J. Uusitalo

Proton radioactivity from highly deformed nuclei.

Proton emission half-lives are calculated within the DWBA formalism for {sup 131}Eu and {sup 141}Ho assuming permanent quadruple deformation. The decay rates are consistent with a decay from either [411 3/2] or [413 5/2] Nilsson states for {sup 131}Eu and [523 7/2] Nilsson state for {sup 141}Ho.

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.

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.

Experiments with radioactive beams at ATLAS

Various beams of short- and long-lived radioactive nuclei have recently been produced at the ATLAS accelerator at Argonne National Laboratory, using either the so-called In-Flight or the Two-Accelerator method. The production techniques, as well as recent results with 44Ti(T1/2=60y) and 17F(T1/2=64s) beams, which are of interest to nucleosynthesis in supernovae and X-ray bursts, are discussed.

Proton and α radioactivity of Bi

Proton and {alpha} emission from {sup 185}Bi have been confirmed and measured with improved statistics. The {sup 185}Bi nuclei were produced via the {sup 95}Mo({sup 92}Mo,pn) reaction at a bombarding energy of 420 MeV. The proton decay energy from the 1/2{sup +} intruder state in {sup 185}Bi to the {sup 184}Pb ground state was measured to be 1.598(16) MeV with a proton branching ratio b{sub p}=0.85(6). An {alpha} decay branch from the same state was measured, b{sub {alpha}}=0.15(6), with an energy of 8.08(3) MeV. The state has a half-life of 50(8) {mu}s. In addition, the {alpha} branching ratio of the ground state of {sup 184}Pb was determined for the first time to be b{sub {alpha}}=0.23(14).

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.

Spectroscopy of the proton emitter109I

Excited states in the proton-unbound nucleus {sup 109}I were populated using the {sup 54}Fe({sup 58}Ni,p2n) reaction at a beam energy of 220 MeV. Gamma rays in {sup 109}I were identified using the recoil-decay tagging technique. The analysis of proton-correlated {gamma}{gamma} coincidence data produced the yrast decay sequence in {sup 109}I, which can be tentatively assigned as built on the h{sub 11/2} proton state based on systematic trends in the neighboring isotopes. This sequence is completely different from that reported previously. A comparison of the h{sub 11/2} band in {sup 109}I with those in heavier iodines shows that {sup 109}I continues the trend of decreasing quadrupole deformation with decreasing neutron number. {copyright} {ital 1999} {ital The American Physical Society}

Unsafe coulomb excitation of 240–244Pu

The high spin states of 240Pu and 244Pu have been investigated with GAMMASPHERE at ATLAS, using Coulomb excitation with a 208Pb beam at energies above the Coulomb barrier. Data on a transfer channel leading to 242Pu were obtained as well. In the case of 244Pu, the yrast band was extended to 34ℏ, revealing the completed πi13/2 alignment, a “first” for actinide nuclei. The yrast sequence of 242Pu was also extended to higher spin and a similar backbend was delineated. In contrast, while the ground state band of 240Pu was measured up to the highest rotational frequencies ever reported in the actinide region (∼300 keV), no sign of particle alignment was observed. In this case, several observables such as the large B(E1)/B(E2) branching ratios in the negative parity band, and the vanishing energy staggering between the negative and positive parity bands suggest that the strength of octupole correlations increases with rotational frequency. These stronger correlations may well be responsible for delaying or supp...

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.