Search

Recoil polarimetry was used to extract the ratio of the proton electromagnetic form factors, μ p G E p / G M p = 0.878 ± 0.064 (stat) ± 0.012 (sys) , at Q 2 = 1.13 ( GeV / c ) 2 from the reaction H 1 ( e → , e p → ) . This was an ancillary measurement in which the proton polarization was determined as part of a larger program utilizing a stand-alone polarimeter designed to measure μ n G E n / G M n . This measurement complements previous recoil polarimetry measurements of μ p G E p / G M p made at the Thomas Jefferson National Accelerator Facility.

The ratio of the electric to the magnetic form factor of the neutron, \(g \equiv G_{En}/G_{Mn}\), was measured via recoil polarimetry (R.G. Arnold, C.E. Carlson, F. Gross, Phys. Rev. C 23, 363 (1981)) from the quasielastic 2H\((\mathop{e}\limits^{\scriptstyle\to},e' \mathop{n}\limits^{\scriptstyle\to} )\)1H reaction at three values of Q2 (viz, 0.45, 1.15, and 1.47 (GeV/c)2) in Hall C of the Thomas Jefferson National Accelerator Facility. The data reveal that GEn continues to follow the Galster parameterization up to Q2 = 1.15 (GeV/c)2 and rises above the Galster parameterization at Q2 = 1.47 (GeV/c)2.

In order to test modern theories of two-nucleon bound and scattering states, we have measured the spin polarization of ejectile protons in the electrodisintegration reaction d( e → ,e′ p → )n . Compared with spin-averaged observations, these observables offer the potential of enhanced sensitivity to details of the reaction process. The experiment was carried out in quasielastic kinematics ( q μ q μ =−2m p ω=−0.38 GeV2/c2). Protons were detected at two angles corresponding to neutron recoil momenta pr of 0 and 100 MeV/c. Full nonrelativistic calculations of the polarization transfer components sl and st, including leading order relativistic contributions, describe our measurements well, but calculations of the induced polarization sn at pr=100 MeV/c underpredict the experimental result.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point (theta_lab = 12.3 degrees and Q^2=0.48 (GeV/c)^2) is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor G_E^s. The result, A=-14.5 +- 2.2 ppm, is consistent with the electroweak Standard Model and no additional contributions from strange quarks. In particular, the measurement implies G_E^s + 0.39G_M^s = 0.023 +- 0.034 (stat) +- 0.022 (syst) +- 0.026 (delta G_E^n), where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor., 10 pages, 4 figures, submitted to Phys. Rev. Lett

Jefferson Lab experiment E94-014 measured the excitation of the Delta (1232) resonance via the reaction p(e,e{prime}p){pi}{sup 0} at Q{sup 2} near 2.8 and 4 Gev{sup 2}. This is the highest Q{sup 2} for which exclusive resonance electroproduction has ever been observed. Decay distributions of the Delta (1232) resonance into the ppi{sup 0} final state were measured over a wide range of barycentric decay angles and energies. The goal of this experiment is to assess the transition in Q{sup 2} from the constituent quark model (CQM) to the regime where hard processes become important. At Q{sup 2} {approx} few GeV{sup 2}/c{sup 2} the ratio E{sub 1+}/M{sub 1+} depends dramatically on the theoretical description, varying from a few 10{sup -2} in the CQM limit, to about 1 in the pQCD limit. Preliminary analysis of the data shows that the ratio E{sub 1+}/M{sub 1+} remains small at Q{sup 2} up to 4 Gev{sup 2}/c{sup 2}. After first pass analysis we obtain E{sub 1+}/M{sub 1+} = -4.1 {+-} 1.2 at Q{sup 2} = 2.8 Gev{sup 2}/c{sup 2} and E{sub 1+}/M{sub 1+} = -7.9 {+-} 0.8 at Q{sup 2} = 4 Gev{sup 2}/c{sup 2} (statistical errors only).

Triple-differential cross sections for neutrons from high-multiplicity La-La collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per nucleon were measured at several polar angles as a function of the azimuthal angle with respect to the reaction plane of the collision. The reaction plane was determined by a transverse-velocity method with the capability of identifying charged-particles with Z=1, Z=2, and Z{gt}2. The flow of neutrons was extracted from the slope at mid-rapidity of the curve of the average in-plane momentum vs the center-of-mass rapidity. The {ital squeeze-out} of the participant neutrons was observed in a direction normal to the reaction plane in the normalized momentum coordinates in the center-of-mass system. Experimental results of the neutron squeeze-out were compared with BUU calculations. The polar-angle dependence of the maximum azimuthal anisotropy ratio r({theta}) was found to be insensitive to the mass of the colliding nuclei and the beam energy. Comparison of the observed polar-angle dependence of the maximum azimuthal anisotropy ratio r({theta}) with BUU calculations for {ital free} neutrons revealed that r({theta}) is insensitive also to the incompressibility modulus in the nuclear equation of state. {copyright} {ital 1999} {ital The American Physical Society}

