D. Nisius

OPENING UP THE A 180 K-ISOMER LANDSCAPE : INELASTIC EXCITATION OF NEW MULTI-QUASIPARTICLE YRAST TRAPS

Abstract New high-spin isomers in 175 Lu, 181 Ta and 186 W target nuclei have been strongly excited by pulsed 238 U beams. The use of inelastic excitation opens up an uncharted region in the neutron-rich K-isomer landscape. First results establish the key role of multi-quasiparticle configurations in the high-angular-momentum structure.

-decay characteristics of neutron-deficient 190, 192Po nuclei and alpha branching ratios of 186, 188Pb isotopes

By using complete fusion reactions and , leading to and compound nuclei, improved -decay characteristics for neutron-deficient isotopes have been obtained. The experiments have been performed at the fragment mass analyzer (Argonne) and at the RITU gas-filled separator (Jyvaskyla). In the l = 0 decay to the excited state in is two times faster than the decay to the ground state. For more precise values of the alpha-decay energy keV and half-life ms were measured. By using correlated -decay chains -branching ratios of 38(9)% and 9.3(8)% were experimentally deduced for the first time for and , respectively. The reduced -width systematics is extended for even Po isotopes, confirming a persistence of the saturation towards the lighter isotopes. Absence of any anomalies in the reduced -widths for the neutron-deficient Pb isotopes is confirmed.

Excitation energies and spins of the yrast superdeformed band inHg191

The excitation energies and spins of the levels in the yrast superdeformed band of {sup 191}Hg have been determined from two single-step {gamma} transitions and the quasicontinuum spectrum connecting the superdeformed and normal-deformed states. The results are compared with those from theoretical mean-field calculations with different interactions. A discussion of pairing in superdeformed states is also included.

Deformed i132 bands and prolate-oblate shape coexistence in 185Tl and 187Tl

Abstract High spin states in the very neutron-deficient isotopes 185Tl and 187Tl have been identified from γ-γ coincidence measurements performed with the reactions 154Gd(35Cl, 4n) and 159Tb(32S, 4n). Unambigous mass assignments were obtained using a Fragment Mass Analyzer and the reactions 154Gd(36Ar, p4n and 155Gd(36Ar, p3n). Struvtures are observed corresponding to both an i 13 2 proton coupled to a prolate deformed core, and also h 9 2 and i 13 2 protons coupled to an oblate defo rmed core. The proposed 13 2 + [606] intrinsic states are isometric, with meanlives of 12±2 ns in 185Tl and 1.0±0.2 ns in 187Tl. Eq uilibrium deformation calculations for the excitation energies of intrinsic states reproduce general trends, but overestimate the bandhead energies of high-j states.

Superdeformed band with a unique decay pattern: possible evidence for octupole vibration in 190Hg

Abstract An excited superdeformed (SD) band has been observed in 190 Hg which decays to the lowest-energy (yrast) SD band rather than to the less deformed states as observed in most known SD bands in the A ∼ 150 and A ∼ 190 regions. The most plausible interpretation of this very unusual decay pattern associates this band with a collective structure built on an octupole-vibrational phonon in the SD well.

Experiments with a radioactive 56Ni beam

Abstract A technique for producing a high-quality radioactive 56 Ni (T 1/2 =6.1 d ) beam via the two-accelerator method has been developed. Beam intensities of 2×10 7 56 Ni/s were extracted from the ion source and 2×10 4 56 Ni/s were delivered to the target. For a study of neutron transfer reactions in inverse kinematics, a high-efficiency detection system was built consisting of a large solid angle (2.8 sr), high-granularity Si detector array for measuring the outgoing protons in coincidence with the heavy reaction products identified with respect to mass A and nuclear charge Z in the focal plane of a recoil mass separator.

E2 POLARIZATION CHARGE IN 102SN

The nucleus {sup 102}Sn, which is the lightest Sn isotope with known excited states, was investigated with the {sup 50}Cr({sup 58}Ni,{alpha}2n) reaction using delayed in-beam {gamma}-ray and conversion-electron spectroscopy. The experimental setup was designed to study the decay of {micro}s-long isomeric states by placing {gamma}-ray and electron detectors behind the focal plane of the Fragment Mass Analyzer at the Argonne National Laboratory. A 44 keV conversion-electron line corresponding to the (6{sup +}){yields}(4{sup +}) transition in {sup 102}Sn was identified and a half-life of 0.62{sup +0.43}{sub -0.19} {micro}s was measured for the (6{sup +}) state. From the available experimental information neutron effective charges from 1.6 to 2.3 e were deduced, based on the comparison with different shell-model calculations.

Relative spins and excitation energies of superdeformed bands in {sup 190}Hg: Further evidence for octupole vibration

An experiment using the Eurogam phase II {gamma}-ray spectrometer confirms the existence of an excited superdeformed (SD) band in {sup 190}Hg and its very unusual decay into the lowest SD band over 3--4 transitions. The energies of the transitions linking the two SD bands have been firmly established, and their angular distributions are consistent with a dipole character. Comparisons with calculations using random-phase approximation indicate that the excited SD band can be interpreted as an octupole-vibrational structure.

Spectroscopy of193,195,197Po

Excited states built on the 13/2{sup +} isomers of the odd-mass {sup 193,195,197}Po isotopes have been observed via in-beam {gamma}-ray spectroscopy. The {alpha} radioactivity of these isotopes has been used to tag {gamma}-ray transitions following the {sup A}Er+164 MeV {sup 32}S reactions, where A=164, 166, 167, 168, and 170. Prompt {gamma} radiation was measured by ten Compton-suppressed Ge detectors at the target position and the Fragment Mass Analyzer was used to select evaporation residues. The results are compared with the first excited states of the heavier odd-mass polonium isotopes and of the even-mass cores. {copyright} {ital 1997} {ital The American Physical Society}

Superdeformed shapes and configurations in thallium nuclei

Superdeformation studies in {sup 189,191,192}Tl at Gammasphere are discussed. New results on {sup 191}Tl are the observation of interband transitions between the signature partner E2 bands and the measurement of an average quadrupole moment (Q{sub 0} = 18 {+-} 1 eb) for these superdeformed structures. These data enable them to derive absolute M1 strengths and confirm that the 81st proton occupies the [642]5/2 orbital. Tentative data for one superdeformed sequence in {sup 189}Tl are consistent with the prediction of a decrease in quadrupole deformation for the lightest nuclei at the limit of this island of superdeformation.