Search

Form-factor measurements in the momentum-transfer range 0.6{le}{ital q}{le}2.7 fm{sup {minus}1} are presented for the 1{sup {minus}} levels in {sup 18}O at 4.46, 6.20, 7.62, and 8.04 MeV, the 3{sup {minus}} levels at 5.10, 6.40, and 8.29 MeV, and the 5{sup {minus}} levels at 7.86 and 8.13 MeV. These are the first measurements of the 5{sup {minus}} states by inelastic scattering and the first electron-scattering measurements of the higher 1{sup {minus}} and 3{sup {minus}} 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 3{h bar}{omega} components in the negative-parity wave functions.

Double differential cross sections for inelastic electron scattering from {sup 4}He have been measured at bombarding energies between 279 and 725 MeV. The longitudinal and transverse response functions were obtained for constant momentum transfer between 300 and 500 MeV/{ital c}. Plane-wave impulse approximation calculations overestimate the longitudinal response and produce fair agreement with the transverse part. The Coulomb sum is determined and compared with exact calculations. Within the experimental error the data exhaust the sum rule; this provides a first measurement of ground-state correlations.

Transverse form factors have been extracted for the low-lying neutron hole (particle) states of /sup 207/Pb from inelastic-electron-scattering data. A systematic, multipolarity-and momentum-transfer-independent quenching of approx.55% in the transverse amplitude is observed when compared with single particle predictions for both electric and magnetic transitions. The magnitude of the observed effect is not readily explained by our present theoretical understanding of this nucleus.

Elastic electron scattering has been used to determine the transverse form factor of the /sup 17/O ground state in the effective momentum-transfer range 0.55 < or = q/sub epsilon/ < or = 2.8 fm/sup -1/. The data show considerable deviation from single-particle predictions; in particular, a sizable suppression of the M3 and an enhancement of the high-q side of the M5. Recent shell-model, core-polarization, and meson-exchange calculations are not adequate to explain these effects.

Natural-parity high-spin states of J/sup ..pi../=12/sup +/ (6.10 MeV) and 10/sup +/ (4.89, 5.07, and 5.92 MeV) were observed and identified via inelastic electron scattering. Dominant single-particle--hole configurations in these excitations were deduced. The measured cross sections indicate a reduction of the transverse transition amplitude to 65% of the shell-model prediction, and the absence of an effective charge for the neutron.

We report high resolution measurements of inelastic electron scattering to all narrow normal-parity states of $^{16}\mathrm{O}$ up to 12.05 MeV, which include states with ${J}_{n}^{\ensuremath{\pi}}$(${E}_{x}$ MeV): ${0}_{2}^{+}$(6.049), ${0}_{3}^{+}$(12.049), ${1}_{1}^{\mathrm{\ensuremath{-}}}$(7.117), ${2}_{1}^{+}$(6.917), ${2}_{2}^{+}$(9.845), ${2}_{3}^{+}$(11.52), ${3}_{1}^{\mathrm{\ensuremath{-}}}$(6.130), ${4}_{1}^{+}$(10.356), and ${4}_{2}^{+}$(11.097). The measurements were performed primarily at 90\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{} and span momentum transfers between 0.6 and 2.6 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. Improved line shape fitting techniques have been developed. These are the first electron scattering measurements for the ${4}_{2}^{+}$ state and of second form-factor maxima for several states. The form factor for excitation of the ${2}_{2}^{+}$ 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 $^{16}\mathrm{O}$. Comparisons have been made with several structure models, including the Brown-Green model, the weak-coupling model, a 2\ensuremath{\Elzxh}\ensuremath{\omega} shell model, and the tetrahedral alpha-cluster model.

The elastic magnetic form factor of $^{17}\mathrm{O}$ has been measured for effective momentum transfers 2.47\ensuremath{\le}${\mathrm{q}}_{\mathrm{eff}}$\ensuremath{\le}3.65 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$ by electron scattering. The form factor drops by almost three decades in this range. The data follow an extreme single-particle Woods-Saxon shell-model calculation. Many-body effects such as meson-exchange currents or core polarization do not seem to play a significant role in the q range of this experiment.

Transverse electron scattering form factors from the ground-state rotational band of /sup 181/Ta have been measured to study the single-particle contribution to the magnetization current density. The data are compared with a Hartree-Fock calculation by use of density matrix expansion with filling approximation.

