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The Polarized Electrons for Polarized Positrons experiment at the injector of the Continuous Electron Beam Accelerator Facility has demonstrated for the first time the efficient transfer of polarization from electrons to positrons produced by the polarized bremsstrahlung radiation induced by a polarized electron beam in a high-$Z$ target. Positron polarization up to 82\% have been measured for an initial electron beam momentum of 8.19~MeV/$c$, limited only by the electron beam polarization. This technique extends polarized positron capabilities from GeV to MeV electron beams, and opens access to polarized positron beam physics to a wide community., Comment: 5 pages, 4 figures

The ratio of the proton's elastic electromagnetic form factors was obtained by measuring the transverse and longitudinal polarizations of recoiling protons from the elastic scattering of polarized electrons with unpolarized protons. The ratio of the electric to magnetic form factor is proportional to the ratio of the transverse to longitudinal recoil polarizations. The ratio was measured over a range of four-momentum transfer squared between 0.5 and 3.5 GeV-squared. Simultaneous measurement of transverse and longitudinal polarizations in a polarimeter provides good control of the systematic uncertainty. The results for the ratio of the proton's electric to magnetic form factors show a systematic decrease with increasing four momentum squared, indicating for the first time a marked difference in the spatial distribution of charge and magnetization currents in the proton.

Differential cross sections for electron scattering from {sup 140}Ce have been measured for excitation energies less than 3.3 MeV over a range of effective momentum transfer of 0.4 to 3.1 fm{sup {minus}1}. The data have been analyzed to extract transition charge densities. These densities are interpreted in terms of a quasiparticle-phonon model and self-consistent finite Fermi system theory to disentangle the collective and noncollective modes of excitation.

A static analysis of measurements of $^{12}\mathrm{C}$ elastic electron-scattering cross sections demonstrates unambiguously the existence of a form-factor energy dependence beyond that due to Coulomb distortion effects. This energy dependence increases smoothly as a function of momentum transfer and incident energy in the region covered by the experiments (1.0${\mathit{q}}_{\mathrm{eff}}$2.3 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$, 240${\mathit{E}}_{0}$690 MeV). Outside the first diffraction minimum the discrepancies between the form factors deduced from data obtained at different energies are as large as 5%; in the minimum discrepancies as large as 18% have been observed. Including dispersion corrections which are reasonably compatible with this observed energy dependence in the analysis of the data increases the rms charge radius deduced for $^{12}\mathrm{C}$ by 0.007 to 2.478(9), fm, which is in excellent agreement with the value from muonic x-ray studies.

The giant multipole resonances in {sup 116}Sn have been studied using the ({ital e},{ital e}{prime}{ital n}) reaction. Data were taken at effective momentum transfers of 0.37, 0.45, and 0.55 fm{sup {minus}1} and a multipole analysis of the data was performed. The inferred multipole strength functions identify the {ital E}2 and {ital E}0 resonances as distinct peaks at 12.2 and 17.9 MeV, respectively. The energy-weighted sum-rule strengths for the {ital E}2 and {ital E}0 resonances, obtained using a Lorentzian fit to the data, are 34{plus minus}13% and 93{plus minus}37%. When compared with results from alpha scattering and pion scattering the sum-rule strengths exhibit approximate agreement, but the {ital E}0 strength identified in this measurement lies at higher excitation energy, consistent with the trend observed in heavier nuclei. The ({ital e},{ital e}{prime}{ital n}) data are compared with a continuum random phase approximation (RPA) calculation of the {ital E}2 and {ital E}0 strengths, and with an open-shell RPA calculation of the {ital E}2 strength. Both calculations disagree with the data in the region of the {ital E}2 resonance.

We have measured the angular and mass distributions of the fragments from photofission of {sup 238}U using tagged photons with energies between 11 and 16 MeV. The fission fragments were detected by a 4{pi} arrangement of position-sensitive parallel-plate avalanche counters. Anisotropic angular distributions have been observed for the first time in the energy range where second-chance fission becomes energetically possible. A consistent assignment of {ital J}{sup {pi}} and {ital K} for the fission channels in {sup 237}U has been deduced from a combined analysis of ({gamma},{ital nf}) and ({ital e},{ital e}{prime}{ital f}) data. A clear relationship between the anisotropies and the fragment mass asymmetry has also been established. This correlation, together with the energy dependence of the angular distribution parameters, points to a possible interpretation of the results in terms of a recent theoretical model incorporating multiple exit channels in fission.

