Search

The ROmanian array for SPectroscopy in HEavy ion REactions (ROSPHERE) has been designed as a multi-detector setup dedicated to γ-ray spectroscopy studies at the Bucharest 9 MV Tandem accelerator. Consisting of up to 25 detectors (either Compton suppressed HPGe detectors or fast LaBr3(Ce) scintillator detectors) together with a state of the art plunger device, ROSPHERE is a powerful tool for lifetime measurements using the Recoil Distance Doppler Shift (RDDS) and the in-beam Fast Electronic Scintillation Timing (FEST) methods. The array's geometry, detectors, electronics and data acquisition system are described. Selected results from the first experimental campaigns are also presented.

Gamma-Beams at the HIgS facility in the USA and anticipated at the ELI-NP facility, now constructed in Romania, present unique new opportunities to advance research in nuclear astrophysics; not the least of which is resolving open questions in oxygen formation during stellar helium burning via a precise measurement of the 12C(a,g) reaction. Time projection chamber (TPC) detectors operating with low pressure gas (as an active target) are ideally suited for such studies. We review the progress of the current research program and plans for the future at the HI{\gamma}S facility with the optical readout TPC (O-TPC) and the development of an electronic readout TPC for the ELI-NP facility (ELITPC)., Comment: Contribution Symposium on Radiation Measurements and Applications (SORMA XVII), June 11 - June 14 , 2018, University of Michigan, Ann Arbor

The parameter-independent (up to overall scale factors) predictions of the X(5)−β 2,X(5)−β 4, and X(3) models, which are variants of the X(5) critical point symmetry developed within the framework of the geometric collective model, are compared to two-parameter calculations in the framework of the interacting boson approximation (IBA) model. The results show that these geometricmodels coincide with IBA parameters consistentwith the phase/shape transition region of the IBA for boson numbers of physical interest (close to 10). 186Pt and 172Os are identified as good examples of X(3), while 146Ce, 174Os, and 158Er, 176Os are identified as good examples of X(5)−β 2 and X(5)−β 4 behavior, respectively

Nuclear level density at low excitation energies is proposed as an indicator of the first order phase transitions in nuclei. The new signature, a maximum value of the level density at the critical point, appears to be sensitive to the detailed way the phase transition takes place in different nuclear regions: it is consistent with phase coexistence in the N=90 region, and with a rapid crossing of the two phases, without their coexistence/mixing at the critical point in the N=60 region, respectively. Candidates for critical point nuclei are proposed for odd-mass and odd-odd nuclei, using correlations between relative excitation energies, and their ratios, for structures (bands) based on unique-parity orbitals., This manuscript has been published in Phys. Rev. C 98, 024301 (2018). This is the preprint version

Background: Many nuclear-structure features have been observed in actinides in recent decades. In particular, the octupole degree of freedom has been discussed lately after the successful measurement of the B(E3;0(1)(+) -> 3(1)(-)) reduced transition strength in Ra-224. Recent results stemming from gamma-spectroscopy experiments and highresolution (p, t) experiments suggested that strong octupole correlations might be observed for some positiveparity states of actinide nuclei. Purpose: This work completes a series of (p, t) experiments on actinide nuclei by adding the data on Pu-240. The (p, t) experiments allow us to study low-spin states up to J(pi) = 6(+). Besides two-nucleon transfer cross sections, spin and parity can be assigned to excited states by measuring angular distributions, and several rotational bands are recognized based on these assignments. Methods: A high-resolution (p, t) experiment at E-p = 24 MeV was performed to populate low-spin states in the actinide nucleus Pu-240. The Q3D magnetic spectrograph of the Maier-Leibnitz Laboratory (MLL) in Munich (Germany) was used to identify the ejected tritons via dE/E particle identification with its focal-plane detection system. Angular distributions were measured at nine different Q3D angles to assign spin and parity to the excited states based on a comparison with coupled-channel distorted-wave Born approximation calculations. Results: In total, 209 states have been excited in Pu-240 up to an excitation energy of 3 MeV. Many previously known states have also been observed and their spin-parity assignments were confirmed. However, many of the populated states have been seen for the first time, e.g., 15 new and firmly assigned J(pi) = 0(+) states. In addition, all low-spin one-octupole phonon excitations, i.e., K-pi = 0(-), 1(-), 2(-), 3(-), could be observed and a new candidate for the K = 3 projection is proposed. Furthermore, the double-octupole or a-cluster structure of the 0+2 state in Pu-240 has been studied in more detail. It is shown that the 0+2 state in Th-230 has a distinctly different structure. In addition, strongly excited 1-states have been observed at 1.5 and 1.8 MeV in Pu-240. The present study suggests that similar states might be observed in Th-230. Conclusions: At least two different and distinct structures for J(pi) = 0(+) states are present in the actinides. These are pairing states and states with enhanced octupole correlations. We have shown that it is crucial to consider negative-parity single-particle states being admixed to some K-pi = 0(2)(+) rotational bands to understand the alpha-decay hindrance factors and enhanced E1-decay rates. Based on our analysis, we have identified the double-octupole or alpha-cluster K-pi = 0(+) candidates from Ra-224 to Pu-240.

