Negative-parity Sequence Research Articles

High spin states in 71Ga * *Supported by the National Natural Science Foundation of China (U1932209, 11975315, U1867210, 11905134), the Leading Innovation Project under Grant (LC192209000701, LC202309000201), the Continuous Basic Scientific Research Project (BJ20002501, WDJC-2019-13), China National Nuclear Corporation (FA18000201)

Excited states in the odd-A nucleus 71Ga have been studied via the 70Zn(7Li, )71Ga fusion-evaporation reaction with incident beam energies of 30 and 35 MeV. The level scheme is established up to spin = (29/2+) and an excitation energy 6.6 MeV. A previously known sequence built on the 9/2+ state is extended as a novel rotational band originating from the alignment. Furthermore, a negative-parity sequence is also reported. The observed energy levels of 71Ga have been interpreted in the framework of the nuclear shell model (SM).

Decoupled and semi-decoupled bands in Hg197 and Hg199

The structure of decoupled positive-parity and semidecoupled negative-parity bands up to high spin in $^{197,199}\mathrm{Hg}$ has been studied using multinucleon transfer reactions with the Gammasphere array. A positive-parity band in $^{199}\mathrm{Hg}$ with the ${(\ensuremath{\nu}{i}_{13/2})}^{3}$ decoupled configuration that deexcites to levels in the negative-parity sequences is newly established. The semidecoupled sequences in both isotopes are extended to higher spin. Rotation-aligned angular momenta and crossing frequencies determined from experiment are in fair agreement with those from cranking calculations. A transition from oblate collective rotation at low spin to triaxial noncollective shapes at high spin is indicated.

Structure of odd- A Pt isotopes along the line of stability

The structure of the odd-$A$ isotopes $^{193,195,197}\mathrm{Pt}$, which lie along the line of stability, has been studied up to high spin through multinucleon transfer reactions. Positive- and negative-parity sequences in $^{193,195}\mathrm{Pt}$ have been considerably extended and multiple band crossings established. An isomer with ${T}_{1/2}=5.0(5)$ ns and ${I}^{\ensuremath{\pi}}=25/{2}^{\ensuremath{-}}$ is present in $^{195}\mathrm{Pt}$. The isotopes $^{193,195}\mathrm{Pt}$ are characterized by moderate oblate deformation, and angular momentum generation at high spin in the yrast, positive-parity sequences is attributed to the rotation alignment of ${i}_{13/2}$ neutrons and ${h}_{11/2}$ protons. A detailed understanding of the observed features is presented using calculations based on tilted axis cranking covariant density functional theory as well as others with the ultimate cranker code.

Collective states and shape competition in 126Te**Supported by National Natural Science Foundation of China (11575018, 11375023, 11305269, 11375267, 11405274, 11205245, 10927507, 10975191, 11075214, 11175259, 11205068)

High-spin states in 126Te have been investigated by using in-beam γ ray spectroscopy with the 124Sn(7Li, 1p4n)126Te reaction at a beam energy of 48 MeV. The previously known level scheme has been enriched, and a new negative-parity sequence has been established. The yrast positive-parity band shows a shape change between triaxial shape and collective oblate shape as a function of spin. In particular, three competitive minima appear in the potential energy surface for the Iπ = 8+ states, with one aligned state at γ = −120° and two triaxial states at γ ∼ 30° and −45°, respectively. The signature splitting behavior of the negative-parity band is discussed. The shape change with increasing angular momentum and the signature splitting can be interpreted well in terms of the Cranked Nilsson-Strutinsky-Bogoliubov and Cranked Nilsson-Strutinsky model calculations.

Structure ofPu240: Evidence for Octupole Phonon Condensation?

The expanded level structure of 240Pu available from the present study highlights the role of strong octupole correlations in this nucleus. In addition to a delayed alignment in the yrast band, the observations include the presence of both I(+)-->(I-1)(-) and I(-)-->(I-1)(+)E1 transitions linking states of the yrast and negative-parity bands at high spin and the presence of an additional even-spin, positive-parity band deexciting exclusively to the negative-parity sequence. The observations appear to be consistent with expectations based on the recently proposed concept of octupole phonon condensation.

Structure change of 156Yb at high-spin states

High-spin states of 156Yb have been studied via the 144Sm(16O,4n)156Yb fusion-evaporation reaction at beam energy 102 MeV. The positive-parity yrast band and negative-parity cascade have been extended up to higher-spin states, respectively. The characteristics of the negative-parity sequence above the 25− state may related to the excitation from the nucleon in the Z = 64, N = 82 core. The E-GOS curve for the positive-parity yrast sequence in 156Yb indicate that this nucleus may undergo an evolution from quasivibrational to quasirotational structure with increasing angular momentum. The Cranked Woods–Saxon–Strutinsky calculations by means of Total–Routhian–Surface (TRS) methods has been made to understand this structure change.

