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Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73) μs have been discovered in the heavy 254Rf nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the Kπ = 8-, ν2(7/2+ [624], 9/2- [734]) two-quasineutron and the Kπ = 16+, 8-ν2(7/22 (7/2- [514], 9 /2+ [624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N= 150 isotones. The four-quasiparticle isomer is longer lived than the 254Rf ground state that decays exclusively∣ by spontaneous fission with a half-life of 23.2(1.1) μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state. [ABSTRACT FROM AUTHOR]

Background: Triplet energy differences (TED) can be studied to yield information on isospin-non-conserving interactions in nuclei. Purpose: The systematic behavior of triplet energy differences (TED) of T=1, Jπ=2+ states is examined. The A=62 isobar is identified as having a TED value that deviates significantly from an otherwise very consistent trend. This deviation can be attributed to the tentative assignments of the pertinent states in 62Ga and 62Ge. Methods: An in-beam γ-ray spectroscopy experiment was performed to identify excited states in 62Ga using Gamma-Ray Energy Tracking In-Beam Nuclear Array with the S800 spectrometer at NSCL using a two-nucleon knockout approach. Cross-section calculations for the knockout process and shell-model calculations have been performed to interpret the population and decay properties observed. Results: Using the systematics as a guide, a candidate for the transition from the T=1, 2+ state is identified. However, previous work has identified similar states with different Jπ assignments. Cross-section calculations indicate that the relevant T=1, 2+ state should be one of the states directly populated in this reaction. Conclusions: As spins and parities were not measurable, it is concluded that an unambiguous identification of the first T=1, 2+ state is required to reconcile our understanding of TED systematics [ABSTRACT FROM AUTHOR]

Forty-six decay chains, assigned to the decay of 288115, were produced using the 243Am)(48,3n)288115 reaction at the Lawrence Berkeley National Laboratory 88-in. cyclotron. The resulting series of α decays were studied using α-photon and α-x-ray spectroscopies. Multiple α-photon coincidences were observed in the element 115 decay chain members, particularly in the third- and fourth-generation decays (presumed to be 280Rg and 276Mt, respectively). Upon combining these data with those from 22 288115 decay chains observed in a similar experiment, updated level schemes in 276Mt and 272Bh (populated by the α decay of 280Rg and 276Mt, respectively) are proposed. Photons were observed in the energy range expected for K x rays coincident with the α decay of both 280Rg and 276Mt. However, Compton scattering of higher-energy γ rays and discrete transitions are present in the K x-ray region preventing a definitive Z identification to be made based on observation of characteristic K x-ray energies. [ABSTRACT FROM AUTHOR]

Recent experimental results related to the quadrupole collectivity in neutron-rich carbon isotopes are analyzed in a phenomenological approach. fi(E2;21+ → 01+) transitions rates derived from lifetime measurements are interpreted in terms of the mixing of basic neutron and proton 2+ excitations. A seniority inspired scheme is used to describe the neutron component. The observed increase in collectivity can be explained with a corresponding increased role of proton excitations. This is likely due to the reduction of the proton p3/2-p1/2 spin-orbit splitting caused by the tensor and two-body spin-orbit components of the force between the protons and the added neutrons in the (d5/2 + S1/2) shells. [ABSTRACT FROM AUTHOR]

A measurement of the direct two-proton removal from 42Si has provided the first structural information on the N = 28 isotone 40Mg. The value for the inclusive cross section for two-proton removal from 42Si of 40+27-17 μb is significantly lower than that predicted by structure calculations using the recent SDPF-MU shell-model effective interaction combined with eikonal reaction theory. This observed discrepancy is consistent with the interpretation that only one of the predicted low-lying 0+ states in 40Mg is bound. A two-state mixing analysis describing two-proton knockout cross sections along N = 28 provides support for the interpretation of a prolate-deformed 40Mg ground state. [ABSTRACT FROM AUTHOR]

