Investigation of pairing vibration states in 88Sr and 90Zr via the (d, 6Li) reaction and search for α-vibration states

With a 45 MeV deuteron beam the ⁹⁴Mo(d, ⁶Li)⁹⁰Zr and ⁹²Zr(d, ⁶Li)⁸⁸Sr reactions have been studied to search for α-vibration states in ⁹⁰Zr and in ⁸⁸Sr. No evidence is found for such a state in both nuclei with a strength > 5 % than that of the ground state. Strongly excited 0⁺ states in ⁸⁸Sr and ⁹⁰Zr are those also seen in two-nucleon stripping and pickup reactions.     

A study of the 90Zr(6Li, 6He)90Nb reaction at E6Li = 93 MeV

The energy spectra of ⁶He nuclei from the ⁹⁰Zr(⁶Li, ⁶He) reaction at E_{⁶Li = 93 MeV} have been measured over the angular range of 7 ° ? θ_lab ? 20 °. The theoretical DWBA analysis of these spectra and angular distributions was performed employing transition densities of bound and resonance states obtained on the basis of the theory of finite Fermi systems. It is shown that up to excitation energies of E_x ≈ 10 MeV in ⁹⁰Nb a direct charge-exchange mechanism dominates in this reaction, while at higher E_x the contribution made by multistep processes increases significantly. It is also shown that the quasi-elastic cross section is determined by a sum of partial contributions of spin-isospin-flip transitions with multipolarities L=0 through L=6.     

Study of the excited 0+ states through conversion electron spectroscopy of neutron rich Zr and Mo fission products

A windowless Si(Li) electron detector has been used in conjunction with the gas filled fission product separator JOSEF to study totally converted transitions in Zr and Mo isotopes. The existence of twoβ-decay modes in⁹⁶Y,⁹⁸Y and¹⁰⁰Nb is confirmed. The half-lives of theβ-decaying levels feeding O ₂ ⁺ states in⁹⁶Zr and⁹⁸Zr are 6.0 ± 0.4 sec and 0.60±0.05 sec respectively. A search for other first excited O ₂ ⁺ states shows that no such states are apparent in¹⁰⁰Zr and¹⁰⁴Mo.     

Discovery of a very low-lying 0+ state in 98Sr and shape coexistence implication in 98Sr

Using the on-line mass separator OSTIS for studies of short-lived fission products, we found two modes for the β-decay of ⁹⁸Rb with respective half-lives of 96 and 114 ms. The latter decay populates a 0⁺ state in ⁹⁸Sr at the unusually low energy of 215.5 keV, which is by far the lowest 0⁺ excited state observed in even-even nuclei. The half-lives of the 0⁺ and the 2₁⁺ states in ⁹⁸Sr were found to be respectively 25 ns and 4 ns. The transition probabilities, the reduced E0 matrix element and the observed level structure suggest shape coexistence for ⁹⁸Sr: the spheroidal 0₁⁺ ground state shows a rotational band with the 2⁺, 4⁺, (6⁺) levels, whereas the presumably spherical 0₂⁺ excited state may be connected with the 2⁺ level of a vibrational band. A simple model is proposed.     

Electric monopole component in the 2+' → 2+ transition in 192Pt

Angular correlation measurements of K- and L-conversion electrons following the decay ¹⁹²Ir → ¹⁹²Pt have been made using a spectrometer with Ge(Li) and Si (Li) detectors. Absolute and relative internal conversion coefficients of transitions in ¹⁹²Pt were measured, using an ICC and prism β-spectrometers, to ≈ 2 % and values of δ_γ determined from the relative ICC. The experimental values measured in the study, A₂₂(K296γ316) = 0.138 ± 0.010, A₂₂(L296γ316) = 0.125 ± 0.013, α_K(296) = 0.0711 ± 0.0011, K/L_III = 9.65 ± 0.13. With δ_γ = + 5.4 ± 0.2, were employed for determining the E0/E2 amplitude ratios of conversion transitions, q(E0/E2), and the penetration parameter λ of the Ml component for the 2⁺' → 2⁺ (296 keV) transition in ¹⁹²Pt. The angular correlation measurements of L-conversion electrons enabled the elimination of one of two ranges of values of q and λ usually obtained. For the first time in our work, analysis of the e_Lγ angular correlation for the determination of the E0 component in the transition was carried out. As a result, q = +0.04 ± 0.05 with λ = ?4.5 ± 3.5 was obtained for the 296 keV transition. In this case, _ρ(E0) = 0.004 ± 0.005 agrees with _ρ(E0) = +0.006 determined theoretically by Kumar and Baranger for the 2⁺' → 2⁺ transition     