We present a measurement of the induced proton polarization ${P}_{n}$ in ${\ensuremath{\pi}}^{0}$ electroproduction on the proton around the $\ensuremath{\Delta}$ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of $W=1231 \mathrm{MeV}$ and ${Q}^{2}=0.126 {\mathrm{GeV}}^{2}{/c}^{2},$ respectively. We measured a large induced polarization, ${P}_{n}=\ensuremath{-}0.397\ifmmode\pm\else\textpm\fi{}0.055\ifmmode\pm\else\textpm\fi{}0.009.$ The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

Neutron fluences were measured from 435 MeV/nucleon Nb ions stopping in a Nb target and 272 MeV/nucleon Nb ions stopping in targets of Nb and Al for neutrons above 20 MeV and at laboratory angles between 3 degrees and 80 degrees. The resultant spectra were integrated over angles to produce neutron energy distributions and over energy to produce neutron angular distributions. The total neutron yields for each system were obtained by integrating over the angular distributions. The angular distributions from all three systems are peaked forward, and the energy distributions from all three systems show an appreciable yield of neutrons with velocities greater than the beam velocity. Comparison of the total neutron yields from the two Nb + Nb systems suggests that the average neutron multiplicity decreases with decreasing projectile energy. Comparison of the total yields from the two 272 MeV/nucleon systems suggests that the total yields show the same dependence on projectile and target mass number as do total inclusive neutron cross sections. The data are compared with Boltzmann-Uehling-Uhlenbeck model calculations.

Double-differential cross sections and analyzing powers A{sub y} for the ({rvec p},n) reactions on {sup 3}He and {sup 4}He were measured at 200 MeV between {theta}{sub lab}=0{degree} and 44{degree}. The neutron spectra from {sup 3}He({rvec p},n) are dominated by the quasifree scattering peak and show no evidence for resonances in the three-proton system. The spectra from {sup 4}He({rvec p},n) exhibit strong resonance behavior in the p{sup 3}He system at low relative p{sup 3}He energies owing to the excitation of known L=1 resonances in {sup 4}Li, but there is no distinct quasifree peak in the measured spectra. The experimental A{sub y} for {sup 3}He({rvec p},n), averaged over the experimental range of neutron energies, are similar to those for free nucleon-nucleon (NN) scattering, whereas for {sup 4}He({rvec p},n) the A{sub y} are generally larger than the free values. The cross sections at far forward angles for both {sup 3}He and {sup 4}He appear to be suppressed relative to the free {ital NN} cross sections by Pauli blocking. At most angles, the shapes of the cross section spectra from both {sup 3}He and {sup 4}He are reproduced by distorted-wave impulse approximation (DWIA) calculations using a quasifree scattering model. Specifically for {sup 4}He({rvec p},n),more » the model requires the use of an optical potential which has a strong L=1 potential resonance corresponding to the low-lying L=1 states in {sup 4}Li. {copyright} {ital 1998} {ital The American Physical Society}« less

We measured the analyzing power and the efficiency of a new neutron polarimeter that was designed to measure G/sub E//sup n/, the neutron electric form factor. The polarimeter calibration was performed as experiment E377 at the Indiana University Cyclotron Facility (IUCF) with the /sup 14/C(p/spl I.oarr/,n/spl I.oarr/)/sup 14/N reaction at proton beam energies of 124.0, 164.6, and 199.7 MeV without any shielding material ahead of the polarimeter; in addition, at 164.6 MeV we measured the analyzing power with 10 cm of lead, sandwiched between 3.5-cm iron plates, ahead of the polarimeter.

The first measurements of the induced proton polarization Pn for the 12C(e,e',p) reaction are reported. The experiment was performed at quasifree kinematics for energy and momentum transfer (w,q) = (294 MeV, 765 MeV/c) and sampled a missing momentum range of 0-250 MeV/c. The induced polarization arises from final-state interactions and for these kinematics is dominated by the real part of the spin-orbit optical potential. The distorted-wave impulse approximation provides good agreement with data for the 1 p3/2 shell. The data for the continuum suggest that both the 1s1/2 shell and underlying l > 1 configurations contribute.

We measured the spin observables ${A}_{y},$ $P$, and ${S}_{{\mathrm{NN}}^{\ensuremath{'}}}$ for the ${}^{208}$Pb${(p,n)}^{208}$Bi reaction at 135 MeV at laboratory angles of 0$\ifmmode^\circ\else\textdegree\fi{}$, 3$\ifmmode^\circ\else\textdegree\fi{}$, 6$\ifmmode^\circ\else\textdegree\fi{}$, and 9$\ifmmode^\circ\else\textdegree\fi{}$. The overall energy resolution was about 1 MeV. Data for ${S}_{{\mathrm{NN}}^{\ensuremath{'}}}$ are compared with distorted-wave impulse-approximation calculations that use random-phase approximation wave functions. Comparisons are also made for the ${}^{48}$Ca${(p,n)}^{48}$Sc reaction. The agreement between these calculations and the data is generally good, after adjustment of the nucleon-nucleon interaction in the random-phase approximation calculations to place the 0${}^{+}$ isobaric-analog states at the correct excitation energies. A single adjustment of the nucleon-nucleon force works for both target nuclei.