This paper was published online on 20 May 2005 without several of the authors' corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16--18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

This Conceptual Design Report (CDR) presents the compelling scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab to 12 GeV. Such a facility will make profound contributions to the study of hadronic matter.

The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ^(16)O(e,e′p) reaction in quasielastic, constant (q,ω) kinematics at Q^2≈0.8(GeV/c)^2, q≈1GeV/c, and ω≈445MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P(LL)-P(TT)/epsilon and P(LT), and the electric and magnetic generalized polarizabilities (GPs) alpha(E)(Q2) and beta(M)(Q2) at momentum transfer Q(2)=0.92 and 1.76 GeV2. The electric GP shows a strong falloff with Q2, and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q2, compensated at higher Q2 by a paramagnetic contribution from piN intermediate states.

Exclusive electroproduction of pi{sup 0} mesons on protons in the backward hemisphere has been studied at Q2 = 1.0 GeV2 by detecting protons in the forward direction in coincidence with scattered electrons from the 4 GeV electron beam in Jefferson Lab's Hall A. The data span the range of the total (gamma*p) center-of-mass energy W from the pion production threshold to W = 2.0 GeV. The differential cross sections sigma{sub T} + epsilon sigma{sub L}, sigma{sub TL}, and sigma{sub TT} were separated from the azimuthal distribution and are presented together with the MAID and SAID parameterizations.

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P(LL)-P(TT)/epsilon and P(LT), and the electric and magnetic generalized polarizabilities (GPs) alpha(E)(Q2) and beta(M)(Q2) at momentum transfer Q(2)=0.92 and 1.76 GeV2. The electric GP shows a strong falloff with Q2, and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q2, compensated at higher Q2 by a paramagnetic contribution from piN intermediate states.

A high precision measurement of the transverse spin-dependent asymmetry A_T' in ^3He(e,e') quasielastic scattering was performed in Hall A at Jefferson Lab at values of the squared four-momentum transfer, Q^2, between 0.1 and 0.6 (GeV/c)^2. A_(T') is sensitive to the neutron magnetic form factor, G_M^n . Values of G_M^n at Q^2 = 0.1 and 0.2 (GeV/c)^2, extracted using Faddeev calculations, were reported previously. Here, we report the extraction of G_M^n for the remaining Q^2 values in the range from 0.3 to 0.6 (GeV/c)^2 using a plane-wave impulse approximation calculation. The results are in good agreement with recent precision data from experiments using a deuterium target.

Polarized beam commissioning has been initiated at TJNAF. Measuring the polarization properties of the accelerator, which consists of two superconducting linacs and more than 2200 magnetic elements at its maximum energy, provide useful information about the beam energy and beam transport through the accelerator. Stability of the output beam polarization depends upon time-dependent orbit and energy oscillations. Modeling indicates that the spin transport dynamics are sensitive (∼10−2) to vertical betatron oscillations in the recirculation arcs. This sensitivity is calculated to be enhanced in the arcs where the spin tune is most nearly matched to the vertical betatron tune. Modeling also suggests that depolarization phenomena associated with the particle trajectory differences for electrons within the accelerator’s nominal transverse phase volume are too small to be measured between the injector and an experimental hall. Preliminary results from measurements obtained during polarization development runs wi...

Low energy (Ek=100 keV) Mott scattering polarimeters are ill-suited to support operations foreseen for the polarized electron injector at Jefferson Lab. One solution is to measure the polarization at 5 MeV where multiple and plural scattering are unimportant and precision beam monitoring is straightforward. The higher injector beam current offsets the lower cross-sections. Recent improvements in the CEBAF injector polarimeter scattering chamber have improved signal to noise.

We have measured the coincidence {ital d}({ital {rvec e}},{ital e}{prime}{ital p}) reaction in quasielastic scattering, detecting the proton in a noncoplanar geometry. The electron helicity asymmetry {ital A}{sub {ital e}} and the imaginary part of the longitudinal-transverse interference structure function {ital f}{sub {ital L}{ital T}}{sup {prime}} have been determined at a four-momentum transfer {ital Q}{sup 2}=3.3 fm{sup {minus}2}. The results are compared with theoretical calculations which use realistic potentials for the {ital NN} interaction.

