E. Hammarén

Structural connections between 148Sm and 149Sm nuclei

Abstract The 146, 148 Nd(α, χn) and 148, 150 Nd( 3 He, χn) reactions at E α = 20–43 MeV and E 3He = 19–27 MeV, are used to study excited states in the 149 Sm 86 and 149 Sm 87 nucleides and consequently the low-spin odd-parity excitation. The mixing ratios and multipolarities of the most prominent transitions are deduced from the combined evidence of angular distribution and electron conversion data. The spin-parity assignments for most of the levels observed are established. In 148 Sm the ground state band extending to I π = 10 + is predominantly populated. A negative-parity odd-spin band extending from I π = 3 − through 11 − is also observed. The bands in 148 Sm are interpreted within the framework of the interacting boson approximation model. In 149 Sm positive-parity levels with spin up to 25 2 and negative-parity levels with spins up to 21 2 are observed. The predominant γ-decay proceeds via transitions associated with i 13 2 , h 9 2 , f 7 2 and h 11 2 intrinsic configurations. The branching ratios B (E1)/ B (E2) are calculated and compared in both 148 Sm and 149 Sm nucleides. The B (E1)/ B (E2) dependence on the value of Z for some N = 86 (as well as 88 and 84) isotones showing a minimum of Z = 64 was noted. A 4 ns high-spin isomer mainly decaying into the positive-parity band based on the i 13 2 state in 149 Sm is found. Experimental evidence is presented to interprete the 1 2 + , 15 2 + , … and 9 2 − , 13 2 − , …, ΔI = 2, sequences in 149 Sm as arising from the coupling of an h 9 2 neutron to the octupole and quadrupole modes of the 148 Sm core nucleus. The absolute reaction cross sections for the 146, 148, 150 Nd( 3 He, χn) reactions have been determined for different bombarding energies. The mixing of the f 7 2 and h 9 2 shells is discussed in the framework of an axial-particle-rotor model calculation.

IBA-1 calculations of E2/M1 mixing ratios

Abstract The interacting-boson approximation is applied in its basic form (IBA-1) to the calculation of γ-ray E2/M1 mixing ratios. The description is tested on a variety of even nuclei with adequate experimental data: 146–152 Sm, 152–156 Gd, 162 Dy, 162–168 Er, 172 Yb, 190 Os. The consistent- Q formalism of Warner and Casten and its extension to the vibrational region are used. The three M1 transition parameters of IBA-1 are chosen so as to display acceptably smooth systematics, in contrast to earlier calculations reviewed for comparison. The calculated M1 matrix elements are found to depend strongly both on the wave functions and on the M1 parameters. Comparison is made with the few available IBA-2 results as well as with the two-fluid Frankfurt model and Kumars microscopic model.

High spin shell model excitations in149Gd

The high spin level structure of the three-neutron nucleus149Gd has been investigated by in-beamγ-ray and electron spectroscopy with (α, xn) reactions. The observed levels are characterized as members of the shell model multipletsνf72/3,νh9/2f72/2,νf72/3×3−,νh9/2f72/2×3−, and tentativelyνf72/3×(3−)2. The energies of theνf72/3 states agree only moderately with those calculated using empirical two-nucleon interactions taken from148Gd, which indicates the importance of long range contributions already atN=85.

Excited states in 146Sm and 147Sm

Abstract The 144, 146 Nd(α, χn) and 146,148 Nd( 3 He, χn) reactions with E α = 20–43 MeV and E 3 He , = 19–27 MeV are used to investigate excited states in the isotopes 146 Sm and 147 Sm. The experiments involve measurements of singles γ-ray spectra and conversion electron spectra, γ-ray angular distributions and three-parameter ( E γ - E γ -time) coincidences. From these experiments information is obtained for states with spin up to I = 13 + and I = 27 2 − , respectively. These states are interpreted within the framework of the cluster-vibration model (CVM) as well as the shell model. In the latter approach, the energies of several well established states, in both isotopes, are calculated using empirical singleparticle energies, empirical two-particle interaction matrix elements and angular momentum algebra. The average deviation between the calculated and the experimental energies is less than 100 keV. The CVM calculations involve the coupling of a three-particle neutron cluster to the quadrupole vibration of the core. For 147 Sm, these calculations reproduce the observed sequence of states based on the I π = 7 2 − ground state, as well as the sequence of states based on the I π = 13 2 + excited state. The CVM calculations also reproduce the ground band in 146 Sm, while for the negative parity states based on the cluster (f 7 2 i 13 2 ) 3 − −10 − an additional shift in energy is expected due to the mixing with octupole phonons.

