Search Results (14 total)

Form-factor measurements in the momentum-transfer range 0.6≤q≤2.7 fm^-1 are presented for the 1^- levels in ¹⁸O at 4.46, 6.20, 7.62, and 8.04 MeV, the 3^- levels at 5.10, 6.40, and 8.29 MeV, and the 5^- levels at 7.86 and 8.13 MeV. These are the first measurements of the 5^- states by inelastic scattering and the first electron-scattering measurements of the higher 1^- and 3^- states. A Rosenbluth separation of the longitudinal and transverse form factors was performed by fitting the data with a phenomenological polynomial-times-Gaussian parametrization motivated by the form of theoretical form factors when harmonic-oscillator wave functions are used. Comparisons are made with structure models. The Coulomb form factors of several levels indicate the significance of small admixtures of 3ħω components in the negative-parity wave functions.

The fourth 2⁺ state in ¹⁸O at an excitation energy of 8.21 MeV has been studied by high-resolution electron scattering. The Coulomb form factor for this state was measured in the momentum-transfer range 0.9≤q≤2.1 fm^-1 and the extracted B(C2)↑ value was determined to be 7.3±4.2 e²fm⁴. The level at 8.21 MeV is identified as a four-particle, two-hole (4p2h) state of the type p^-2(sd)⁴. Form-factor measurements in the range 1.0≤q≤1.9 fm^-1 are also presented for a strongly excited normal-parity state at 9.36 MeV. Preliminary measurements of the differential cross section for exciting this state by 135-MeV protons suggest a 2⁺ assignment; the measured Coulomb form factor of this state allows it to be identified as the best experimental candidate for a predicted K=2 4p2h state with the four sd-shell nucleons coupled to T=0. The excitation of these states and other members of rotational bands in ¹⁸O are discussed within the framework of structure models. Shell-model calculations for the C4 form factors of the first three 4⁺ states in ¹⁸O are compared to existing data.

The angular distributions of cross section and analyzing power for the radiative capture reaction ¹⁵N(p→,γ₀)¹⁶O have been measured at excitation energies of 35.0 and 39.0 MeV in ¹⁶O. Gamma rays were detected in a large, self-shielded NaI spectrometer at eight angles from 23° to 155°. The complex reaction amplitudes associated with the capture channels leading to dipole, quadrupole, and octupole radiation have been extracted. The data at 39 MeV excitation indicate a quadrupole component σ_E2/σ_E1=0.124±0.015, and an octupole component σ_E3/σ_E1=0.0051±0.0026. The observation of high multipolarity radiation is discussed within the context of microscopic calculations which locate large components of the isovector quadrupole and octupole resonances within this region of excitation in ¹⁶O.

Differential cross sections and analyzing powers for scattering of 135-MeV protons by ¹⁶O have been measured for all narrow states below 12.1 MeV of excitation up to a momentum transfer of 3.2 fm^-1. Calculations that employ accurate transition densities fitted to electroexcitation data are used to study medium modifications to the two-nucleon effective interaction with little residual uncertainty from nuclear structure. Definitive evidence for strong density dependence in the isoscalar spin-independent central component of the two-nucleon effective interaction has been found. The differential cross sections show that as the density increases, the strength of the central interaction is suppressed at low momentum transfer and enhanced at high momentum transfer. The analyzing powers exhibit strong negative excursions near 2.5 fm^-1, which support enhanced repulsion at high density. The data are well described by the local-density approximation, which employs the nuclear-matter effective interaction appropriate to the density in the vicinity of the interacting nucleons. We find that the qualitative results are insensitive to ambiguities in the local-density prescription, the local-exchange approximation, and the choice of distorted waves. However, effective interactions based upon the Paris, Bonn, and Hamada-Johnston potentials do give substantially different results. Of these, the Paris-Hamburg effective interaction provides the best description of normal-parity isoscalar transitions. The analysis also supports a rearrangement contribution to the effective interaction for inelastic scattering.

Excited states of ¹⁷O up to an excitation energy of 15 MeV have been studied by high-resolution electron scattering for momentum transfers between 0.8 and 2.6 fm^-1. Previous electron-scattering measurements in this excitation region were confined to momentum transfers below 1.1 fm^-1 and were of lower resolution. Form factors were measured and reduced transition probabilities determined for most excited states below 9.5 MeV. The present data are interpreted within the framework of weak-coupling models to facilitate a simple description of the observed spectrum. Particular emphasis is placed upon understanding states that are excited strongly by electric quadrupole or octupole transitions. Spins and parities have been assigned for several levels that have very narrow widths and, hence, are not amenable to partial-wave analyses involving the ¹⁶O+n and ¹³C+α reactions. Levels at 5.87 MeV ((3/2⁺), 6.86 MeV ((5/2⁺), 7.58 MeV ((7/2⁺), and 8.47 MeV ((9/2⁺) are suggested as candidates for predominantly 5p-4h members of a K^π=(3/2⁺ rotational band. A narrow state (Γ≤20 keV) at 12.22±0.02 MeV was observed for the first time and the existence of narrow states at 8.90±0.02 and 14.72±0.02 MeV was confirmed.