Structure ofHe7by proton removal fromLi8with the (d,3He) reaction

We report on a study of the structure of the unbound nucleus {sup 7}He utilizing the proton-removal reaction {sup 2}H({sup 8}Li,{sup 3}He){sup 7}He. Combining the present results with those of our prior measurements of the neutron-adding reaction {sup 2}H({sup 6}He,p){sup 7}He, a consistent picture emerges for the low-lying excitations in {sup 7}He. Specifically, the negative-parity sequence of resonances, in order of excitation energies, is consistent with 3/2{sup -},1/2{sup -}, and 5/2{sup -}. The stable-beam reactions {sup 2}H({sup 7}Li,t){sup 6}Li and {sup 2}H({sup 7}Li,{sup 3}He){sup 6}He were also measured. The results are compared with the predictions of nuclear structure models, including those of ab initio quantum Monte Carlo calculations.

High-spin states inYb156and structure evolutions at large angular momenta in even-AYb isotopes

High-spin states of (156)Yb have been studied via the (144)Sm((16)O,4n)(156)Yb fusion-evaporation reaction at beam energy 102 MeV. The positive-parity yrast band and negative-parity cascade have been extended up to higher-spin states, respectively. In the present work, the negative-parity sequence above the 25(-) state was found to be irregular and fragment into many parallel branches. This pattern may related to the excitation from the nucleon in the Z=64,N=82 core. The characteristics of alignment plot and E-GOS curve for the positive-parity yrast sequence in (156)Yb indicate that this nucleus may undergo an evolution from quasivibrational to quasirotational structure with increasing angular momentum. Based on a systematic summary of the available experimental alignments for the even-A (156,158,160,162,164)Yb isotopes, the structural evolutions induced by the increase in angular momentum, as well as by the change in neutron numbers, in these even-A Yb isotopes have been discussed in comparison with the cranked Woods-Saxon-Strutinsky calculations by means of total-Routhian-surface (TRS) methods.

High-spin structure and band termination inCd103

Excited states of the neutron-deficient {sup 103}Cd nucleus have been investigated via the {sup 72}Ge({sup 35}Cl, p3n) reaction at beam energy of 135 MeV by use of in-beam spectroscopic methods. Gamma rays depopulating the excited states were detected using the Gammasphere spectrometer with high-fold {gamma}-ray coincidences. A quadrupole {gamma}-ray coincidence analysis ({gamma}{sup 4}) has been used to extend the known level scheme. The positive-parity levels have been established up to J=35/2({Dirac_h}/2{pi}) and E{sub x}=7.071 MeV. In addition to the observation of a highly fragmented level scheme belonging to the positive-parity sequences at E{sub x}{approx}5 MeV, the termination of a negative-parity sequence connected by E2 transitions has been established at J=47/2({Dirac_h}/2{pi}) and E{sub x}=11.877 MeV. The experimental results corresponding to both the positive- and negative-parity sequences have been theoretically interpreted in the framework of the core particle coupling model. Evidence is presented for a shape change from collective prolate to noncollective oblate above the J{sup {pi}}=39/2{sup -} (8011-keV) level and for a smooth termination of the negative-parity band.

Experimental study of nuclear structure ofMo91at high spin

The high spin structure of $^{91}\mathrm{Mo}$ with neutron and proton number close to magicity has been investigated using the $^{63}\mathrm{Cu}(^{31}\mathrm{P},\mathit{xnyp})$ reaction at an incident beam energy of $125\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The level at ${E}_{x}=3811\phantom{\rule{0.3em}{0ex}}\text{keV}$ has been assigned a negative parity based on our polarization measurements. In view of this change, the observed level structure for $^{91}\mathrm{Mo}$ so far does not have any negative parity sequence. More than 30 new transitions were observed and placed in the decay scheme, which has been extended up to spin of $J\ensuremath{\sim}39∕2\ensuremath{\hbar}$ and an excitation energy of ${E}_{x}\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The energy spectrum bears signature of core breaking viz. presence of high energy $({E}_{\ensuremath{\gamma}}\ensuremath{\sim}2\phantom{\rule{0.3em}{0ex}}\text{MeV})$ $\ensuremath{\gamma}$ rays originating due to excitation of nucleons across the shell gaps. The proposed level scheme is compared with large basis spherical shell model calculations. Excitation of protons across $Z=38$ shell dominates the level structure up to moderate spins i.e., $J\ensuremath{\sim}27∕2\ensuremath{\hbar}$. Configurations arising due to excitation of a single $\ensuremath{\nu}{g}_{9∕2}$ across $N=50$ shell gap dominate the higher angular momentum $J\ensuremath{\geqslant}35∕2\ensuremath{\hbar}$ states.

