Shape coexistence in neutron-rich nuclei near N=40

Recent data show that both the 2+ and 4+ levels in the even neutron-rich Cr and Fe isotopes decrease in excitation energy toward N=40. This observation, along with Coulomb excitation and lifetime data, strongly indicates an increase in collectivity near N=40 in contradiction with expectations based on first principles. A straightforward two-band mixing model is used to investigate the structure of these neutron-rich Cr and Fe nuclei. The approach takes advantage of the extensive data available for 60Fe to provide the parameter values with which to reproduce the experimental observations in the 58-64Cr and 60-68Fe isotopic chains. Comparisons between the model and the data suggest marked structural differences for the ground-state configurations of N=40 Cr and Fe

New results on octupole collectivity

Octupole correlations play an important role in determining the level structure of nuclei throughout the periodic chart. Microscopically, octupole correlations are the result of the long-range, octupole-octupole interaction between nucleons occupying pairs of orbitals which differ in both orbital and total angular momentum by 3 units. A review of some of the most recent findings on octupole correlations is given. Emphasis is placed on new results from the actinide region, where two distinct collective modes have long been identified: octupole vibration and octupole deformation. These new results include negative-parity structures which appear to evolve from an octupole vibration into a static octupole deformed mode. In addition, newly observed rotational structures built on an excited 0+ state have been tentatively associated with a double-octupole phonon excitation. These newly observed properties can be successively described by calculations based on the concept of rotational-aligned octupole phonon condensation

Octupole Deformation in the Odd-Odd Nucleus 224-Ac

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Rotational bands with identical transition energies in actinide nuclei

We point out the existence of ground-state rotational bands with identical transition energies (up to spin 8Latin small letter h with stroke) in Pu240, Cm244, Cm246, and Cf250. The corresponding transitions in the ground-state bands of U236 and U238 have identical energies (within 2 keV) up to spin 24Latin small letter h with stroke. These features are very similar to those recently observed for superdeformed bands in the mass-150 and mass-190 regions and suggest that the phenomenon of identical bands is not restricted to superdeformed bands

Identification of key astrophysical resonances relevant for the Al26g(p,γ)Si27 reaction in Wolf-Rayet stars, AGB stars, and classical novae

A γ-ray spectroscopy study of Al26g+p resonant states in Si27 is presented. Excitation energies were measured with improved precision and first spin-parity assignments made for excited states in Si27 above the proton threshold. The results indicate the presence of low-lying resonances with lp=0 and lp=2 captures that could strongly influence the Al26g(p,γ)Si27 reaction rate at low stellar temperatures, found in low-mass asymptotic giant branch (AGB), intermediate-mass AGB, super AGB, and Wolf-Rayet stars

Identification of analog states in the T=1/2 A=27 mirror system from low excitation energies to the region of hydrogen burning in the 26Alg ,m(p,γ)27Si reactions

The reactions 26Alg(p,γ)27Si and 26Alm(p,γ)27Si are important for influencing the galactic abundance of the cosmic γ-ray emitter 26Alg and for the excess abundance of 26Mg found in presolar grains, respectively. Precise excitation energies and spin assignments of states from the ground state to the region of astrophysical interest in 27Si, including the identification and pairing of key astrophysical resonances with analog states in the mirror nucleus 27Al, are reported using γ rays observed in the 12C + 16O fusion reaction. The detailed evolution of Coulomb energy differences between the states in 27Si and 27Al is explored, including the region above the astrophysical reaction thresholds

Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

The 92-keV resonance in the 25Mg(p,γ)26Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γ rays. It is therefore important to precisely determine the ground-state branching fraction f0 of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat(6)syst. The f0 value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γ rays.peerReviewe

Analog E1 transitions and isospin mixing

We investigate whether isospin mixing can be determined in a model-independent way from the relative strength of E1 transitions in mirror nuclei. The specific examples considered are the A=31 and A=35 mirror pairs, where a serious discrepancy between the strengths of 7/2--->5/2+ transitions in the respective mirror nuclei has been observed. A theoretical analysis of the problem suggests that it ought to be possible to disentangle the isospin mixing in the initial and final states given sufficient information on experimental matrix elements. With this in mind, we obtain a lifetime for the relevant 7/2- state in 31S using the Doppler-shift attenuation method. We then collate the available information on matrix elements to examine the level of isospin mixing for both A=31 and A=35 mirror pairs

Rotational bands in neutron-rich 169,171,172Er

The neutron-rich 169,171,172Er nuclei were populated by few-neutron transfer reactions between 170Er and 238U at a near barrier energy. The spectroscopy of these Er isotopes was studied using prompt γ rays correlated with delayed transitions or events involving at least three prompt transitions. The ground-state band of 172Er was populated up to spin 22+ at an excitation energy of 5528 keV. Rotational bands built on the 1/2-[521], 5/2-[512], and 7/2 +[633] neutron configurations in 169,171Er were extended to substantially higher spins than previously known. The signature splitting observed in these rotational bands is addressed within the framework of the particle-rotor model in terms of triaxiality and Coriolis attenuation. The signature inversion observed in the 5/2-[512] band is well reproduced by including the triaxial degree of freedom in the calculation. Attenuating the Coriolis interaction in the calculation is found to be necessary to reproduce the signature splitting observed in the 7/2+[633] band. A similar Coriolis attenuation also is needed to account for the signature splitting as well as the B(M1)/B(E2) ratios in the 7/2+[633] ground-state band in the neighboring N=99 isotones, 167Er and 169Yb