International audience; The European Strategy Forum on Research Infrastructures (ESFRI) has selected in 2006 a proposal based on ultra-intense laser fields with intensities reaching up to 1022–1023 W cm−2 called 'ELI' for Extreme Light Infrastructure. The construction of a large-scale laser-centred, distributed pan-European research infrastructure, involving beyond the state-of-the-art ultra-short and ultra-intense laser technologies, received the approval for funding in 2011–2012. The three pillars of the ELI facility are being built in Czech Republic, Hungary and Romania. The Romanian pillar is ELI-Nuclear Physics (ELI-NP). The new facility is intended to serve a broad national, European and International science community. Its mission covers scientific research at the frontier of knowledge involving two domains. The first one is laser-driven experiments related to nuclear physics, strong-field quantum electrodynamics and associated vacuum effects. The second is based on a Compton backscattering high-brilliance and intense low-energy gamma beam (

Extreme Light Infrastructure–Nuclear Physics (ELI–NP) is aiming to use extreme electromagnetic fields for nuclear physics research. The facility, currently under construction at Magurele–Bucharest, will comprise a high power laser system and a very brilliant gamma beam system. The technology involved in the construction of both systems is at the limits of the present-day’s technological capabilities. The high power laser system will consist of two 10 PW lasers and it will produce intensities of up to 10 23 –10 24 W/cm 2 . The gamma beam, produced via Compton backscattering of a laser beam on a relativistic electron beam, will be characterized by a narrow bandwidth (

The ( J , T ) = ( 1 , 1 ) parity doublet in 20 Ne at 11.26 MeV is a good candidate to study parity violation in nuclei. However, its energy splitting is known with insufficient accuracy for quantitative estimates of parity violating effects. To improve on this unsatisfactory situation, nuclear resonance fluorescence experiments using linearly and circularly polarized γ -ray beams were used to determine the energy difference of the parity doublet Δ E = E ( 1 − ) − E ( 1 + ) = − 3.2 ( ± 0.7 ) stat ( − 1.2 + 0.6 ) sys keV and the ratio of their integrated cross sections I s , 0 ( + ) / I s , 0 ( − ) = 29 ( ± 3 ) stat ( − 7 + 14 ) sys . Shell-model calculations predict a parity-violating matrix element having a value in the range 0.46–0.83 eV for the parity doublet. The small energy difference of the parity doublet makes 20 Ne an excellent candidate to study parity violation in nuclear excitations.

The evolution of level structures due to the unique parity orbitals ${g}_{9/2}$, ${h}_{11/2}$, and ${i}_{13/2}$ in odd-mass nuclei from Zn to Am is studied within a unified framework, by correlations between ratios of excitation energies in both odd-mass nuclei and their even-even core nuclei. These plots reveal regularities that can be understood in terms of the particle-plus-rotor model, as evolutions along its three limiting coupling schemes: weak coupling, decoupling, and strong coupling, and transitions between them. Peculiar transitions between the decoupling and strong coupling schemes are found in both ${i}_{13/2}$ structures of neutron-odd nuclei and ${h}_{11/2}$ structures of proton-odd nuclei, at neutron numbers around 90 and 70, respectively. These are correlated with the critical shape phase transitions from vibrator to rotor from the even-even nuclei in the same regions and are characterized as critical phase transitions too. This behavior is corroborated with a nonmonotonic behavior of the differential variation of the two-neutron separation energies in the same nuclear regions.

