Search

Abstract Self-conjugate nuclei in the A ≈ 70 – 80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N = Z line. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B (E 2 ; 2 1 + → 0 1 +) transition strength in 74Kr. The method is then applied to determine the B (E 2 ; 2 1 + → 0 1 +) transition strength in 74Rb, the heaviest odd–odd N = Z nucleus for which this quantity has been determined. This result and extended systematics along N = Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in 74Rb. [ABSTRACT FROM AUTHOR]

Interfacial morphology evolution in FeCrAl alloys with varying Al content has been investigated by conducting a series of directional solidification experiments. A comparison between the experimental data and the Hunt-Lu model confirms that cellular spacing widens while dendritic spacing narrows as Al content of the alloy increases. The ratio of upper and lower limit cellular/dendritic spacing λ max /λ min is only weakly dependent on growth velocity and Al content. Under the experimental conditions in the present study, cell to dendrite transition (CDT) occurs at a growth velocity between 25 μm/s and 50 μm/s, and dendrite to cell transition (DCT) is observed at a growth velocity less than 100 μm/s. Critical spacing of CDT λ cd and local solidification time (LST) t f are used to discuss the influence of Al content on CDT and DCT respectively. The transition between non-planar and planar interface of the alloy is investigated based on the interface stabilization theory of Mullins and Sekerka. [ABSTRACT FROM AUTHOR]

We have carried out a detailed study of scaling region using detrended fractal analysis test by applying different forcing likewise noise, sinusoidal, square on the floating potential fluctuations acquired under different pressures in a DC glow discharge plasma. The transition in the dynamics is observed through recurrence plot techniques which is an efficient method to observe the critical regime transitions in dynamics. The complexity of the nonlinear fluctuation has been revealed with the help of recurrence quantification analysis which is a suitable tool for investigating recurrence, an ubiquitous feature providing a deep insight into the dynamics of real dynamical system. An informal test for stationarity which checks for the compatibility of nonlinear approximations to the dynamics made in different segments in a time series has been proposed. In case of sinusoidal, noise, square forcing applied on fluctuation acquired at P = 0.12 mbar only one dominant scaling region is observed whereas the forcing applied on fluctuation (P = 0.04 mbar) two prominent scaling regions have been explored reliably using different forcing amplitudes indicating the signature of crossover phenomena. Furthermore a persistence long range behavior has been observed in one of these scaling regions. A comprehensive study of the quantification of scaling exponents has been carried out with the increase in amplitude and frequency of sinusoidal, square type of forcings. The scalings exponent is envisaged to be the roughness of the time series. The method provides a single quantitative idea of the scaling exponent to quantify the correlation properties of a signal. [ABSTRACT FROM AUTHOR]

A new high-spin isomer in the neutron-rich nucleus 128 Cd was populated in the projectile fission of a 238 U beam at the Radioactive Isotope Beam Factory at RIKEN. A half-life of T 1 / 2 = 6.3 ( 8 ) ms was measured for the new state which was tentatively assigned a spin/parity of ( 15 − ). The experimental results are compared to shell model calculations performed using state-of-the-art realistic effective interactions and to the neighbouring nucleus 129 Cd. In the present experiment no evidence was found for the decay of a 18 + E 6 spin-trap isomer, based on the complete alignment of the two-neutron and two-proton holes in the 0 h 11 / 2 and the 0 g 9 / 2 orbit, respectively, which is predicted to exist by the shell model. [ABSTRACT FROM AUTHOR]

Undoped and co-doped zinc oxide thin films were deposited on glass substrates using a chemical spray technique. The experimental Lorentz profiles of co-doped ZnO thin films were measured in the spectral region 720–1750 nm. A single oscillator model of the resulted materials is constructed by calculation of: zero-frequency refractive index ( n o ), average interband oscillator wavelength ( λ o ), oscillator strength ( S o ), dispersion energy ( E d ), single-effective oscillator energy ( E o ), plasma oscillation frequency ( ω p ) and interband transition strength moments ( M −1 , M −3 ). [ABSTRACT FROM AUTHOR]