We have measured a new electron scattering observable, the fifth'' structure function, [ital f][sub 01][sup [prime]], which is the imaginary part of the transverse-longitudinal interference response. Its observation requires a longitudinally polarized beam and coincident, out-of-plane particle detection. [ital f][sub 01][sup [prime]] arises from interference between reaction channels and provides an additional means for their disentanglement. In the quasielastic [sup 12]C([ital [rvec e]],[ital e][prime],[ital p]) measurements reported here, [ital f][sub 01][sup [prime]] is driven by the interference of the direct knockout and rescattering amplitudes.

The new generation of electron accelerators provides higher energies, luminosities, and/or duty factors, and presents important physics opportunities for electro‐weak interaction studies and experiments that exploit the spin degrees of freedom in electron scattering. Recent developments in polarized electron source technology promise dramatic improvements over earlier experiments. We review the status of these sources, with emphasis on those best suited for accelerator applications.

We compare the elastic-scattering cross sections for electrons and positrons from {sup 208}Pb and {sup 12}C. The results demonstrate that the electron-nucleus interaction can be described to good accuracy as a single hard-photon exchange process, and validate the theoretical framework used to interpret electron-scattering data in terms of ground-state charge densities of nuclei.

The isoscalar monopole quadrupole giant multipole resonances of /sup 208/Pb have been studied in the excitation region of 9 to 16 MeV by coincident electron scattering. Concentrations of strength have been found at 10.4 and 14.2 MeV, in reasonable agreement with the predictions of mean-field theory. The measurements reported are the first nuclear-structure investigation carried out using the (e,e'n) reaction.

Our investigation of the energy dependence of dispersive effects in elastic electron scattering from {sup 12}C has been extended to a beam energy of 690 MeV. The cross section observed in the minimum of the form factor is larger than that predicted using a static charge density obtained by fitting lower-energy data with the customary phase-shift analysis. This filling-in'' of the minimum at 690 MeV is even larger than that observed previously at 430 MeV. This indicates an increase in the relative strength of higher-order processes in the minimum with an increase of beam energy; the presence of dispersive effects is the most likely explanation for this effect.

The elastic photon scattering cross sections of $^{208}\mathrm{Pb}$ and $^{206}\mathrm{Pb}$ were measured at 90\ifmmode^\circ\else\textdegree\fi{} and 135\ifmmode^\circ\else\textdegree\fi{} in the energy range from 9.5 to 12 MeV with a tagged photon beam whose energy spread was about 125 keV. The $^{206}\mathrm{Pb}$ cross section rises monotonically with energy, and is consistent with a total photon interaction cross section which has a Lorentzian energy dependence with a peak cross section of 650 mb at 13.6 MeV and a width $\ensuremath{\Gamma}\ensuremath{\simeq}3.8$ MeV. The $^{208}\mathrm{Pb}$ scattering cross section is larger and has some rapid variations with energy; there is a narrow extra peak near 10.04 MeV and there are abrupt increases in the cross section just below 10.6 and 11.3 MeV. The relative scattering observed at the two angles indicates that all of the scattering, including the rapid variations with energy, is dominated by dipole interactions. This dipole assignment for the fine structure is important for the proper interpretation of inelastic electron scattering by $^{208}\mathrm{Pb}$. Some of the observed fine structure in inelastic electron scattering must be dipole; the fine structure previously reported as being due to electric quadrupole excitation should be considered as tentative until the correct dipole contributions are included.

We measured protons and alpha particles emitted from the giant electric dipole resonance of $^{28}\mathrm{Si}$ using tagged photons; discrete final states in the residual nuclei were resolved. The proton decay was found to be dominated by the ground state branch. Comparisons with Hauser-Feshbach calculations and spectroscopic factors from nucleon pickup reactions indicate that most proton decay channels have a significant, nonstatistical component. The alpha decay channels appear to be statistical.

A significant energy dependence was observed in the region of the first diffraction minimum of the form factor for elastic electron scattering from /sup 12/C between 238 and 419 (and 431) MeV. This effect might be attributed to dispersive (two-step) processes.

The Nuclear Physics Laboratory at the University of Illinois currently operates a 6-traversal Microtron Using a Superconducting Linac, MUSL-2, which provides 100% duty factor electron beams with energies between 2 and 80 MeV. We have submitted to the National Science Foundation a proposal to construct a 750 MeV cascade microtron capable of delivering 100% duty factor beams with currents of up to 100 ?A. Subharmonic RF splitting of the output beam will provide up to three simultaneous beams for experiments, with independently controllable currents (summing to 100 ?A). The proposal also includes two large building additions and the major new experimental equipment needed to exploit fully the research potential of this new accelerator.

