement with both theoretical calculations and previous experimental results obtained in direct kinematics

The results obtained from electron and in-beam spectroscopy experiments reveal that the 44 S nucleus is located in a transitional region between the spherical 48 Ca and the oblate 42 Si. The comparison of the results with Large Scale Shell Model calculations points towards prolate-spherical shape coexistence where the ground state becomes the intruder configuration due to quadrupole excitations across the Z = 14 and N = 28 shell gaps.

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The yrast decay sequence of 41 Cl, populated in deep inelastic processes produced by the interaction of 234 MeV 37 Cl ions with a 160 Gd target, has been studied using the highly sensitive EUROBALL IV ␥-ray detector array. Yrast states are established for the first time at energies of 130 and 891 keV.Studies of the properties of nuclei far from stability provide a unique opportunity to increase our understanding of nuclear interactions in extreme conditions and often challenge our theoretical models. An example is the well-known deformation near singly magic 32 Mg ͓1͔. Theoretical calculations by Werner et al. have indicated the possibility of deformation in the region near singly magic (Nϭ28) 44 S ͓2͔.The experimental quadrupole deformation ␤ 2 values obtained from B(E2:0 ϩ →2 ϩ ) measurements for 40 S ͑0.284͒ ͓3͔ and 42 Ar ͑0.273͒ ͓4͔ provide evidence for the deformation of these isotones of 41 Cl. However, experimental information on the nuclear structure in this region has been quite limited, primarily due to the inability of most traditional methods to produce these nuclei. There is little spectroscopic information available currently for the neutron-rich NϾ23 isotopes of Cl. In particular, for the Tϭ7/2 41 Cl nucleus, no published work exists on the experimental level structure ͓5͔. Spectroscopic measurements can reveal details of the underlying microscopic structures and have proved essential for understanding properties of nuclei far from stability. Furthermore, they potentially provide a stringent test of modern large scale shell-model calculations.Neutron-rich 17 41 Cl 24 has three proton holes in the sd shell and four neutrons in the f p shell. It is thus a good candidate for testing cross-shell shell-model interactions that have a strong implication on the ordering of the first 3/2 ϩ and 1/2 ϩ states ͓6͔. The ground state of 41 Cl was assigned a J value ͓7͔ of either 1/2 ϩ or 3/2 ϩ by Gurach et al. on the basis of an allowed and dominant ␤ Ϫ decay branch to the J ϭ1/2 ϩ 1.87 MeV level in 41 Ar. Considering the 41 Cl excited states, we have observed in the present work two decay transitions at energies of 130 and 761 keV.A number of neutron-rich nuclei were populated in the interaction of a 234 MeV beam of 37 Cl ions, delivered by the VIVITRON at IReS, Strasbourg, with a 160 Gd target. The target, isotopically enriched to 98.2% in 160 Gd, was of thickness 12 mg/cm 2 and was backed with 40 mg/cm 2 of isotopically enriched 208 Pb (99.47%). The backed target was sufficiently thick to stop all forward-recoiling reaction fragments. The ␥ decay of excited states with lifetimes comparable with or smaller than the slowing-down time (ϳ1 ps) of recoiling nuclei in the target material will not readily be observed because of Doppler effects. For projectilelike species the v/c value is estimated to be of the order of 10%. The present experiment is thus sensitive to the ␥ decay of states with longer lifetimes. Study of the ␥ deexcitation of the products of deep-inelastic reactions is difficult because yields are gen...