Collective Octupole Research Articles

Nuclear Schiff moment in nuclei with soft octupole and quadrupole vibrations

Nuclear forces violating parity and time reversal invariance (P,T-odd) produce P,T-odd nuclear moments, for example, the nuclear Schiff moment. In turn, this moment can induce the electric dipole moment in the atom. The nuclear Schiff moment is predicted to be enhanced in nuclei with static quadru pole and octupole deformation. The analogous suggestion of the enhanced contribution to the Schiff moment from the soft collective quadrupole and octupole vibrations in spherical nuclei is tested in this article in the framework of the quasiparticle random phase approximation with separable quadrupole and octupole forces applied to the odd {sup 217-221}Ra and {sup 217-221}Rn isotopes. In this framework, we confirm the existence of the enhancement effect due to the soft modes, but only in the limit when the frequencies of quadrupole and octupole vibrations are close to zero. According to the QRPA, in realistic cases the enhancement in spherical nuclei is strongly reduced by a small weight of the corresponding ''particle + phonon'' component in a complicated wave function of a soft nucleus. The perspectives of a better description of the structure of heavy soft nuclei are discussed.

High-spin octupole yrast levels inRn21686

The yrast level structure of $^{216}\mathrm{Rn}$ has been studied using in-beam spectroscopy \ensuremath{\alpha}-\ensuremath{\gamma}-\ensuremath{\gamma} coincidence techniques through the $^{208}\mathrm{Pb}$($^{18}\mathrm{O}$, $2\ensuremath{\alpha}2n$) reaction in the 91--93 MeV energy range, using the 8\ensuremath{\pi} GASP-ISIS spectrometer at Legnaro. The level scheme of $^{216}\mathrm{Rn}$ resulting from this study shows alternating parity bands only above a certain excitation energy. From this result, the lightest nucleus showing evidence of octupole collectivity at low spins is still $^{216}\mathrm{Fr}$, thereby defining the lowest-mass corner for this kind of phenomenon as $N\ensuremath{\ge}129$ and $Z\ensuremath{\ge}87$.

Octupole collectivity in the Sm isotopes

Microscopic models suggest the occurrence of strong octupole correlations in nuclei with $N\ensuremath{\approx}88$. To examine the signatures of octupole correlations in this region, the spdf interacting boson approximation model is applied to Sm isotopes with $N=86\text{\ensuremath{-}}92$. The effects of including multiple negative-parity bosons in this basis are compared with more standard one negative-parity boson calculations and are analyzed in terms of signatures for strong octupole correlations. It is found that multiple negative-parity bosons are needed to describe properties at medium spin. Bands with strong octupole correlations (multiple negative-parity bosons) become yrast at medium spin in $^{148,150}\mathrm{Sm}$. This region shares some similarities with the light actinides, where strong octupole correlations were also found at medium spin.

Complete spectroscopy of the 162Dy nucleus

States in 162Dy have been populated by ( n , γ ) , ( n , e − ) , and ( α , 2 n ) reactions in order to characterize a complete set of low K bands below 2 MeV. The ( n , γ ) and ( n , e − ) measurements were made using high precision curved crystal and β − spectrometers. Gamma–gamma coincidence and DCO measurements were made following the ( α , 2 n ) reaction using an array of Compton-suppressed Ge detectors. Using all these data and employing the Ritz combination principle, a level scheme was developed which represents one of the most complete and extensive spectra of levels available for comparison with nuclear models. The observed positive and negative parity bands are discussed in terms of collective quadrupole and octupole vibrational excitations as well as 2 quasi-particle excitations.

Polarization measurement and γ-ray spectroscopy ofCs122

High-spin states in $^{122}\mathrm{Cs}$ have been studied via $^{107}\mathrm{Ag}$($^{19}\mathrm{F}$, p3n)$^{122}\mathrm{Cs}$ fusion evaporation reaction at beam energy of 93 MeV. Fifteen new transitions belonging to $^{122}\mathrm{Cs}$ have been observed and placed in the level scheme. The level structure of $^{122}\mathrm{Cs}$ has been extended up to ${E}_{x}\ensuremath{\approx}7$ MeV and $J\ensuremath{\approx}28\ensuremath{\hbar}$. We have performed the linear polarization measurements using the Clover detectors to assign the unknown spins and parities of a few bands. We have also observed a few linking transitions between negative and positive parity bands indicating the octupole collectivity in this nucleus. The structure of the various bands are discussed in the frame work of the cranked Hartree-Fock-Bogoliubov model and the microscopic projected Hartree-Fock model calculations.