The perspectives for photonuclear experiments at the new Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility are discussed in view of the need to accumulate novel and more precise nuclear data. The parameters of the ELI-NP gamma beam system are presented. The emerging experimental program, which will be realized at ELI-NP, is presented. Examples of day-one experiments with the nuclear resonance fluorescence technique, photonuclear reaction measurements, photofission experiments and studies of nuclear collective excitation modes and competition between various decay channels are discussed. The advantages which ELI-NP provides for all these experiments compared to the existing facilities are discussed.

We report on the measurement of the half-life of the yrast I π = 5 − state in the transitional nucleus 136 Ce using a combined HPGe-LaBr3(Ce) scintillator gamma-ray detection array. The measured value for the E 1 decay is approximately half a nanosecond, which corresponds to an E 1 decay strength of approximately 2 × 10 − 6 Wu. This value is in line with single-particle type E 1 decays in this mass region and suggests no sign of additional K-hindrance associated with axially symmetric quadrupole deformations observed for lighter cerium isotopes.

Low-energy isomeric states of 86Y were populated in the reaction 73Ge + 16O at 57MeV and were investigated by means of delayed n $ \gamma$ and $ \gamma$ $ \gamma$ coincidences. A half-life of 70(7)ns was measured for the 5- state at 208keV, yielding an exceptionally small B(M1) value of 2.0(7)×10-5 W.u. and a B(E2) value of 0.34(+24 -13) W.u. For the other three known isomeric states at 218, 243, and 302keV, the half-lives extracted from the present experimental data are in very good agreement with previous measurements. Given the newly observed isomeric character of the 5- 208keV state, the re-analysis of earlier experimental data on the 302keV isomer led to a new spin-parity assignment, 6+, for this state. In addition, this re-evaluation provided two g -factors, -0.083(3) and +0.63(2) , for the 208 and 302keV states, respectively. The results are discussed in terms of spherical-shell model calculations performed with a truncated space of configurations built on the f 5/2 , p 3/2 , p 1/2 , and g 9/2 valence orbitals. Effective spin, orbital, and “tensor” g -factors were determined empirically for protons and neutrons in the considered configuration space.

We present a new study of the low-lying states in 124 Te nucleus by γ-ray spectroscopy following 124 I β+/ε decay. The β radioactive sources were produced in the 124 Te ( p, n )124 I reaction induced by 11 MeV protons, delivered by the Bucharest FN Tandem Accelerator. The γ-rays were measured in a low background area with three large volume HPGe detectors. A total number of 276 milion double coincidence events were recorded in a six-day run. Most of the gamma line intensities previously measured were confirmed with improved accuracy and several gamma lines were obtained for the first time. Our results, combined with those from a recent ( n , γ) study are compared with the predictions of the E(5) critical point symmetry model and numerical IBA-1 model calculations at the critical point of the U(5)–O(6) phase transition.

For an accurate determination of the absorbed doses in complex radiation fields (e.g. mixed neutron-gamma fields), a better interpretation of the response of ionisation chambers is required. This study investigates a model of the ionisation chambers using a different approach, analysing the collected charge per minute as a response of the detector instead of the dose. The MCNPX Monte Carlo code is used. In this paper, the model is validated using a well-known irradiation field only: a (60)Co source. The detailed MCNPX models of a Mg(Ar) and TE(TE) ionisation chamber is investigated comparing the measured charge per minute obtained free-in-air and in a water phantom with the simulated results. The difference between the calculations and the measurements for the TE(TE) chamber is within +/-2% whereas for the Mg(Ar) chamber is around +7%. The systematic discrepancy in the case of Mg(Ar) chamber is expected to be caused by an overestimation of the sensitive volume.

The presentation will focus on the Status of Implementation of the project Extreme Light Infrastructure – Nuclear Physics (ELI-NP), project co-financed by the European Regional Development Fund, and the research opportunities at the future European Center for Scientific Research.

The low-lying positive- and negative-parity states in $^{168}\mathrm{Yb}$ have been investigated by means of the $(\ensuremath{\alpha},2\mathrm{n}\ensuremath{\gamma})$ fusion evaporation reaction. Using the coincidence method, the level scheme was corrected and extended up to 3 MeV, for both the positive- and negative-parity states. Using the new branching ratios determined in the present experiment, the $K$ quantum number was proposed for two negative-parity bands by direct comparison with the Alaga rule. Like in some other nuclei, one negative-parity band was established, decaying predominantly to the $\ensuremath{\gamma}$-vibrational band. In a second experiment, the lifetimes of the low-lying excited states up to ${J}^{\ensuremath{\pi}}={6}^{+}$ in the ground-state band were measured by using the in-beam fast-timing method with the Bucharest mixed high-purity germanium (HPGe) and ${\mathrm{LaBr}}_{3}$:Ce detector array using the triple-$\ensuremath{\gamma}$ coincidence method. Reduced $E2$ transition probabilities were extracted from the measured lifetimes and compared with the corresponding observables in neighboring isotopes, showing a smooth behavior with increasing mass. The positive- and negative-parity states as well as $E1$ and $E2$ transition probability ratios revealed by these experiments are compared with the interacting boson model in the $sd$ and $spdf$ boson space, and with the confined beta-soft rotor model, and are found to be in good agreement.

