N. Fotiades

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}

GAMMASPHERE + FMA: A journey beyond the proton drip-line

The majority of experiments performed during the 2-year long stay of GAMMAS-PHERE at the Argonne National Laboratory aimed to study proton-rich nuclei far from the line of stability at and beyond the proton drip-line. A high reaction channel selectivity was required to assign in-beam {gamma}-ray transitions to weakly populated exotic nuclei in the presence of background from strong reaction channels. In many of the experiments this was achieved by using the Argonne fragment mass analyzer to separate heavy-ion fusion-evaporation reaction products from scattered beam and disperse them according to their mass-over-charge-state ratio. For medium mass and heavy a and proton emitters the Recoil-Decay Tagging method was implemented. In-beam {gamma}-ray transitions were observed in several proton emitters between Z=50 and Z=82. Among others, rotational bands were assigned to {sup 141}Ho and {sup 131}Eu. A quadruple deformation of {beta}=0.25(4) was deduced for the ground state in {sup 141}Ho from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7/2{sup {minus}} [523] and 1/2{sup +}[411] configurations were proposed for the ground state and the isomeric state, respectively. Comparison with particle-rotor calculations indicates, however, that {sup 141}Ho may have significant hexadecapole deformation and could be triaxial.

High-spin states in 71As, 72Se, and 72Br

The 16O+58Ni reaction was used to study yrast and non-yrast excitations in 71As, 72Se, and 72Br. High-spin yrast and negative-parity non-yrast bands were observed in 72Se. The f7/2 proton extruder orbital was identified in 71As. The odd-even staggering in the πg9/2νg9/2 decoupled band in 72Br is compared with similar structures in heavier Br isotopes.

Rotational bands in the proton emitters 131Eu and 141Ho

Proton-decay studies led in recent years to the observation of about 20 new proton emitters. Among others, first two cases of highly deformed proton emitters, namely 131Eu and 141Ho, were reported [1]. Also, the first case of proton-decay fine structure was found in 131Eu [2]. In this work the studies of the deformed proton emitters 131Eu and 141Ho were extended to their excited states using in-beam spectroscopic methods. The experiments were performed at the Argonne National Laboratory using the GAMMASPHERE array of Ge detectors coupled to the Fragment Mass Analyzer. Partial results from this study has been published in [3]. There were no data on excited states in 131Eu and 141Ho prior to this work. Nothing is known on their immediate neighbors. The quadrupole deformation calculated by Moller and Nix for 141Ho is about β2 = 0.29. According to the proton-decay rate calculations [4] half-lives of the two know proton-decaying states in 141Ho are consistent with the 7/2−[523] and 1/2+[411] Nilsson orbitals at this deformation. 131Eu is expected to be even more deformed with β2 = 0.33. Its proton-decay rate and branching ratio to the 2+ state are consistent with the 3/2+[411] orbital.

Quasicontinuous Decay of Superdeformed 195Pb

The quasicontinuous decay spectrum of superdeformed excitations in 195Pb has been extracted. The slow increase of intensity as Eγ decreases is consistent with a level density of normal deformed excitations at ∼ 11ℏ with no gap parameter.

In-beam studies of proton emitters using the Recoil-Decay Tagging method

The last five years have witnessed a rapid increase in the volume of data on proton decaying nuclei. The path was led by decay studies with recoil mass separators equipped with double-sided Si strip detectors. The properties of many proton-decaying states were deduced, which triggered renewed theoretical interest in the process of proton decay. The decay experiments were closely followed by in-beam {gamma}-ray studies which extended ones knowledge of high-spin states of proton emitters. The unparalleled selectivity of the Recoil-Decay Tagging method combined with the high efficiency of large arrays of Ge detectors allowed, despite small cross sections and overwhelming background from strong reaction channels, the observation of excited states in several proton emitters. Recently, in-beam studies of the deformed proton emitters {sup 141}Ho and {sup 131}Eu have been performed with the GAMMASPHERE array of Ge detectors and the Fragment Mass Analyzer at ATLAS. Evidence was found for rotational bands in {sup 141}Ho and {sup 131}Eu. The deformations and the single-particle configurations proposed for the proton emitting states from the earlier proton-decay studies were confronted with the assignments deduced based on the in-beam data. It should be noted that the cross section for populating {sup 131}Eu is only about 50 nb, and it represents the weakest channel ever studied in an in-beam experiment.

Odd-even staggering in the {pi}g{sub 9/2} {nu}g{sub 9/2} band in {sup 72}Br

High-spin positive-parity states in {sup 72}Br have been studied using the {sup 16}O+{sup 58}Ni reaction. The {pi}g{sub 9/2}{nu}g{sub 9/2} decoupled band in {sup 72}Br has been observed up to {approximately}10 MeV excitation energy and the expected odd-even staggering has been delineated. A larger signature splitting is observed for this band in {sup 72}Br than in the same collective structures in the heavier {sup 74,76,78}Br. No signature inversion at low spin is observed for this band in {sup 72}Br, in contrast to the heavier isotopes, {sup 74,76,78}Br, in which signature inversion is observed below {approximately}10{h_bar}. The observations are in general agreement with theoretical models in this mass region which predict no signature inversion for nuclei with less than 39 protons and neutrons. {copyright} {ital 1999} {ital The American Physical Society}

Odd-even staggering in the {phi}g9/2vg9/2 band in {sup 72}Br.

High-spin positive-parity states in {sup 72}Br have been studied using the {sup 16}O+{sup 58}Ni reaction. The {pi}g{sub 9/2}{nu}g{sub 9/2} decoupled band in {sup 72}Br has been observed up to {approximately}10 MeV excitation energy and the expected odd-even staggering has been delineated. A larger signature splitting is observed for this band in {sup 72}Br than in the same collective structures in the heavier {sup 74,76,78}Br. No signature inversion at low spin is observed for this band in {sup 72}Br, in contrast to the heavier isotopes, {sup 74,76,78}Br, in which signature inversion is observed below {approximately}10{h_bar}. The observations are in general agreement with theoretical models in this mass region which predict no signature inversion for nuclei with less than 39 protons and neutrons. {copyright} {ital 1999} {ital The American Physical Society}

Odd-even staggering in theπg9/2νg9/2band in72Br

High-spin positive-parity states in {sup 72}Br have been studied using the {sup 16}O+{sup 58}Ni reaction. The {pi}g{sub 9/2}{nu}g{sub 9/2} decoupled band in {sup 72}Br has been observed up to {approximately}10 MeV excitation energy and the expected odd-even staggering has been delineated. A larger signature splitting is observed for this band in {sup 72}Br than in the same collective structures in the heavier {sup 74,76,78}Br. No signature inversion at low spin is observed for this band in {sup 72}Br, in contrast to the heavier isotopes, {sup 74,76,78}Br, in which signature inversion is observed below {approximately}10{h_bar}. The observations are in general agreement with theoretical models in this mass region which predict no signature inversion for nuclei with less than 39 protons and neutrons. {copyright} {ital 1999} {ital The American Physical Society}

Structure and formation mechanism of the transfermium isotope 254No

The ground-state band of the Z=102 isotope 254No has been identified up to spin 14, indicating that the nucleus is deformed. The deduced quadrupole deformation, β=0.27, is in agreement with theoretical predictions. These observations confirm that the shell-correction energy responsible for the stability of transfermium nuclei is partly derived from deformation. The survival of 254No up to spin 14 means that its fission barrier persists at least up to that spin.