J. Kownacki

The single-closed-shell nucleides 14462Sm82 and 14664Gd82

Abstract Alpha-particle irradiation of enriched 142 Nd and 144 Sm targets has been used to populate states in the N = 82 nucleides 144 62 Sm and 146 64 Gd. Singles γ-ray spectra, angular distributions, γ-γ coincidence spectra as well as time distributions with respect to the α-beam bursts, have been measured at various bombarding energies. Properties of the levels in the two nucleides are compared. The existence of excited states with the spin sequence 6 + → 4 + → 2 + → 0 + is established in both of them, similarly to all the other known N = 82, even- Z isotones. The energy spacing of these states in 146 Gd differs from those found in the lighter isotones, which may be attributed to the subshell effect at Z = 64 (filling of both the g 7 2 and d 3 2 orbitals). The 6 + → 4 + E2 transitions are found to be retarded. The systematic occurrence of the 6 + isomeric states in all the N = 82 even- Z isotones is discussed. The lowest excited state with negative parity (3 − , 1810 keV) in 144 Sm is found to be very strongly populated in the γ-ray cascade following the (α, 2n) reaction. The evidence for the existence of a rather weakly populated low-lying 3 − state in 146 Gd (1584 keV) is discussed. A number of higher excited states with excitation energies up to about 5.3 MeV and with spin values up to 10 or 11 are established for both nucleides. In a qualitative discussion, the majority of these states can be explained as primarily two-quasiparticle configurations. Strong population of the highest excited states in both nucleides (at about 5.3 MeV) is noted. This fact is difficult to explain on the basis of simple statistical considerations of the ( α , x n) reaction mechanism.

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.

Rotational bands in 107Cd

Abstract High-spin states in 107Cd were populated by the reaction 94Zr(17O, 4n) at 80 MeV and their decay was studied by γ-spectroscopic methods using the NORDBALL multi-detector array. The experiment included γ-ray yields, γ-coincidences and γ-ray angular correlation measurements. Rotational bands were observed up to spin 39 2 and ( 51 2 + ) in the negative- and positive-parity bands, respectively. Calculations of total routhian surfaces and quasi-particle routhians have been performed and are used for assigning quasiparticle configurations to the bands. In the g 7 2 neutron band, a h 11 2 neutron alignment is observed. A large signature spliting is expected in the ν g 7 2 band before the band crossing. For the band built on a single ν h 11 2 quasiparticle, an alignment of either g 7 2 or h 11 2 neutrons is proposed. Shape change effects are discussed.

Multi-detector arrangements for on-line measurements of γ-ray and neutron multiplicities

Abstract This paper describes multi-detector arrangements for measuring the number of quanta emitted in a cascade. Two set-ups are described, one with NaI(Tl) detectors and another with liquid scintillators. The latter system has the advantage of being able to separate between neutron and γ-ray events, which is of particular importance for reactions with a large number of neutrons emitted. The set-ups are used on-line the Stockholm cyclotron. Examples of experiments with 51 MeV α-particles and 118 MeV 12 C-ions are presented.

IN-BEAM SPECTROSCOPY AT THE PROTON-DRIP LINE - FIRST OBSERVATION OF EXCITED-STATES IN SB-106 AND SB-107

Neutron deficient nuclei close to Sn-100 have been investigated in-beam using the NORDBALL detector array. A target of Fe-54 was bombarded with a beam of Ni-58 at 270 MeV. Evaporation residues were identified by detecting protons and alpha particles in a 4 pi charged particle multi-detector set-up and neutrons in a 1 pi neutron detector wall in coincidence with gamma rays. Excited states of the proton drip line nucleus Sb-106 and of Sb-107 were identified for the first time. The yrast level schemes constructed from gamma-gamma-particle coincidence and gamma-gamma angular correlation analysis are presented and discussed within the framework of the nuclear shell model.

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.

Gamma-gamma energy correlations and moment of inertia in 130Ce

Abstract High-spin properties of 130Ce have been studied by using the reaction (12C, 6n). The γ-rays following this reaction were detected by six NaI(Tl) detectors in a two-dimensional coincidence arrangement. An experimental γ-ray energy-energy correlation spectrum was extracted from the original coincidence matrix. The matrix of the correlated spectrum shows a valley along the 45° diagonal extending up to Eγ ≈ 1.2 MeV, thus indicating a collective behaviour beyond the region of known discrete lines. From the width of the valley a collective moment of inertia is deduced.

High-spin states and evidence for hole-core coupling in the 143Sm81 and 145Gd81 nuclei

Abstract The level structure of the 143 Sm and 145 Gd nuclei has been studied using the 142 Nd (α, 3nγ) 143 Sm and 144 Sm(α, 3nγ) 145 Gd reactions. Singles γ-ray, γ-γ coincidence spectra, angular and time distributions with respect to the beam burst have been measured. Gamma transitions between excited states with spin values up to 29 2 in 143 Sm and up to 21 2 in 145 Gd have been observed. The observed level structure in both nuclei can be explained if the neutron-hole is coupled to the doubly even core excitations. The coupling of the h 11 2 neutron-hole with the 2 + and 3 − vibrations is calculated in the weak and intermediate coupling models. The results obtained in these calculations are in reasonably good agreement with the experimental data. The energies of the three-particle states, being the result of the coupling of the h 11/2 neutron-hole with the two-proton excitations in the core, are nicely reproduced in the calculations. Empirical values of the two-body interaction matrix elements have been used. However, the three-particle calculation does not explain the existence of the 30 ms 23 2 − isomeric level in the 143 Sm nucleus.

Collective excitations in 106Cd

High Spin states in 106Cd were populated by the reactions Zr-94(O-17, 5n) and Ge(S-32, 4n) at 80 MeV and at 148 MeV, respectively. The gamma-decay was studied by gamma-spectroscopic methods using t ...

Gamow-Teller decay of 118Pd and of the new isotope 120Pd

Abstract With the use of the proton-induced fission of 238 U and the ion guide-fed on-line mass separation, even- A isotopes of palladium with high neutron excess were studied by means of γ-ray and conversion-electron spectroscopy. The decay of 118 Pd was reinvestigated in detail, and evidence for the new isotope 120 Pd, with a half-life T 1 2 = 0.5 ± 0.1 s , was found. As established for 118 Pd and tentatively shown for 120 Pd, the β-decay of these two isotopes proceeds mainly through 0 + → 1 + Gamow-Teller transitions with log ft values between 4.2 and 4.7. The strength of individual transitions has been compared with the predictions of the shell-correction model with a deformed Woods-Saxon potential and pairing residual interaction. The total GT strength was found to be quenched with respect to both the deformed shell-model and spherical-quasiparticle random-phase-approximation predictions.