The half-lives o fthe I=5- and 7- yrast stateys with Ex=1978 kev and Ex=2307 keV in the N=78 isotone 136Ce have been measured to be 496(23) ps and 270(24) ps respectively , using the concident fast-timing spectroscopy technique following population by a 124Sn(16O,4n) fusion-evporation reaction at an incident beam energy of 68 MeV.

The Project Extreme Light Infrastructure (ELI) is part of the European Strategic Forum for Research Infrastructures (ESFRI) Roadmap. ELI will be built as a network of three complementary pillars at the frontier of laser technologies. The ELI-NP pillar (NP for Nuclear Physics) is under construction near Bucharest (Romania) and will develop a scientific program using two 10 PW lasers and a Compton back-scattering high-brilliance and intense gamma beam, a marriage of laser and accelerator technology at the frontier of knowledge. In the present paper, the technical description of the facility, the present status of the project as well as the science, applications and future perspectives will be discussed.

The evolution of the structure of the gamma-vibrational band head 2+ γ in well-deformed rare-earth nuclei is studied within the framework of RPA calculations using a quasiboson approximation based on the Nilsson model. The obtained evolution of the quasiparticle structure of the gamma-vibrational band head is shown to correlate well with the evolution of the same nuclei within the IBA symmetry triangle when described in the extended consistent-Q formalism. An empirical relation is presented that links the IBA parameter χ with the quasiparticle structure of the gamma-vibrational band head.

The (J,T)=(1,1) parity doublet in 20Ne at 11.26 MeV is a good candidate to study parity violation in nuclei. However, its energy splitting is known with insufficient accuracy for quantitative estimates of parity violating effects. To improve on this unsatisfactory situation, nuclear resonance fluorescence experiments using linearly and circularly polarized γ-ray beams were used to determine the energy difference of the parity doublet ΔE=E(1−)−E(1+)=−3.2(±0.7)stat(−1.2+0.6)sys keV and the ratio of their integrated cross sections Is,0(+)/Is,0(−)=29(±3)stat(−7+14)sys. Shell-model calculations predict a parity-violating matrix element having a value in the range 0.46–0.83 eV for the parity doublet. The small energy difference of the parity doublet makes 20Ne an excellent candidate to study parity violation in nuclear excitations. Keywords: Parity doublet, Parity violation

Lifetimes for decays linking near-yrast states in 107Cd have been measured using the recoil distance method (RDM). The nucleus of interest was populated via the 98Mo(12C,3n)107Cd fusion–evaporation reaction at an incident beam energy of 60 MeV. From the measured lifetimes, transition probabilities have been deduced and compared with the theoretical B(E2) values for limiting cases of harmonic vibrational and axially deformed rotational systems. Our initial results suggest a rotor-like behaviour for the structure based on the unnatural-parity, h11/2 orbital in 107Cd, providing further evidence for the role of this 'shape-polarizing' orbital in stabilizing the nuclear deformation in the A ~ 100 transitional region.

Recent theoretical and experimental work focusing on nuclei in spherical-deformed transition regions has sparked considerable interest in phase transitional nuclei. A review of searches and experimental results on critical point nuclei in the A = 130 and A = 150 regions is presented.