Structure of positive-parity yrast band in [formula omitted] Br

The states of the positive-parity yrast band in 80 Br have been investigated using the 76 Ge( 7 Li, 3n γ ) reaction at a beam energy of 32 MeV. The band has been extended up to 4450 keV with a spin of (14 + ). Unambiguous spin assignments are made for states up to 2944.0 keV and γ -ray multipole mixing ratios are determined for several ΔJ=1 transitions. Lifetimes studies for the 10 + , 11 + and 12 + states provide an estimate of the quadrupole deformation in this band. Cranking model analysis of the experimental data reveals a signature inversion at a spin of 12 ℏ and a probable neutron alignment at ℏω≈0.7 MeV. The results are discussed within the framework of the systematics of similar bands in the lighter Br isotopes and the cranked-shell model. In addition, a new positive-parity and two negative-parity sequences have been identified.

Spin ∼60 ℏ in 156Dy: competition between collective and terminating structures at very high-spin

The highest-spin discrete states ( I∼60 ℏ and E x ∼30 MeV) in normal deformed nuclei have been observed in the rare-earth isotope 156Dy using the 124Sn( 36S,4n) reaction in conjunction with the GAMMASPHERE spectrometer. The positive parity yrast sequence appears to evolve smoothly from a prolate (collective) towards an oblate (non-collective) shape, in contrast to the sudden shape change observed in the isotone 158Er. Terminating states are identified in the negative parity sequences at I π =52 − and 53 − and specific multi-particle-hole configurations are assigned by comparison with cranked Nilsson-Strutinsky calculations. An order of magnitude increase in the interaction strength between close lying high-spin weakly collective structures is determined compared with that found in the lower-spin strongly collective domain. These results give valuable insight into the interplay between collective and terminating structures, and their competition for yrast status in the 40−60 ℏ spin range.

High-spin spectroscopy near the deformed N ≈ Z ≈ 38 shell gap: The light Rb isotopes

Nuclei near the prolate single-particle energy gap at proton number Z = 38 ( ε 2 ≈ +0.38) and neutron number N ≈ Z offer unique possibilities to study the pn-interaction at large quadrupole deformation. In the yrast band of the Z = N = 37 nucleus 74Rb, we have identified a crossing of the T = 1 pn-paired band (even spins) with the T = 0 pn-unpaired band (odd spins). In 77Rb, the positive-parity yrast and yrare bands have been followed over large spin ranges and assigned different 1qp-proton 2qp-neutron structures. These band crossings do not follow the usual pattern of two identical particles aligning in high-j orbits. In addition, an aligned g 9 2 proton-neutron negative-parity sequence with K= 11 2 has been identified in 77Rb Rotational bands with more conventional band crossings have also been found.

High-K structure in 77As.

High-spin states in {sup 77}As have been studied via the {sup 76}Ge({alpha},{ital p}2{ital n}) reaction. Proton-gated excitation functions extracted from measured {gamma}-ray yields at beam energies of 32, 36, and 40 MeV and an additional {gamma}-{gamma} coincidence measurement at 40 MeV have been used to assign new {gamma}-ray decay sequences to {sup 77}As. In particular, a positive-parity sequence up to spin 25/2 and a high-lying {Delta}{ital I}=1 sequence up to 21/2 and of probably negative parity have been found to decay into the known 9/2{sup +} isomer at 475.5 keV. Nuclear-shape calculations show a triaxial shape for the positive-parity states with moderate quadrupole deformation. The high-lying negative-parity sequence is interpreted as a high-{ital K} three-quasiparticle band similar to bands in adjacent nuclei. {copyright} {ital 1996 The American Physical Society.}

High-spin spectroscopy of 109Te.

Excited states of the neutron deficient nucleus $^{109}\mathrm{Te}$ were identified for the first time in the $^{54}\mathrm{Fe}$${(}^{58}$Ni,2pn) reaction by in-beam \ensuremath{\gamma}-spectroscopic methods. The nordball detector array, equipped with a charged-particle and a neutron detector system for reaction channel identification, was used to detect the evaporated particles and \ensuremath{\gamma} rays. A level scheme was constructed on the basis of \ensuremath{\gamma}\ensuremath{\gamma}-coincidence relations, and the angular momenta of the states were determined from the measured angular correlation intensity ratios. Two favored bands built on the \ensuremath{\nu}${\mathit{g}}_{7/2}$ and \ensuremath{\nu}${\mathit{h}}_{11/2}$ neutron quasiparticle states were identified. The structure of the nucleus is discussed in the framework of the particle-vibration coupling model using the interacting boson-fermion formalism up to spin 27/2. The negative parity sequence is compared to Total Routhian Surface calculations.

