H. Ai

Isomer and In‐Beam Spectroscopy of Medium‐Spin States in 91,92Zr

Near‐yrast states have been identified in the stable 91,92Zr isotopes using the fusion evaporation reaction 82Se(13C,xn)95−xZr at an incident beam energy of 50 MeV. Gamma‐ray spectroscopy of states above the reported τ = 6 μs, Iπ=212+ isomer in 91Zr are reported for the first time with consequences for stimulating new shell model calculations which incorporate either the breaking of the N=50 shell closure and/or the inclusion of the negative parity h11/2 neutron intruder orbital.

Surrogate Nuclear Reactions using STARS

The results from two surrogate reaction experiments using the STARS (Silicon Telescope Array for Reaction Studies) spectrometer are presented. The surrogate method involves measuring the particle and/or γ‐ray decay probabilities of excited nuclei populated via a direct reaction. These probabilities can then be used to deduce neutron‐induced reaction cross sections that lead to the same compound nuclei. In the first experiment STARS coupled to the GAMMASPHERE γ‐ray spectrometer successfully reproduce surrogate (n,γ), (n,n′γ) and (n,2nγ) cross sections on 155,156Gd using 3He‐induced reactions. In the second series of experiments an energetic deuteron beam from the ESTU tandem at the Wright Nuclear Structure Lab at Yale University was used to obtain the ratio of fission probabilities for 238U/236U and 237U/239U populated using the 236,238U(d,d′f) and 236,238U(d,pf) reactions. Results from these experiments are presented and the implications for the surrogate reaction technique are discussed.

A search for low‐lying mixed symmetry states in 140Nd

Multipole mixing ratios for transitions between excited 2+ states and the 21+ state in 140Nd are measured using angular correlation techniques for beta decay reactions. Two transitions with dominant M1 components are identified, and the higher‐lying 2+ states involved in these transitions are presented as candidates for the one‐phonon mixed‐symmetric 2+ state in 140Nd. Assuming pure two‐state mixing between the one‐phonon 2+ state and a presumed three‐phonon symmetric state yields a mixing matrix element between these states, which is discussed in terms of shell effects on mixed‐symmetric behavior in the N = 80 isotones.

A First Test of the E(5/4) Bose‐Fermi Symmetry : 135Ba

There has been intense recent interest in equilibrium shape / phase transitions in nuclei and the concept of critical point symmetries to describe them. The first critical point symmetry for an odd‐mass nucleus has been recently proposed, namely, the E(5/4) Bose‐Fermi critical point symmetry, it corresponds to coupling between an odd particle in a j = 3/2 orbit and the E(5) critical point symmetry, at the transition between the O(6) gamma‐soft and the U(5) vibrator symmetries. Since 134Ba is a candidate for the E(5) critical point symmetry, we carried out a β‐decay experiment on 135Ba whose last neutron can occupy a 2d3/2 orbit as a first test of E(5/4). The experimental results were compared to E(5/4) and also with the Interacting Boson‐Fermion Approximation Model (IBFA) and Shell Model calculations. We see fair agreement with E(5/4) for B(E2) values but not for all energies. The IBFA shows better agreement with data than E(5/4) and the Shell model shows the best agreement.

Measurement of the g factor of the 4{sub 1}{sup +} state in {sup 70}Ge

The g factors of the 2 + 1 , 3 - 1 , and 4 + 1 states of 70 Ge have been measured by the transient field technique. The measured value, g(4 + 1 ) = +0.5(2) is in good agreement with the results of shell-model calculations within the full fp configuration space and with the nuclear g-factor systematics in this region. The measured g(4 + 1 ) is consistent with the recently reported positive g(4 + 1 ) value for 68 Zn, the N = 38 isotone of 70 Ge. The determination of the logarithmic slope of the particle-y angular correlations is discussed for two typical arrangements of transient field experiments.

Sign of the g factor of the 4{sub 1}{sup +} state in {sup 68}Zn

In two recent papers a negative g factor was reported for the 4{sub 1}{sup +} state in {sup 68}Zn. The negative sign is unexpected. It is not consistent with the systematics of g factors in the neighboring Zn and Ge isotopes and could not be explained by shell-model calculations even when significant contributions of the 0g{sub 9/2} neutrons were included. Therefore, an independent g factor measurement was performed, using {sup 68}Zn projectiles which were accelerated to a higher energy in order to obtain a higher yield for the 4{sub 1}{sup +} state. The new measurement yielded a positive g factor, g(4{sub 1}{sup +})=+0.6(3), which agrees with the results of full fp spherical shell model calculations, as well as with Z/A, the collective model prediction.

Revisiting anomalous B(E2;4{sub 1}{sup +}{yields}2{sub 1}{sup +})/B(E2;2{sub 1}{sup +}{yields}0{sub 1}{sup +}) values in {sup 98}Ru and {sup 180}Pt

Recently, a set of nine nonmagic nuclei with anomalous values of the B(E2) ratio B 4/2 ≡ B(E2; 4 + 1 → 2 + 1)/B(E2; 2 + 1 → 0 + 1) < 1 were identified. Such values are outside the range allowed by current collective models. In the present work, the B(E2; 4 + 1 → 2 + 1) values for two of these nuclei, 98 Ru and 180 Pt, were re-measured to determine if the current literature values for these nuclei are correct. 98 Ru was studied in a 27 Al(98 Ru, 98 Ru *) Coulomb excitation experiment in inverse kinematics, while the lifetime of the 4 + 1 state in 180 Pt was measured in a 122 Sn(62 Ni, 4n) 180 Pt recoil distance method (RDM) experiment. For both nuclei, the remeasured B 4/2 values are well above 1, removing the deviations from collective models.Recently, a set of nine nonmagic nuclei with anomalous values of the B(E2) ratio B{sub 4/2}{identical_to}B(E2;4{sub 1}{sup +}{yields}2{sub 1}{sup +})/B(E2;2{sub 1}{sup +}{yields}0{sub 1}{sup +}) < 1 were identified. Such values are outside the range allowed by current collective models. In the present work, the B(E2;4{sub 1}{sup +}{yields}2{sub 1}{sup +}) values for two of these nuclei, {sup 98}Ru and {sup 180}Pt, were re-measured to determine if the current literature values for these nuclei are correct. {sup 98}Ru was studied in a {sup 27}Al({sup 98}Ru,{sup 98}Ru*) Coulomb excitation experiment in inverse kinematics, while the lifetime of the 4{sub 1}{sup +} state in {sup 180}Pt was measured in a {sup 122}Sn({sup 62}Ni, 4n){sup 180}Pt recoil distance method (RDM) experiment. For both nuclei, the remeasured B{sub 4/2} values are well above 1, removing the deviations from collective models.