Low-energy excited states of 210 Ra and 208 Ra were investigated at the Wright Nuclear Structure Laboratory of Yale University. Fusion evaporation recoils were selected using the gas-filled spectrometer, SASSYER. Delayed γ-rays, following isomeric decays, were detected at the focal plane of SASSYER with a small array of HPGe detectors. Transitions following the proposed J π = 8 + isomers were observed, and the half-lives measured. The experiments are discussed and results compared to expectations from the seniority scheme.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation and single-nucleon transfer. The B(E2;0+ → 2+) values for first 2+ excited states of even-even neutron-rich 132-136Te and 126-130Sn have been measured by Coulomb excitation in inverse kinematics. Neutron transfer onto a 134Te beam from 9Be and 13C targets, to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the νi13/2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2+ excited states of even-even neutron-rich 132–136Te and 126–134Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134Te beam, from 9Be and 13C targets to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the ν i 13 / 2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

A systematic study of nuclei with Z, N~64, 88 and 88, 134 in terms of the spdf-Interacting Boson Approximation model is presented. Various properties are reproduced with a very simple Hamiltonian which predicts an onset of strong octupole correlations at medium spin.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of exciting measurements around the 132 Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2 + excited states of neutron-rich 132,134,136 Te and 126,128,130,132,134 Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134 Te beam, from 9 Be and 13 C targets to populate single-particle states in 135 Te, has also been studied. Results, and plans for future experiments with these neutron-rich beams, are presented.

Shape/phase transitions in low-energy nuclear spectra, the new critical point symmetries E(5) and X(5), and their empirical realization have recently been the subject of many experimental and theoretical investigations. With a set of polar coordinates, the precise location of the critical phase transition region and of X(5)-type nuclei can be mapped in the interacting boson model symmetry triangle. An empirical mapping of the symmetry triangle for the N=82–104 rare-earth nuclei is also obtained.

The Wright Nuclear Structure Laboratory has recently acquired a gas-filled recoil separator previously used at Berkeley National Laboratory for heavy-element synthesis. The separator will be used to separate reaction recoils from primary beam particles and fission products following target bombardment. Commissioning of the separator has recently been completed, and the structure of 203 Rn investigated.

B(E2;0+1→2+1) values for the unstable, neutron-rich nuclei 132,134Te were determined through Coulomb excitation, in inverse kinematics, of accelerated beams of these nuclei. The systematics of measured B(E2) values from the ground state to the first excited state have been extended to the N=82 shell closure in the Te nuclei and have been compared with the predictions of different theories. The measurements were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) using the GRAFIK detector. The success of this approach, which couples a 5.7% efficient through-well NaI(Tl) γ-ray detector with thin foil microchannel plate beam detectors, also demonstrates the feasibility for Coulomb excitation studies of neutron-rich nuclei even further from the valley of beta stability, both at present-generation ISOL facilities and at the proposed Rare Isotope Accelerator.

Dynamical symmetries played a major role in interpreting collective nuclear spectra. Recent studies show that the nuclear system presents a phase transitional behavior in its evolution from spherical to deformed shape and new symmetries describe the structure at the critical point.

Extreme Light Infrastructure-Nuclear Physics (ELI-NP), to become operational in 2019, is a new Research Center built in Romania that will use extreme electromagnetic fields for nuclear physics research. The ELI-NP facility will combine two large equipments with state-of-the-art parameters, namely a 2 × 10 PW high-power laser system and a very brilliant gamma-beam system delivering beams with energies up to 19.5 MeV. The laser and gamma-beam systems under construction and typical proposed first-phase experiments are described.

The octupole bands in the Z∼88, N∼134 region, with their abundance of experimental information, offer a real challenge for different nuclear structure models. In this region, quasimolecular bands, which are thought to indicate the presence of stable octupole deformation, have been observed. The quantities most often reported are the B(E1)/B(E2) branching ratios from these levels. The calculations in the framework of the spdf model show that the 1-pf boson limit (dipole-octupole vibrations) can describe the essential experimental features related to the octupole bands in the Rn-Th region.

New sideband partners of the yrast bands built on the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\nu}{h}_{11/2}$ configuration were identified in ${}_{55}\mathrm{Cs}$, ${}_{57}\mathrm{La}$, and ${}_{61}\mathrm{Pm}$ $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}75$ isotones of ${}^{134}\mathrm{Pr}$. These bands form with ${}^{134}\mathrm{Pr}$ unique doublet-band systematics suggesting a common basis. Aplanar solutions of 3D tilted axis cranking calculations for triaxial shapes define left- and right-handed chiral systems out of the three angular momenta provided by the valence particles and the core rotation, which leads to spontaneous chiral symmetry breaking and the doublet bands. Small energy differences between the doublet bands suggest collective chiral vibrations.

