Search

The measurement was performed at the National Superconducting Cyclotron Laboratory (NSCL).; International audience; Background: Neutron-rich, even-mass chromium and iron isotopes approaching neutron number N = 40 havebeen important benchmarks in the development of shell-model effective interactions incorporating the effectsof shell evolution in the exotic regime. Odd-mass manganese nuclei have received less attention, but provideimportant and complementary sensitivity to these interactions.Purpose:We report the observation of two newγ -ray transitions in 63Mn,which establish the (9/2−) and (11/2−)levels on top of the previously known (7/2−) first-excited state. The lifetime for the (7/2−) and (9/2−) excitedstates were determined for the first time, while an upper limit could be established for the (11/2−) level.Method: Excited states in 63Mn have been populated in inelastic scattering from a 9Be target and in thefragmentation of 65Fe. γγ coincidence relationships were used to establish the decay level scheme. A Dopplerline-shape analysis for the Doppler-broadened (7/2−) → 5/2−, (9/2−)→ (7/2−), and (11/2−) → (9/2−)transitions was used to determine (limits for) the corresponding excited-state lifetimes.Results: The low-lying level scheme and the excited-state lifetimes were compared with large-scale shell-modelcalculations using different model spaces and effective interactions in order to isolate important aspects of shellevolution in this region of structural change.Conclusions: While the theoretical (7/2−) and (9/2−) excitation energies show little dependence on the modelspace, the calculated lifetime of the (7/2−) level and calculated energy of the (11/2−) level reveal the importanceof including the neutron g9/2 and d5/2 orbitals in the model space. The LNPS effective shell-model interactionprovides the best overall agreement with the new data.

The neutron-rich nuclei 34, 35, 36, 37, 38P were populated in binary grazing reactions initiated by a beam of 36S ions at 215 MeV incident on a thin 208Pb target. The combination of the PRISMA magnetic spectrometer and the CLARA array of escape-suppressed Ge detectors was used to detect gamma rays in coincidence with projectile-like nuclear species. Further evidence to support the observed gamma decay of excited states of the populated phosphorus isotopes is provided by gamma-gamma coincidence measurements based on the results of an earlier thick-target deep-inelastic experiment. Level schemes for 34, 35, 36, 37, 38P are presented and discussed within the context of state-of-the-art shell-model calculations. In particular, new spin and parity assignments are proposed for a number of excited states of 36P. The evolution with neutron number of the energy difference of the low-lying 3/2+ and 1/2+ states in the odd-A isotopes of phosphorus, and the relationship to nuclear collectivity, is discussed. The evolution with neutron number of the shell-model wave functions of the first 1/2+, 3/2+, and 5/2+ states of the odd-A phosphorus isotopes and of the first 3−, 4−, and 5− states of the even-A isotopes is also explored.

The structure of 58Fe was investigated at Gammasphere using 48Ca(13 ,14C,xn) fusion-evaporation reactions at a beam energy of 130 MeV. The level scheme has been revised and extended to J∼17 and an excitation energy of 16.6 MeV. Regular band structures consisting of low-energy ΔJ=1 transitions have been observed at moderate spin (J∼8-15) and are candidates for magnetic rotational bands. Self-consistent tilted-axis-cranking calculations within a relativistic mean-field theory were applied to investigate these bands and were found to reproduce the experimental results well. In other parts of the level scheme, quasirotational bands composed of stretched-E2 transitions have been extended to high spin, and other new bands have been identified. Positive-parity experimental states were compared to predictions of the spherical shell model using the GXPF1A, KB3G, and FPD6 effective interactions in the fp model space. The projected shell model, with a deformed quasiparticle basis including the neutron νg9/2 orbital, was applied to interpret regular ΔJ=2 band structures that extend beyond the maximum spin available for π[(f7/2)-2]- ν[(p3/2f 5/2p1/2)4] configurations and exhibit features characteristic of rotational alignment. It is clear that the νg9/2 intruder orbital plays a crucial role in describing the quasirotational structures in this nucleus, even starting as low as J∼5.

