[en] Beta-delayed fission (βDF) is a rare decay process which couples beta decay and fission. In this two-step process, the parent nucleus undergoes beta decay populating an excited state in the daughter nucleus close to or above the fission barrier. The daughter nucleus may then fission in competition with gamma decay to the ground state. βDF is ideally suited to study low-energy fission and is experimentally accessible only in the well-studied uranium region, and lesser studied (by fission) neutron-deficient lead region. A recent study of the βDF of ¹⁸⁰Tl showed an unexpected asymmetric fission fragment split for ¹⁸⁰Hg instead of the expected symmetric split to semi-magic ⁹⁰Zr. This result identified a new region of asymmetric fission in addition to the well known asymmetry in trans-uranium nuclei

[en] In the last two decades, through technological, experimental and theoretical advances, the situation in experimental low-energy fission studies has changed dramatically. With the use of advanced production and detection techniques, much more detailed fission information can be obtained for traditional regions of fission research and, very importantly, new regions of nuclei have become accessible for fission studies. This contribution will review the recent fission experiments in very neutron-deficient nuclei in the lead region, spanning from Pt to Bi isotopes. Three complementary methods of fission studies in this region will be discussed: low-energy beta-delayed fission and Coulex-induced fission, and higher-energy fusion-fission reactions induced by charged particles. This document is made up of an abstract and the slides of the presentation. (author)

[en] The contribution reviews the results of our recent experiments at SHIP (GSI, Darmstadt) aimed at the study of very neutron-deficient Pb-Rn nuclides in the vicinity of N=104. The first part presents the unambiguous identification of the EC-delayed fission (ECDF) in the odd-odd isotopes ^192,194At. The ECDF data allow us to study the fission properties of the nuclei which do not decay via spontaneous fission at all. The preliminary analysis indicate unusually high ECDF probabilities for these nuclides. The second part discusses the new data on the shape coexistence in the Pb region. Identification of the new isotopes ^193,194Rn and first experimental evidence for the long-sought ground state deformation in lightest Rn isotopes will be presented. The strong retardation (by a factor of ∝10) of the half-life of the new isotope ¹⁸⁶Po relative to the classical Geiger-Nuttall rule for alpha decay will be also discussed. This indicates that the Geiger-Nuttall rule is broken in the lightest Po isotopes

[en] Beta-delayed fission is a rare process in which the beta-decaying precursor populates relatively low-excited state in its decay daughter, which may then fission. This allows to study low-energy fission properties e.g. the isospin dependence of the fission barriers. It is currently believed that the beta-delayed fission process is crucial for understanding the r-process path and for the production of the heaviest elements. In the presentation, the recent experiments performed at the velocity filter SHIP (GSI) and at the mass-separator ISOLDE (CERN) will be discussed. In these experiments, the ECDF decay was unambiguously observed in several very neutron-deficient nuclides in the Pb region (^192,194At and ¹⁸⁰Tl). ECDF probability and total kinetic energy were determined. Surprisingly, an asymmetric mass distribution of fission fragments was observed in ECDF of ¹⁸⁰Tl. Preliminary analysis also shows that the cold fission (no neutron emission) might be the main decay channel in the ECDF decay of this nucleus.

[en] Detailed α-decay studies of the neutron-deficient isotope ¹⁹⁴At have been performed in the complete fusion reaction ⁵⁶Fe+¹⁴¹Pr→¹⁹⁴At+3n at the velocity filter SHIP. Two α-decaying isomeric states with half-lives of T_1/2(¹⁹⁴At^m1)=310(8) ms and T_1/2(¹⁹⁴At^m2)=253(10) ms were identified in this nucleus. Their complex decays to the states in the daughter nucleus ¹⁹⁰Bi are discussed in the article. We propose that similar to the case of the neighboring ^{191,192,193,195}At isotopes, the oblate-deformed configurations based on the proton 1/2⁺[440] and/or 7/2^-[514] Nilsson orbitals become important in ¹⁹⁴At. A new isomeric state with the half-life of 175(8) ns was observed in ¹⁹⁰Bi.

