Search Results (17 total)

Excited states in ¹⁸²Pt have been studied via the heavy-ion reactions ¹⁷⁰Yb(¹⁶O,4n), ¹⁶²Dy(²⁴Mg,4n), and ¹⁶³Dy(²⁴Mg,5n). γ-ray coincidence measurements were performed with arrays of HPGe detectors at the McMaster University Tandem Accelerator Laboratory (¹⁶O-induced reaction) and the Institut de Physique Nucléaire, Orsay (²⁴Mg-induced reactions). The ground-state rotational band has been extended to I=26ħ, and six new band structures have been identified and assigned quasiparticle configurations. The γ-vibrational band and the band built upon the first excited 0⁺ state have also been extended. Properties of the rotational bands are compared with cranked-shell-model and total-Routhian-surface calculations. Evidence concerning shape coexistence at low spin and band crossings at high spin is discussed.

High-spin states have been populated in odd-odd ^186,188Au via the heavy-ion ^171,173Yb(¹⁹F,4n) reactions at 93–94 MeV. Rotational bands built on 11^- oblate νi_13/2^-1⊗πh_11/2^-1 structures have been observed in both nuclei, similar to those reported in ^190-194Au. Two new strongly coupled bands in ¹⁸⁶Au are interpreted as prolate πh_9/2⊗νi₁₃2 and πi_13/2⊗νi_13/2 configurations. Based on blocking arguments a band crossing in the latter structure is attributed to the alignment of h_9/2 protons. Measured B(M1;I→I-1)/B(E2;I→I-2) ratios are compared to predictions of the Dönau and Frauendorf geometrical formalism extended to odd-odd cases.

The reactions ⁴⁰Ca+¹⁴N at E_N=40 MeV and ²⁷Al+²⁷Al at E_Al=90 MeV are used to populate high-spin levels in a number of f_7/2 nuclei, from ⁴⁵Sc to ⁵²Fe. In ⁵¹Cr, the f_7/2 band-terminating state at J ^π=23/2^- is identified at 5.711 MeV, and further transitions, from the pf band, are found. Similar evidence for interband transitions in ⁵¹Mn is sought without success. In ⁴⁸V, new transitions are found within the positive-parity band and between the negative- and positive-parity bands.

Prescission charged-particle multiplicities, following fusion of ^164,167,170Er+²⁸Si, have been measured. The multiplicities at the lowest bombarding energies limit the statistical model level density parameters. More importantly, the α data restrict the time spent near equilibrium and suggest evaporation occurs predominantly from larger deformations.

A total of ten previously observed γ rays have been assigned to the N=Z+1 nucleus ⁶³Ga; these provide the first information on excited states reported in this neutron-deficient system. ⁶³Ga has been populated using the ⁴⁰Ca(²⁸Si, αp), ⁴⁰Ca(³²S, 2αp), and ⁴⁰Ca(²⁹Si, αpn) reactions at beam energies of 80–100 MeV. Particle-γ coincidence techniques provide a definitive identification of γ transitions in ⁶³Ga. An yrast-level scheme is proposed on the basis of charged-particle-γ, charged-particle-γ-γ, and neutron-γ-γ coincidences. Spin and parity assignments are based on neutron-gated γ-ray angular distributions, directional correlation with oriented nuclei ratios and systematics. Nine new energy levels are identified in addition to the previously known ground state, with the highest level at an excitation energy of 7.7 MeV. The results are interpreted in terms of the relevant single-particle orbitals and the systematics of the neighboring nuclei. In addition, a new determination has been made of the E2/M1 mixing ratio for the (5/2^-)₁→(3/2^-)₁ transition in ⁶⁵Ga, resolving a disagreement between two previous measurements.

The temperature dependence of fission barriers has been interpolated from the results of recent theoretical calculations and included in a statistical model code. It is shown that the inclusion of temperature dependence causes significant changes to the values of the statistical model parameters deduced from fits to experimental data.

Charged-particle-γ and charged-particle-neutron-γ coincidences have been used to identify in-beam γ-ray transitions in ⁶⁹Se produced in the reaction ⁴⁰Ca(³²S,2pn)⁶⁹Se. Charged-particle-γ-γ coincidences, neutron-γ-γ coincidences, and neutron-gated γ-ray angular distributions were used to deduce the level scheme. A long-lived isomer attributed to the g_9/2 state was found; its half-life was measured with charged-particle-γ timing as 1023±110 ns. The presence of strong ΔJ=1 and ΔJ=2 transitions in the band built on the νg_9/2 neutron single-particle state and the measured sign of the E2/M1 mixing ratio for the (11/2⁺→(9/2⁺ transition are consistent with a theoretically predicted oblate shape for this nucleus.

High-spin states in ¹⁸⁵Pt have been studied with the ¹⁷³Yb(¹⁶O,4n)¹⁸⁵Pt reaction at 90 MeV using a multidetector array consisting of five Ge detectors and a multiplicity filter of six NaI counters. Rotational bands built on the Nilsson configurations (9/2⁺[624], (7/2^-[503], and (1/2^-[521] have been identified and interpreted within the framework of the cranked shell model. Additional band structures built on levels at 3.131 and 3.294 MeV are observed which arise from more complex configurations. Experimental values of B(M1;I→I-1)/B(E2;I→I-2) ratios have been extracted and compared to predictions of the semiclassical Dönau and Frauendorf approach.