The heavy-ion inelastic scattering of the neutron-rich nucleus 29Ne to its excited states was studied using a 100.1 MeV/u 29Ne rare isotope beam on 181Ta and 9Be targets. The combined setup consisting of the GRETINA array, the TRIPLEX device and the S800 Spectrograph facilitates the simultaneous measurements of the two inelastic reactions, providing the first measurement of the transition strengths for this isotope. A sizable E2 strength B(E2↑) which amounts to 163(30) e2 fm4 was determined in the excitation to the 931-keV state, demonstrating a large degree of collectivity. The present results of B(E2↑) are compared to various shell-model calculations, confirming the role of intruder configurations in 29Ne at the boundary of the island of inversion. [ABSTRACT FROM AUTHOR]

I have used a simple two-state model to fit E2 strengths connecting the first two 0 + states to the first two 2 + states in 98 Sr. Results for mixing parameters are in excellent agreement with those from a recent analysis. Perhaps surprisingly, they are also in remarkable agreement with results from 1980, despite the wide variation reported in the intervening years. [ABSTRACT FROM AUTHOR]

Unique optical properties of semiconductor nanoparticles (SN) make them very promising in the multitude of applications including lasing, light emission and photovoltaics. In many of these applications it is imperative to understand the physics of interaction of electrons in a SN with external electromagnetic fields on the quantitative level. In particular, the strength of electron–photon coupling determines such important SN parameters as the radiative lifetime and absorption cross section. This strength is often assumed to be fully encoded by the so called Kane momentum matrix element. This parameter, however, pertains to a bulk semiconductor material and, as such, is not sensitive to the quantum confinement effects in SNs. In this work we demonstrate that the quantum confinement, via the so called band mixing, can result in a significant suppression of the strength of electron interaction with electromagnetic field. Within the envelope function formalism we show how this suppression can be described by introducing an effective energy-dependent Kane momentum. Then, the effect of band mixing on the efficiencies of various photoinduced processes can be fully captured by the conventional formulae (e.g., spontaneous emission rate), once the conventional Kane momentum is substituted with the renormalized energy-dependent Kane momentum introduced in here. As an example, we evaluate the energy-dependent Kane momentum for spherical PbSe and PbS SNs (i.e., quantum dots) and show that neglecting band mixing in these systems can result in the overestimation of absorption cross sections and emission rates by a factor of ∼ 2. [ABSTRACT FROM AUTHOR]

Lineshapes for the electromagnetically induced absorption (EIA) of thermal 85 Rb atoms in a degenerate two-level system have been investigated using matching ( σ ∥ σ , π ∥ π ) and orthogonal ( σ ⊥ σ , π ⊥ π ) polarization configurations of coupling and probe beams. EIA signals, which result from coherence population oscillation and transfer of coherence of the excited state, are obtained in detail theoretically and experimentally. The observed EIA linewidths, which are limited due to the decoherence rate between the magnetic sublevels in the ground state from transit-time relaxation, match well with the calculated ones. Decompositions of the absorption signals analyzed with respect to magnetic sublevels of the ground state show that enhanced or decreased absorption signals for each component of magnetic sublevels in the ground state depend on several factors. These factors include the decay rates and transition strengths, which determine the overall absorption spectral profile. [ABSTRACT FROM AUTHOR]

The rheological characterization of drilling fluids is crucial to ensure an adequate performance of drilling operations. These materials show elastoviscoplastic and thixotropic behaviors simultaneously so that its gel strength is highly affected by the aging time to which the sample is subjected. This work presents experimental evidences of a correlation between the stress overshoot observed in flow start-up rheometric experiments after different aging times and the storage modulus, G′, monitored during a low amplitude oscillatory sweep over the aging time. A comparison of the correlation with the measured data at five different shear rates show that results lie within error bands of 10%. The results also suggest that the stress overshoot for different aging times can be estimated by using only two rheometric tests: an oscillatory sweep and a controlled shear rate experiment. [ABSTRACT FROM AUTHOR]

Excited low-spin states in 96 Mo and 98 Mo have been studied in γγ angular correlation experiments in order to determine spins and multipole mixing ratios. Furthermore, from a Doppler lineshape analysis effective lifetimes τ in the femtosecond range were obtained. The experimental data show a complex spectrum due to configuration mixing, which is confirmed by Interacting Boson Model calculations based on a Skyrme energy density functional. The M 1-transition strengths of transitions depopulating excited 2 + states to the first 2 + state are discussed in terms of the proton–neutron mixed symmetry. [ABSTRACT FROM AUTHOR]