We report the results of a study of high-resolution inelastic electron scattering from /sup 17/O and /sup 18/O at excitation energies between 15 and 23 MeV. Measurements were performed at 90/sup 0/ for momentum transfers of 1.4, 1.5, 1.7, and 1.9 fm/sup -1/ and at 160/sup 0/ for a momentum transfer of 1.7 fm/sup -1/. Several prominent narrow peaks were observed in the spectra of both nuclei. The measured form factors for levels in /sup 17/O at 15.78, 17.06, 20.14, and 20.70 MeV and in /sup 18/O 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/sub 3/2/..-->..1d/sub 5/2/. A simple weak-coupling model is used to assign spins to the levels in /sup 17/O. 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 /sup 154/Gd have been measured for states below 1 MeV of excitation. Levels of three major collective bands are studied: the 0/sup +/, 2/sup +/, 4/sup +/, and 6/sup +/ rotational states; the 2/sup +/ gamma vibration; and the 0/sup +/ and 2/sup +/ 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/sup +/ vibrations, substantiating their geometrical interpretations. Once transition strength of the beta band is set, however, the strength and shape of the 0/sup +/ beta is not correctly predicted, indicating a more complicated configuration.

Excited states of $^{17}\mathrm{O}$ 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 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. Previous electron-scattering measurements in this excitation region were confined to momentum transfers below 1.1 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$ 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 $^{16}$O+n and $^{13}$C+\ensuremath{\alpha} 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}^{\ensuremath{\pi}}$${=(3/2}^{+}$ rotational band. A narrow state (\ensuremath{\Gamma}\ensuremath{\le}20 keV) at 12.22\ifmmode\pm\else\textpm\fi{}0.02 MeV was observed for the first time and the existence of narrow states at 8.90\ifmmode\pm\else\textpm\fi{}0.02 and 14.72\ifmmode\pm\else\textpm\fi{}0.02 MeV was confirmed.

States at 6.42-, 6.75-, and 706-MeV excitation have been observed in electron scattering on /sup 2/08Pb. The transverse character of the excitation cross sections has been established. The states have been interpreted as the v (i/sub 13/2//sup -/1j/sub 15/2/)/sub 12hyphen/,/sup 14hyphen/ and the ..pi.. (h/sub 11/2//sup -/1i/sub 13/2/)/sub 12hyphen/ single-particle hole excitations of the /sup 2/08Pb ground state, on the basis of the measured momentum-transfer dependence and the magnitude of the cross section.

The electromagnetic form factors for the stronger transitions to negative parity states in /sup 24/Mg were measured for electron energies 90--280 MeV and scattering angles of 90/sup 0/ and 160/sup 0/. The isoscalar K/sup ..pi../ = 0/sup -/ and 3/sup -/ bands show form factors in agreement with open-shell random-phase approximation calculations, even though the parentages of these two bands are radically different. For the isovector negative parity states, a quenching of magnetic strength is observed; its origins are discussed.

The unique high-resolution and high-energy features of the electron-scattering facility at the MIT-Bates Accelerator were used to locate a dominant narrow resonance at 15.045 +- 0.035 MeV in /sup 24/Mg. A spin-parity assignment of 6/sup -/ and an isospin T=1 assignment were made. The M6 form factor was measured and compared to the prediction of a theoretical calculation which uses the open-shell random-phase approximation on a shell-model ground state for /sup 24/Mg.

Electron scattering cross section measurements on $^{32}\mathrm{S}$ have been made at incident electron energies between 34 and 74 MeV and at scattering angles of 162.4\ifmmode^\circ\else\textdegree\fi{} and 180\ifmmode^\circ\else\textdegree\fi{}. Form factors were deduced for transitions to states at 8.11, 9.68, 10.05, 10.78, 11.12, and 11.63 MeV. Additional peaks at 7.12, 12.02, and 13.36 MeV were observed in some spectra. Comparisons of cross sections at different angles show that the above six transitions are transverse. Comparison of the experimental form factors with those calculated using an oscillator shell model indicate that the 8.11, 9.68, 11.12, and 11.63 MeV transitions are $M1$. Transition probabilities $B(M1)\ensuremath{\uparrow}=1.14\ifmmode\pm\else\textpm\fi{}0.18$, $0.69\ifmmode\pm\else\textpm\fi{}0.20$, $2.40\ifmmode\pm\else\textpm\fi{}0.22$, and $1.26\ifmmode\pm\else\textpm\fi{}0.20 {\ensuremath{\mu}}_{0}^{2}$, respectively, were determined for these four transitions. The $M1$ form factors and transition probabilities are also compared with other theoretical shell model calculations. The transition at 10.78 MeV is probably $M2$, or a mixture of $M2$ and transverse $E2$ transitions to unresolved states at about that energy.