Excited states in the neutron-rich N = 38, 36 nuclei 60Ti and 58Ti were populated in nucleon-removal reactions from 61V projectiles at 90 MeV/nucleon. The γ-ray transitions from such states in these Ti isotopes were detected with the advanced γ-ray tracking array GRETINA and were corrected event by event for large Doppler shifts (v/c∼0.4) using the γ-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N = 36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, 58,60Ti provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly magic nucleus 60Ca. [ABSTRACT FROM AUTHOR]

The 257Rf isotope has beéri populated via the 208Pb(50)Ti, n) fusion-evaporation reaction and delayed y-ray and electron decay spectroscopy has been performed. The existence of a high- K isomeric state in 257Rf has been confirmed. The isomeric state decays into a rotational band based on the 11/2-[725] excitation, which was observed up to spin of (23/2-). Three multipolarity-El y transitions depopulating the isomeric state have been observed, which fixes the spin for that state to (21/2+). This assignment agrees with theoretical predictions calculated with the microscopic-macroscopic approach, which suggest the isomeric state to be formed by coupling an unpaired 11/2- [725] quasineuiron to the (1/2-[521] ⊗ 9/2+[624])5- two-quasiproton state. The same two-quasiproton excitation is possible for the lowest isomer in 256Rf. [ABSTRACT FROM AUTHOR]

Intermediate-energy Coulomb excitation measurements are performed on the N ≥ 40 neutron-rich nuclei 66,68Fe and 64Cr. The reduced transition matrix elements providing a direct measure of the quadrupole collectivity B(E2;21+ → 01+) are determined for the first time in 68Fe42 and 64Cr40 and confirm a previous recoil distance method lifetime measurement in 66Fe40. The results are compared to state-of-the-art large-scale shell-model calculations within the full fpgd neutron orbital model space using the Lenzi-Nowacki-Poves-Sieja effective interaction and confirm the results of the calculations that show these nuclei are well deformed. [ABSTRACT FROM AUTHOR]

Medium- and high-spin states of 132La have been studied based on the data obtained from 100Mo(36S, p3n) and 116Cd(23Na, α3n) fusion-evaporation reactions using the EUROBALL and Gammasphere detector arrays, respectively. Triple-γ coincidence relations, angular correlations, and linear polarizations of the observed γ transitions have been deduced. The level scheme of 132La has been considerably extended, and unambiguous spin and parity values have been assigned to most of the excited states. The configuration of one of the bands is πg7/2(h11/2)²vh11/2 instead of the previously thought πh11/2vh11/2; thus its previously suggested magnetic rotational character cannot be upheld. The observed similarities between the level structures of 132La and 134Pr suggest the possible existence of a third πh11/2vh11/2 band in 132La that may show chiral features. [ABSTRACT FROM AUTHOR]

Over a period of several years we have performed three separate experiments at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron in which 235U (thick target) was Coulomb-excited. The program involved standalone experiments with Gammmasphere and with the 8pi Spectrometer using 136Xe beams at 720 MeV, and a CHICO-Gammasphere experiment with a 40Ca beam at 184 MeV. In addition to extending the known negative-parity bands to high spin, we have assigned levels in some seven positive-parity bands which are in some cases (e.g., [631]l/2, [624]7/2, and [622]5/2) strongly populated by E3 excitation. The CHICO data have been analyzed to extract E2 and E3 matrix elements from the observed yields. Additionally, many M1 matrix elements could be extracted from the γ-ray branching ratios. A number of new features have emerged, including the unexpected attenuation of magnetic transitions between states of the same Nilsson multiplet, the breakdown of Coriolis staggering at high spin, and the effect of E3 collectivity on Coriolis interactions. [ABSTRACT FROM AUTHOR]