Cross-section excitation-energy spectra and angular distributions were measured for the $^{2}\mathrm{H}$(p,n)2p reaction at 135 MeV in 6\ifmmode^\circ\else\textdegree\fi{} steps from 0\ifmmode^\circ\else\textdegree\fi{} to 30\ifmmode^\circ\else\textdegree\fi{} (laboratory), using the beam swinger facility at the Indiana University Cyclotron Facility. The target was a 12.8 mg/${\mathrm{cm}}^{2}$ foil of ${\mathrm{CD}}_{2}$. Neutron energies were measured by the time-of-flight method using large-volume plastic scintillator arrays at flight paths of 91 m. The overall energy resolution was 260 keV. The 0\ifmmode^\circ\else\textdegree\fi{} spectrum is dominated by a large peak near 0 MeV of relative energy in the final 2p system; this peak corresponds to the two protons in the $^{1}$${\mathit{S}}_{0}$ state. The wider-angle spectra are dominated by a broad peak centered at 10 to 20 MeV of excitation which is the quasifree scattering peak. The spectra are compared with impulse approximation and three-body Faddeev calculations. \textcopyright{} 1996 The American Physical Society.

Gamow-Teller (1{sup +}) strength was studied in {sup 38}K with the analog reactions {sup 38}Ar({ital p},{ital n}){sup 38}K and {sup 38}Ca({beta}{sup +}){sup 38}K. The ({ital p},{ital n}) reaction was performed at 135 MeV using the beam swinger facility at the Indiana University Cyclotron Facility. Excitation-energy spectra were measured at 15 angles between 0{degree} and 63{degree}. Neutron energies were measured by the time-of-flight method using a large-volume plastic scintillator array at a flight path of 131.0 m. The overall energy resolution was 280 keV. Gamow-Teller (GT) strength was extracted from the measured angular distributions to discrete 1{sup +} final states. The {beta}-decay experiment was performed with the ISOLDE on-line mass separator facility at CERN. The {beta}-decay branching ratios were determined by observing the delayed {gamma} decays of {sup 38}K. These decay measurements provide an increased sensitivity over earlier measurements and are able to extract transitions down to {approximately} 10{sup {minus}4} of the strongest branches. The {ital B}(GT) values obtained from the two experiments are generally in good agreement, except for the transition to the first 1{sup +} state at 0.46 MeV, which is observed to be much weaker in the ({ital p},{ital n}) measurements. The {beta}-decay measurements provide good resolution andmore » high sensitivity while the ({ital p},{ital n}) measurements extend the {beta}-decay measurements to higher excitation energies. The summed {ital B}(GT) strength is {approximately}50{percent} of the simple Ikeda sum rule. The distribution of GT strength is in reasonable agreement with that predicted from a shell-model calculation using {open_quote}{open_quote}effective{close_quote}{close_quote} GT operators. {copyright} {ital 1996 The American Physical Society.}« less

We describe how polarization-transfer data can be used to identify J π = 0 − , 1 − and 2 − components of the Δ T = 1, Δ L = 1, Δ S = 1 isovector “spin-dipole” resonance seen in charge-exchange reactions at medium energies. Data for 40 Ca(p,n) are compared with distorted-wave impulse approximation (DWIA) calculations with theory of finite Fermi systems (TFFS) wave functions.

Double-differential cross sections, d2σ(dΩdEx), for the inclusive 4He(p, n) reaction were measured at 100 and 200 MeV. The data contradict previous evidence for narrow states in 4Li and are in qualitative agreement with predictions using continuum shell model wave functions. These data are needed to constrain models of neutron production in solar flares and in the atmospheres of accreting neutron stars.

We measured the analyzing power {ital A}{sub {ital y}} and the triple differential cross section {ital d}{sup 3}{sigma}/{ital d}{Omega}{sub {ital pd}}{Omega}{sub {ital ndE}}{sub {ital p}} for the {sup 2}H({ital {rvec p}},{ital pn}){sup 1}H reaction at 200 MeV. Coplanar coincidence data were taken for all combinations of neutron angles {theta}{sub {ital n}}=35{degree}, 45{degree}, or 55{degree} with proton angles {theta}{sub {ital p}}=35{degree}, 45{degree}, or 52{degree}. Protons were detected with a {Delta}{ital E}-{ital E} telescope with a 1000-{mu}m silicon surface barrier {Delta}{ital E} detector and a plastic scintillator {ital E} detector. Neutrons were detected with large-volume plastic scintillators at flight paths of 17.5 or 18 m. The overall neutron separation-energy resolution was about 3 MeV. Data are compared with plane-wave impulse-approximation calculations with a Hulthen deuteron wave function and {ital p}-{ital n} cross sections and analyzing powers obtained from {ital N}-{ital N} phase shifts. The agreement between these calculations and the data is generally good for the cross sections. The agreement for the analyzing powers is good near the point of zero recoil momentum. Our results suggest that the deuteron is a good ``neutron target`` for recoil momenta {lt}100 MeV/{ital c}.

The {sup 20}Ne({ital p},{ital n}){sup 20}Na reaction at 135 MeV was studied experimentally using the beam-swinger neutron time-of-flight facility at the Indiana University Cyclotron Facility. Backgrounds {similar_to}10{times} lower than an earlier experiment were obtained using the ``stripper loop`` storage ring to achieve {similar_to}2 {mu}s between beam bursts. We were able to see a weakly excited state at 2.65 MeV in {sup 20}Na with a {Delta}{ital l}=2 angular distribution. The angular distribution is consistent with a DWIA calculation assuming a 3{sup +} final state. The characteristics of this state are important to know for considerations of possible breakout in the hot CNO cycle in supernovae explosions.

