Search

Prompt {gamma}-ray cascades in N=83 fission product nuclei near {sup 132}Sn have been studied at Eurogam II using a {sup 248}Cm source. Cross coincidences observed between {gamma} rays from complementary light and heavy fission fragments were vital for isotopic assignments. Yrast states in the N=83 isotones {sup 134}Sb, {sup 135}Te, and {sup 136}I are reported. The interpretation of the level schemes is based mainly on results of shell model calculations using empirical proton-proton interaction energies from {sup 134}Te, and proton-neutron interactions estimated from the well-known {sup 210}Bi level spectrum. {copyright} {ital 1997} {ital The American Physical Society}

Neutron-rich nuclei ${}^{147,149,151}$Ce have been studied via the prompt $\ensuremath{\gamma}$-ray spectroscopy of ${}^{248}$Cm fission fragments. A partial level scheme for ${}^{151}$Ce is presented for the first time, along with the high-spin structure in ${}^{147,149}$Ce. A rapidly changing yrast structure with neutron number is observed and it is suggested that bands in ${}^{147}$Ce and ${}^{151}$Ce are built on the $\ensuremath{\nu}{i}_{13/2}$ Nilsson state. The level scheme of ${}^{149}$Ce displays two bands with quite similar transition energies. There is no experimental evidence for octupole deformation in any of the three nuclei.

A = 140–148 even-even barium nuclei produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Spins and parities of excited levels have been firmly established from triple-γ angular correlation and direction-polarization correlation measurements. Known band structures have been extended and new bands were found. Band-crossing phenomena have been studied, which suggest termination of an alternating-parity band by particle alignment in 146Ba. The new data for 148Ba confirm predictions of a local decrease of electric dipole moment in 146Ba.

Prompt $\ensuremath{\gamma}$-ray cascades in neutron-rich nuclei around doubly magic ${}^{132}\mathrm{Sn}$ have been studied at Eurogam II using a ${}^{248}\mathrm{Cm}$ fission source. Yrast states to above 5.5 MeV in the two- and three-proton $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}82$ isotones ${}^{134}\mathrm{Te}$ and ${}^{135}\mathrm{I}$ are reported. They are interpreted in terms of valence proton and particle-hole core excitations with the help of shell model calculations employing empirical nucleon-nucleon interactions from both ${}^{132}\mathrm{Sn}$ and ${}^{208}\mathrm{Pb}$ regions. A serious inconsistency in the accepted masses of $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}82$ isotones near ${}^{132}\mathrm{Sn}$ is discovered but not resolved.

Triple angular correlations between γ rays following spontaneous fission of 248 Cm are studied using the EUROGAM array. Results, for transitions in even-even fission-product nuclei, agree with predictions of angular correlation theory. Efficient detector systems combined with new analysis techniques offer for the first time a convenient tool to determine multipolarities of prompt γ transitions in the fission-product nuclei.

An investigation has been made into the population of states in 241Pu following the alpha decay of 245Cm. These studies focused on the weakly fed 12+[631] rotational band. States populated in this band are expected to decay by electromagnetic transitions to the band head 12+ state at an excitation energy of 161.7 keV. This state de-excites to the ground state of 241Pu with a half-life of 0.88 μs. The alpha-particle decay feeding these rotational band members was investigated by performing γ-γ and α-γ coincidence experiments using a 56.3 kBq 245Cm radioactive source. The alpha population of the more strongly populated bands was also investigated, and resulted in two previously observed gamma rays being confidently placed within the 241Pu level scheme. Four gamma rays that were previously unassigned can now be excluded from the alpha decay of 245Cm as a consequence of the present study.

Excited states in the 134Sb nucleus, populated in the β−-decay of 134Sn, have been studied at the mass separator OSIRIS. The 134Sn activity was produced via fast neutron-induced fission of 238U target. A main result was the discovery of a very low-lying first-excited state of 134Sb, at 13 keV, which has led to a strong revision of the level scheme. The new results are compared with different theoretical calculations and with the known data for the analogous neutron and proton two-particle nucleus in the 208Pb region. On the basis of this comparison, the energy of the (πg 7/2νf 7/2)7− isomer is estimated to be about 250 keV, some 100 keV lower than previously reported.