Decay of 0 2 + states in88,92,94Zr andE0 systematics of Zr isotopes

Branching ratios ofE0 andE2 transitions depopulating the 0 ₂ ⁺ states of^{88, 92, 94}Zr have been determined using conversion-electron and γ-ray spectroscopy. Two different lifetime-measurement methods were applied in the remeasurement of the half-lives of the 0 ₂ ⁺ states in^{92, 94}Zr, yielding consistent results ofT _1/2(0 ₂ ⁺ ,⁹²Zr)=85(15) ps andT _1/2(0 ₂ ⁺ ,⁹⁴Zr)=280(40) ps. The monopole strengths extracted areρ ²(⁹²Zr)=8.4(17) ×10^?3 andρ ²(⁹⁴Zr)=11.9(20)×10^?3. The proton configurations of the 0 ₂ ⁺ states are discussed in view of these values and proton transfer data.     

A study of the 14C(6Li, 6He)14N reaction at 93 MeV

The reaction ¹⁴C(⁶Li, ⁶He)¹⁴N was investigated with 93 MeV ⁶Li ions in an angular interval of 7–26°. Angular distributions were analysed for the four most intense groups of ⁶He nuclei, corresponding to transitions to the ground (1₁⁺) and the excited (1₂⁺, 2₁^?, 4₁^?) states of ¹⁴N. In the theoretical analysis a mechanism of the spin-isospin excitation was suggested in the DWBA frame with the finite range of interaction and recoil in the light system (⁶Li⁶He) taken into account. In the calculations both shell-model wave functions and transition densities obtained in the theory of finite Fermi systems (FFS) were used. From the comparison between theory and experiment the Landau-Migdal force constant g′ is estimated in order to obtain some information on the degree of nuclear proximity to the threshold of pion condensation.     

Electromagnetic decay of excited 0+ states in 64, 66, 68Zn

The E0 and E2 transitions depopulating the excited 0⁺ states in ^64,66,68Zn are investigated. Several methods of γ-ray, conversion-electron and internal-pair spectrometry are employed, including a new time measurement technique. A total of 5 E0 transitions are observed and the monopole strengths ρ²(0_i⁺ → 0_i⁺) for most of them are extracted. The results and the nature of the excited 0⁺ states are discussed in terms of several nuclear models.     

0+ states and E0 and E2 transition rates in even Sn nuclei

0⁺ states and their depopulating E0 and E2 transitions have been studied in ^112–124Sn. Several methods of γ-ray and electron spectrometry have been employed, including special coincidence techniques for lifetime, conversion electron and double Coulomb excitation measurements. For ^114–118Sn the E2 transition probabilities from the first excited 0⁺states (0₂⁺) are about 20 W.u., which is compatible with a vibrational two-phonon character. Also the E0 transition probabilities from these states are within a factor of two from the vibrational values. For the second excited 0⁺ states (0₃⁺) the corresponding E2 and E0 transition probabilities are considerably smaller. A total of 12 E0 transitions have been observed. The 0₃⁺ → 0₂⁺ transition is observed in ^114–120Sn and has in ¹¹⁶Sn a reduced transition probability 1–2 orders of magnitude larger than those of the groundstate E0 transitions, which indicates that the 0₃⁺ and 0₂⁺ states are strong mixtures of components with different 〈r²〉. The 0⁺ states in ¹¹⁶Sn are discussed as possible rotational band heads associated with a deformation β₂ ≈ 0.2. The Coulomb excitation cross section of the 0₂⁺ state is found to be sensitive to an interference term including matrix elements with the 2₂⁺ state. The relative sign of this term has been determined.     