Virtual Compton Scattering (VCS) on the proton has been studied at Jefferson Lab using the exclusive photon electroproduction reaction (e p --> e p gamma). This paper gives a detailed account of the analysis which has led to the determination of the structure functions P{sub LL}-P{sub TT}/epsilon and P{sub LT}, and the electric and magnetic generalized polarizabilities (GPs) alpha{sub E}(Q{sup 2}) and beta{sub M}(Q{sup 2}) at values of the four-momentum transfer squared Q{sup 2} = 0.92 and 1.76 GeV{sup 2}. These data, together with the results of VCS experiments at lower momenta, help building a coherent picture of the electric and magnetic GPs of the proton over the full measured Q{sup 2}-range, and point to their non-trivial behavior.

We determined the electric form factor [ital G][sup [ital n]][sub [ital E]] of the neutron from the quasielastic [sup 2]H([ital [rvec e]],[ital e][sup [prime]][ital [rvec n]])[sup 1]H reaction at a central squared four-momentum transfer [ital Q][sup 2]=0.255 (GeV/[ital c])[sup 2] with a longitudinally polarized electron beam of 868 MeV and a low ([similar to]0.8%) duty factor. A neutron polarimeter designed and constructed specifically for this experiment was used to measure the sideways polarization of the recoil neutron, which was detected in coincidence with the scattered electron. Theoretical calculations have established that this polarization-transfer technique for quasielastic scattering produces a value of [ital G][sup [ital n]][sub [ital E]] that shows little sensitivity to the influence of final-state interactions, meson-exchange currents, isobar configurations, and deuteron structure. The value for [ital G][sup [ital n]][sub [ital E]] from this measurement is 0.066[plus minus]0.036[plus minus]0.009.

The spin observable D NN ′ ( θ ) was measured for ( p , n ) at 135 MeV on 16 O, 40 Ca, 48 Ca, and 208 Pb. Data for 16 O and 40 Ca are compared with distorted-wave impulse approximation (DWIA) calculations with Tamm-Dancoff (TDA) wave functions; the D NN ′ data generally agree with the J π distribution (O − , 1 − , 2 − ) predicted for the “spin-dipole” resonance.

We measured the first azimuthal distributions of triple--differential cross sections of neutrons emitted in heavy-ion collisions, and compared their maximum azimuthal anisotropy ratios with Boltzmann--Uehling--Uhlenbeck (BUU) calculations with a momentum-dependent interaction. The BUU calculations agree with the triple- and double-differential cross sections for positive rapidity neutrons emitted at polar angles from 7 to 27 degrees; however, the maximum azimuthal anisotropy ratio for these free neutrons is insensitive to the size of the nuclear incompressibility modulus K characterizing the nuclear matter equation of state., Comment: Typeset using ReVTeX, with 3 ps figs., uuencoded and appended

The [sup 2]H([ital e],[ital e][prime][ital n])[sup 1]H quasielastic cross section was measured at [ital Q][sup 2] values of 0.109, 0.176, and 0.255 (GeV/[ital c])[sup 2]. The neutron detection efficiency was determined by the associated particle technique with the [sup 2]H([gamma],[ital pn]) reaction for each of the three neutron kinetic energies. These [sup 2]H([ital e],[ital e][prime][ital n]) measurements of the coincidence cross sections are the first at low [ital Q][sup 2]. The cross sections are sensitive primarily to the neutron magnetic form factor [ital G][sub [ital M]][sup [ital n]] at these kinematics. The extracted [ital G][sub [ital M]][sup [ital n]] values have smaller uncertainties than previous data and are consistent with the dipole parametrization at the two higher momentum transfers; at the lowest momentum transfer, the value of [ital G][sub [ital M]][sup [ital n]] is [similar to]10% higher than the dipole value.

Differential cross sections were measured and compared to distorted-wave impulse-approximation calculations for the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${4}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{d}}_{5/2}$,\ensuremath{\pi}${\mathit{p}}_{3/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{12}\mathrm{C}$(n,p${)}^{12}$B (4.52 MeV) reaction, the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${6}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu},${\mathit{f}}_{7/2}$\ensuremath{\pi}${\mathit{d}}_{5/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{28}\mathrm{Si}$(n,p${)}^{28}$Al (5.175 MeV) reaction, and the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{h}}_{11/2}$,\ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$) state excited in the $^{120}\mathrm{Sn}$(n,p${)}^{120}$In (\ensuremath{\simeq}0.0 MeV) reaction at 300 MeV. We obtained a normalization factor of 0.68 for the ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${10}^{\mathrm{\ensuremath{-}}}$(\ensuremath{\nu}${\mathit{h}}_{11/2}$,\ensuremath{\pi}${\mathit{g}}_{9/2}^{\mathrm{\ensuremath{-}}1}$) state excited in $^{120}\mathrm{Sn}$(n,p${)}^{120}$In (\ensuremath{\simeq}0.0 MeV) reaction; this is the first ``1\ensuremath{\Elzxh}\ensuremath{\omega}'' stretched state known in this mass region.