Soft octupole vibrations on superdeformed states in nuclei around 40Ca suggested by Skyrme-HF and self-consistent RPA calculations

We present the results of fully self-consistent RPA calculation for low-frequency negative-parity modes built on superdeformed states in the 40Ca region. The RPA calculation was carried out using the mixed representation on the three-dimensional Cartesian mesh in a box. The SD shell structure provides a very favorable situation for octupole shape fluctuations, and we show that the coherent excitation of protons and neutrons play an important role in the emergence of strongly collective octupole vibrations buit on the SD states in the N = Z nuclei, 32S, 36Ar, 40Ca and 44Ti. In particular, the calculation suggests that a low-frequency, strongly collective K Δ = 1− octupole vibration appears on the SD state in 40Ca.

Systematic failure of the Woods-Saxon nuclear potential to describe both fusion and elastic scattering: Possible need for a new dynamical approach to fusion

A large number of precision fusion excitation functions, at energies above the average fusion barriers, have been fitted using the Woods-Saxon form for the nuclear potential in a barrier passing model of fusion. They give values for the empirical diffuseness parameter $a$ ranging between 0.75 and $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, compared with values of about $0.65\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ which generally reproduce elastic scattering data. There is a clear tendency for the deduced $a$ to increase strongly with the reaction charge product ${Z}_{1}{Z}_{2}$, and some evidence for the effect of nuclear structure on the value of $a$, particularly with regard to the degree of neutron richness of the fusing nuclei, and possibly with regard to deformation. The measured fusion-barrier energies are always lower than those of the bare potentials used, which is expected as a result of adiabatic coupling to high energy collective states. This difference increases with increasing ${Z}_{1}{Z}_{2}$ and calculations show that about $1∕3$ of it may be attributed to coupling to the isoscalar giant-quadrupole resonances in the target and projectile. Coupling to all giant resonances may account for a significant part. Fluctuations about the trend line may be due to systematic errors in the data and/or structure effects such as coupling to collective octupole states. Previously suggested reasons for the large values of $a$ have been related to departures from the Woods-Saxon potential and to dissipative effects. This work suggests that the apparently large values of $a$ may be an artifact of trying to describe the dynamical fusion process by use of a static potential. Another partial explaination might reside in fusion inhibition, due for example to deep-inelastic scattering, again a process requiring dynamical calculations.

0+states in deformed actinide nuclei by the(p,t)reaction

By means of the $(p,t)$ reaction we study the excitation spectra of ${0}^{+}$ states in the deformed nuclei $^{228}\mathrm{Th}$, $^{230}\mathrm{Th}$, and $^{232}\mathrm{U}$, using the Q3D magnetic spectrograph facility at the Munich tandem accelerator. At small reaction angles the ${0}^{+}$ transfer angular distributions have steeply rising cross sections which allow us to identify these states in otherwise very complicated and dense spectra. For each of these nuclei we resolve typically about ten excited states with safe ${0}^{+}$ assignments. The studied excitation energies range up to 2.5, 2.7, and $2.3\phantom{\rule{0.3em}{0ex}}\text{MeV}$, respectively, and the summed transfer strengths add to more than $60%$ of the ground state strength. As in a recent study of $^{158}\mathrm{Gd}$ we compare with interacting boson approximation (IBA) calculations in the $spdf$ boson space. This highly schematic collective model description, including octupole collectivity, but neglecting other relevant degrees of freedom, gives numbers of excited ${0}^{+}$ states in these actinide nuclei that are rather close to the observed ones.

Gyromagnetic ratios and octupole collectivity in the structure of theZr90–96isotopes

Shell model calculations have been performed for low-excitation states in the Zr isotopes between $^{90}\mathrm{Zr}$ and $^{96}\mathrm{Zr}$ with an emphasis on the $g$ factors and electromagnetic decay rates for the lowest ${2}^{+}$ and ${3}^{\ensuremath{-}}$ states. Overall the ${2}^{+}$ states are reasonably well described. In contrast, the ${3}^{\ensuremath{-}}$ states present a puzzle because the measured $g$ factors imply a single-particle configuration whereas the experimental $E3$ transition rates imply collective structures that cannot be explained by shell model calculations. A consistent description of the ${3}^{\ensuremath{-}}$ states in $^{90}\mathrm{Zr}$ and $^{96}\mathrm{Zr}$ is sought in terms of coupling between the single-particle structure and a collective octupole vibration.

Enhancement of nuclear Schiff moments and time-reversal violation in atoms due to soft nuclear octupole vibrations

Parity and time invariance violating $(\mathcal{P},\mathcal{T}$-odd) nuclear forces produce $\mathcal{P},\mathcal{T}$-odd nuclear moments, for example, the nuclear Schiff moment. In turn, this moment can induce electric dipole moments (EDMs) in atoms. We estimate the contribution to the Schiff moment from the soft collective octupole vibrations existing in many heavy nuclei. Two possible mechanisms are considered, mixing of the ground state of an odd-$A$ nucleus with the octupole phonon state, and $\mathcal{P},\mathcal{T}$-odd admixture to the single-particle wave function of the valence nucleon. We found practically the same contribution to the Schiff moment as that of the static octupole deformation calculated earlier. This confirms a suggestion that the soft octupole vibrations can replace the (controversial) static octupoles in development of the collective Schiff moments. The values of atomic EDM predicted for ${}^{223,225}\mathrm{Ra}$ and ${}^{223}\mathrm{Rn}$ are enhanced by factors ${10}^{2}--{10}^{3}$ compared to experimentally studied spherical nuclei ${}^{199}\mathrm{Hg}$ and ${}^{129}\mathrm{Xe}.$

Octupole vibration in superdeformed 152(66)Dy86.