Inelastic electron scattering from the low-lying even-parity states of $^{18}\mathrm{O}$ has been performed. The measurements span a range in momentum transfer from 0.6 to 2.7 ${\mathrm{fm}}^{\ensuremath{-}1}$. Form factors have been determined for the two lowest ${0}^{+}$ excitations, the three lowest ${2}^{+}$ excitations, and the three lowest ${4}^{+}$ excitations. Transition densities for the ${2}^{+}$ and ${4}^{+}$ excitations have been obtained from a Fourier-Bessel analysis of these data. Comparisons were made with the predictions of various theoretical calculations and the agreement was found generally to be poor. A coexistence model was used to decompose the measured form factors into their single-particle and collective components, which in turn were compared with their counterparts in $^{16}\mathrm{O}$ and $^{17}\mathrm{O}$. The results indicate that this model provides a useful framework within which to understand the structure of the oxygen isotopes. Transition densities also were obtained within the context of this model.NUCLEAR REACTIONS $^{18}\mathrm{O}$($e$,${e}^{\ensuremath{'}}$) low-lying even-parity excited states; measured form factors at 90\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{}, $0.6\ensuremath{\le}q\ensuremath{\le}2.7$ ${\mathrm{fm}}^{\ensuremath{-}1}$; comparison to shell and coexistence models; transition densities extracted.

A cryogenic tritium ( 3 H 2 ) gas target system was constructed and used for a program of electron scattering studies. The 3 H 2 was supplied and safely stored as solid U 3 H 3 . For the experiments, it was transferred to a thin walled cylindrical cell, where it was exposed to 25 μA electron beams with energies up to 800 MeV. During operation, the target cell contained 115000 Ci of tritium at 225 psia and 45 K. Multiple safety enclosures surrounded both target and gas transfer systems. A microprocessor-based control and monitoring system presented target parameters to the operators, identifying those out of range. The target system operated safely and effectively for about 2000 h, enabling completion of comprehensive elastic and elastic electron scattering study of the three nucleon system.

A simple model for nuclear surface vibrations in permanently deformed nuclei does well in reproducing electron scattering cross sections of rotational levels built on a K/sup ..pi../= 0/sup -/ intrinsic octupole vibration in /sup 238/U.

Recent electron scattering results on the 0.850 MeV level of /sup 150/Nd, when analyzed in terms of the interacting boson model, are inconsistent with the interpretation of this level as a pure J/sup ..pi../(K) = 2/sup +/(0) state. Very recent (n,n'..gamma..) work has shown this level to be a 1/sup -/, 2/sup +/ doublet. Assuming this level to be the band head of a ''K/sup ..pi../ = 0/sup -/'' octupole band, a simple model is used to predict electron scattering form factors for the 0.850 MeV state and a 3/sup -/ octupole level observed at 0.931 MeV. Comparison is made between these predicted form factors and recent electron scattering data.

We are constructing a polarized electron source in order to study parity violation in elastic electron‐carbon scattering at the MIT Bates Linac. The source uses a GaAs photocathode illumiated by light from an infrared krypton ion laser. Our design, which uses a multi‐chamber vacuum system, meets the special requirements for operation at Bates.

An experiment is being performed at the MIT Bates Linear Accelerator Center to measure the parity‐violating asymmetry in the elastic scattering of longitudinally polarized electrons from Carbon. The asymmetry, which is due to the isoscalar component of the neutral weak current, is predicted to be about 0.6×10−6. We have recently obtained preliminary data with polarized electrons. The status of the experiment is discussed.

We are performing an experiment at the MIT Bates Linear Accelerator Center to measure the parity‐violating asymmetry in the elastic scattering of polarized electrons from carbon. The asymmetry, which is due to the isoscalar piece of the neutral weak current, is predicted to be about one part per million for our experimental conditions. We have recently obtained preliminary data with polarized electrons. This paper discusses the status of the experiment, with emphasis on our understanding of the most important systematic errors.

The reaction $^{12}\mathrm{C}(e,{e}^{\ensuremath{'}}p)$ in the dip region ($\ensuremath{\omega}=200 \mathrm{MeV}, \mathrm{q}=400 \mathrm{Mev}c$) has been measured in parallel kinematics for missing energies up to 160 MeV. A coincidence yield considerably larger than that expected for a one-body reaction process is observed, though the one-body contribution from $p$- and $s$-shell knockout is also present. A uniform continuum strength extends from beyond the $p$ shell to the highest measured missing energies. This continuum strength is the dominant contribution to the ($e,{e}^{\ensuremath{'}}p$) reaction process in the dip region.