Excited states in 16C were populated via the 9Be(17N,16C + γ)X one-proton knockout reaction. The lifetime of the 21+ state in 16C was measured using the recoil distance method. The extracted lifetime of τ21+ = 11,4-0.9+0.8(stat) ± 0.7(systBp)-1.5+0.0(systfeeding) ps yields a deduced B(E2;21+ → O1+ ) = 4.21-0.26+0.34(stat)-0.24+0.28(systsBp)-0.00+0.64(systfeeding) e²fm4 value in good agreement with a previous measurement. The one-proton knockout cross section is used to extract the proton amplitude of the 16C 21+ state, which confirms the neutron dominant character of this state. Gamma-ray branching ratios between the 22+ state and the 21+ and ground states were also determined. The results are compared with p-sd shell model and no-core shell model (with NN and NN + NNN) calculations. The inclusion of three-body forces are essential in order for the no-core shell model calculations to reproduce the experimental findings on the γ-ray branching ratios. [ABSTRACT FROM AUTHOR]

Medium- and high-spin states of 134Pr were populated using the 116Cd(23Na, 5n) reaction and studied with the GAMMASPHERE spectrometer. Several new bands have been found in this nucleus, one of them being linked to the previously observed chiral-candidate twin-band structure. The ground state of 134Pr could be determined through establishing a level structure that connects the two previously known long-lived isomeric states. Unambiguous spin-parity assignments for the excited states could be performed based on the known 2- spin-parity of the ground state combined with the present experimental data. Intrinsic single-particle configurations have been assigned to the newly observed bands on the basis of the measured B(M1)/B(E2) ratios, alignments, band-crossing frequencies, bandhead spins, the observed single-particle configurations in the neighboring nuclei, and taking into account the predictions of total Routhian surface and tilted-axis cranking calculations. [ABSTRACT FROM AUTHOR]

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2+I state in the near-dripline nucleus 20C. The deduced value of r2. = 9.8 ± 2.8(stat)+0.5-1.1syst) ps gives a reduced transition probability of B(E2; 2+I →. O+g) = 7.5 +3.0-1.7(stat)+1.0-0.4(stat) e2 fm4 in good agreement with a shell model calculation using isospin-dependent effective charges. [ABSTRACT FROM AUTHOR]

The superfluid tunneling model is applied to the calculation of ground-state-to-ground-state α decay in the even-even neutron-deficient Te-Ba nuclei. We show that there is a larger α-particle formation probability in nuclei of this region above 100Sn when compared to analogous nuclei above 208Pb. This is consistent with the expected systematic variation of the pair gap Δ as a function of mass number. The recent experimental data on the α decay of the N=Z nuclei 104Te and 108Xe are shown to leave open the possibility of enhanced α-particle formation involving nucleon correlations beyond the standard treatment of like-nucleon pairing, which is the mechanism suggested as underlying "superallowed" α decay. [ABSTRACT FROM AUTHOR]

The superfluid tunneling model is applied to the calculation of half-lives of the observed a decays in N=84 isotones. Results of our calculations are compared to experimental data on the ground-state a decays along the isotonic chain from 144Nd to 159Re. Good agreement is found. The a decays of the known high-spin isomers in 155Lu, 156Hf, 157Ta, and 158W are also well reproduced, once a reduction in the pairing strength is taken into account. This includes reproduction of the main features of the recently observed fine structure from 155Lu(25/2-) and 156Hf(8+). Predictions for the α-decay fine structure of 157Ta(25/2-) and 158W(8+) high-spin isomers are presented. [ABSTRACT FROM AUTHOR]

High-spin states in 124Cs were populated in the 64Ni(64Ni,p3n) reaction and the Gammasphere detector array was used to measure γ-ray coincidences. Both positive- and negative-parity bands, including bands with chiral configurations, have been extended to higher spin, where a shape change has been observed. The configurations of the bands before and after the alignment are discussed within the framework of the cranked Nilsson-Strutinsky model. The calculations suggest that the nucleus undergoes a shape transition from triaxial to prolate around spin I≃22 of the positive-parity states. The alignment gain of 8ℏ, observed in the positive-parity bands, is due to partial alignment of several valence nucleons. This indicates the absence of band crossing due to paired nucleons in the bands. [ABSTRACT FROM AUTHOR]