The level structure of odd-proton ¹⁸⁷Au was studied using the ¹⁷²Yb(¹⁹F,4n) reaction. Gamma-gamma coincidence, angular distributions and γ-γ-time measurements were used to establish the energies, spins, parities, and lifetimes for the states in the decay scheme of ¹⁸⁷Au. Both prolate and oblate structures have been established. The prolate bands are built on the πh_9/2 (α=±(1/2) and the πi_13/2 (α=(1/2) particle states. An oblate, strongly coupled band built on a 90±10 ns isomeric state is reported for the first time. A second oblate structure built on the h_11/2 proton-hole state was established to much higher spin than before. The backbend present in the πi_13/2 band has been interpreted as the alignment of a pair of h_9/2 protons.

Excited states in ¹¹⁹Xe have been populated via the ⁹⁵Mo(²⁷Al,p2n) reaction at a beam energy of 105 MeV. The high-spin level structure is interpreted as consisting of rotational bands built on quasineutron h_11/2, d_5/2, and g_7/2 states. Although these positive-parity configurations differ from previous interpretations, most notably an earlier g_9/2 assignment, they are supported by experimental B(M1;I→I-1)/B(E2;I→I-2) ratios plus additional levels presented here. A backbend in the g_7/2 band is attributed to the rotation alignment of a pair of νh_11/2 quasiparticles.

Evidence is presented for a πh_{9 / 2} band crossing at surprisingly low rotational frequency in the odd-neutron ¹⁸⁵Pt and odd-proton ¹⁸³Ir nuclei. The proton nature of the crossing is indicated by measured B(M1;I→I-1) / B(E2;I→I-2) ratios, strongly supported by theoretical values from both the semi-classical Dönau and Frauendorf approach and a variable-core particle-rotor calculation employing cranked-shell-model matrix elements. However, the actual crossing frequency is greatly overestimated by the cranking method using generally accepted model parameters.

A two-photon technique is reported for the measurement of relative cross sections for the photolysis of simple molecules into particular product channels. In a demonstration of this method, two independently tunable dye lasers were used to dissociate molecules of Cs₂ sequentially for wavelengths in the range 445 to 505 nm, and then to excite the resulting products to determine the relative cross sections for the photolysis of the Cs₂ molecules into each of the energetically possible product states.

Space-charge amplification of the indirect ionization resulting from photoabsorption in cesium vapor has been shown to offer to conventional techniques of absorption spectroscopy a useful complement of highest sensitivity. It was used in this work together with a xenon continuum source filtered to a linewidth of 0.14 Å to measure the relative absorption probabilities for transitions of the cesium-principal-series (6²S-n²P_{1 / 2,3 / 2}) doublets. Anomalous pressure effects on the resulting line-strength ratios, resulting in part from polarization molecules of Cs₂*, were examined over a range of cesium-atom concentration varying from 1.2×10¹⁴ to 1.5×10¹⁵ cm^-3. From the corrected doublet line-strength ratios for 11≤n≤16, the segment of the curve of Fano X parameters corresponding to discrete energies was constructed. Interpolation between these results and those from electron-polarization experiments served to locate the pole in the principal-series, doublet line-strength ratio near 19²P.

The multiphoton excitation of cesium through potentially resonant continuum molecular states is investigated over the 6200-5000-Å wavelength region with a tunable dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. Conditions are established under which the lifetime of the resonant intermediate state against photoexcitation exceeds the lifetime against dissociation. Single- and double-photon resonances occur for the same wavelength and give absorption maxima corresponding to the line spectrum from the intermediate state following dissociation. These lines are modulated in amplitude as a function of wavelength by the more slowly varying absorption resonance from the initial to the intermediate continuum state. As a consequence, the resulting dispersion curve for two-photon absorption in cesium shows what appear to be resonant intermediate dπ(³Π_g) terms dissociating to give a 5²D atom and resulting in the strong development of features corresponding to the fundamental series (5²D→n²F) of atomic cesium in absorption. Components to n=50 were observed and recorded to a precision sufficient to determine the average quantum defect for F states to be 〈n-n^*〉=0.033 in the limit of large n. More complex structure is attributed to pπ(³Π_g) and pσ(¹Σ_u) terms dissociating to give a 6 ²P_{3 / 2} atom and resulting in the development of the n=8 to 32 components of the 6 ²P_{3 / 2}→n ²D_{3 / 2, 5 / 2} part of the diffuse series in absorption. Terms dissociating into 6 ²P_{1 / 2} were found only at higher photon energies (∼2.27 eV) corresponding to the n≥10 members of the 6 ²P_{1 / 2}→n ²D_{3 / 2} series.

The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-Å wavelength region with a tunable-dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 6²S_{1 / 2} level to resonant n²D and n²S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9 to 13 have been recorded for the 6²S_{1 / 2}-n²D_{5 / 2,3 / 2} series and from n=11 to 14 for the 6²S_{1 / 2}-n²S_{1 / 2} series.

This paper reports the observation of the two-photon ionization of molecular cesium as a continuous function of laser wavelength in the 6200-6600-Å region with a tunable dye laser having a 0.18-Cm^-1 linewidth. To the author's knowledge, it serves to illustrate the first extension to neutral molecular species of the important techniques of resonant multiphoton photodetachment spectroscopy. The resulting two-photon photoionization spectrum is intepreted in terms of the vibrational structure of the resonant intermediate molecular state.

Space-charge amplification of the direct and indirect ionization resulting from photoabsorption in alkali-metal vapor is shown to provide a useful complement to the conventional techniques of absorption spectroscopy. Because of its inherently greater sensitivity, this technique is particularly useful in spectroscopic applications involving weak transitions such as those to highly excited states or absorptive transitions from excited states of low population. This paper reports the use of this technique to extend prior measurements of the oscillator strengths of the (6²S→n ²P) series in Cs, the n ²D doublet separations, and the photoionization spectrum of Cs₂.