V. Berg

Negative parity states in odd-mass Ir and au nuclei

Abstract The decays of 193 Os → 193 Ir , 191 Pt → 191 Ir , 189 Pt → 189 Ir and 193 Hg → 193 Au have been studied in γ-γ coincidences, internal conversion and electron-electron delayed coincidences. New levels with the following spin-parity assignments and half-lives are proposed: 193Ir: 299.45±0.09keV, I π = 7 2 − , T 1 2 = 0.19±0.03 ns ; 598.26±0.l5keV, I π = 3 2 − ; 740.46±0.15keV, I π = 5 2 − . 191Ir: 391.14±0.05 keV, I π = 7 2 − , T 1 2 = 0.24±0.02 ns ; 659.06±0.08 keV, I π = 3 2 − , T 1 2 ns . 189Ir: 615.70±0.08 keV, I π = 7 2 − , T 1 2 = 0.18±0.02 ns ; 1184.54±0.11 keV, I π = 5 2 − , T 1 2 ns . The 508 keV 7 2 − level in 193Au was found to have T 1 2 = 0.29±0.02 ns . In each nucleus these negative parity states and the 11 2 − isomeric state are strongly connected with E2 and Ml transitions and only weakly connected to the low-lying positive parity states. The B(E2) values are close to the values of B(E2; 2+ → 0+) in the neighbouring even nuclei. We propose these negative parity level systems to be associated with an essentially spherical shape of the nuclei in contrast to the low-lying positive parity states (including the ground state) of these nuclei, which in Ir are likely to be deformed states.

Quasi-prolate and quasi-oblate bands in soft 189Au

Abstract The decay of 189Hg is studied using mass separated sources. Internal conversion coefficients are determined for 57 transitions and the half-lives of four excited states in 189Au are measured. A decay scheme with 55 states is proposed. It is found that 189Au has many features in common with the heavier gold isotopes. However ≈ 1 3 of the total 189Hg decay proceeds via a new system of negative parity states to the 11 2 − isomer. The new states are suggested as resulting from the Coriolis decoupling of a proton in the h 9 2 orbital. This interpretation implies that a tendency for oblate as well as prolate shapes exists in 189Au.

The level structure in 193Ir

Abstract The decay of 193 Os has been studied using Ge(Li) detectors and a 50 cmπ√2 spectrometer. Multipolarities and E2/M1 mixing ratios have been determined for 23 transitions. Half-life measurements applying the delayed coincidence technique have given the following mean lives: τ (138.97 keV level) = 108 ± 15 ps and τ (180.17 keV level) = 80 +11 −21 ps. The M1 and E2 transition probabilities obtained are discussed in terms of the core excitation model and the Nilsson model.

The low-energy level structure of 191Ir

Abstract The decay of 191 Pt to 191 Ir has been investigated using Ge(Li) detectors and a double focusing beta spectrometer. Thirty-five transitions were observed and most of them were placed in a level scheme. Special attention was given to the low energy level band structure. Several multipolarity mixing ratios were determined from L-subshell ratio measurements. Using the delayed coincidence technique the half-life of the 179.05 keV level was measured to 40±12 psec. The low-level decay properties are discussed in terms of the Nilsson model with the inclusion of Coriolis coupling.

The decay of 187Pt: Comparison of the levels of 187Ir with those of heavier odd mass iridium isotopes

Abstract The decay scheme of 2.35 h 187 Pt has been investigated using isotopically separated samples produced by the ISOLDE facility at CERN. A total of 20 excited levels in 187 Ir have been established. For many of these levels, spin, parity and absolute probabilities of depopulating transitions have been determined. The results indicate that the 187 Ir nucleus can be described in terms of a single particle Nilsson model including Coriolis coupling between bands ( 3 2 + [402], 1 2 + [400] ).