Nine transitions of dipole character have been identified linking an excited superdeformed (SD) band in 152Dy to the yrast SD band. As a result, the excitation energy of the lowest level in the excited SD band has been measured to be 14 238 keV. This corresponds to a 1.3 MeV excitation above the SD ground state. The levels in this band have tentatively been determined to be of negative parity and odd spin. The measured properties are consistent with an interpretation in terms of a rotational band built on a collective octupole vibration.

Level structure of141Baand139Xeand the level systematics ofN=85even-odd isotones

New level schemes of {sup 141}Ba and {sup 139}Xe are proposed from the analyses of spontaneous-fission gamma data from our {sup 252}Cf spontaneous fission Gammasphere runs of 1995 and 2000. By analogy with the N = 85 even-odd isotones {sup 149}Gd, {sup 147}Sm, and {sup 145}Nd, spins and parities were assigned to the observed excited states in {sup 141}Ba and {sup 139}Xe. It appears that spherical shell model neutron excitations plus octupolephonons are an appropriate basis at the lower end of the bands. Going to higher spins it is clear that the soft rotor involving valence protons as well as neutrons becomes increasingly important in the configurations. Level systematics in the N = 85 even-odd isotones from Gd(Z=64) through Te(Z=52), are discussed. The excitation systematics and smooth trends of the analogous levels support the spin and parity assignment for excited levels observed in {sup 141}Ba and {sup 139}Xe. The level systematics and the comparison with neighboring even-even isotopes indicate that quadrupole and octupole collectivity play roles in {sup 141}Ba and {sup 139}$Xe. From Gd(Z=64) through Te(Z=52), increasing excitation energies of the 13/2{sup +} states and lowering relative intensities of the positive parity bands in the N = 85 even-odd isotones may indicate that the octupole strength is becoming weaker for the isotones when approaching the Z = 50 closed shell.

Coherent proton–neutron contribution to octupole correlations in the neutron-deficient 114Xe nucleus

Gamma ray linear polarization and picosecond lifetimes have been measured for levels in the neutron deficient nucleus 114Xe using the EUROBALL IV spectrometer and the Cologne plunger device. The EUCLIDES Si-ball was used to improve the reaction channel selectivity. The linear polarization results have, for the first time, unambiguously determined the electromagnetic character of the dipole transitions de-exciting the negative parity level sequence, providing clear evidence for enhanced octupole collectivity. The discovery of two E3 transitions and the measurement of the lifetimes of the states depopulated by these transitions have allowed a quantitative determination of the octupole collectivity in the A≈112 mass region. The large measured B(E3) values, close to ≈70 W.u., are among the strongest observed hitherto and indicate a coherent proton–neutron contribution to the octupole moment.

Electromagnetic properties of the rotationally aligned band in162Dy

Rotational band structures in ${}^{162,164}\mathrm{Dy}$ were studied by projectile inelastic excitation on a ${}^{118}\mathrm{Sn}$ target at near-barrier energies. States with spin up to ${24}^{+}{(22}^{+})$ of the ground-state band and ${18}^{+}{(18}^{+})$ of the $\ensuremath{\gamma}$-vibrational band in ${}^{162}\mathrm{Dy}{(}^{164}\mathrm{Dy})$ were populated in these reactions. States with spin spanning from ${8}^{+}$ to ${20}^{+}$ of the rotationally aligned S band in ${}^{162}\mathrm{Dy}$ were populated by way of its band-crossing point with the $\ensuremath{\gamma}$-vibrational band at spin ${12}^{+},$ where strong mixing occurred due to an accidental degeneracy. This S band eventually crosses the ground-state band at spin ${18}^{+}$ and becomes the yrast at spin ${20}^{+}.$ From the peripheral-collision $\ensuremath{\gamma}$-ray yield data, sets of electromagnetic matrix elements for both the intraband and interband transitions were determined by using the Coulomb excitation code, GOSIA. The measured intraband $E2$ matrix elements were used to derive a quadrupole deformation of the S band that is $\ensuremath{\approx}20%$ larger than those of the nearby low-lying collective bands. This represents the first measurement made for the quadrupole deformation of the S band for states well below the crossing point with the ground-state band. The two-phonon $\ensuremath{\gamma}$-vibration harmonic strength in ${}^{162}\mathrm{Dy}$ is highly fragmented, 36% of the strength is located in two ${K}^{\ensuremath{\pi}}{=4}^{+}$ bands at excitation energies 1535.9 and 2181.0 keV, respectively. Significant octupole collectivity was observed coupling the ground-state band to the ${K}^{\ensuremath{\pi}}{=2}^{\ensuremath{-}}$ band in ${}^{162}\mathrm{Dy}.$