We report the first neutron triple-differential cross-sections measured in multiplicity-selected heavy-ion collisions. We examine the maximum azimuthal anisotropy ratio of these neutron cross-sections from 400 AMeV Nb-Nb and Au-Au collisions as a probe of the nuclear equation of state. Experiment reveals that this ratio does not depend on the mass of the colliding nuclei, contrary to the predictions of the Quantum Molecular Dynamics (QMD) transport theory. We compare the Nb-Nb data also with theoretical calculations done within the Boltzmann-Uehling-Uhlenbeck framework with both momentum-dependent and momentum-independent interactions. We find that the experiment is sensitive to the size of the nuclear incompressibility modulus K in the nuclear matter equation of state, and that the data demand a K value compatible with state-of-the-art nuclear matter calculations.

High-spin, stretched-state excitations were studied in the $^{38}\mathrm{Ar}$(p,n${)}^{38}$K reaction at 135 MeV with the beam-swinger system at the Indiana University Cyclotron Facility. Neutron kinetic energies were measured by the time-of-flight technique with large-volume plastic-scintillator neutron detectors at flight paths ranging from 81 to 131 m. Overall time resolutions of about 825 ps provided energy resolutions from 320 to 450 keV. The target was a 4-cm-long gas cell filled to \ensuremath{\sim}3 atm absolute. Angular distributions were extracted for the low-lying excitations observed at wide angles. A known ${7}^{+}$ state at ${\mathit{E}}_{\mathit{x}}$=3.5 MeV was observed, plus two states at 5.3 and 5.9 MeV, which are tentatively identified as ${6}^{\mathrm{\ensuremath{-}}}$ excitations, based on comparisons with distorted wave Born approximation and sdpf shell-model calculations. The $^{38}\mathrm{K}$ spectrum is in reasonable agreement with the shell-model calculations. In contrast to the analysis of 80 MeV $^{40}\mathrm{Ca}$(d,\ensuremath{\alpha}${)}^{38}$Ar measurements, these results do not indicate a significant problem for the nuclear shell model near A=40.

The polarization ({ital P}) and the analyzing power ({ital A}) were measured for the {sup 48}Ca({ital p},{ital n}){sup 48}Sc reaction at 135 MeV. Of special interest is the difference ({ital P}{minus}{ital A}) in these two quantities for the transitions to the two 1{sup +} states at 2.52 and 16.8 MeV. Because of the difference in the predominant one-particle--one-hole configurations for these two states, viz., ({ital f}{sub 7/2},{ital f}{sub 7/2}{sup {minus}1}) and ({ital f}{sub 5/2},{ital f}{sub 7/2}{sup {minus}1}), respectively, qualitative differences in {ital P}{minus}{ital A} are predicted based on nonlocal exchange contributions. The experimental results agree qualitatively with these predicted differences, but there are significant quantitative differences indicating that other effects contribute as well.

Isovector stretched-state excitations were studied in the (p,n) reaction at 136 MeV on the self-conjugate s-d shell nuclei $^{20}\mathrm{Ne}$, $^{24}\mathrm{Mg}$, $^{28}\mathrm{Si}$, and $^{32}\mathrm{S}$. The measurements were performed in three separate experiments using the beam-swinger neutron time-of-flight facility at the Indiana University Cyclotron Facility. Neutrons were detected in large-volume plastic scintillation detectors located in three detector stations at 0\ifmmode^\circ\else\textdegree\fi{}, 24\ifmmode^\circ\else\textdegree\fi{}, and 45\ifmmode^\circ\else\textdegree\fi{} with respect to the undeflected beam line; the flight paths varied from 75 to 134 m. Overall time resolutions of about 825 ps provided energy resolutions of about 320 keV in the first two stations and about 425 keV in the third station. Both (${\mathit{d}}_{5/2}$,${\mathit{d}}_{5/2}^{\mathrm{\ensuremath{-}}1}$)${5}^{+}$ 0\ensuremath{\Elzxh}\ensuremath{\omega} and (${\mathit{f}}_{7/2}$,${\mathit{d}}_{5/2}^{\mathrm{\ensuremath{-}}1}$)${6}^{\mathrm{\ensuremath{-}}}$ 1\ensuremath{\Elzxh}\ensuremath{\omega} stretched-state excitations are observed in the (p,n) reaction on $^{20}\mathrm{Ne}$, $^{24}\mathrm{Mg}$, and $^{28}\mathrm{Si}$.For the (p,n) reaction on $^{32}\mathrm{S}$, only the ${6}^{\mathrm{\ensuremath{-}}}$ strength is observed. The states are identified by comparison of the extracted angular distributions with distorted-wave impulse-approximation calculations and by comparison with known analog states observed in inelastic scattering. In the $^{20}\mathrm{Ne}$ and $^{24}\mathrm{Mg}$(p,n) reactions, the majority of the ${5}^{+}$ strength is observed in single states at low excitation energy; the strengths are described well by full s-d shell-model calculations. A weak ${5}^{+}$ state is observed in the $^{28}\mathrm{Si}$(p,n${)}^{28}$P reaction that corresponds to the strength expected if there is \ensuremath{\sim}1/3 of a hole in the ${\mathit{d}}_{5/2}$ proton orbital. The ${6}^{\mathrm{\ensuremath{-}}}$ 1\ensuremath{\Elzxh}\ensuremath{\omega} strength is fragmented in all four reactions. Less than 60% of the strength expected in the extreme single-particle-hole model is observed in each case. The observed ${6}^{\mathrm{\ensuremath{-}}}$ strengths and distributions in the $^{28}\mathrm{Si}$ and $^{32}\mathrm{S}$(p,n) reactions are described well by large-basis shell-model calculations.