Gamma-ray transitions have been observed in the proton-emitting N=Z+2 (Tz=1) isotope 112Cs. The transitions have been unambiguously assigned to 112Cs by correlation with the characteristic proton decay, using the method of recoil-decay tagging with mass selection. The measured proton-decay energy and half-life are Ep=810(5) keV and T1/2=470(50) μs, respectively, which are consistent with previous measurements. Five γ-ray transitions have been observed which appear to form a rotational sequence. The energy differences between excited states in the sequence are consistent with an assignment as the favored signature of the ν(h 11/2) π(h11/2) structure. Tentative evidence for fine structure in the 112Cs proton decay is also observed.

Excited states in V55 and Cr55 have been populated via pn and 2n evaporation channels, respectively, following the fusion of a Ca48 beam at 172 MeV with a Be9 target. Level schemes have been deduced for the two nuclides to excitation energies of 7467 (V55) and 12226 keV (Cr55), with spins of 27/2 + and 33/2+, respectively. Negative-parity states are compared with shell-model calculations using three different effective interactions in the full fp model space. Negative-parity levels of Cr55 are explained in terms of single-particle fp-shell configurations outside N=28 and N=32 cores. Positive-parity states in both isotopes show evidence for the involvement of neutron g9/2 configurations. In the case of Cr55, a quasirotational structure based on the 1/2+[440] Nilsson orbital is observed up to the terminating state. In V55, positive-parity states do not exhibit well-developed collective features, and the observation of octupole decays is an indication of their importance in transitions from neutron g 9/2 configurations to the fp shell. Experimental results are compared with the predictions of a traditional shell model, the projected shell model, and total-Routhian-surface calculations.

Excited states in the neutron-rich isotopes Mn57-60 have been studied with fusion-evaporation reactions induced by Ca48 beams at 130 MeV on C13,14 targets. Level schemes have been deduced reaching spins of ∼16□ and ∼27□/2 in the odd-odd and odd-even isotopes, respectively. States with natural parity within an fp model space are compared to the predictions of large-scale shell-model calculations using the recently developed GXPF1A effective interaction. Quasirotational structures are evident in all of the isotopes and are discussed in terms of the deformation-driving potential of the ν1g9/2 intruder orbital. It is apparent that an enlarged model space, incorporating at least the 1g9/2 intruder state, is necessary to reproduce the observed experimental systematics in a more satisfactory manner.

Yrast states up to (6+) in the neutron-rich 40S nucleus have been studied using binary grazing reactions produced by the interaction of a 215 MeV beam of 36S ions with a thin 208Pb target. The novel experimental setup that combines the large acceptance magnetic spectrometer, PRISMA, and the high-efficiency γ -ray detection array, CLARA, was used. A new γ -ray transition at an energy of 1572 keV was observed and tentatively assigned to the (6+) → (4+) transition. A comparison of experimental observations and the results of large-scale 0hω sd-pf shell-model calculations indicates that one- and two-proton excitations from the 2s1/2 to the 1d3/2 orbitals play an important role in reproducing the 40S yrast level structure and the published B(E2 ; 0+g.s.→ 2+) value. The structure of the yrast states of the even-A isotopes of sulfur is interpreted in terms of the configurations of valence protons and neutrons within the context of large-scale 0hω sd-pf shell-model calculations.

Yrast states in the neutron-rich 33Si nucleus have been studied using binary grazing reactions produced by the interaction of a 215 MeV beam of 36S ions with a thin 208Pb target. An experimental setup which combines the large acceptance magnetic spectrometer, PRISMA, and the high efficiency gamma-ray detection array, CLARA, was used in the experiment. Four new gamma-ray photopeaks at energies of 971, 1724, 1772, and 2655 keV were observed and assigned to the 33Si level scheme. The experimental level scheme is compared with the results of p-sd-pf large-scale shell-model calculations using the recently developed PSDPFB effective interaction ; good agreement is obtained. The structure of the populated states of 33Si is discussed within the context of an odd-neutron coupled to states of the 32Si core.