Absolute photofission cross sections of 235,238U have been measured in the energy range 40–105 MeV with “tagged” photons, using parallel plate avalanche counters for a direct detection of the fission fragments. The measured cross sections are nearly constant in the investigated energy range and amount to about 17 mb for both isotopes. The results are compared with recent photoneutron data.

The elastic magnetic form factor of /sup 3/He has been measured up to q/sup 2/ = 32 fm/sup -2/. Few-body theory fails to explain the location of the diffraction minimum unless nonnucleonic degrees of freedom are explicitly introduced.

The ground-state magnetization densities of $^{207}\mathrm{Pb}$ and $^{205}\mathrm{Tl}$ have been studied by 180\ifmmode^\circ\else\textdegree\fi{} elastic electron scattering. The magnetic form factors were found to be quenched substantially relative to the predictions of the independent-particle model. Microscopic calculations indicate that core polarization can account for part of the observed suppression but it is not sufficient to reproduce the data. The quenching observed at high momentum transfers is comparable to that observed in related electron-scattering studies in the lead region, and can be explained in terms of partial occupancy of shell orbits in $^{208}\mathrm{Pb}$.

High resolution inelastic electron scattering measurements on /sup 140/Ce have led to the identification of a J/sup ..pi../ = 12/sup -/ state at 6.31 MeV. This transition has only 8% of the calculated single-particle strength. The absence of strong transverse excitations is interpreted as evidence for fragmentation of the single-particle strength.

The second 6 pass microtron using a superconducting linac has been completed and has operated on 24 hour schedules for nuclear physics for about 5000 hours during the past year. The Q of the linac has remained at 3x109 and the C.W. energy gain has remained at 2.3 MeV/m as in its initial tests in 1975. Single pass C.W. currents up to 20 ?A with energies up to 14.6 MeV have been available for resonance fluorescence experiments. Six pass beams with energies up to 67 MeV have been available to other experimental areas but useful currents have been limited to 0.3 ?A by the excitation of transverse beam blowup modes around 2.3 GHz. The multiple pass currents, however, have been more than sufficient for all tagged photon experiments. Work is proceeding to replace our MUSL-2 linac with another Stanford linac in which the loading of the 2.3 GHz modes is increased by a factor of 100 or more by hybrid electric-magnetic loading probes. Plans to reach higher energies by using MUSL-2 as an injector into a second microtron continue to be attractive.

We report some alternative plans for obtaining continuous electron beams with energies of about 450 MeV by using two microtrons in series. The first stage could be the accelerator we are now operating, which is a six transversal Microtron Using a Superconducting Linac, MUSL-2. The second stage would include another linac which can sustain an r.f. field continuously and a higher energy recirculation system. The recirculation that is best understood and seems to have the least distortion is the conventional microtron with a pair of 180° bending magnets such as are used in MUSL-2. The alternative recirculation system uses four 90° bending magnets, and could be extended to higher energy more easily.

Recent theoretical and experimental investigations of the (e, e’γ) reaction show that it provides a probe of unparalleled precision and selectivity. Experiments aimed towards the isolation of multipole form factors in mixed transitions, the study of continuum excitations in nuclei, and the measurement of the response of the proton are underway at several laboratories.

The second 6 pass microtron using a superconducting linac (MUSL-2) has been completed and has operated on 24 hour schedules for nuclear physics for about 5000 hours during the past year. The Q of the linac has remained at 3x109 and the C.W. energy gain has remained at 2.3 MeV/m as in its initial tests in 1975. Single pass C.W. currents up to 20 ~A with energies up to 14.6 MeV have been available for resonance fluorescence experiments. Six pass beams with energies up to 67 MeV have been available to other experimental areas but useful currents have been limited to 0.3 ~A by the excitation of transverse beam blowup modes around 2.3 GHz. The multiple pass currents, however, have been more than sufficient for all tagged photon experiments. ~7ork is proceeding to replace our MUSL-2 linac with another Stanford linac in which the loading of the 2.3 GHz modes is increased by a factor of i00 or more by hybrid electric-magnetic loading probes. Plans to reach higher energies by using MUSL-2 as an injector into a second microtron continue to be attractive.

The measurement of fragment angular and mass distributions in photofission represents an important tool to investigate the properties of the double humped fission barrier, in particular the spectrum of transition states, which provides direct information on nuclear shapes and possible collective modes. Photoexcitation has the unique feature to be specific to low spin states. The ratio of quadrupole to dipole photofission is directly connected to the relative heights of the bumps of the double humped fission barrier.