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The position of the K 17 2 shell orbital plays a significant role in determining the shell correction energy and hence the stability of the superheavy elements. A deformed component of this orbital, 1 2 + [880] , comes down in energy with increasing prolate deformation, and is expected to have an excitation energy of less than 2 MeV in nuclei with N = 153. We have performed the reaction 248Cm(4He,3He) using 98.5-MeV alpha particles from the IUCF cyclotron to populate high-l states in 249Cm. this reaction preferentially excites high-l states because of the angular momentum mismatch. The emerging 3He ions were momentum analyzed with the K600 magnetic spectrometer. Spectra were measured at θcm = 4.1°, 6.1°, 10.2°, 12.2°, and 16.2° The spectrometer was calibrated with the 208Pb(4He,3He) and 232Th(4He,3He) reactions at the same incident energy. Three strong peaks were identified in the 248Cm(4He,3He) spectrum and these have been given tentative assignments: 593 keV, 15 2 11 2 − [725] ; 1560 keV and 1898 keV, 17 2 1 2 + [880] , 15 2 13 2 − [716] . Our data do not distinguish between the 17 2 + and 15 2 − assignments.

High-spin states in {sub 94}{sup 240}Pu and {sub 96}{sup 248}Cm have been populated with Coulomb excitation at beam energies slightly above the Coulomb barrier. In each nucleus a new side band has been identified in {gamma}-{gamma} coincidence measurements and has been interpreted as the lowest-lying octupole excitation. The alignments and Routhians of these bands, as well as the branching ratios of E1 decays to the ground-state bands, are in good agreement with results from cranked RPA calculations. {copyright} {ital 1998} {ital The American Physical Society}

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.

Prompt γ radiation following spontaneous fission has been measured using the EUROGAM 2 array. Using these data and newly developed analysis techniques a number of subjects were studied, which are briefly discussed. One of these subjects, shape transition in A≈100 Sr and Zr nuclei is discussed in more detail. Quadrupole moments were determined for rotational bands in the N=58,59,60,62 and 64 Sr and Zr isotopes. Deformation parameters, which increase from β2 ≈ 0.1 at N=56 to β2 ≈ 0.4 at N=64, suggest that in strontium and zirconium isotopes shape change happens gradually between N=56 and N=62 and is probably due to an occupancy of three or more deformation-driving orbitals of h11/2 parentage.

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.

The Eurogam-2 array has been used to study {gamma} rays emitted following the spontaneous fission of a {sup 248}Cm source. Analysis of Doppler-broadened line shapes corresponding to decays from excited rotational states in neutron-rich Sr, Zr, and Mo fission fragments has enabled the first measurements of state lifetimes in these nuclei at around {ital I}=10. The results are consistent with a predicted rotation-induced change in the triaxiality of the yrast states of the Mo isotopes that contrasts with the more stable behavior observed in the Zr and Sr nuclei. {copyright} {ital 1996 The American Physical Society.}

The level structures of the neutron-rich {sup 104}Mo and {sup 108}Mo nuclei have been investigated by observing prompt {gamma} rays emitted in the spontaneous fission of {sup 248}Cm with the EUROGAM spectrometer. Levels with spins up to 12{h_bar} have been observed and {gamma} branching obtained. The data can be satisfactorily described when {sup 104,108}Mo are considered as axially symmetric nuclei: in {sup 104}Mo, rotational bands based on the ground state, the one-phonon and the two-phonon {gamma}-vibrational states and a quasiparticle state have been observed, whereas in {sup 108}Mo the information is limited to the yrast band and the one phonon {gamma} band. {copyright} {ital 1996 The American Physical Society.}

Rotational bands based on two-quasi-neutron excitations have been observed for the first time in neutron-rich isotopes. The bandhead excitation energies have been used within the BCS model to determine the strength {ital G} of the neutron pairing interaction. Values of {ital G}=0.21 and 0.19 MeV have been obtained for {sup 100}Zr and {sup 102}Zr, respectively. For these neutron-rich nuclei the strength {ital G} is thus close to 20/{ital A} MeV, smaller than the value 23/{ital A} MeV used to describe even-even nuclei near stability in this region.

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.