Spectroscopic factors in 10Be, 11Be, and 12Be, extracted from (d,p), one-neutron knockout, and (p,d) reactions, are interpreted within the rotational model. Assuming that the ground state and first excited state of 11Be can be associated with the ½[220] and ½[101] Nilsson levels, the strong-coupling limit gives simple expressions that relate the amplitudes of these wave functions (in the spherical basis) with the measured cross sections and derived spectroscopic factors. We obtain good agreement with both the measured magnetic moment of the ground state in 11Be and the reaction data. [ABSTRACT FROM AUTHOR]

Spectroscopic factors, extracted from one-neutron knockout and Coulomb dissociation reactions, for transitions from the ground state of 33Mg to the ground-state rotational band in 32Mg, and from 32Mg to low-lying negative-parity states in 31Mg, are interpreted within the rotational model. Associating the ground state of 33Mg and the negative-parity states in 31Mg with the 3/2[321] Nilsson level, the strong coupling limit gives simple expressions that relate the amplitudes (Cjℓ) of this wave function with the measured cross sections and derived spectroscopic factors (Sjℓ). To obtain a consistent agreement with the data within this framework, we find that one requires a modified 3/2[321] wave function with an increased contribution from the spherical 2p3/2 orbit as compared to a standard Nilsson calculation. This is consistent with the findings of large-scale shell model calculations and can be traced to weak binding effects that lower the energy of low-ℓ orbitals. [ABSTRACT FROM AUTHOR]

Background: Competing interpretations of the results of a 30Mg (t,p)32Mg measurement populating the ground state and 0+2 state in 32Mg, both limited to a two-state mixing description, have left an open question regarding the nature of the 32Mg ground state. Purpose: Inspired by recent shell-model calculations, we explore the possibility of a consistent interpretation of the available data for the low-lying 0+ states in 32Mg by expanding the description from two-level to three-level mixing. Methods: A phenomenological three-level mixing model of unperturbed 0p0h, 2p2h, and 4p4h states is applied to describe both the excitation energies in 32Mg and the transfer reaction cross sections. Results: Within this approach, self-consistent solutions exist that provide good agreement with the available experimental information obtained from the 30Mg(t,p)32Mg reaction. Conclusion: The inclusion of the third state, namely the 4p4h configuration, resolves the "puzzle" that results from a two-levelmodel interpretation of the same data. In our analysis, the 32Mg ground state emerges naturally as dominated by intruder (2p2h and 4p4h) configurations, at the 95% level. [ABSTRACT FROM AUTHOR]

There is significant evidence supporting the existence of deformed ground states within the neutron-rich N≈20 neon, sodium, and magnesium isotopes that make up what is commonly called the "island of inversion." However, the rotational band structures, which are a characteristic fingerprint of a rigid nonspherical shape, have yet to be observed. In this work, we report on a measurement and analysis of the yrast (lowest lying) rotational band in 32Mg up to spin I=6+ produced in a two-step projectile fragmentation reaction and observed using the state-of-the-art γ-ray tracking detector array, GRETINA (γ-ray energy tracking in-beam nuclear array). Large-scale shell-model calculations using the SDPF-U-MIX effective interaction show excellent agreement with the new data. Moreover, a theoretical analysis of the spectrum of rotational states as a function of the pairing gap, together with cranked-shell-model calculations, provides intriguing evidence for a reduction in pairing correlations with increased angular momentum, also in line with the shell-model results. [ABSTRACT FROM AUTHOR]

One of the most exotic light neutron-rich nuclei currently accessible for experimental study is ^{40}Mg, which lies at the intersection of the nucleon magic number N=28 and the neutron drip line. Low-lying excited states of ^{40}Mg have been studied for the first time following a one-proton removal reaction from ^{41}Al, performed at the Radioactive Isotope Beam Factory of RIKEN Nishina Center with the DALI2 γ-ray array and the ZeroDegree spectrometer. Two γ-ray transitions were observed, suggesting an excitation spectrum that shows unexpected properties as compared to both the systematics along the Z=12, N≥20  Mg isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.

