High-spin states in Sn-112 were investigated using the Rh-103(C-12, 1p2n) reaction at a beam energy of 62 MeV. Six electric dipole E1 transitions linking positive- and negative-parity intruder bands have been identified in Sn-112(50) through gamma -ray spectroscopy. The experimental reduced E1 transition probabilities B(E1), derived from level lifetimes by means of the Doppler-shift attenuation method, are of the order of 10(-4) W.u., which gives evidence for the octupole correlation in Sn-112. The systematics of the reduced E1 transition probabilities and neutron energy levels of 5/2(+) and 11/2(-) in Xe isotopes suggest an enhanced octupole correlation at neutron number N = 62, which is supported by the relativistic Hartree-Bogoliubov calculations.

High-spin states of 67Ga have been studied via the 58Ni(12C, 3p) 67Ga fusion-evaporation reaction at a beam energy of 50.4 MeV. Three negative-parity bands and three positive-parity bands in 67Ga are established. The observation of one new E3 transition linking the positive-parity pi 1g9/2 band and negative-parity pi 2p3/2 band provides evidence of octupole correlations in 67Ga. The characteristics of octupole correlations in the 67Ga are discussed in terms of the reflection-asymmetric triaxial particle rotor model and microscopic relativistic mean field + Bardeen-Cooper-Schrieffer model.

A new isotope 203Ac was produced in the reaction 40Ca + 169Tm at beam energy of 226 MeV. One correlated α-decay chain was attributed to 203Ac. The α-particle energy and half-life were determined to be 8217(16) keV and 56−26+269 μs, respectively. The reduced α-decay width (δ2) of 203Ac was deduced to be 1096−906+921 keV, which indicates a favored α decay. The systematics of δ2 and single-particle energy levels indicate that the new state probably arises from the intruder s1/2 configuration, which shows an onset of deformation in Ac isotopes. The proton separation energy Sp of 203Ac was extracted as −1214(22) keV and the systematics of Sp for odd-A isotopes beyond proton drip line were discussed.

The isotopes 261,262Bh were reinvestigated by using the cold fusion reaction 54Cr+209Bi. The evaporation residues were separated by the gas -filled recoil separator SHANS2 (Spectrometer for Heavy Atoms and Nuclear Structure -2) and identified by means of the recoil-alpha-correlation method. The alpha-decay properties of 261,262Bh were measured and the results are consistent with the previously reported values. Based on the measured half-lives, the existence of two distinct states of 262Bh was confirmed, whereas 261Bh lacked any isomer with alpha-decay properties. Two spontaneous fission events potentially from 262Bh and/or 261Bh were detected and the production cross sections at beam energies of 264.1 and 270.4 MeV were obtained.

Received 7 March 2024DOI:https://doi.org/10.1103/PhysRevC.109.049902©2024 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsElectromagnetic transitionsEnergy levelsFissionNuclear structure & decaysProperties90 ≤ A ≤ 149Nuclear Physics

The fusion-evaporation reaction 40Ca + 175Lu aimed at synthesizing new neutron-deficient protactinium isotopes has been studied at the gas-filled recoil separators SHANS and SHANS2. The cross sections for the xn and pxn evaporation channels were measured. The calculation by the statistical model HIVAP was performed to reproduce the experimental data. The quasifission process in the 40Ca + 175Lu reaction was also studied with time-dependent Hartree-Fock theory and the improved quantum molecular dynamics model. It is shown that the influence of quasifission in the analysis of evaporation residue cross sections of this reaction can be neglected.

The new isotope Pu-227 has been produced in the fusion-evaporation reaction Os-192(Ar-40, (5)n) Pu-227 at the gas-filled recoil separator SHANS, and identified using a well-established implantation-decay correlation technique. From the nine observed decay chains, the alpha-particle energy and half-life of Pu-227 were measured to be 8191(28) keV and 0.78(-0.19)(+0.39) s, respectively. The alpha decay, when analyzed using the reduced decay width, was tentatively assigned to be a hindered alpha transition with angular-momentum change of Delta L = 2.

Using the fusion-evaporation reaction ^{106}Cd(^{58}Ni,4n)^{160}Os and the gas-filled recoil separator SHANS, two new isotopes _{76}^{160}Os and _{74}^{156}W have been identified. The α decay of ^{160}Os, measured with an α-particle energy of 7080(26) keV and a half-life of 201_{-37}^{+58}  μs, is assigned to originate from the ground state. The daughter nucleus ^{156}W is a β^{+} emitter with a half-life of 291_{-61}^{+86}  ms. The newly measured α-decay data allow us to derive α-decay reduced widths (δ^{2}) for the N=84 isotones up to osmium (Z=76), which are found to decrease with increasing atomic number above Z=68. The reduction of δ^{2} is interpreted as evidence for the strengthening of the N=82 shell closure toward the proton drip line, supported by the increase of the neutron-shell gaps predicted in theoretical models.