[en] A He-filled on-line separator system was built at the SuperHILAC and used to study the fusion products in the reaction ⁵⁶Fe + ¹⁴¹Pr. The new neutron-deficient isotopes ¹⁹⁴At and ¹⁹⁵At were produced in this bombardment as three- and two-neutron-out products, respectively, and were identified by the α-α time-correlation technique. The measured α energies and half lives are 7.20 +/- 0.02 MeV and 180 +/- 80 msec for ¹⁹⁴At, and 7.12 +/- 0.02 MeV and 200 +/- 100 msec for ¹⁹⁵At

[en] Nuclear spectroscopic information for known nuclides of mass number 194 (Re,Os,Ir,Pt,Au,Hg,Tl,Pb,Bi, Po,At) with Z = 75 to 85 and N = 119 to 109 have been evaluated and presented together with adopted energies and Jπ of levels in these nuclei. No excited state data are yet available for ¹⁹⁴Re and ¹⁹⁴At. Many superdeformed structures are known in A = 194: three SD bands in ¹⁹⁴Hg, two of which are connected to the normal-deformed structures; six SD bands in ¹⁹⁴Tl; and three SD bands in ¹⁹⁴Pb, one of which is connected to the normal-deformed structure. In addition four magnetic-rotational (MR) dipole bands are known in ¹⁹⁴Pb together with many other dipole bands which are probable multi-quasiparticle structures. This evaluation supersedes previous full evaluations of A = 194 published by 1996Br26, 1989Si01, 1977Ha46 and 1972Au11

[en] Experimental data pertaining to all nuclei with mass A=194 were evaluated. Level schemes from both radioactive decay and reaction studies are presented, along with associated tables of experimental data and adopted properties for levels and γ rays. The literature cutoff date for this revision is September 30, 1996. Extensive high-spin experiments using large gamma-detector arrays have revealed superdeformed structures in ¹⁹⁴Hg (three SD bands: Yrast and pair of excited signature partners). ¹⁹⁴Tl (six SD bands: Three pairs of signature partners), and ¹⁹⁴Pb (three SD bands: Yrast and pair of excited signature partners). For yrast bands (SD-1) in ¹⁹⁴Hg and ¹⁹⁴Pb, connecting transitions between the SD band and normal bands have recently been reported. Several dipole (oblate) bands in ¹⁹⁴Pb, some based on high-K (oblate) states have been reported from similar in-beam γ-ray experiments. New γγ coincidence measurements using the ¹⁹³Ir(n,γ) reaction helped to place several transitions in the level scheme and established the 31.85-ms ¹⁹⁴Ir isomer at 147.078 keV. Results from the first study of the ¹⁹³Ir(d,p) reaction agree with those from (n, γ). Unpublished γγ coincidence results (using HERA, an array of 20 intrinsic germanium detectors) revealed a new level in ¹⁹⁴Pt at 1737.4 keV. It also provided correct placement in the decay scheme for many γ rays from ¹⁹⁴Au ε decay. Recent measurements of the α-decay chain ²⁰²Fr-¹⁹⁸At-¹⁹⁴Bi confirm the existence of ¹⁹⁴Bi isomers, and show population to previously unknown levels at 218 and 396 keV in ¹⁹⁴Bi. 480 refs

[en] During November 1994 and January 1995, systematic mass measurements in the neutron deficient region of Z element of (56..85) were performed at the Experimental Storage Ring ESR of the GSI. The masses of more than 300 different nuclides were measured directly, about 100 of them had not been measured before. We present the method of evaluating atomic masses from the Schottky spectra, its limitations and accuracy. Some of the measured masses are compared to the results of nuclear mass models. As we have measured masses of several isotopes which belong to α-decay chains with well known decay energies, we determined additionally 62 new masses of isotopes in those chains up to the proton drip line. (orig.)