Search Results (33 total)

An experimental study of the ¹⁶O(e,e^′K⁺)_Λ¹⁶N reaction has been performed at Jefferson Lab. A thin film of falling water was used as a target. This permitted a simultaneous measurement of the p(e,e^′K⁺)Λ,Σ⁰ exclusive reactions and a precise calibration of the energy scale. A ground-state binding energy of 13.76±0.16  MeV was obtained for _Λ¹⁶N with better precision than previous measurements on the mirror hypernucleus _Λ¹⁶O. Precise energies have been determined for peaks arising from a Λ in s and p orbits coupled to the p_1/2 and p_3/2 hole states of the ¹⁵N core nucleus.

The interference response function f_LT(R_LT) of the ²H(e,e^'p)n reaction has been determined at squared four-momentum transfer Q²=0.33 (GeV/c)² and for missing momenta up to p_m=0.29 GeV/c. The results have been compared to calculations that reproduce f_LT quite well but overestimate the cross sections by 10–20% for missing momenta between 0.1 GeV/c and 0.2 GeV/c.

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a ¹²C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed _Λ¹²B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Λ peak and the 11 MeV p-wave Λ peak.

BL11, the most recently installed wiggler in the SPEAR storage ring at the Stanford Synchrotron Radiation Laboratory, produces a large nonlinear perturbation of the electron beam dynamics, which was not directly evident in the integrated magnetic field measurements. Measurements of tune shifts with betatron oscillation amplitude and closed orbit shifts were used to characterize the nonlinear fields. Because of the narrow pole width in BL11, the nonlinear fields seen along the wiggling electron trajectory are dramatically different from the magnetic measurements made along a straight line with a stretched wire. This difference explains the tune shift measurements and the observed degradation in dynamic aperture. Because of the relatively large dispersion (1.2 m) at BL11, the nonlinearities particularly reduced the off-energy dynamic aperture. Because of the nature of these nonlinear fields, it is impossible, even theoretically, to cancel them completely with short multipole correctors. Magic finger corrector magnets were built, however, that partially correct the nonlinear perturbation, greatly improving the storage ring performance.

The ¹⁶O(γ,pn) reaction was measured at E_γ∼72 MeV with resolution sufficient to distinguish the low-lying states of the residual ¹⁴N nucleus. Cross sections averaged over the acceptance of the detector system were determined for each of the resolved states. The relative population of residual states indicates that proton-neutron pairs coupled to (J^π,T)  =  (0⁺,1) play a minor role in the photon absorption process compared to (1⁺,0) pairs and that both L  =  0 and L  =  2 pairs participate in the reaction.

The reaction p(e,e^′p)π⁰ has been studied very close to threshold ΔW≤4 MeV and at 4-momentum transfer Q²  =  0.1 (GeV/c)². The scattered electron and the kinematically focused recoil proton were detected in coincidence using the 3-magnetic spectrometer setup at the Mainz Microtron MAMI. The transverse and longitudinal as well as the longitudinal-transverse and the transverse-transverse interference structure functions have been determined. From the data the s-wave multipoles E₀₊ and L₀₊ have been extracted. The experimental results are compared to recent calculations in chiral perturbation theory.

The (γ,d) and (γ,t) reactions on ⁶Li were studied at average (tagged) photon energies of 59 and 75 MeV. Differential cross sections were obtained at five different angles (30°<~θ_lab<~150°), from which the integrated-over-angle cross sections for the d₀ and t₀ channels were determined. The experimental results are discussed in terms of a cluster model calculation.

Forward-angle coincident electroproduction cross sections of charged pions from ³He have been measured at electron energies E₀  =  855, 675, 600, and 555 MeV. The overall features of the data for energy transfers of ω  =  370 to 430 MeV with pions detected along the momentum transfer axis are reproduced in terms of a microscopic model, including pole terms, final state rescattering and produced and preformed Δ resonances. Separation of the cross section into its longitudinal and transverse parts was performed at Q²  =  0.045  (GeV/c)². The longitudinal part of the cross section in the π⁺ channel does not contradict with the assumption of a preformed Δ⁺⁺.

Inclusive inelastic electron scattering cross sections for ³H and ³He were measured for excitation energies below 18 MeV. Longitudinal and transverse response functions were determined from these cross sections for six values of the three-momentum transfer q in the range 0.88 <q<2.87 fm^-1. A previously observed C0 multipole enhancement near threshold in the two-body channel for ³He is confirmed but is not observed for ³H. The experimental data are found to be in good agreement with two recent calculations of the longitudinal and transverse response functions. The first uses variational ground-state wave functions and the orthogonal correlated states method to describe the two- and three-body breakup channels. The second uses bound and continuum Faddeev wave functions obtained for a simple central potential. Agreement with the data is good for the first model and better for the second. The inclusion of final-state interactions (FSI) in the Faddeev continuum is found to be very important in these threshold breakup kinematics; in many cases inclusion of FSI changes the response functions by factors of two or three giving excellent agreement with the data. The transverse response functions are well reproduced, even though neither model includes meson exchange currents. Ratios of the response functions for the two nuclei are also well described.

Excitation-energy spectra resulting from the ¹²C, ¹⁶O, and ⁴⁰Ca(γ,p) reactions at 90° have been measured with tagged photons of about 61 and 77 MeV. Decay to several discrete states is discussed. Evidence is shown for an absorption process where the photon interacts with a T=1 proton-neutron pair, instead of the T=0 p-n pair implicit in the quasideuteron model.

The coincident electrofission (e,e’f) cross sections of ^233,238U were measured in the excitation range 150–550 MeV. The probability of fission was found to be near unity in the delta resonance region (250–550 MeV), but less than unity in the quasifree scattering region (150–250 MeV). The data are analyzed with a model based on the quasifree and delta reaction mechanisms. An excitation energy of 25 MeV is found for the residual nucleus in the quasifree region and over 40 MeV in the delta region.