A gas-filled recoil separator, SHANS2 (Spectrometer for Heavy Atoms and Nuclear Structure-2), with five magnets arranged in a Qv-D-Qh-Qv-D configuration is constructed at the China Accelerator Facility for Superheavy Elements (CAFE2) of the Institute of Modern Physics (IMP) in Lanzhou. It is dedicated to the study of the heavy nuclei produced in heavy-ion-induced fusion evaporation reactions, especially to the experiments on the superheavy elements with cross-sections lower than 1 pb. The maximum magnetic rigidity of the main dipole magnet is designed to be 2.5 T m. The detailed design of the SHANS2 and the results of several test experiments are presented.

The & alpha; decay of 222U was reinvestigated using a general least-square superpulse fitting algorithm dedicated to resolving pileup signals in the decay of very short-lived nuclei. The & alpha;-particle energy of 222U was revised to be 9246(8) keV, and the precision was improved significantly compared with previous result. Using the present & alpha; energy, the anomaly in the systematics of the & alpha;-decay reduced width 82 observed at 222U in the NpNn scheme is solved, all the 82 values converge into a smooth and narrow band with NpNn up to & AP;90 in the northeast of 208Pb.

The electromagnetic character of the ΔI=1 transitions connecting the 1- to 0-phonon and the 2- to 1-phonon wobbling bands should be dominated by an E2 component, due to the collective motion of the entire nuclear charge. In the present work it is shown, based on combined angular correlation and linear polarization measurements, that the mixing ratios of all analyzed connecting transitions between low-lying bands in 135Pr interpreted as 0-, 1-, and 2-phonon wobbling bands, have absolute values smaller than one. This indicates predominant M1 magnetic character, which is incompatible with the proposed wobbling nature. All experimental observables are instead in good agreement with quasiparticle-plus-triaxial-rotor model calculations, which describe the bands as resulting from a rapid re-alignment of the total angular momentum from the short to the intermediate nuclear axis.

β decay of proton-rich nuclei plays an important role in exploring isospin mixing. The β decay of ^{26}P at the proton drip line is studied using double-sided silicon strip detectors operating in conjunction with high-purity germanium detectors. The T=2 isobaric analog state (IAS) at 13 055 keV and two new high-lying states at 13 380 and 11 912 keV in ^{26}Si are unambiguously identified through β-delayed two-proton emission (β2p). Angular correlations of two protons emitted from ^{26}Si excited states populated by ^{26}P β decay are measured, which suggests that the two protons are emitted mainly sequentially. We report the first observation of a strongly isospin-mixed doublet that deexcites mainly via two-proton decay. The isospin mixing matrix element between the ^{26}Si IAS and the nearby 13 380-keV state is determined to be 130(21) keV, and this result represents the strongest mixing, highest excitation energy, and largest level spacing of a doublet ever observed in β-decay experiments.

The isomer depletion of ^{93m}Mo was recently reported [Chiara et al., Nature (London) 554, 216 (2018)NATUAS0028-083610.1038/nature25483] as the first direct observation of nuclear excitation by electron capture (NEEC). However, the measured excitation probability of 1.0(3)% is far beyond the theoretical expectation. In order to understand the inconsistency between theory and experiment, we produce the ^{93m}Mo nuclei using the ^{12}C(^{86}Kr,5n) reaction at a beam energy of 559 MeV and transport the reaction residues to a detection station far away from the target area employing a secondary beam line. The isomer depletion is expected to occur during the slowdown process of the ions in the stopping material. In such a low γ-ray background environment, the signature of isomer depletion is not observed, and an upper limit of 2×10^{-5} is estimated for the excitation probability. This is consistent with the theoretical expectation. Our findings shed doubt on the previously reported NEEC phenomenon and highlight the necessity and feasibility of further experimental investigations for reexamining the isomer depletion under low γ-ray background.

The alpha decays of 207-209Ac, located near the proton drip-line, were reinvestigated by means of alpha-decay spectroscopy. They were produced in the fusion reaction 36Ar +176Hf and separated in flight using the gas-filled recoil separator SHANS. A new alpha-decay line at 7483(15) keV was observed and assigned as the decay from the ground state of 208Ac to the excited (2+) or (4+) state in 204Fr. In addition, the previously known alpha-decay properties of 207-209Ac were measured with improved precision. The half-lives obtained, together with the existing decay data of other N 126 actinium isotopes, were compared with the values calculated with the Wentzel-Kramers-Brillouin approximation and without considering the alpha-preformation factors. It is shown that the calculated half-lives exhibit a distinct odd-even staggering (OES) following the same trend as the experimental data, whereas the amplitudes of the exhibited OES are 30-60 % of the real ones. This implies that apart from the alpha-preformation factors, the alpha-decay energies also contribute significantly to the OES of alpha-decay half-lives.