Nucleon scattering analysis from90,92,594Zr and isospin decomposition of transition matrix elements for low-lying collective transitions

Simultaneous analysis of neutron and proton inelastic scattering to low-lying collective states in^{90, 92, 94}Zr are used to derive an isospin decomposition of the corresponding transition matrix elements. The deduced (M _n/M_p) ratios for the strongest transitions are in sharp disagreement with values derived from Coulomb-nuclear interference observed in 35.4 MeV inelastic α-scattering from even Zr isotopes.     

Traces of Landau-Zener transitions in the fusion cross section of heavy nuclei?

Nucleus-nucleus collisions with center-of-mass energies E_c.m. close to the Coulomb barrier are investigated within the framework of time-dependent two-center shell-model states. The influence of Landau-Zener transitions on the nucleus-nucleus potential is studied in detail as a function of E_c.m. Dynamical barrier shifts and barrier fluctuations are evaluated for central collisions of ⁹⁰Zr + ⁹⁰Zr, ⁹²Zr + ⁹²Zr, ⁹⁶Zr + ⁹⁶Zr and their effect on the fusion probability is discussed explicitly. While for ⁹⁰Zr + ⁹⁰Zr no modification of the barrier penetration is found a relative enhancement (reduction) of the fusion probability up to a factor of 4 is obtained for ⁹²Zr + ⁹²Zr and⁹⁶Zr + ⁹⁶Zr.     

A study of the γ-ray decays from single particle states in 91Zr and 93Mo

Lifetimes have been measured in ⁹¹Zr and ⁹³Mo by the Doppler shift attenuation method through the ⁹⁰Zr(¹³C, ¹²C)⁹¹Zr and the ⁹²Mo(¹³C, ¹²C)⁹³Mo reactions. Gamma rays were observed in coincidence with the back scattered particles and the reactions took place at sub-Coulomb energies. The lifetimes measured in ⁹¹Zr are τ(1205) = 250⁺⁹⁰_?70 fs, τ(1467) = 280⁺¹⁶⁰_?110 fs, τ(2042) < 30 fs, τ(2558) = 180⁺⁹⁰_?60 fs. The lifetimes measured in ⁹³Mo are τ(1361) = 170⁺⁸⁰_?40 fs, τ(1494) = 60 ± 40 fs, τ(1697) = 440⁺²³⁰_?130 fs, τ(2705)=160⁺⁸⁰_?60fs These results are compared with the expectations of the shell model and the $\text{3}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{→ 2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{E}\text{2}$ transition is shown to be very enhanced.     

Analysis of (6Li,d) and (d, 6Li) reactions in the nickel and tin regions

The (⁶Li, d) and (d, ⁶Li) L = 0 transitions in the mass regions $\text{A ? 60}\text{and}\text{A ? 116}$ are analyzed utilizing microscopic form factors. The cross sections follow the pattern given by the two-proton and two-neutron transfer cross sections.     

Study of the 92,96Zr(d, α)90,94Y reactions

The hole-hole structure of ⁹⁴Y was studied via the reaction ⁹⁶Zr(d, α)⁹⁴Y and compared to the particle-hole structure of ⁹⁰Y, which was populated by the reaction ⁹²Zr(d, α)⁹⁰Y. The deuteron beam energy was 28 MeV. Angular distributions of both reactions were obtained for the prominent lines. New states of ⁹⁴Y were observed at 0.44, 1.17, 1.39, 1.53, 1.82, 1.90, 2.17, 2.33, 2.46 and 2.77 MeV. Our data are consistent with the previously reported 2^? assignment of the ground state, and we suggest J^π = 3^? for the 0.44 MeV state, these being members of the $\text{(π2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{, ν2}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{)}$ doublet. The 1.17 state is suggested to be a member of the $\text{(πp}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{, ν}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}{}^{\mathrm{?1}}\text{)}$ multiplet. The Q-value of the ⁹⁶Zr(d, α)⁹⁴Y reaction was measured to be 7.609 ± 0.020 MeV. The reaction ⁹⁴Zr(d, α) was performed at two angles. Several new states of ⁹²Y were observed at 0.31, 0.78, 1.03, 1.31, 1.49, 1.69 and 1.89 MeV.     