We report electron scattering form factors for known 4^- states in ¹⁶O at 17.79 and 18.98 MeV. We also report statistical upper bounds for the 4^- state at 19.80 MeV. The ratios of these form factors are consistent with the isospin amplitudes derived from pion scattering data. The form factors of levels observed in (e,e’) at 17.880±0.015, 18.635±0.020, and 20.510±0.025 MeV are compatible with 4^- T=1 assignments. Existing (π,π’) and (p,p’) spectra do not contradict these identifications. The 4^- form factors are well described by a harmonic-oscillator density with oscillator length b=1.58 fm. This value is substantially smaller than the value of 1.77 fm obtained from the ground state charge density. The form factor of the 18.98 MeV state can also be fitted by a Woods-Saxon transition density, with parameters fitted to the elastic M5 multipole of ¹⁷O. The fit is improved by inclusion of meson-exchange currents. However, the isoscalar-isovector differences in the Woods-Saxon plus meson-exchange current form factors result in a poor fit to the 17.79 MeV state. The total isovector and isoscalar strengths observed for the established 4^- states are 41% and 23%, respectively, of a pure single-particle prediction.

We report the results of a study of high-resolution inelastic electron scattering from ¹⁷O and ¹⁸O at excitation energies between 15 and 23 MeV. Measurements were performed at 90° for momentum transfers of 1.4, 1.5, 1.7, and 1.9 fm^-1 and at 160° for a momentum transfer of 1.7 fm^-1. Several prominent narrow peaks were observed in the spectra of both nuclei. The measured form factors for levels in ¹⁷O at 15.78, 17.06, 20.14, and 20.70 MeV and in ¹⁸O at 18.70, 20.36, and 22.39 MeV are, within experimental uncertainties, completely transverse. These strongly excited states are assumed to arise from isovector M4 transitions of the type 1p_3/2→1d_5/2. A simple weak-coupling model is used to assign spins to the levels in ¹⁷O. Finally, we also report measurements for several narrow states in both nuclei, for which the spins and parities have not yet been identified.

Differential cross sections for elastic and inelastic scattering of electrons by ¹⁵⁴Gd have been measured for states below 1 MeV of excitation. Levels of three major collective bands are studied: the 0⁺, 2⁺, 4⁺, and 6⁺ rotational states; the 2⁺ gamma vibration; and the 0⁺ and 2⁺ beta vibrational levels. This study includes the first electron scattering measurements of a beta vibration. Extraction of these small inelastic cross sections required improvements in momentum resolution and detector response uniformity. Data from the rotational band are compared with axially symmetric Hartree-Fock predictions. Disagreement between the measurements and Legendre projections of the calculated density, particularly for higher multipolarity states, indicates a breakdown in the Hartree-Fock approximation or the rotational model. Small amplitude vibrations of the nuclear shape derived from the measured rotational densities agree well in shape when independently normalized to the cross section strength for both the beta and gamma 2⁺ vibrations, substantiating their geometrical interpretations. Once transition strength of the beta band is set, however, the strength and shape of the 0⁺ beta is not correctly predicted, indicating a more complicated configuration.

We report high resolution measurements of inelastic electron scattering to all narrow normal-parity states of ¹⁶O up to 12.05 MeV, which include states with J_n^π(E_x MeV): 0₂⁺(6.049), 0₃⁺(12.049), 1₁^-(7.117), 2₁⁺(6.917), 2₂⁺(9.845), 2₃⁺(11.52), 3₁^-(6.130), 4₁⁺(10.356), and 4₂⁺(11.097). The measurements were performed primarily at 90° and 160° and span momentum transfers between 0.6 and 2.6 fm^-1. Improved line shape fitting techniques have been developed. These are the first electron scattering measurements for the 4₂⁺ state and of second form-factor maxima for several states. The form factor for excitation of the 2₂⁺ state is strikingly different from that of the other 2⁺ states and indicates a transition density peaked in the interior. Transition charge densities were extracted from a combined data set that includes earlier electron scattering data renormalized to a recent analysis of elastic scattering from ¹⁶O. Comparisons have been made with several structure models, including the Brown-Green model, the weak-coupling model, a 2ħω shell model, and the tetrahedral alpha-cluster model.