The low-lying states of Eu-146 populated in the decay of the 9(+) isomer were studied. The gamma-ray intensities were reanalyzed employing germanium detectors, and the lifetimes of the 6(1)(-) and 6(2)(-) states were measured using the mirror symmetric centroid difference (MSCD) method with fast-timing LaBr3(Ce) scintillator detectors. The B(M1) values of the 6(1)(-) -> 5(1)(-) and 6(2)(-) -> 5(1)(-) transitions were deduced, and all observed states were interpreted as members of the pi d(5/2)(-1)nu f(7/2) and pi g(7/2)(-1)nu f(7/2) multiplets. In particular, the 5(1)(-) level is shown to be dominated by the pi d(5/2)(-1)nu f(7/2) configuration, solving the discrepancy in its configuration assignment proposed in previous works. These experimental results were compared with the shell model calculations using several different effective interactions. The systematics of low-lying structure in the N = 83 isotones Pr-142, Pm-144, and Eu-146 was established.

A new proton-unbound isotope 204Ac was produced in the fusion-evaporation reaction Tm(40169Ca,5n)204Ac and identified via the recoil-α correlation at the gas-filled recoiled separators SHANS2 and SHANS. The excitation function for producing 204Ac was measured and the optimal beam energy was evaluated to be 212 MeV. Nineteen ER - α1 - α2 - α3 decay chains were observed for this isotope. Based on these chains, the α-particle energy and half-life of 204Ac were determined to be 7948(15) keV and 7.4−1.4+2.2 ms, respectively. Both are consistent with the theoretical prediction. The reduced α-decay width was deduced to be 49−15+10 keV, which indicates a favored α decay for 204Ac. In addition, more precise decay properties of Eα = 7883(15) keV and T1/2 = 7.7−1.6+2.7 ms were obtained for 205Ac.

The new thorium isotope Th-207 has been produced in the 5n evaporation channel of the fusion reaction Ar-36 + Hf-176. It was separated in flight by the gas-filled recoil separator SHANS and identified on the basis of a correlated alpha-decay chain. The alpha decay of Th-207, measured with an alpha-particle energy of 8167(21) keV and a half-life of 9.7(-4.4)(+46.6) ms, is assigned to originate from ground state. By combining with existing data, we find that the a-decay energies of nuclei with Z > 82 and N < 126 show a regular and distinct odd-even staggering (OES) rather than the commonly supposed smooth pattern. A theoretical analysis has been performed within relativistic Hartree-Fock-Bogoliubov and large-scale shell-model approaches. It is found that the OES originates from both pairing correlations and blocking of particular orbitals by unpaired nucleons. Of particular importance is that pairing correlations result in the OES not only through the contribution of pairing energy to binding energy, but also by configuration mixing induced by scattering nucleons to orbitals away from Fermi levels.

A new α-emitting isotope ^{214}U, produced by the fusion-evaporation reaction ^{182}W(^{36}Ar,4n)^{214}U, was identified by employing the gas-filled recoil separator SHANS and the recoil-α correlation technique. More precise α-decay properties of even-even nuclei ^{216,218}U were also measured in the reactions of ^{40}Ar, ^{40}Ca beams with ^{180,182,184}W targets. By combining the experimental data, improved α-decay reduced widths δ^{2} for the even-even Po-Pu nuclei in the vicinity of the magic neutron number N=126 are deduced. Their systematic trends are discussed in terms of the N_{p}N_{n} scheme in order to study the influence of proton-neutron interaction on α decay in this region of nuclei. It is strikingly found that the reduced widths of ^{214,216}U are significantly enhanced by a factor of two as compared with the N_{p}N_{n} systematics for the 84≤Z≤90 and N<126 even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the π1f_{7/2} and ν1f_{5/2} spin-orbit partner orbits, which is supported by the large-scale shell model calculation.

Two new excited bands built on the π h 11 / 2 configuration have been identified in 135 Nd in addition to the known π h 11 / 2 band. The energy spectra of the excited bands and the available electromagnetic transition probabilities are in good agreement with theoretical results obtained using quasiparticle-plus-triaxial-rotor model calculations. The properties of the bands identify them as tilted precession bands instead of wobbling bands. Our results give a new insight into the interpretation of the low-lying bands in odd-A mass nuclei, and can stimulate future studies to address the nuclear triaxiality.