The neutron-rich ${}^{106}$Mo nucleus has been studied by prompt \ensuremath{\gamma}-ray spectroscopy following the spontaneous fission of ${}^{248}$Cm. The characteristics of the rotational band built on a state at 1435 keV identifies the band head as the best candidate for a harmonic double-phonon ${K}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4}^{+}$ \ensuremath{\gamma}-vibrational state observed so far.

This paper reports a new synthesis of pure, microcrystalline Am(OH){sub 3}, its characterization by x-ray powder diffraction and infrared spectroscopy, and the calorimetric determination of its enthalpy of solution in dilute hydrochloric acid. From the enthalpy of solution the enthalpy of formation of Am(OH){sub 3} has been calculated to be {minus}1371.2{plus_minus}7.9 kj{center_dot}mol{sup {minus}1}, which represents the first experimental determination of an enthalpy of formation of any actinide hydroxide. The free energy of formation and solubility product constant of Am(OH){sub 3} (K{sub sp} = 7 {times} 10{sup {minus}31}) have been calculated from our enthalpy of formation and entropy estimates and are compared with literature measurements under near-equilibrium conditions. Since many properties of the tripositive lanthanide and actinide ions (e.g., hydrolysis, complex-ion formation, and thermochemistry) change in a regular manner, these properties can be interpreted systematically in terms of ionic size. This paper compares the thermochemistry of Am(OH){sub 3} with thermochemical studies of lanthanide hydroxides. A combined structural and acid-base model is used to explain the systematic differences in enthalpies of solution between the oxides and hydroxides of the 4f{sup n} and 5f{sup n} subgroups and to predict solubility-product constants for the actinide(III) hydroxides of Pu through Cf.

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.

We have assigned [gamma]-ray transitions to [sup 136]Te by examining the prompt gamma radiation following spontaneous fission of [sup 248]Cm. The transitions were identified in spectra gated on the Ru complementary fragments, and confirmed by studying prompt [gamma] rays following spontaneous fission of [sup 252]Cf. The results are examined in terms of the systematics of other tellurium and [ital N]=84 nuclei.

Excited levels of {sup 141}Xe, populated in spontaneous fission of {sup 248}Cm, were studied by means of prompt {gamma}-ray spectroscopy, using the EUROGAMM2 array. Level scheme of {sup 141}Xe obtained in this work shows patterns characteristic of simplex symmetry with s = +i and s = -i bands present but low value of D{sub 0} moment indicates that octupole correlations in Xe isotopes are systematically lower than in Ba nuclei.

Rotational bands up to spin of ≈10 h have been studied in even-even and odd- A nuclei in the mass 100 region, by observing prompt γ-rays from the spontaneous fission of 248 Cm. Transitions in the neutro-rich nuclei 103 Zr, 104 Zr, 107 Mo and 108 Mo have been observed for the first time, and evidence for a band in 105 Nb is presented. Rotational bands are also reported in 98–100 Sr, 99,101 Y, 100–102 Zr, 101,103 Nb and 102–106 Mo. In most cases these bands have been extended to much higher spins than previously known. The data on the even-even nuclei provide information on deformation trends in this region. The level structures of the odd- A nuclei indicates which Nilsson orbitals lie close to the Fermi level. Bands based on the 5 2 + [422] and 5 2 − [303] configurations are identified in the odd- A nuclei with Z = 39 and 41. Evidence is presented that certain bands observed in N = 61 and 63 nuclei are built on the 5 2 − [532] configuration, and the role of the neutron h 11 2 orbital in stabilizing the deformation in this region is discussed.

Excited levels in 133Sn and 134Sb, populated in spontaneous fission of 248Cm, were studied by means of prompt-γ spectroscopy, using the EUROGAM 2 multidetector array. The neutron i13/2 single-particle energy has been determined for the first time in the 132Sn region. The energy of the h9/2 neutron level, proposed previously, has been confirmed.

Excited states in the neutron-rich nucleus {sup 142}Xe have been determined by observing prompt {gamma} rays in fragments from {sup 248}Cm spontaneous fission. Estimates of the lowest excited-state energies in {sup 142}Xe based on normal systematics differ considerably from those derived from the {ital N{sub p}N{sub n}} scheme. Experimental values are in good agreement with the {ital N{sub p}N{sub n}} scheme and illustrate its predictive power in this mass region.