Nuclear structure studies at Saha Institute of Nuclear Pysics using gamma detector arrays

In-beam gamma-ray spectroscopy, carried out at the Saha Institute of Nuclear Physics in the recent past, using heavy-ion projectiles from the pelletron accelerator centres in the country and multi-detector arrays have yielded significant data on the structure of a large number of nuclei spanning different mass regions. The experiments included the study of two-fold γγ-coincidence events for establishing decay schemes, directional correlation of oriented nuclei (DCO) for help in spin assignments and Doppler shift attenuation for lifetime information. The studies have led to the observation of rotational sequences of states in nuclei near closed shell in the mass A=110 region, vibrational spectra in nuclei with A ∼ 60, interplay between single-particle and collective modes of excitation in the doubly-odd bromine isotopes, decoupled bands with large quadrupole deformation in 77Br, shape transition with rotational frequency within a band in 135Pm and octupole collectivity in 153Eu. Particle-rotor-model and cranked-shell-model calculations have been carried out to provide an understanding of the underlying nuclear structure.

High-spin study of neutron-deficient [formula omitted]Xe

High-spin states have been populated in 114 54 Xe via the 58 Ni( 58 Ni, 2p) reaction at 210 MeV, using the Jurosphere γ-ray spectrometer to record γ-ray coincidences. The known level scheme has been significantly extended and includes two positive-parity and three negative-parity structures. At the highest spins, one of the negative-parity bands becomes yrast and shows the characteristics of a smoothly terminating band. Quadrupole moments for two of the bands at high spin have been estimated through a Doppler-broadened lineshape analysis; the terminating band has a reduced quadrupole moment implying that it is not far from full termination into a noncollective oblate state. Extracted B( E1) strengths are discussed in terms of possible octupole collectivity.

Sub-Nanosecond Lifetime Measurement Using the Recoil-Distance Method.

The electromagnetic properties of low-lying nuclear states are a sensitive probe of both collective and single-particle degrees of freedom in nuclear structure. The recoil-distance technique provides a very reliable, direct and precise method for measuring lifetimes of nuclear states with lifetimes ranging from less than one to several hundred picoseconds. This method complements the powerful, but complicated, heavy-ion induced Coulomb excitation technique for measuring electromagnetic properties. The recoil distance technique has been combined with heavy-ion induced Coulomb excitation to study a variety of problems. Examples discussed are: study of the two-phonon triplet in 110Pd, coupling of the β and γ degrees of freedom in 182,184W, highly deformed γ bands in 165Ho, octupole collectivity in 96Zr, and opposite parity states in 153Eu. Consistency between the Coulomb excitation results and the lifetime measurements confirms the reliability of the complex analysis often encountered in heavy-ion induced Coulomb excitation work.

Generator coordinate method and superdeformation in A=190 nuclei

The Generator Coordinate Method with particle number projection using a set of Hartree-Fock plus BCS states is applied to the superdeformed even-even Hg and Pb isotopes. The q 30 and q 32 octupole vibrations are investigated in even-even Hg and Pb isotopes. These one-dimensional calculations predict that the collective octupole K π — 0 − excitations are the lowest mode in energy. The electric monopole E 0 decay out of superdeformed states is also compared to the electric quadrupole E2 transition rates.

Octupole Vibration and Influence of Shell Effects on theE1Transition Rates in140Xe

Spectroscopic studies of the $\ensuremath{\beta}$ decay of ${}^{140}\mathrm{I}$ have firmly established octupole collectivity in ${}^{140}\mathrm{Xe}$ and evinced unique theoretical predictions of a quenched dipole moment. The first detailed investigations of an exotic Xe nucleus, including measurements of half-lives of levels in the ground and octupole bands, were made possible due to a new approach to produce beams of iodine with ISOL sources.

Octupole collectivity and spin dependence of the E1 transition moments in 150Sm

The lifetimes of the lowest 1 −, 3 −, 5 − and 7 −states in 150Sm are measured in the ( n th, γ) reaction with the GRID method. Above these up to spin J=18, B(E1) values are estimated from experimental B(E1)/ B(E2) ratios. The E1 transition moments peak at J=5–7 and are lowest in the region 11≤ J≤15 where previous studies suggest enhanced octupole collectivity. Observed is a staggering of the E1 moments with odd and even J values.