Search

$\ensuremath{\beta}$-decay half-lives of new neutron-rich isotopes ${}^{167}\mathrm{Tb}$ and ${}^{168}\mathrm{Tb}$ produced in the 20 MeV proton-induced fission of ${}^{238}\mathrm{U}$ have been determined to be 19.4(27) s and 8.2(13) s, respectively, using a gas-jet coupled on-line isotope separator. The present half-lives and those of the recently identified nuclei ${}^{159}\mathrm{Pm},$ ${}^{161}\mathrm{Sm},$ ${}^{165}\mathrm{Gd},$ ${}^{166}\mathrm{Tb}$ were compared with theoretical predictions. The recent calculation by the gross theory with the new one-particle strength functions shows quite good agreement with the experimental half-lives. Excited states of the daughter nuclides ${}^{167}\mathrm{Dy}$ and ${}^{168}\mathrm{Dy}$ have been established for the first time. Level energies of the first ${2}^{+}$ states in even-even Dy isotopes were found to show an irregular behavior at ${}^{164}\mathrm{Dy}$ and increasing deformation toward the neutron midshell around $N\ensuremath{\approx}104$.

We have measured in-beam {gamma} rays in the neutron-rich {sup 246}Pu{sub 152} and {sup 245}Pu{sub 151} nuclei by means of {sup 244}Pu({sup 18}O, {sup 16}O){sup 246}Pu and {sup 244}Pu({sup 18}O, {sup 17}O){sup 245}Pu neutron transfer reactions, respectively. The {gamma} rays emitted from {sup 246}Pu ({sup 245}Pu) were identified by selecting the kinetic energy of scattered {sup 16}O ({sup 17}O) detected by Si {delta}E-E detectors. The ground-state band of {sup 246}Pu was established up to the 12{sup +} state. We have found that the shell gap of N=152 is reduced in energy with decreasing atomic number by extending the systematics of the one-quasiparticle energies in N=151 nuclei into those in {sup 245}Pu. This reduction of the shell gap clearly affects the 2{sup +} energy of the ground-state band of {sup 246}Pu.

The ground-state bands of the neutron-rich $^{236}\mathrm{Th}$ and $^{242}\mathrm{U}$ nuclei were established up to spin 10 and 8, respectively, by in-beam \ensuremath{\gamma}-ray spectroscopy using the ($^{18}\mathrm{O}$, $^{20}\mathrm{Ne}$) two-proton pickup reaction with a $^{238}\mathrm{U}$ and a $^{244}\mathrm{Pu}$ target. Deexcitation \ensuremath{\gamma} rays in $^{236}\mathrm{Th}$ and $^{242}\mathrm{U}$ were identified by selecting the kinetic energies of $^{20}\mathrm{Ne}$ using Si $\ensuremath{\Delta}E\text{\ensuremath{-}}E$ detectors. The excitation energies of the first ${2}^{+}$ states in U and Pu isotopes have local minima at $N\ensuremath{\simeq}146$, suggesting the possibility that nuclei with $Z\ensuremath{\simeq}92$ have a spherical shell closure of $N=164$. Calculation using the Koura-Yamada single-particle potential gives an energy gap of 1.8 MeV at $N=164$ for $^{256}\mathrm{U}$.

Alpha decays of $^{237}\mathrm{Cm}$ and $^{238}\mathrm{Cm}$ have been studied using a gas-jet coupled on-line isotope separator. The new isotope $^{237}\mathrm{Cm}$ has been identified through the detection of $6656\ifmmode\pm\else\textpm\fi{}10$ keV \ensuremath{\alpha} particles. The \ensuremath{\alpha} energy of $^{238}\mathrm{Cm}$ has been revised more precisely than the previous one. The \ensuremath{\alpha} transition to the first excited ${2}^{+}$ state in $^{234}\mathrm{Pu}$ has also been observed. It was found that the ${2}^{+}$ energy in $^{234}\mathrm{Pu}$ is much higher than those in heavier Pu isotopes.

{alpha}-{gamma} and {alpha}-electron coincidence spectroscopy for a short-lived heavy actinide nucleus {sup 257}No (T{sub 1/2}=24.5 s) has been performed using a gas-jet transport system and an on-line isotope separator. Spin-parities of excited states in {sup 253}Fm fed by the {alpha} decay of {sup 257}No have been identified on the basis of the measured internal conversion coefficients. The {nu}3/2{sup +}[622] configuration has been assigned to the ground state of {sup 257}No as well as to the 124.1 keV level in {sup 253}Fm. It was found that the ground-state configuration of {sup 257}No is different from that of lighter N=155 isotones.

We measured, for the first time, the mass distribution of ${}^{240}\mathrm{Pu}$ fission fragments following the \ensuremath{\beta}-vibrational resonance, whose level is formed on the second minimum of the double-humped fission barrier. The distribution shows an asymmetric mass distribution similar to the one observed for thermal neutron-induced fission of ${}^{239}\mathrm{Pu}$ and isomeric fission of ${}^{240}\mathrm{Pu}.$ This indicates that the ${}^{240}\mathrm{Pu}$ system following the \ensuremath{\beta}-vibrational resonance descends into a fission valley which is identical to the fission valley of the ${}^{240}\mathrm{Pu}$ isomer and ${}^{239}\mathrm{Pu}{(n}_{\mathrm{th}},f).$

Decay spectroscopy using on-line mass separators has the advantage of allowing the study of level properties in the low energy region, including band head information, since g transitions between low-spin states can be measured more intensively under lower background conditions than can those with in-beam spectroscopy measurements. Neutron-deficient nuclides of 125,127 Ba have been studied by means of in-beam spectroscopy, and the level structures for high-spin states were successfully interpreted within the framework of the IBFM model @1‐5#. Decay studies of these nuclides have rarely been reported. Therefore, the last evaluations in Refs. @6, 7# were mainly based on in-beam studies. In 1991, the possibility of static octupole deformation, similar to that of the A5145 and 225 regions proposed, was proposed for the A5130 region by Cottle @8#. The presence of static octupole deformation is represented by parity doublets, which have equal spins and opposite parity levels of rotational bands with alternating bands. The E1 transitions between parity doublets are characterized by a two to four orders of magnitude enhancement compared to those of more normal cases. The 127‐130 Ba isotopes were successively studied by in-beam conversion electron measurements to investigate for parity doublets, and no evidence of them was observed @9,10#. These studies, however, focused only on the conversion electron measurements; the transition probabilities were not measured. It is necessary to measure not only the conversion electrons, but also the half-lives of the excited states in order to explain this feature. The aim of this investigation was to study the level properties of 125,127 Ba in the low energy region, focusing on a search for E1 transitions, and to study their properties by measuring the half-lives of low-lying excited states. The half-lives and conversion electrons were measured for the first time through the decays of 125,127 La, by means of a delayed coincidence technique and a cooled Si~Li! detector, respectively. The expected enhanced E1 transitions were not observed. Level properties based on the Nilsson model are proposed.