E2 and M1 transition probabilities in 193Au and 195Au

High-resolution conversion electron- and γ-ray spectroscopy has been made on the transitions depopulating the three lowest excited states in 193Au and 195Au. The half-lives of these levels have been measured with the delayed coincidence method. The multipole assignments are for the transitions in 193Au: 38.22 keV M1+(17.7 ± 1.0) % E2, 219.75 keV E2, 257.97 keV M1+(21 ± 7)% E2 and for 195Au: 61.44 keV M1+(16.6 ± 1.0) % E2, 180.32 keV M1 + < 3% E2, 200.38 keV E2, 261.74 keVM1+(21.5 ± 4.0) % E2. The obtained half-lives are for 193Au: 38.22 keV level (3.81 ± 0.18) ns, 224.84 keV level < 0.03 ns, 257.97 keV level (45 ± 20) ps and for 195Au: 61.44 keV level (3.0 ± 0.2)ns, 241.72 keV level < 0.03 ns, 261.79 keV level (54 ± 10) ps. A systematic compilation of absolute E2 and M1 transition rates in odd-mass Au nuclei is given and compared to the predictions of the pairing-plus-quadrupole force model and the core-excitation model.

Transition probabilities in 189Os

Abstract The level structure of 189 OS has been studied from the decay of 189 Ir (13.3 d) produced in proton spallation at CERN and mass separated in the ISOLDE on-line facility. Theγ-ray spectrum has been recorded both with a high-resolution Si(Li) detector and Ge(Li) detectors. Three previously unreported transitions were observed defining a new level at 438.5 keV. Special attention was given to the low-energy level band structure. Several multipolarity mixing ratios were deduced from measured L-subshell ratios which, together with measured level half-lives, gave absolute transition probabilities. The low-level decay properties are discussed in terms of the Nilsson model with the inclusion of Coriolis coupling.

E2 and M1 transition probabilities in odd-mass Hg nuclei

Abstract L- and M-subshell ratios have been measured for the 39.45 keV transition in 193Hg and the 37.13 and 16.2 keV transitions in 193Hg yielding 0.38±0.12,

E2 and M1 transition probabilities between low-energy states in 189Ir

Half-lives have been measured for the lowest excited states in 189Ir and the transitions between the states have been studied using high-resolution electron and γ-spectroscopy. The following half-lives were obtained: 94.34 keV, 11.6±0.6 nsec; 113.82 keV, 76±18 psec; 176.53 keV, 22±10 psec; 300.51 keV, < 20 psec. A new transition was found at 62.65 keV. From L Subshell ratios the following multipolarity mixtures were obtained: 62.65 keV, M1 + (8.5 −3+6 % E2; 71.69 keV, M2; 82.22 keV, M1 + (2.8±0.7) % E2; 94.34 keV, E2 + (5.5±1.0) % M1; 113.82 keV, M1 + (23±2)% E2; 141.18 keV, M1 + (1.5+1.5−0.8) % E2; 176.53 keV, M1 + (44±3) % E2; 186.70 keV, M1 + (21±4) % E2; 203.78 keV, M1 + < 30% E2; 223.34 keV, E2; 243.50 keV, E2; 258.37 keV, E3; 300.51 keV, E2; 317.65 keV, M1 + (23±10) % E2. Reduced E2 and M1 transition probabilities are calculated and compared to the result of a two-band Coriolis coupling calculation. A deformation parameter δ = 0.23 is obtained in 189Ir. This represents a sharp increase from the value δ = 0.12 obtained in 191Ir and 193Ir. The B(E2) and B(M1) values in all odd-AIr nuclei are presented systematically and compared to current nuclear models.

On the low-energy band structure in 187Os

Abstract The level structure of 187 Os has been investigated in the decay of 187 Ir (11 h) produced in 600 MeV proton spallation at CERN and mass-separated in the ISOLDE on-line facility. Special emphasis has been put on the study of absolute transition rates from low-lying levels in 187 Os by means of high-resolution electron and γ-ray spectroscopy and half-life measurements using the delayed-coincidence technique. The observed transition rates have been used to assign Nilsson quantum numbers to different levels. The large B (E2) values for some interband transitions have been accounted for through a Coriolis coupling calculation. Through an extensive investigation of the γ-ray spectrum from 187 Ir three new levels at 190.70, 332.94 and 586.40 keV have been proposed.