Uncertainties in absolute plunger distances are an often overlooked component in measurements with the Recoil Distance Doppler-shift (RDDS) method when the Differential Decay Curve (DDC) method cannot be used for the analysis. It is shown how these uncertainties arise when the capacitance method is used to determine absolute distances and how they influence the obtained lifetime values of a plunger experiment. Furthermore, a new approach to obtain absolute foil separations for plunger measurements using a high precision optical distance probe is introduced, which can reduce these uncertainties by a large margin. The performance of this method is demonstrated by comparisons to the capacitance method as well as to a precise measurement of absolute distances using known lifetimes in 181Ta. [ABSTRACT FROM AUTHOR]

Lifetimes of the states in the quadrupole structure in 143 Eu have been measured using the Doppler shift attenuation method and the parity of the states in the sequence has been firmly identified from polarization measurements using the Indian National Gamma Array. The decreasing trends of the deduced quadrupole transition strength B ( E 2 ) with spin, along with increasing J ( 2 ) / B ( E 2 ) values before the band crossing, conclusively establish the origin of these states as arising from antimagnetic rotation. The abrupt increase in the B ( E 2 ) values after the band crossing in the quadrupole band, a novel feature observed in the present experiment, may possibly indicate the crossing of different shears configurations resulting in the re-opening of a shears structure. The results are reproduced well by numerical calculations within the framework of a semi-classical geometric model. [ABSTRACT FROM AUTHOR]

The deformed proton–neutron quasiparticle random phase approximation (QRPA) has been developed and applied to evaluate Gamow–Teller (GT) transition strength distributions, including high-lying excited states. The data of high-lying excited states are recently available beyond one or two nucleon threshold by charge exchange reactions using hundreds of MeV projectiles. Our calculations started with single-particle states calculated using a deformed, axially symmetric Woods–Saxon potential. The neutron–neutron and proton–proton pairing correlations are explicitly taken into account at the deformed Bardeen–Cooper–Schriffer theory. Additionally, the ground state correlations and two-particle and two-hole mixing states were included in the deformed QRPA. In this work, we used a realistic two-body interaction, given by the Brueckner G -matrix based on the CD Bonn potential to reduce the ambiguity on the nucleon–nucleon interactions inside nuclei. We applied our formalism to the GT transition strengths for 76 Ge, 76,82 Se, and 90,92 Zr, and compared the results with the available experimental data. The GT strength distributions were sensitive to the deformation parameter as well as its sign, i.e. , oblate or prolate. The Ikeda sum rule, which is usually thought to be satisfied under the one-body current approximation, regardless of nucleon models, was used to test our numerical calculations and shown to be satisfied without introducing the quenching factor, if high-lying GT excited states were properly taken into account. Most of the GT strength distributions of the nuclei considered in this work have the high-lying GT excited states beyond one-nucleon threshold, which are shown to be consistent with the available experimental data. [ABSTRACT FROM AUTHOR]

The distribution of the total transition strength, i.e. the right hand side of the integral form of Thomas–Reiche–Kuhn sum rule, into individual absorption processes is described for crystalline silicon containing interstitial oxygen. Utilization of the sum rule allows the construction of a dispersion model covering all elementary excitations from phonon absorption to core electron excitations. The dependence of transition strength of individual electronic and phonon contributions on temperature and oxygen content is described. [ABSTRACT FROM AUTHOR]

The decay of an isomeric state in the neutron-rich nucleus 129 Cd has been observed via the detection of internal conversion and Compton electrons providing first experimental information on excited states in this nucleus. The isomer was populated in the projectile fission of a 238 U beam at the Radioactive Isotope Beam Factory at RIKEN. From the measured yields of γ -rays and internal conversion electrons, a multipolarity of E 3 was tentatively assigned to the isomeric transition. A half-life of T 1 / 2 = 3.6 ( 2 ) ms was determined for the new state which was assigned a spin of ( 21 / 2 + ) , based on a comparison to shell model calculations performed using state-of-the-art realistic effective interactions. [ABSTRACT FROM AUTHOR]

To improve the bonding strength and the oxidation resistance of SiC coating by chemical vapor deposition (CVD) for carbon/carbon (C/C) composites, C/C composites were modified by pre-oxidation treatment. It is found that an inlaid transition structure was obtained at the interface between SiC coating and pre-oxidized C/C composites. After pre-oxidation treatment, the interface adhesion strength increases from 23.5±3.2 to 37±2.5N, and the weight loss of the coated C/C samples is only 0.50% after oxidation at 1773K for 12h. The obvious improvement of the bonding strength and the oxidation resistance was mainly attributed to the pre-oxidation of the C/C substrate, forming an inlaid transition layer at the C/C–SiC interface during CVD process. [ABSTRACT FROM AUTHOR]