We have made the first measurements of the virtual Compton scattering (VCS) process via the H$(e,e'p)\gamma$ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the $W$-dependence at fixed $Q^2=1$ GeV$^2$, and for the $Q^2$-dependence at fixed $W$ near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed $Q^2$-dependence is smooth. The measured ratio of H$(e,e'p)\gamma$ to H$(e,e'p)\pi^0$ cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to Real Compton Scattering (RCS) at high energy and large angles, our VCS data at the highest $W$ (1.8-1.9 GeV) show a striking $Q^2$- independence, which may suggest a transition to a perturbative scattering mechanism at the quark level., Comment: 20 pages, 8 figures. To appear in Phys.Rev.C

Pion absorption by 3He was studied at Tπ=165 MeV in a kinematically complete experiment. The cross section for absorption on a (pn) pair of nucleons, σpn(π+), was found to be 17.0±2.6 mb; that for absorption on a (pp) pair, σpp(π-), 0.91±0.20 mb. The angular distribution in the πNN center-of-mass system for σpn(π+) resembles that for the π++d→p+p reaction while the angular distribution for σpp(π-) is strongly backward peaked. Evidence that a significant fraction of the absorptions involves all three target nucleons is seen in the angular correlation between the two detected nucleons as well as in the momentum distribution of the unobserved nucleon. For π+ and π- absorption, the three-body cross sections were found to be 9.6±2.1 and 4.2±01.2 mb, respectively. Neither initial- nor final-state interactions appear to be major contributors to the observed three-body absorption, though initial-state interactions may be contributing to the enhancement of the three-body π+ absorption at the Δ resonance.

A new equation for the total net retention volume is used to describe pressuredependent retention measurements for cyclohexane and cyclohexene on wide-pore and narrow-pore silicas. This equation gives a good representation of the above experimental data. Parameters of this equation permit evaluation of the distribution functions that characterize the energetic heterogeneity of solid adsorbents.

Gamow-Teller (GT) strength distributions were studied in the ({ital p},{ital n}) reaction at 136 MeV on the self-conjugate {ital s}-{ital d} shell nuclei {sup 20}Ne, {sup 24}Mg, and {sup 28}Si. The measurements were performed in two separate experiments with the beam-swinger neutron time-of-flight facility at the Indiana University Cyclotron Facility. The flight paths were 91 and 131 m, respectively, for the two experiments. The neutrons were detected in large-volume plastic-scintillation detectors. The overall time resolutions were about 825 ps; this provided energy resolutions from 300 to 400 keV. GT strength was identified as {Delta}{ital l}=0 contributions in transitions to discrete final states and also in the background and continuum. The 0{degree}, {Delta}l=0 cross sections were converted to {ital B}(GT) units using a universal'' conversion formula calibrated to ({ital p},{ital n}) reactions on other even-even {ital s}-{ital d} shell nuclei. The resulting {ital B}(GT) distributions were compared with full {ital s}-{ital d} shell-model predictions. The distribution for {sup 24}Mg is described well, but the distributions for {sup 20}Ne and {sup 28}Si are described poorly. The total {ital B}(GT) strength observed in discrete states (up to 12 MeV of excitation) for each reaction is 65{plus minus}10 % of that predicted. If onemore » considers {ital B}(GT) strength observed in the continuum above a calculated quasi-free-scattering background, the strength increases to 70--100 % of that predicted. If one considers {ital B}(GT) strength in an analysis of the full continuum (up to {similar to}20 MeV), the entire amount predicted may be observed. These results are consistent with that observed in other light nuclei.« less

A theoretical description of gas adsorption on heterogeneous microporous solids is compared with that based on the fractal nature of these solids. It is shown that the structural parameters of a microporous solid correlate with the fractal dimension that characterizes the geometrical irregularities of microporous solids. Numerical studies performed for model solids with different degrees of microporosity show that the root-mean-square value of micropore size depends linearly on the fractal dimension.

Background: The strength of electron capture for medium mass nuclei has a significant effect on the evolution of supernovae. There is insufficient knowledge of these strengths and very little data for important radioactive nuclei. Purpose: Determine whether it is feasible to obtain EC strength from studies of ${T}_{o}+1$ excitations in ($p$, $n$) reactions, and whether this might yield information for radioactive nuclei. Methods: Cross sections for the $^{58,60,62,64}\mathrm{Ni}$($p$, $n$)$^{58,60,62,64}\mathrm{Cu}$ reactions were measured over the angular range of $0.3{}^{\ensuremath{\circ}}$ to $11.6{}^{\ensuremath{\circ}}$ at 134.3 MeV using the IUCF neutron time-of-flight facility. Results: The ${T}_{o}+1$ excitations in $^{60,62}\mathrm{Ni}$ were identified by comparison with inelastic proton scattering spectra, their $B$(GT) were extracted, and the corresponding electron capture rates in supernovae were calculated. Data from the TRIUMF ($n$, $p$) experiments at 198 MeV were reanalyzed; the electron capture rates for the reanalyzed data are in moderately good agreement with the higher resolution ($p$, $n$) results, but differ in detail. The possibility of future measurements with radioactive nuclei was considered. Conclusions: It may be possible to obtain low-lying electron capture strength for radioactive nuclei by studying ($p$, $n$) reactions in inverse kinematics.