$J^{\pi}$= 0$^+$ states have been investigated in the actinide nucleus ${}^{240}$Pu up to an excitation energy of 3 MeV with a high-resolution (p,t) experiment at $E_{p}$= 24 MeV. To test the recently proposed $J^{\pi}$= 0$^+_2$ double-octupole structure, the phenomenological approach of the spdf-interacting boson model has been chosen. In addition, the total 0$^+$ strength distribution and the $0^+$ strength fragmentation have been compared to the model predictions as well as to the previously studied (p,t) reactions in the actinides. The results suggest that the structure of the 0$^+_2$ states in the actinides might be more complex than the usually discussed pairing isomers. Instead, the octupole degree of freedom might contribute significantly. The signature of two close-lying 0$^+$ states below the 2-quasiparticle energy is presented as a possible manifestation of strong octupole correlations in the structure of the 0$^+_2$ states in the actinides., Comment: 6 pages, 5 figures, published in Phys. Rev. C 88, 041303(R) (2013)

The Yale Rochester Array for SpecTroscopy, YRAST Ball, is described. Containing up to 30 Compton suppressed Ge detectors YRAST Ball is a powerful new array for c-ray spectroscopy. Several of its auxiliary detectors, including a multi-element solar cell array for heavy charged fragment detection and a new state of the art plunger system for recoil distance measurements are also described. ( 2000 Elsevier Science B.V. All rights reserved.

Different classes of evolutionary tracks in nuclear spherical-deformed transition regions are discussed. For each trajectory we associate a characteristic behavior for simple observables based on low-lying energy levels. The usefulness of this morphological classification for the study of exotic nuclei is stressed.

The study of spherical-deformed ground--state phase transitions in finite nuclei as a function of N and Z is hindered by the discrete values of the nucleon number. A resolution of the integer nucleon number problem, and evidence relating to phase transitions in finite nuclei, are discussed from the experimental point of view and interpreted within the framework of the interacting boson model., Comment: 8 pages Latex + 8 figs (postscript). In Phys Rev Lett, June 1999

The high-spin structure of {sup 205}Rn has been investigated for the first time following the {sup 170}Er({sup 40}Ar,5n) and {sup 197}Au({sup 14}N,6n) reactions at beam energies of 183 MeV and 90{endash}110 MeV, respectively, using the Jurosphere and YRAST Ball arrays. Two new cascades have been identified which dominate the high-spin decay. One of these, consisting of ten stretched M1 transitions with unobserved E2 crossover transitions, is interpreted as a shears structure based on the {nu}i{sub 13/2}{sup {minus}1}{circle_times}{pi}i{sub 13/2}{sup 2} (or {nu}i{sub 13/2}{sup {minus}1}{circle_times}{pi}h{sub 9/2}i{sub 13/2}) configuration. {copyright} {ital 1999} {ital The American Physical Society}

Lifetimes of the 0 2 + and 2 γ + states in 154 Sm were measured using the Doppler-shift Attenuation Method following Coulomb excitation. The experiment employed the gamma-ray detector array YRAST Ball at Yale University in conjunction with an array of photo-electric cells. 154 Sm was identified as one of the few deformed nuclei where the first excited 0 + state is the β -vibration of the ground state. The 0 3 + level has no collective decay to the ground state band and is interpreted as a spherical shape coexisting state based on its decay properties and calculations in the framework of the interacting boson model (IBM) and geometric collective model (GCM).

Ultraprecise energies and absolute B(E2) values associated with K=0{sup +} excitations in {sup 158}Gd, measured with the GRID technique, reveal significant information relating to the possible existence or absence of two-phonon vibrational excitations in deformed nuclei. A strong B(E2:K=0{sub 2}{sup +}{r_arrow}{gamma}) is observed but reflects band mixing rather than a {gamma}{gamma} mode. For the K=0{sub 3}{sup +} band, measurement of precise {gamma} transition energies shows that suggested existing placements of strong deexcitation transitions are incorrect and that there is, again, no evidence for {gamma}{gamma}-phonon character in this band either. A weak B(E1:K=1{sup {minus}}{r_arrow}K=0{sub 1}{sup +}) value is interpreted in terms of the microscopic structure of K=0{sup {minus}} and K=1{sup {minus}} excitations. {copyright} {ital 1999} {ital The American Physical Society}

The decay properties of the lowest band in the SU(3) dynamical limit of the interacting boson model exhibit properties which are not compatible with the classical picture of a -vibration. Induced by this doubt, recent experimental work on and shows that the lowest band can be indeed of various origins in different nuclei.