High-resolution study of E0 internal pair decay of excited 0+ states in 58,60,62Ni

The recently developed magnetic plus Si(Li)-Si(Li) sum-coincidence technique is employed to measure E0 internal-pair-formation (IPF) branching ratios of excited 0⁺ states in ^58,60,62Ni. The $\text{X(}\text{E}\text{0}\text{E}\text{2}\text{)}$ values are obtained for a new 0⁺₄ state in ⁶⁰Ni at 3588.0 keV, for the 0⁺₂ and 0⁺₃ states at 2942.3 keV and 3530.9 keV in ⁵⁸Ni, for the corresponding states at 2284.8 keV and 3318.3 keV in ⁶⁰Ni, and for the 0⁺₂ state in ⁶²Ni at 2048.4 keV. The results are combined with the available lifetimes of these states to extract the monopole strengths ?²(0⁺_i ? 0⁺₁). The results and the nature of the 0⁺ states are discussed.     

High-spin states in the isotones 89Zr, 91Mo and 93Ru populated in α-particle-induced reactions

The reactions (α, 3nγ) and (α, α′nγ) on ⁸⁸Sr, ⁹⁰Zr and ⁹²Mo were used to populate excited states in ⁸⁹Zr, ⁹¹Mo and ⁹³Ru. The de-excitation of these states was studied by in-beam γ-ray spectroscopy. The short-lived radioactivity of the ⁹²Mo target was measured in order to study the decay of ⁹³Ru. Many new levels were observed, particularly in ⁹¹Mo. They are interpreted as due to the coupling of a $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ neutron hole with known excited states in ⁹⁰Zr, ⁹²Mo and ⁹⁴Ru.     

Coupled-channels study of projectile and target excitation in the scattering of 6Li from 12C and 16O

Coupled-channels calculations are presented tor elastic and inelastic ⁶Li + ¹²C scattering at E_c.m. = 16 MeV and 20 MeV, and for ⁶Li + ¹⁶O at 18.7 MeV. Excitation of states within ⁶Li, ¹²C and ¹⁶O are treated with rotational, rotation-vibration and vibrational models only. The 3⁺⁶Li and 2⁺¹²C states are strongly coupled to the elastic scattering and reduce the strengths of both the real and imaginary potentials. The 3^?16O state reduces only the strength of the imaginary potential. All other states are weakly coupled and have little effect on each other or the potential. The data are reasonably well described, with there being some preference for the 3^? state in ¹²C to be K = 0. Excitation of the 0₂⁺ state in ¹²C requires a combination of β-vibration and monopole breathing-mode form factors. The deformation lengths found are in poor agreement with those deduced from electron or proton scattering.     

Angular correlation studies in 144Nd

The decays of ¹⁴⁴Pr and ¹⁴⁴Pm populate ten excited states in ¹⁴⁴Nd. We have carried out detailed γγ angular correlation measurements in order to establish spin assignments for these levels. The measurements were performed with Ge(Li)-Ge(Li) and NaI(Tl)-Ge(Li) systems. Eleven sets of A₂₂ and A₄₄ values were obtained. The J^π assignments for levels in ¹⁴⁴Nd (level energies in keV) are as follows: g.s.,0⁺; 696, 2⁺; 1314, 4⁺ ; 1511, 3^?; 1561, 2⁺ ; 1791, 6⁺; 2084, 2⁺ ; 2093, 5^?; 2186, 1^?; 2204, 4; 2644, 1. Mixing ratios were determined for six γ-transitions.     

Vibrational transitions in 182W

Directional correlation measurements (e-e, e-γ) have been performed on ¹⁸²W. The multipole mixing ratios have been measured for the 2_γ⁺2?2_g⁺0 and 4_γ⁺2?4_g⁺0 transitions and a 6⁺6-6_g⁺0 transition in the decay of ¹⁸²Ta and ¹⁸²Re. The mixing ratio of the 2_β⁺0-2_g⁺0 transition is discussed. A strong mixing of the 4_β⁺ and 4_γ⁺ states is suggested.