High-spin g-factors in 154Dy

g-factors of excited states in 154Dy were measured up to high spins. A method that combines the recoil-distance technique with the use of the large transient magnetic fields was applied. The measurement was sensitive only to states which were populated ≈ 13.5 ps after the nuclear reaction. Hence the influence of unobserved quasi-continuum feeding states on the measured precession angles was minimized. The results are compared with calculated g-factors. They can be explained by the neutron i 13 2 alignment in the positive- and negative-parity sequences at medium spins. They also confirm the predicted alignment of protons in the band termination region of spins I ⩾ 30kh.

Evidence for weak coupling in 145Pm.

States in {sup 145}Pm were studied in the reaction {sup 130}Te({sup 19}F,4{ital n}) at a beam energy of 75 MeV using techniques of in-beam {gamma}-ray and conversion electron spectroscopy. Measurements were made of {gamma}-{gamma} coincidences, {gamma}-ray angular distributions, conversion electrons, and excitation functions between 70 and 85 MeV. The reduced {ital B}({ital E}3) transition strength for the 795 keV (11/2{sup {minus}}{r arrow}5/2{sub g.s.}{sup +}) transition was determined to be 7.8(18) Weisskopf units, supporting the interpretation of the 11/2{sup {minus}} state as a {ital h}{sub 11/2} proton state. A negative-parity sequence built on the 11/2{sup {minus}} state as well as eleven new states above the 1503 keV 15/2{sup +} state were found. A weak coupling model can explain the structure of the deduced decay scheme up to an energy of 2.5 MeV.

Structure of 75Se.

The low-lying low-spin levels of {sup 75}Se have been investigated. These levels were populated via the {sup 75}As({ital p},{ital n}{gamma}){sup 75}Se reaction with proton energies between 3.0 and 4.0 MeV. Angular distribution measurements have been used to assign spin values and to determine multipole mixing ratios via compound statistical theory of nuclear reactions. Lifetimes for 17 states in {sup 75}Se were obtained by the Doppler-shift attenuation method for the first time. For several of the transitions in {sup 75}Se, values or limits for {ital B}({ital M}1) and {ital B}({ital E}2) were obtained. The negative-parity sequence of levels up to about 2.5 MeV could be predicted well on the basis of the collective model with Coriolis perturbation. An effective moment-of-inertia parameter under the effect of Coriolis perturbation is used to predict these states. The results compare quite well with experimental values.

Negative-parity excitations in 144Nd.

High angular momentum states in $^{144}\mathrm{Nd}$ are studied via the \ensuremath{\gamma} rays produced in the $^{130}\mathrm{Te}$${(}^{18}$O, 4n${)}^{144}$Nd reaction. In addition to the previously known sequence of states built on the ${3}_{1}^{\mathrm{\ensuremath{-}}}$ state, we have observed another negative-parity sequence built on the 3395 keV ${9}^{\mathrm{\ensuremath{-}}}$ state, which is apparently of a two quasiparticle nature. These two negative-parity sequences probably interact and mix significantly. The present $^{144}\mathrm{Nd}$ results raise questions regarding the interpretation of medium spin states of $^{146}\mathrm{Nd}$ in terms of strong octupole correlations.

High-spin states in 75Kr

Levels in the 75Kr nucleus were populated in the inverse 24Mg( 54Fe, 2pn) 75Kr reaction at beam energies of 177 and 190 MeV. In the reaction study at 177 MeV, the γ-rays were detected in coincidence with neutrons and mass-75 nuclei and in the reaction at 190 MeV only γγ-coincidence data were collected. Using the γγ-neutron gated γγ- and Recoil-γγ-coincidence techniques, we were able to develop the level scheme up to spins ( 37+ 2 ) and ( 31− 2 ) in the positive- and negative-parity bands, respectively. These spins are high enough to allow us to study alignment effects in this mass region. The signature splitting at low spin can be reproduced in a cranked shell model calculation assuming a quadrupole deformation β 2 = 0.37 and a slightly triaxial ( γ≈ −10°) shape for positive-parity states but an axial shape for the negative-parity sequence. The spin alignment as a function of rotational frequency has been studied; in the positive-parity band alignment is observed at a rotational frequency of h ̵ ω ≈ 0.63 MeV and in the negative-parity band the alignment occurs at h ̵ ω ≈ 0.57 MeV . The observed alignment in both bands is associated with a pair of g 9 2 protons, and the difference in the alignment frequencies is thought to be due to differing intrinsic shapes.