Most of the superconducting oxides are nonstoichiometric. The range of homogeneity varies, but in YBa2Cu3O7-δ it is extensive, 0 ≤ δ ≤ 1, so that the phase can serve as a prototype to illustrate the roles of chemical equilibria among the nominal valence states and defects in predicting the normal state properties. Through models which are essentially nearest-neighbor lattice statistics in the Hamiltonian, one can describe quantitatively the variation of the partial pressure of oxygen with temperature and composition. And one can predict the role of localized thermodynamic equilibrium in the transport of electrical charge.

Partial decay schemes in the neutron-rich nuclei $^{103}\mathrm{Zr}$ and $^{104}\mathrm{Zr}$ have been measured for the first time and rotational bands in $^{100--102}\mathrm{Zr}$ have been extended to spins of up to 10\ensuremath{\Elzxh} by observing prompt \ensuremath{\gamma} rays from the spontaneous fisson of $^{248}\mathrm{Cm}$. These nuclei are among the most deformed known at low spin and excitation energy. The level structures in the odd-A nuclei show that the ${\mathit{h}}_{11/2}$ intruder orbital plays an important role in stabilizing the deformation in this region.

In this chapter we have shown that the thermochemistry of the rare-earth oxides is well established for the majority of the stoichiometric compounds. The thermodynamic properties follow clear trends that can be understood in terms of valence states and electronic configurations of the lanthanide ions and metals. For the sesquioxides, the principal group of rare earth compounds, the data are reliable up to 2000 K, an interval in which the A, B and C phases are stable. However, for the high-temperatures, where the H and X phases are stable, no experimental data exist.

As part of the spent fuel treatment program at Argonne National Laboratory (ANL), a ceramic waste form is being developed for disposition of the salt waste stream generated during the treatment process. Ceramic waste form (CWF) degradation and radionuclide release modeling is being carried out for the purpose of estimating the impact of the CWF on the performance of the proposed repository at Yucca Mountain. The CWF is composed of approximately 75 wt% salt-loaded sodalite encapsulated in 25 wt% glass binder. Most radionuclides are present as small inclusion phases in the glass. Since the release of radionuclides can only occur as the glass and sodalite phases dissolve, the dissolution rates of the glass and sodalite phases are modeled to provide an upper bound to radionuclide release rates from the CWF. Transition-state theory for the dissolution of aluminosilicate minerals provides a mechanistic basis for the CWF degradation model, while model parameters are obtained by experimental measurements. Performance assessment calculations are carried out using the engineered barrier system model from the Total System Performance Assessment--Viability Assessment (TSPA-VA) for the proposed repository at Yucca Mountain. The analysis presented herein suggests that the CWF will perform in the repository environment in a manner thatmore » is similar to other waste forms destined for the repository.« less

Tests were conducted with ceramic waste form (CWF) materials that contained small amounts of uranium and plutonium to study their release behavior as the CWF corroded. Materials made using the hot isostatic press (HIP) and pressureless consolidation (PC) methods were examined and tested. Four different materials were made using the HIP method with two salts having different U:Pu mole ratios and two zeolite reagents having different residual water contents. Tests with the four HIP U,Pu-loaded CWF materials were conducted at 90 and 120 C, at CWF-to-water mass ratios of 1:10 and 1:20, and for durations between 7 and 365 days. Materials made using two PC processing conditions were also tested. Tests with the two PC U,Pu-loaded CWF materials were conducted at 90 and 120 C, at a CWF-to-water mass ratio of 1:10, and for durations between 7 and 182 days. The releases of matrix elements, U, and Pu in tests conducted under different test conditions and with different materials are compared to evaluate the effects of composition and processing conditions on the release behavior of U and Pu and the chemical durabilities of the different materials. The distributions of released elements among the fractions that were dissolved, in colloidal form in the solution, and fixed to test vessel walls were measured and compared. Characterization of Pu-bearing colloidal particles recovered from the test solutions using solids analysis techniques are also reported. The principal findings from this study are: (1) The release of U and Pu is about 10X less than the release of Si and 50X less than the release of B under all test conditions. This implies that U and Pu are in a phase that is less soluble than the sodalite and binder glass matrix. (2) Almost all of the plutonium that is released from U,Pu-loaded CWF is present either as colloidal-sized particles in the size range between 5 and 100 nm in the test solution (about 15% of the total) or becomes fixed on stainless steel test vessel walls (about 85% of the total). Work is in progress to determine the nature of the material fixed to the steel. (3) The Pu in the colloidal particles is present as the dioxide PuO{sub 2} or a mixed oxide (U,Pu)O{sub 2}. These particles are of dimensions and composition similar to the (U,Pu)O{sub 2} particles that are present as inclusions in the CWF. They are often found associated with larger (100-nm size) aluminosilicate colloids. (4) The degradation behaviors of U,Pu-loaded CWF processed by HIP and PC are the same with regard to the release of Si, U, and Pu. One difference is that B is released faster from the HIP CWF materials than from the PC CWF materials. (5) The presence of U and Pu does not affect the durability of CWF materials.