Elastic and inelastic electron-scattering cross sections of an Os-Pt transition region nucleus, ¹⁹²Os, have been measured in a momentum transfer range from 0.6 to 2.9 fm^-1. The data for the ground and the J^π=2⁺, 2^+′, 4⁺, and 3^- states were analyzed model independently with a Fourier-Bessel parametrization of the ground state and transition charge densities. The normalization of the (e,e′) cross sections was obtained from a combined analysis with muonic-atom data for the ground and first 2⁺ states. The densities and their radial moments are compared with theoretical predictions of the Davydov model and with axially symmetric deformed density-matrix-expansion Hartree-Fock calculations (including the Legendre expansion and the small-amplitude vibration model extensions).

The distribution of Gamow-Teller strength in the ¹⁸O(p, n)¹⁸F reaction was studied at a bombarding energy of 135 MeV. Five 1⁺, T=0 states are identified below E_x=7 MeV and a concentration of 1⁺ states of presumed T=1 character is observed between E_x=9.5 and 12 MeV. Approximately 82% of the 1⁺ strength is concentrated into the ground-state transition and only 5.5% is seen in the T=1 component. Normalization of the ground-state transition to the known Gamow-Teller matrix element from the analogous beta decay of ¹⁸Ne allows the (p, n) cross sections to be related to the Gamow-Teller strength. The resulting total Gamow-Teller strength observed in the (p, n) reaction is about two-thirds of the minimum value required by the sum rule for a T=1 nucleus. This result is in reasonable agreement with the total Gamow-Teller strength predicted from a shell-model calculation which uses empirically renormalized single-particle Gamow-Teller matrix elements. The concentration of the T=0 strength predominantly into the ground state and the observed ratio of T=1 to T=0 strength also are consistent with these calculations.

NUCLEAR REACTIONS ¹⁸O(p, n)¹⁸F, E=135 MeV; neutron spectra measured in ∼3° steps between 0° and 69°; angular distributions extracted for separate transitions. Strengths of forward-peaked transitions compared with shell-model predictions of Gamow-Teller strength.

We measured neutron energy spectra and extracted angular distributions for eleven separate transitions for the ¹⁶O(p,n)¹⁶F reaction at 99.1 and 135.2 MeV. Several new spin and parity assignments are obtained for states in ¹⁶F by comparison of the excitation energy spectra with known analog states in ¹⁶O and with a shell-model prediction and by analysis of the neutron angular distributions. The most strongly-excited states are two 2^- states at E_x=0.40±0.05 and 7.6±0.1 MeV, a 4^- state at 6.37±0.05 MeV, and two broad 1^- states at 9.4±0.1 and 11.5±0.1 MeV. These states are analogs of known 2^- (M 2) states, a 4^- "stretched" state, and 1^- (E 1) strength, respectively, in ¹⁶O. Three weakly excited 1⁺ states are observed at E_x=3.75±0.05, 4.65±0.05, and 6.23±0.05 MeV. These states are analogs of known 1⁺ (M 1) states in ¹⁶O and directly indicate correlations in the ground state of ¹⁶O. A weakly excited 4^- state is seen at 5.93±0.05 MeV in good agreement with a 4^- state observed in ¹⁶O (e,e^′) measurements. All of the most strongly excited states align (to within ±200 keV) with known T=1 analog states in ¹⁶O for a common net displacement energy of 12.6 MeV. The (p,n) reaction at medium energies is shown to be an important spectroscopic tool.

NUCLEAR REACTIONS ¹⁶O(p,n)¹⁶F, E=99.1 and 135.2 MeV; measured neutron spectra in ∼3° steps between 0° and 69°; extracted σ(θ) for eleven separate transitions to discrete states in ¹⁶F. Compared excitation energies with analog states in ¹⁶O and with a shell-model prediction. Deduced several new J^π assignments.

We measured the analyzing power for the ¹⁶O(p→,n)¹⁶F (4^-,6.37 MeV) reaction at 134.0 MeV and the differential cross section for the same reaction at 135.2 MeV. The shape of the cross section for the transition to this unnatural parity stretched state is described well by a distorted-wave impulse-approximation calculation using a (πd_{5 / 2},νp_{3 / 2}^-1)_4^- configuration and the effective interaction derived by Love and Franey from nucleon-nucleon phase shifts. The analyzing power from this calculation reproduces all of the qualitative features of the data and supports the use of the impulse approximation as an excellent starting point for describing the reaction mechanism. Quantitative agreement between the experimental and theoretical analyzing power can be improved by eliminating the imaginary tensor term of this interaction and taking the real part to be that derived by Love from the Sussex matrix elements. The sensitivity of the calculations to the choice of optical potentials and the importance of spin-orbit distortion is explored.

NUCLEAR REACTIONS ¹⁶O(p→,n)¹⁶F, E=134 MeV; measured neutron spectra at 12 angles between θ=0° and 62.9°; extracted σ(θ) and A(θ) to J^π=4^-,6.37 MeV state of ¹⁶F. Compared angular distributions of σ(θ) and A(θ) with calculations based on a nucleon-nucleon effective interaction.