A Study of the Nuclear Structure at High Energy and Low Spin

A novel method to study nuclear structure at low spin as a function of temperature is developed and used on 146,148Sm, 154,156Gd and 160,162Dy, by means of the (3He, α) reaction. The nuclear level density for a wide energy range is also studied. The γ-multiplicities and the first generation γ-ray spectra indicate a structure change in deformed nuclei at about 6 MeV.

The (3He, α) strength functions in rare-earth nuclei

Abstract The 156 Gd( 3 He, α) 155 Gd, 157 Gd( 3 He, α) 156 Gd and 171 Yb( 3 He, α) 170 Yb reactions have been studied over a wide region in excitation energy. The bombarding energy was 26 MeV, and the reaction products were analyzed by means of a particle telescope. Strength functions and angular distributions have been measured. All strength functions contain a strong low-energy peak which is shown to be the result of the Coriolis coupling. Additional αγ coincidence experiments were performed in order to study the population of the yrast levels as a function of excitation energy. These population patterns, as well as the α-particle angular distributions, are consistent with a dominating large- l transfer in the excitation region up to the neutron binding energy. Comparisons are made with calculated strength functions based on the particle-rotor model. Only a part of the total yield observed at high excitation energy can be accounted for by the model.

Microscopic and phenomenological analysis of the Alaga rule for dipole states

Abstract We discuss the relation between K -mixing and gamma-decay branching ratios (BR) for dipole excitations using a microscopic, number and spin-projected mean-field model and two phenomenological approaches based on the former: a generalized two-level rotational model and a model with general projected matrix elements. The microscopic model shows that most of the unnatural parity excitations with strong ground state B (MI)↑ strength are associated with BRs to the 0 + and 2 + members of the ground band in agreement with the Alaga-rule. However, BRs calculated for other states do not correlate with the intrinsic K -values in a manner, which would allow one to predict K -mixing from them. The phenomenological models, with randomly selected, intrinsic transition matrix elements, reproduce qualitatively the experimental distribution of the BRs with no decisive information about the K -content of the initial states.

In-beam study and a rotational description of low-lying levels in the N = 88 nucleus 149 Pm

Abstract Levels of 149 Pm were studied by methods of in-beamy-ray spectroscopy using the 150 Nd(p, 2nγ) reaction. About ten new medium- and high-spin states (from 7 2 to 15 2 ) were observed below 1 MeV. An axial particle-rotor model was applied to the N = 88 nucleus 149 Pm and the calculations were found to predict satisfactorily properties of most levels. Both the positive and the negative-parity band structures are strongly Coriolis-mixed. The results indicate larger deformation for the negative- parity states than for the positive-parity states.

Nanosecond lifetimes of the low-spin states in the nucleus 147Nd: A particle-rotor model interpretation of the results

Abstract Nanosecond lifetimes of several states in 147 Nd have been studied using the reaction 146 Nd(d, pγ) 147 Nd with 10 MeV deuterons. The following lifetimes were observed: the 7 2 − level at 49.9 keV, 2.5±0.5 ns; the 5 2 − level at 127.9 keV, ≦ 0.8 ns; the 9 2 − level at 190.3 keV, 1.1±0.3 ns and the 1 2 − level at 214.6 keV, 5.8±0.8 ns. The wave functions of the states were constructed using an axial particle-plus-rotor model. The free parameters used are compared to the systematics observed in the neighbouring heavier N = 87 isotones as well as in the N = 89 and 91 isotones. Transition rates within the f 7 2 and h 9 2 based excitations, separately, are reasonably well reproduced, but the connecting transitions indicate too strong a mixing of the shells in the calculation.

Deep-lying νh112 hole states in 145Nd

Abstract The level scheme of 145 Nd has been studied by measuring γ-rays following the ( 3 He, α) pick-up reaction. Up to the neutron binding energy of B n = 5.8 MeV the γ-ray multiplicity has been measured as a function of excitation energy. Assuming that Fermi gas conditions are appropriate in the continuum part of the level scheme the level density parameter is determined to be a = (14.5 ± 2.0) MeV −1 . The ν h 11 2 spectroscopic strength function has been extracted by means of a coincidence filter technique. An almost uniform ν h 11 2 strength function in the region 3.2–5.8 MeV of excitation energy and a summed strength of only 50% of the shell-model strength indicate a fragmentation to higher energies.