The neutron-rich nucleus ^{144}Ba (t_{1/2}=11.5  s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV ^{144}Ba beam on a 1.0-mg/cm^{2} ^{208}Pb target. The measured value of the matrix element, ⟨3_{1}^{-}∥M(E3)∥0_{1}^{+}⟩=0.65(+17/-23) eb^{3/2}, corresponds to a reduced B(E3) transition probability of 48(+25/-34)  W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73) μs have been discovered in the heavy ^{254}Rf nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the K^{π}=8^{-}, ν^{2}(7/2^{+}[624],9/2^{-}[734]) two-quasineutron and the K^{π}=16^{+}, 8^{-}ν^{2}(7/2^{+}[624],9/2^{-}[734])⊗8^{-}π^{2}(7/2^{-}[514],9/2^{+}[624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. The four-quasiparticle isomer is longer lived than the ^{254}Rf ground state that decays exclusively by spontaneous fission with a half-life of 23.2(1.1) μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state.

Excited states in the neutron-rich N = 38, 36 nuclei (60)Ti and (58)Ti were populated in nucleon-removal reactions from (61)V projectiles at 90 MeV/nucleon. The γ-ray transitions from such states in these Ti isotopes were detected with the advanced γ-ray tracking array GRETINA and were corrected event by event for large Doppler shifts (v/c ∼ 0.4) using the γ-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N = 36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, (58,60)Ti provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly magic nucleus (60)Ca.

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2(1)+ state in the near-dripline nucleus 20C. The deduced value of τ(2(1)+)=9.8±2.8(stat)(-1.1)(+0.5)(syst)  ps gives a reduced transition probability of B(E2; 2(1)+→0(g.s.)+)=7.5(-1.7)(+3.0)(stat)(-0.4)(+1.0)(syst)  e2  fm4 in good agreement with a shell model calculation using isospin-dependent effective charges.

The lifetime of the 2_+(1) state in 16C has been measured with the recoil distance method using the 9Be(9Be,2p) fusion-evaporation reaction at a beam energy of 40 MeV. The mean lifetime was measured to be 11.7(20) ps corresponding to a B(E2;2_+(1)-->0+) value of 4.15(73)e_2 fm_4 [1.73(30) W.u.], consistent with other even-even closed shell nuclei. Our result does not support an interpretation for "decoupled" valence neutrons.

A new frontier of discrete-line gamma-ray spectroscopy at ultrahigh spin has been opened in the rare-earth nuclei (157,158) Er. Four rotational structures, displaying high moments of inertia, have been identified, which extend up to spin approximately 65 variant Planck's over 2pi and bypass the band-terminating states in these nuclei which occur at approximately 45 variant Planck's over 2pi. Cranked Nilsson-Strutinsky calculations suggest that these structures arise from well-deformed triaxial configurations that lie in a valley of favored shell energy which also includes the triaxial strongly deformed bands in (161-167) Lu.

High-spin states in 58Ni have been investigated by means of the fusion-evaporation reaction 28Si(32S, 2p)58Ni at 130 MeV beam energy. Discrete-energy levels are observed in 58Ni at record-breaking 42 MeV excitation energy and angular momenta in excess of 30h. The states form regular rotational bands with unprecedented high rotational frequencies. A comparison with configuration dependent cranked Nilsson-Strutinsky calculations reveals an exceptional two-band crossing scenario, the interaction strength of which is strongly shape dependent.