The 137I nucleus, populated in the spontaneous fission of 248Cm has been studied by means of prompt γ-ray spectroscopy using the EUROGAM2 array. Medium-spin yrast excitations in 137I were observed for the first time. The experimental level scheme is compared to the shell model calculation with modified Kuo-Herling interaction. The theoretical predictions differ significantly from the experimental results, indicating that the excitation pattern in 137I deviates significantly from the shell model scheme.

Excited levels in 139Cs, populated in spontaneous fission of 248Cm, have been studied by means of prompt gamma spectroscopy, using the EUROGAMM 2 array. New level scheme of 139Cs, different from the one proposed recently, has been established. To confirm the new scheme, additional study of 139Cs has been performed, where γ-rays following β-decay of 139Xe to 139Cs have been measured at the mass separator OSIRIS.

Neutron-rich, odd-{ital Z} nuclei {sup 145}La and {sup 147}La populated in spontaneous fission of {sup 248}Cm were studied using the EUROGAM array. The experiment indicates the presence of similar octupole correlations in the studied La isotopes as found in the barium core nuclei. Alternating-parity structures were found in {sup 145}La, indicating enhanced octupole correlations, as observed in {sup 144}Ba, while in {sup 147}La octupole effects are weakened by alignment phenomena, as seen in the core nucleus {sup 146}Ba. {copyright} {ital 1996 The American Physical Society.}

A search for octupole deformation in neutron-rich Xe isotopes has been conducted through prompt gammaray spectroscopy of secondary fragments produced in the spontaneous fission of248Cm. The spectrometer consisted of the Eurogam 1 array and a set of 5 LEPS detectors. Level schemes were constructed for Xe isotopes with mass number ranging from 140 to 144 and excited states for143,144Xe nuclei were observed for the first time. None of the level schemes exhibit an alternating parity quasimolecular band, a feature usually expected in nuclei in which octupole correlation effects are strong enough to produce stable octupole deformation. For several isotopes, structures observed in the level schemes are consistent with an octupole softness of the nuclei.

Partial level schemes of the very neutron-rich nuclei 141,143,145 Cs have been determined from the study of γ-γ-γ coincidences in 248 Cm fission fragments. Excited states of 143,145 Cs nuclei were observed for the first time. The possible existence of octupole deformation in the Cs nuclear ground states is discussed.

High-spin structures in the neutron-rich nuclei138Xe and139Xe have been investigated by observing prompt γ -rays from the spontaneous fission of248Cm with the Eurogam spectrometer. The partial level schemes were constructed on the basis of double-, triple- and quadruple-γ coincidences. The ground state band in138Xe has been extended up to 5 MeV excitation energy. The structure of the newly observed medium-spin states in139Xe is similar to the one of heavier N=85 isotones where vf72/3, vh9/2f72/2 and vf72/3 ⊗ 3− multiplets have been observed.

The Eurogam-1 array has been used to study $\ensuremath{\gamma}$ rays emitted following the spontaneous fission of a $^{248}\mathrm{Cm}$ source. Yrast level schemes for the neutron-rich nuclei $^{152,154,156}\mathrm{Nd}$ have been constructed. The stopping of the Nd fragments in the source material leads to Doppler-broadened line shapes for those states that have lifetimes comparable to the stopping time. This paper describes the first measurements of lifetimes of medium-spin states in very neutron-rich nuclei obtained from the analysis of these line shapes. The transition quadrupole moments for the yrast states in the spin range $(10\ensuremath{-}16)\ensuremath{\hbar}$ have been deduced to be 5.88 \ifmmode\pm\else\textpm\fi{} 0.08 e b ($^{152}\mathrm{Nd}$), 5.75 \ifmmode\pm\else\textpm\fi{} 0.10 e b ($^{154}\mathrm{Nd}$), and 5.33 \ifmmode\pm\else\textpm\fi{} 0.16 e b ($^{156}\mathrm{Nd}$).

By analysis of fission product gamma-ray data measured at Eurogam II using a Cm-248 source, yrast levels up to about 2 MeV in the N=84 three-particle nucleus Sb-135 have been identified. These levels are interpreted as pi g(7/2)nu f(7/2)(2) and pi g(7/2)nu f(7/2)h(9/2) states with the help of shell model calculations using empirical nucleon-nucleon interactions.