The one-proton knockout reaction Be9(Ti54,Sc53+γ)X at 72 MeV/nucleon has been measured. The location of the first 3/2- state at 2110(3) keV was confirmed, and new γ-ray transitions were observed at 1111(2), 1273(2), 1539(4), and 2495(5) keV. Large spectroscopic strength to excited states in Sc53 was found and attributed to the knockout of sd-shell protons.

Be9-induced inelastic scattering of Fe62,64,66 and Cr60,62,64 was performed at intermediate beam energies. Excited states in Cr64 were measured for the first time. Energies and population patterns of excited states in these neutron-rich Fe and Cr nuclei are compared and interpreted in the framework of large-scale shell-model calculations in different model spaces. Evidence for increased collectivity and for distinct structural changes between the neighboring Fe and Cr isotopic chains near N=40 is presented.

Excited states of 38Cl were populated in binary grazing reactions during the interaction of a beam of 36S ions of energy 215 MeV with a 208Pb target. The combination of the PRISMA magnetic spectrometer and the CLARA γ-ray detector array was used to identify the reaction fragments and to detect their decay via γ-ray emission. A level scheme for 38Cl is presented and discussed within the context of the systematics of neighboring nuclei and is compared with the results of state-of-the-art shell-model calculations.

Gamma rays from Ti52 have been studied with Gammasphere and the Fragment Mass Analyzer using reactions induced by a Ca48 beam on a Be9 target. The data have been used in combination with information from deep-inelastic reactions of Ca48 beams on a thick U238 target at an energy about 25% above the Coulomb barrier. The Ti52 level scheme was expanded considerably, and the lifetimes of some of the identified states were determined for the first time. The excitation of two protons and two neutrons outside the Ca48 core provide new tests of effective interactions in the full pf-shell model space. The positive-parity states in Ti52 were compared to theoretical predictions obtained with the GXPF1A, FPD6, and KB3G effective interactions. The comparisons favor, to a degree, the results computed with the GXPF1A interaction.

The excited states of the neutron-rich nucleus $_{15}^{37}P_{22}$ have been populated in grazing reactions, using a beam of $^{36}$S ions (at 215 MeV) delivered onto a thin $^{208}$Pb target. Emitted $\gamma$ rays from excited projectile-like nuclei were detected using the CLARA array of 25 escape-suppressed Ge clover detectors in coincidence with reaction products detected and identified with the PRISMA magnetic spectrometer. A level scheme is presented for $^{37}$P together with proposed spin assignments. The level structure of $^{37}$P is discussed within the context of shell-model calculations by using an improved sdpf effective interaction.

The neutron-rich Fe isotopes from A=61 to 66 were studied through multinucleon transfer reactions by bombarding a $^{238}$U target with a 400 MeV $^{64}$Ni beam. Unambiguous identification of prompt $\gamma$ rays belonging to each nucleus was achieved using coincidence relationships with the ions detected in a high-acceptance magnetic spectrometer. The new data extend our knowledge of the level structure of Fe isotopes, which is discussed in terms of the systematics of the region and compared with large-scale shell-model calculations.

The level structures of 55V and 55Ti, the two nuclei with a single nucleon outside the semi-magic 5422Ti32 core, have been investigated in order to provide new tests of full pf-shell calculations with the GXPF1A interaction. The addition of a proton does not appear to affect the N = 32 shell gap significantly, although comparisons between calculations and experiment at high spins (Iπ ≥ 21 / 2-) indicate the need for a larger model space for an accurate description of the data in this regime. The energy separation between the ν p1 / 2 and ν f5 / 2 orbitals in neutron-rich Ti isotopes is not large enough to result in an N = 34 shell gap. However, comparisons between the 55Ti data and the calculations argue for the presence of a sizable N = 34 gap in 54Ca.

The structure of the neutron-rich isotopes Fe59,60 has been studied with the Gammasphere detector array using fusion-evaporation reactions. Level schemes for these nuclei are presented which have been extended to spins of ∼20. Both isotopes exhibit regular, near-yrast γ-decay sequences which are generated by the intrusion of the g9/2 orbital into the fp shell-model space. Lower-spin, natural-parity levels are discussed within the context of shell-model calculations using the GXPF1A interaction in the full fp model space. Experimental features of the high-spin bands are compared with total Routhian surface calculations.