Abstract: Transition strengths for decays from low-lying states in A ∼ 70 nuclei have been deduced from lifetime measurements using the recoil distance Doppler shift technique. The results confirm the collectivity previously reported for the decay in 68Se and reveal a relative decrease in collectivity in 70Br. This trend is reproduced by shell model calculations using the GXPF1A interaction in an fp model space including the Coulomb, spin-orbit and isospin non-conserving interactions. The decay in 70Br is found to have a very small B(M1) value, which is consistent with the configuration of the state being dominated by the coupling of protons and neutrons. The results suggest that the orbit does not play an important role at low spin in these nuclei. The B(E2) values for the decays of the (T = 1) states in 70Br and 70Se are almost identical, suggesting there is no major shape change between the two nuclei at low spin. [Copyright &y& Elsevier]

Abstract: Details of Gamow–Teller (GT) and Fermi (F) strength extraction from charge-exchange reactions at intermediate energies, as well as the concept of using the phenomenological effective interaction for the description of the reactions are reviewed. The focus lies on high-resolution (3He, t) data, yet new data on (d, 2He) charge-exchange reactions are presented as well to indicate the connection and the similarity of both probes to the elementary , resp. reactions. We show that for the extraction of and strength a proper extrapolation of the charge-exchange cross section to the zero momentum transfer point, , gets to be increasingly important for medium to high-mass target nuclei and at moderate excitation energies of a few MeV. Numerous (3He, t) and spectra are being compared, and, once adjusted for the different momentum transfers, they reveal a remarkable similarity up to the highest measured excitation energies of . Further, we remark that the precision, with which GT and F transition strengths can be extracted, is limited by the relatively poor knowledge of the absorption processes, and a more thorough theoretical understanding of the nuclear structure leading to absorption in charge-exchange reactions, especially in the restricted kinematic region of low-momentum transfers, is warranted. Finally, values for the 136Xe(3He, t) are being re-evaluated in the light of the present discussion. [Copyright &y& Elsevier]

Abstract: Nine lifetimes have been determined for the first time in the and bands of 167Yb by means of Recoil distance Doppler-shift measurements carried out at the Cologne FN tandem. For the data analysis, a dedicated version of the Differential decay curve method was used. The newly deduced transition strengths and the level scheme are reasonably described by the Particle plus triaxial rotor model (PTRM) calculations except for the behavior of the quadrupole collectivity in the two signatures of the band. The stretched B(E2)ʼs in the favored signature are appreciably larger than those in the unfavored signature. The effect increases with spin. In the PTRM calculations, the wave functions of the favored signature levels contain larger contribution of low-Ω orbitals from parentage than those of the unfavored levels. This could selectively increase the deformation of the favored signature band members. If so, rotation and Coriolis interaction would give rise to a dynamic shape coexistence taking place between the two signatures. [Copyright &y& Elsevier]

Abstract: Excited states in 124Xe have been studied via the 12C(124Xe, 124Xe∗) Coulomb excitation reaction. Their population cross-sections relative to the state have been determined from the γ-ray yields observed with Gammasphere. More than twenty absolute E2 strengths for seven off-yrast, low-spin states of 124Xe have been deduced for the first time. The absolute values indicate pronounced symmetry, even for the off-yrast states with high quantum number τ, while the symmetry is substantially broken. [Copyright &y& Elsevier]

The lifetimes of several negative-parity states in 134Ce excited via the reaction 122Sn(16O, 4 n)134Ce at an incident beam energy of 76 MeV were measured for the first time by means of the recoil-distance Doppler-shift technique. The small B (E 2) transition strengths deduced from the lifetime data at the bottom of the negative-parity yrast states show spherical shape, in qualitative accordance with predictions of the shell model. At higher excitation energy along the negative-parity yrast line, the experimental B (E 2) transition strengths indicate an apparent gain of collectivity with increasing spin. The quadrupole deformation derived from the B (E 2) values at the bottom of the negative-parity yrast band confirms the predictions of the cranked Nilsson-Strutinsky Bogoliubov calculations. The systematic evolution in structure of the negative yrast states in Ce isotopes and N = 76 isotones is discussed. [ABSTRACT FROM AUTHOR]