Cross sections for the (p,t) reaction to ground states of even-even nuclei from Zr to Hg are interpreted in a simple approach. They are shown to be sensitive both to the degree of shell filling (which is related to the number of L=0 pairs in the ground state) and to the difference in the number of these pairs between target and final nuclei (that is, to the overlap of the initial and final systems). The cross sections separate according to the two broad classes of nuclei{emdash}vibrational and rotational. The analysis is carried out in both a model-independent way and within the framework of the interacting boson approximation. thinsp {copyright} {ital 1998} {ital The American Physical Society}

Octupole states have been studied systematically in the deformed actinide nuclei ${}^{230}\mathrm{Th},$ ${}^{234,236,238}\mathrm{U},$ ${}^{240}\mathrm{Pu},$ ${}^{246}\mathrm{Cm},$ and ${}^{250}\mathrm{Cf}$ using the interacting boson approximation 1. Parameters for the positive parity cores were set by reproducing data from the ground state and \ensuremath{\gamma}-vibration bands with the extended consistent-$Q$ formalism. A prescription for setting the parameter ${\ensuremath{\epsilon}}_{f}$ systematically was adopted for the calculations of the octupole bands. The ordering of the ${K}^{\ensuremath{\pi}}{=0}^{\ensuremath{-}},$ ${1}^{\ensuremath{-}},$ and ${2}^{\ensuremath{-}}$ bands and the behavior of many $E1$ and $E3$ transitions are well reproduced with a narrow range of parameters, particularly when the existence of a $N=164$ subshell gap is assumed. It is predicted that in ${}^{246}\mathrm{Cm}$ and ${}^{250}\mathrm{Cf}$ the ${K}^{\ensuremath{\pi}}{=3}^{\ensuremath{-}}$ octupole states built on the ground state are located above 6 MeV and are strongly fragmented.

Recent data provide evidence for coexisting phases at low energy in the spherical-deformed transitional nucleus {sup 152}Sm . The nature of the wave functions at the spherical-deformed phase transition in nuclei is analyzed within the framework of the interacting-boson model. It is shown that in the U(5)-SU(3) transition, two phases coexist in a very small region of parameter space around the critical value of the control parameter {xi} . The coexistence region with two minima in the potential shrinks to zero as one moves to the U(5)-O(6) transition. Implications for other systems are briefly mentioned. {copyright} {ital 1998} {ital The American Physical Society }

The properties of ${K=0}^{+}$ excitations in deformed and transitional nuclei have recently been of intense interest. We present results of a study of the deexcitations of the lowest excited ${0}^{+}$ state in ${}^{152}$Sm from the $\ensuremath{\epsilon}$-decay of ${}^{152}$Eu, yielding one of the few precisely known values of the branching ratio ${R}_{0g}^{\ensuremath{\gamma}}{=B(E2;2}_{\ensuremath{\gamma}}^{+}\ensuremath{\rightarrow}{0}_{2}^{+}{)/B(E2;2}_{\ensuremath{\gamma}}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ =0.048(4), which is extraordinarily small. From ${T}_{1/2}{(2}_{\ensuremath{\gamma}}^{+})$ we also obtain ${B(E2;2}_{\ensuremath{\gamma}}^{+}\ensuremath{\rightarrow}{0}_{2}^{+})$=0.17 W.u. Values of ${R}_{0g}^{\ensuremath{\gamma}}$ calculated in the interacting boson model (IBA) go to zero extremely rapidly, changing by orders of magnitude for a narrow range of parameter values. ${}^{152}$Sm is a rare case of a transitional nucleus that lands almost at the minimum. ${}^{152}$Sm and ${}^{154}$Gd are the only nuclei from $90l~Nl~114$ where the $B(E2)$ values for all four transitions ${2}_{\ensuremath{\gamma}}^{+}\ensuremath{\rightarrow}{0}_{2}^{+}$, ${2}_{\ensuremath{\gamma}}^{+}\ensuremath{\rightarrow}{0}_{1}^{+}$, ${0}_{2}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}$, and ${2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+}$ are now known. In ${}^{152}$Sm these $B(E2)$ values span three orders of magnitude, from 144 to 0.17 W.u. and are reproduced to within a factor of 2--3 by the IBA. The rather strong ${B(E2;0}_{2}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ value of 33 W.u. suggest that the ${0}_{2}^{+}$ level is an example of a good low energy $\ensuremath{\beta}$-vibration.