Excited states in ${}^{139}\mathrm{Xe},$ populated in the spontaneous fission of ${}^{248}\mathrm{Cm},$ were studied by means of prompt-$\ensuremath{\gamma}$ spectroscopy, using the EUROGAM2 multidetector array. Spins and parities of excited levels were determined experimentally. New information was also obtained for the ${}^{141}\mathrm{Ba}$ nucleus. These data and a reevaluation of the data available in the literature on ${}^{141}\mathrm{Ba}$ and ${}^{143}\mathrm{Ce}$ show that the near-yrast structures of the ${}^{139}\mathrm{Xe},$ ${}^{141}\mathrm{Ba},$ and ${}^{143}\mathrm{Ce}$ nuclei are similar to that in the heavier $N=85$ isotones. The presence of strong octupole correlations in the ${}^{139}\mathrm{Xe}$ and ${}^{141}\mathrm{Ba}$ nuclei, reported in another recent study, is not supported by the present work. The observed excitations in the $N=85$ isotones are interpreted as being due to quadrupole and octupole vibrations coupled to the valence-neutron levels.

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 states in ${}^{139}\mathrm{Te}$ were observed for the first time, using EUROGAM2 multidetector array to measure prompt-$\ensuremath{\gamma}$ radiation following the spontaneous fission of ${}^{248}\mathrm{Cm}.$ The systematic behavior of excitation energies in the $N=87$ isotones and the multipolarity measurements suggest spin ${7/2}^{\ensuremath{-}}$ for the ground state and ${9/2}^{\ensuremath{-}}$ for the 271.0 keV level in ${}^{139}\mathrm{Te}.$ The 271.0 keV level most likely corresponds to the $\ensuremath{\nu}{h}_{9/2}$ single-particle excitation. A shape transition from spherical to prolate Te isotopes is observed at the neutron number $N=87,$ in accord with the Hartree-Fock plus BCS, PES calculations. Predictions for excitations in the ${}^{137}\mathrm{Sn}$ nucleus are made based on the extended systematics obtained for the $N=87$ isotones.

Excited states in the nucleus {sup 133}Sb, populated in spontaneous fission of {sup 248}Cm, were studied with EUROGAM2. Medium-spin structure, described as the {nu}(f{sub 7/2}h{sub 11/2}{sup -1}) multiplet of the {sup 132}Sn core coupled to the odd proton in the g{sub 7/2} orbital, has been identified in this nucleus. Levels corresponding to the octupole excitations of the {sup 132}Sn core were also identified. Some uncertainties concerning isomeric decays in {sup 133}Sb, observed in previous works, have been resolved. (c) 2000 The American Physical Society.

Excited states in ${}^{137}\mathrm{Te},$ populated in spontaneous fission of ${}^{248}\mathrm{Cm},$ were studied by means of prompt-$\ensuremath{\gamma}$ spectroscopy, using the EUROGAM2 multidetector array. This is the first observation of excited states in ${}^{137}\mathrm{Te}.$ The yrast excitations of ${}^{137}\mathrm{Te}$ are due to the three valence neutrons, occupying the $\ensuremath{\nu}{f}_{7/2}$ and $\ensuremath{\nu}{h}_{9/2}$ orbitals, similarly as observed in its heavier $N=85$ isotones. Systematic comparison of excited levels in the $N=85$ isotones shows inconsistencies in spin and parity assignments in ${}^{139}\mathrm{Xe}$ and ${}^{141}\mathrm{Ba}$ nuclei. The new data for ${}^{137}\mathrm{Te}$ do not confirm earlier suggestions that octupole correlations increase in the $N=85$ isotones, close to the $Z=50$ closed shell.