Inclusive neutron spectra were measured at 0\ifmmode^\circ\else\textdegree\fi{}, 4\ifmmode^\circ\else\textdegree\fi{}, 8\ifmmode^\circ\else\textdegree\fi{}, 15\ifmmode^\circ\else\textdegree\fi{}, 30\ifmmode^\circ\else\textdegree\fi{}, and 42\ifmmode^\circ\else\textdegree\fi{} from Nb-Nb and Au-Au collisions at 800 MeV/nucleon. A peak that originates from neutron evaporation from the projectile appears in the spectra at angles out to 8\ifmmode^\circ\else\textdegree\fi{}. The shapes and magnitudes of the spectra are compared with those calculated from models of nucleus-nucleus collisions. The differential cross sections for Au-Au collisions are about four times those for Nb-Nb collisions. The predictions of the Vlasov-Uehling-Uhlenbeck (VUU) and QMD theories agree with the angular distributions of the differential cross sections except at small angles; the VUU prediction overestimates the angular distributions from a few degrees to about 20\ifmmode^\circ\else\textdegree\fi{}, whereas the QMD prediction underestimates the angular distributions below 8\ifmmode^\circ\else\textdegree\fi{}. The Firestreak model overestimates the angular distribution for Nb-Nb collisions and underestimates it for Au-Au collisions. Also, the VUU and QMD models agree with the measured double-differential cross sections in more angular and energy regions than the Firestreak and intranuclear cascade models; however, none of the models can account for the peaks at small angles (\ensuremath{\theta}\ensuremath{\le}15\ifmmode^\circ\else\textdegree\fi{}).

The Jaroniec-Choma (JC) isotherm equation for adsorption of gases and vapors on heterogeneous microporous solids is associated with a gamma distribution function F(z = 1/E0), where z is the inverse characteristic energy that appears in Dubinin-Astakhov (DA) equation. The distribution F(z) characterizes uniquely a microporous solid because its evaluation does not require the knowledge of the relationship between the micropore dimension x and the characteristic energy E0; however, this relationship is necessary to calculate the micropore-size distribution function J(x), which can be obtained easily from the distribution F(z) for any arbitrary relationship x(z). It is shown that the shape of the micropore-size distribution J(x) depends significantly on the relationship x(z), and that the relationship x(z) found experimentally by Stoeckli leads to a decreasing function J(x), which coincides with that obtained for fractally microporous solids over a large region of micropore sizes. The benzene adsorption isotherm on a microporous activated carbon is used to illustrate the proposed procedure for evaluating structural heterogeneities and surface irregularities of microporous activated carbons.

The reaction plane of high-multiplicity Au-Au collisions at 650, 400, 250, 150, and 75 MeV/nucleon was determined by measuring the transverse velocity of charged fragments. Measurements of the collective flow of charged fragments reveal that the onset of flow in high-multiplicity Au-Au collisions occurs at a finite beam energy below 60 MeV/nucleon.

Thermodynamic equations are derived for physical adsorption of gases and vapors on fractal surfaces of heterogeneous microporous solids. The influence of surface geometry on gas adsorption on a microporous solid is studied by analyzing the dependence of the differential molar enthalpy ΔH, the immersion enthalpy ΔHim, and the differential molar entropy ΔS on the fractal dimension D, which is used to characterize the surface irregularity. It is shown that −ΔH, −ΔHim, −ΔS, and the average adsorption potential A increase as the fractal dimension of the surface accessible for adsorption increases. The dependence of these thermodynamic quantities on the fractal dimension D is attributed to the fact that the fraction of small micropores in microporous solids increases as the fractal dimension D increases.

The well-known BET isotherm equation is used to describe benzene adsorption on microporous activated carbons of different degrees of microporosity. It is shown that the parameters of this equation evaluated for benzene-activated carbon systems correlate with the micropore parameters that are obtained by means of the Jaroniec-Choma equation. This correlation indicates that the BET parameters provide also some information about the microporosity of the solid adsorbents.

The outlet concentration as a function of time for pulsed and stepped inputs of 100 ppm concentrations of methane, acetylene, and acetaldehyde adsorptives in helium at 1 atm pressure through columns of activated carbon at 25°C were measured and correlated with the solutions to a longitudinal-diffusion model with a linear isotherm. The solutions provided good fits to the experimental data for adsorptives with linear and almost linear isotherms, viz., methane and acetylene, respectively. The nonlinear isotherm of acetaldehyde precludes the determination of the longitudinal diffusion coefficient.