Double differential cross sections for inelastic electron scattering from ⁴He 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/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.

The pion angular distribution for ¹⁴N(γ,π⁺)¹⁴C_g.s. has been measured at photon energies of 230 and 260 MeV. These data are consistent with the trend of our earlier data at 200 and 320 MeV. Additional forward angle data were obtained at 320 MeV. The results are compared to several calculations based on the distorted-wave impulse approximation and on the delta-hole model. The data are best fit by the calculations of Tiator et al. which use the delta-hole model in the resonance channel. These calculations fit the 230 MeV data well and underestimate the 260 and 320 MeV data by about 25%.

The ¹²C(e,e’p) reaction at the delta resonance has been measured for protons detected parallel to the momentum transfer at two kinematical situations: at the maximum and the low-energy-loss side of the peak. The cross section was measured for missing energies up to 320 and 220 MeV, respectively. In the missing energy spectrum two distinct structures are observed, which correspond to multinucleon knockout and quasifree pion production processes. In addition, there is a remnant of the quasifree knockout process.

We present new results for inclusive electron scattering in ²H, ³He, and ⁴He in order to test the reaction mechanism for quasielastic scattering as a function of nuclear density. Radiative corrections are applied to the cross section data and Rosenbluth separations are made for three-momentum transfer (q) between 300 and 600 MeV/c. The A and q dependencies of the data are discussed for the quasielastic peak and the region between the quasielastic peak and the Δ resonance peak (dip region). Comparisons are shown between the data and models based on a quasielastic reaction mechanism. The models give a reasonable representation of the peak at q∼500 MeV/c, but the longitudinal data for the helium isotopes are significantly suppressed with respect to the quasielastic predictions at q<400 MeV/c. None of the calculations predict the rapid rise with q and A in the transverse strength in the dip region seen in the data. A significant breakdown of the quasielastic picture is seen in the data as A increases from 2 to 4.

The cross sections for inelastic electron scattering from ²H have been measured over a wide kinematic range covering momentum transfers q from approximately 250 to 600 MeV/c. To make longitudinal transverse separations of the response functions, both forward angle (60°) and backward angle (134.5°) data were obtained. The observations are in excellent agreement with calculations of Arenhövel and Leidemann and in slightly less good agreement with the calculations of Laget throughout the quasielastic peak and the ‘‘dip’’ regions.

A longitudinal/transverse separation of the ¹²C(e,e’p) reaction in the quasielastic region has been performed in parallel kinematics. Above the two-particle emission threshold a broad bump is seen which has been attributed to knockout from the deeply bound s shell. The s-shell region shows a large transverse enhancement compared to distorted-wave impulse-approximation calculations based on free-proton form factors. This enhancement increases with missing energy, indicating that a nonquasielastic reaction mechanism plays a significant role. The p shell is consistent with the free-proton behavior.

Inclusive cross sections for the proton and nuclear targets of A=4, 9, 12, and 16 were measured for 537 and 730 MeV electrons scattered at 37.1 deg. Systematic features of the continuum scattering data are compared with other electron scattering data and with photoabsorption measurements. A model calculation based on the isobar-hole formalism is compared with the data in the delta resonance region.

The pion angular distribution for the ¹⁶O(γ,π⁺) ¹⁶N reaction leading to the lowest quartet of states in ¹⁶N has been measured at 320 MeV photon energy. The results are compared with a distorted-wave impulse approximation calculation.

The reaction ¹²C(e,e^′p) in the dip region (ω=200 MeV, q=400 Mevc) 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^′p) reaction process in the dip region.

Differential cross sections for ¹⁴N(γ, π⁺)¹⁴C(g.s.) (0⁺1) have been measured for an incident photon energy E_γ=200 MeV at laboratory angles of 45°, 60°, 75°, 90°, 120°, and 140°. Momentum-space distorted-wave-impulse-approximation calculations, using a complete treatment of a one-body pion-photoproduction operator, are found to be in excellent agreement with the experimental data. The effect of the momentum-dependent terms in the nuclear photoproduction operator is observed. The reaction is found to be sensitive to input nuclear structure elements not observable in the (e, e^′) form-factor data.

The inclusive electroproduction of 30 MeV π⁺ and π^- mesons has been observed at a laboratory angle of 90° for electron energies from 185 MeV (the kinematic threshold) to 235 MeV. Measurements of the yield were performed and double differential cross sections for photoproduction were obtained using virtual-photon theory. A significant discrepancy between experiment and distorted-wave impulse approximation calculations was found. This is particularly surprising since these calculations agree with the total cross sections for the ²H(γ,π⁺)nn reaction from threshold to 20 MeV above threshold.

The inelastic electron-scattering cross sections of H, He, Be, C, and O were measured for 730-MeV electrons scattered at 37.1° for energy transfers to the nucleus up to 550 MeV. The nuclear response in the delta region is found to be quite similar for the A>1 targets. Although the differential cross section per nucleon at the peak of the delta region (380 MeV) is suppressed compared to that of the free nucleon, the area in the 230-550-MeV region is enhanced for the A>1 targets.

The nuclear response to 200-350 MeV electrons inelastically scattered at 20° for six nuclei ranging from A=9 to 181 is given. An excitation energy integral is formed and compared with three theoretical calculations of the total inelastic scattering cross section.

NUCLEAR STRUCTURE Z(e, e^′) with Z=4, 6, 13, 23, 40, and 73, measured d²σ / dΩdE at θ=20° vs E, determined S_inel, compared with model predictions.