Excited states in the neutron-rich, N=84 nuclei 134Sn, 136Te and 138Xe, populated in the spontaneous fission of 248Cm, were studied to medium spins using the EUROGAM2 array. OXBASH code calculations support the experimental identification of maximum aligned configurations in these isotopes. Empirical shell model calculations agree with the proposed excitation energy of the neutron h9/2 excitation in the 132Sn region. A discrepancy between the observed and calculated excitation energy of the Iπ= 12+ level in 136Te indicates possible admixtures of collective excitations in this nucleus. Clear signs of collective excitations are observed in 138Xe.

Neutron-rich, 88-94Kr nuclei, populated in spontaneous fission of 248Cm, have been studied with EUROGAM 2, by measuring prompt γ-rays. Many new excited states in even-even Kr isotopes have been identified. For the first time spins and parities were determined experimentally in these nuclei. Our results indicate that the quadrupole deformation of Kr isotopes will appear only above N = 58, as observed in Sr and Zr nuclei. The newly found 3- level at 1506.4 keV in 90Kr suggests the exsistence of a new region of increased octupole correlations, probably associated with the ν(d 5/2 h 11/2) pair of Δl = Δj = 3 orbitals.

The very neutron-rich nucleus 138Te produced in the spontaneous fission of 248Cm has been investigated using the EUROGAM II γ-ray multidetector array. The excited states of 138Te observed for the first time indicate that the region of ground state deformation beyond the doubly magic nucleus 132Sn has yet not been reached.

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.

The authors have performed the reaction {sup 248}Cm({sup 4}He, {sup 3}He) using 98.5-MeV alpha particles from the IUCF cyclotron to populate high-j states in {sup 249}Cm. A tentative assignment of the K{sub 17/2} component of the 1/2{sup +}[880] Nilsson state has been made.

Prompt {gamma}-ray cascades in neutron-rich nuclei around doubly magic {sup 132}Sn have been studied at Eurogam II using a {sup 348}Cm fission source. Here they report results for the four-valence-proton N = 82 nucleus {sup 136}Xe and for its N = 83 neighbor {sup 137}Xe. For both nuclei, the yrast level spectra have been considerably extended, and empirical nucleon-nucleon interactions have been used to assign probable shell model configurations for most of the observed levels.

Neutron-rich, N=84 nuclei from the 132Sn region, populated in spontaneous fission of 248Cm, have been studied with EUROGAM 2. Excited states and their spins and parities in the 136Te nucleus were established up to 17ħ. OXBASH code calculations support the experimental identification of maximum aligned configurations in 136Te. Empirical shell model calculations for the Iπ= 14+ level in 136Te indicate that the adopted mass of the 134Te nucleus should be lowered by 200(80) keV.

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.

The nuclei 143,145,147 Ba produced in the spontaneous fission of 248 Cm have been studied using the EUROGAM phase II array. The parities and multipolarities of γ rays in these nuclei have been assigned from triple angular correlation, directional linear-polarisation correlation and internal conversion coefficient measurements. These data confirm an alternating-parity sequence connected by strong electric dipole transitions in 143 Ba. They also reveal a more weakly populated, complementary alternating-parity sequence. The two sequences together exhibit some characteristics of parity-doublet bands. Interpreting the E1 strengths in terms of a rotating intrinsic dipole moment gives a moment similar in size to that observed in 142 Ba and 144 Ba. No well-developed similar structure is seen in 145 Ba, although E1 transitions are observed, from which a value of the dipole moment is obtained. This value is smaller than the moment observed in 143 Ba. No features appropriate to strong octupole correlations were observed in 147 Ba.

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.

Partial decay schemes for the very neutron-rich nuclei 108,110,112 Ru and 114 Ru, about which nothing was previously known, have been determined from the study of prompt, triple-γ coincidences in 248 Cm fission fragments. Levels with probable spin up to 10ħ have been observed and γ-ray branching ratios obtained. Ratios of reduced electric quadrupole transition probabilities obtained from the branching ratios are in good agreement with predictions of a simple model of a rigid triaxial rotor.