The $^{54}\mathrm{Fe}$(p,n${)}^{54}$Co reaction was studied at 135 MeV by the time-of-flight technique. Neutrons were detected with an energy resolution of 245 keV in large-volume, mean-timed counters at a flight path of 125 m. The forward-angle spectra are dominated by the excitation of ${1}^{+}$ states with characteristic \ensuremath{\Delta}l=0 angular distributions peaked at 0\ifmmode^\circ\else\textdegree\fi{}. The strengths of the ${1}^{+}$ excitations are interpreted as being equivalent to Gamow-Teller (GT) strengths excited in beta decay. This strength is observed to be highly fragmented; GT strength is identified in more than 30 states. The B(GT-) strength is obtained relative to the Fermi strength B(F) assumed to be concentrated in the ${0}^{+}$, isobaric-analog ground state. The total B(GT-) strength observed in discrete states, combined with the B(GT+) strength obtained from the (n,p) reaction, yields a lower limit of 48 percent of the 3(N-Z) sum rule. Inclusion of \ensuremath{\Delta}l=0 strength in the background and continuum above a quasifree scattering background increases this lower limit to 73 percent. If one considers the \ensuremath{\Delta}l=0 strength observed in the full background and continuum up to ${\mathit{E}}_{\mathit{x}}$=25 MeV, the entire sum rule may be satisfied. The observed distribution of GT strength is in good agreement with a truncated 1f-2p shell-model calculation.

The $^{45}\mathrm{Sc}$(p,n${)}^{45}$Ti reaction was studied at 135 MeV with a beam-swinger system. Neutron kinetic energies were measured by the time-of-flight technique, with neutron detectors located in three detector stations at 0\ifmmode^\circ\else\textdegree\fi{}, 24\ifmmode^\circ\else\textdegree\fi{}, and 45\ifmmode^\circ\else\textdegree\fi{} with respect to the undeflected beam. Flight paths of 131.0, 131.1, and 81.3 m were used. The overall timing resolution was about 825 ps, providing energy resolutions of 320 keV in the first two stations and 520 keV in the third. The wide-angle spectra are dominated by a complex at ${\mathit{E}}_{\mathit{x}}$=4.3 MeV. The angular distribution for this complex is fitted well by a distorted-wave impulse approximation calculation for an assumed ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=17/${2}^{\mathrm{\ensuremath{-}}}$,19/${2}^{\mathrm{\ensuremath{-}}}$ (\ensuremath{\pi}${\mathit{f}}_{7/2}^{2}$,\ensuremath{\nu}${\mathit{f}}_{7/2}^{\mathrm{\ensuremath{-}}1}$) doublet. The spectra and angular distribution are consistent with (p,${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$) excitations of the same states and are in good agreement with a full 1f-2p shell-model calculation. The 17/${2}^{\mathrm{\ensuremath{-}}}$,19/${2}^{\mathrm{\ensuremath{-}}}$ strength predicted by distorted-wave impulse approximation calculations using these shell-model wave functions is in good agreement with the experimental measurements.

The adsorption excess of a component from a binary nonelectrolytic liquid mixture on a porous solid is expressed as a simple sum of the adsorption excesses referring to fine pores (micropores) and large pores (mesopores and macropores). Because the energetic state of molecules adsorbed in the micropores differs from that of molecules adsorbed on the mesopore surface, a comparison of the excess adsorption isotherm on a porous solid with that measured on a reference nonporous solid permits evaluation of the adsorption capacities for the micropores and the mesopores. Experimental examples indicate that this comparative method is promising for studying adsorption from liquid mixtures on microporous solids such as microporous activated carbons.

Ethane and benzene adsorption isotherms on two commercial activated carbons were measured and analyzed in terms of the Dubinin—Radushkevich theory of micropore filling. Ethane isotherms were obtained by a dynamic method, whereas benzene isotherms were measured by a gravimetric method. Comparative studies of these adsorption isotherms showed that the adsorption isotherms measured in the low-concentration region are sufficient for obtaining information about the microporous structure of activated carbons.

Expressions for the enrichment factor, the critical recycle ratio, and the extent of recovery are discussed for a pressure swing adsorption system with a Langmuir—Freundlich isotherm. The enrichment factor is defined as the ratio of the mole fraction of the adsorbate gas in the low-pressure product stream to that in the high-pressure feed stream, the critical recycle ratio is defined as the ratio of the number of moles of the purge gas to the total number of moles of high-pressure product gas, and the extent of recovery is defined as the ratio of the net amount of the carrier gas in the product stream to the total amount of the carrier gas that enters the bed during a cycle. Effects of the non-linear isotherm parameters on these quantities are discussed. It is shown that the enrichment factor increases with increasing adsorbent heterogeneity, whereas the extent of recovery and the critical recycle ratio depend only slightly on the heterogeneity parameter.

Etude de l'adsorption du benzene sur du charbon actif microporeux par determination des isothermes d'adsorption

We report values for the neutron electric tomagnetic form factor ratio,G_(En)/G_(Mn), deduced frommeasurements of the neutron’s recoil polarization in the quasielastic ^2H(e, e'n)^1H reaction, at three Q^2 values of 0.45, 1.13, and 1.45 (GeV/c)^2. The data at Q^2 = 1.13 and 1.45 (GeV/c)^2 are the first direct experimental measurements of G_(En) employing polarization degrees of freedom in the Q^2 > 1 (GeV/c)^2 region and stand as the most precise determinations of G_(En) for all values of Q^2.

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.