The odd-odd fp-shell nucleus Sc52 was investigated using in-beam γ-ray spectroscopy following secondary fragmentation of a V55 and Cr57 cocktail beam. Aside from the known γ-ray transition at 674(5) keV, a new decay at Eγ=212(3) keV was observed. It is attributed to the depopulation of a low-lying excited level. This new state is discussed in the framework of shell-model calculations with the GXPF1, GXPF1A, and KB3G effective interactions. These calculations are found to be fairly robust for the low-lying level scheme of Sc52 irrespective of the choice of the effective interaction. In addition, the frequency of spin values predicted by the shell model is successfully modeled by a spin distribution formulated in a statistical approach with an empirical, energy-independent spin-cutoff parameter. © 2006 The American Physical Society.

Excited states up to Iπ =8+ in the neutron-rich nucleus 58Cr have been identified by using a new experimental setup composed of the large acceptance magnetic spectrometer PRISMA and the highly efficient γ -detector array CLARA. Interestingly, the excitation energy sequence of the ground-state band follows the one expected by the E(5) dynamical symmetry for a nucleus at the critical point of the shape phase transition from a spherical vibrator (U(5)) to a γ -soft rotor (O(6)). For the first time, in the same physical system, large scale shell-model calculations in the full fp shell are compared to the E(5) analytical model results and to the Interacting Boson Model. The theoretical results are in excellent agreement with the present data.

The structure of the Cr56,58,60 nuclei was investigated at Gammasphere using both deep inelastic and fusion-evaporation reactions. As a result of this work, expanded level schemes are now available for these three neutron-rich, even-even isotopes. Experimental signatures were found for both single-particle and collective excitations whose relative importance varies with neutron number. Together with recent structural information on the odd-A Cr55,57,59 neighbors, the present data highlight the changing role of the g9/2 orbital with neutron number. Comparisons with shell-model calculations using the recently developed GXPF1A interaction and with total Routhian surface calculations are presented.

The one-neutron knockout reaction Be9(Cr57,Cr56+γ)X has been measured in inverse kinematics with an intermediate-energy beam. Cross sections to individual states in Cr56 were partially untangled through the detection of the characteristic γ-ray transitions in coincidence with the reaction residues. The experimental inclusive longitudinal momentum distribution and the yields to individual states are compared to calculations that combine spectroscopic factors from the full fp shell model and nucleon-removal cross sections computed in a few-body eikonal approach.

A study of high spin states in the odd-neutron isotones 239Pu and 237U is reported. Striking differences were found in the high-spin properties of rotational bands built on the 1/2+[631] ground states in these two nuclei. These differences mirror those observed in the even-even Pu and U immediate neighbors and appear to be related to the strength of octupole correlations.

The even 52-56Ti isotopes have been studied with intermediate-energy Coulomb excitation and absolute B(E2;0+ → 21+) transition rates have been obtained. These data confirm the presence of a subshell closure at neutron number N = 32 in neutron-rich nuclei above the doubly magic nucleus 48Ca and provide no direct evidence for the predicted N = 34 closure. Large-scale shell model calculations with the most recent effective interactions are unable to reproduce the magnitude of the measured strengths in the semimagic Ti nuclei and their strong variation with neutron number.

Excited states in 57Cr have been populated to high spin in the 14C(48Ca,αn) reaction at a beam energy of 130 MeV. A regular sequence of stretched quadrupole transitions has been established above the yrast 9/2+ level. This sequence is interpreted as a rotational band associated with prolate deformation induced by the excitation of the odd neutron into the 1/2+[440] orbital. Total Routhian surface calculations, which follow this configuration to high spin, reproduce the observed features of the band. They are also able to account for a similar, but less well developed structure in 55Cr. In contrast, the isomeric yrast 9/2+ state in 59Cr appears to be a band-head state dominated by the 9/2+[404] configuration, which is favoured at oblate deformations. Such a marked difference presents a significant challenge for theoretical descriptions, but is consistent with features exhibited by the low-lying negative-parity states in 59Cr.