L. Lapikás

The 500 MeV electron-scattering facility at NIKHEF-K

The 500 MeV electron-scattering facility, designed and built in conjunction with the 500 MeV, 2.5% duty factor electron linear accelerator (MEA), has been completed. It allows high-resolution (1 × 10−4) single-arm (e,e′) experiments with a QDD spectrometer and single-arm hadron detection [(γ, π) and (γ,p) reactions] with a large solid angle (17 msr) QDQ spectrometer. With both specrometers coincidence experiments (e,e′x) can be performed with a 225 keV missing energy resolution and a 1 ns timing resolution. A comprehensive description is given of the beam-handling system, spectrometers, detection system, data handling techniques and other instrumentation. Finally, test results obtained at energies ranging from 70 to 460 MeV are presented.

A consistent analysis of (e,e′p) and (d,3He) experiments

Abstract The apparent discrepancy between spectroscopic factors obtained in (e,e′p) and (d, 3 He) experiments is investigated. This is performed first for 48 Ca(e,e′p) and 48 Ca(d, 3 He) experiments and then for other nuclei. It is shown that the discrepancy disappears if the (d, 3 He) experiments are reanalyzed with a nonlocal finite-range DWBA analysis with a bound-state wave function that is obtained from (e,e′p) experiments.

Magnetization distribution of the 7Li nucleus as obtained from electron scattering through 180°. The electric quadrupole moment of 7Li

Abstract Cross sections for scattering of electrons from the ground state doublet of 7Li have been measured in the energy range of 25 to 90 MeV with an energy resolution better than 200 keV. From the cross sections for transverse elastic scattering measured at 180° the rms radius of the ground state magnetization density distribution has been determined: 〈r 2 〉 M 1 2 = 2.98 ± 0.05 fm . The value of the reduced transition probability for M1 excitation of the first excited level has been determined from the ratio of transverse inelastic and elastic cross sections at 180°. The result [B(M1, ↑; 478 keV)=2.50±0.12 μN2] agrees very well with the result of lifetime measurements. The cross sections for longitudinal (C2) excitation of this level have been extracted from scattering through angles of 90° to 150°. The measured reduced transition probability [B(C2, ↑; 478 keV)=7±4 fm4] is compared with the values obtained by several other experimental methods. In a shell-model analysis our data can be considered to measure the size of the 1p shell. The resulting value (a1p=1.90±0.03 fm) is much larger than the one determined from earlier electron scattering experiments, where the 478 keV level was not resolved. A relative measurement of the longitudinal elastic scattering from 7Li and from 6Li showed no isotopic effect on the rms charge radius: 〈r 2 〉 7 1 2 /〈r 2 〉 6 1 2 =1.001±0.008 . Accepting the measured rms charge radius for 6Li this corresponds to a1s=a1p=1.90 fm for 7Li reconfirming the above value. The data are compared with shell-model and projected Hartree-Fock calculations.

Proton ground-state correlations in 40Ca studied with the reaction 40Ca(e, e′ p)49K

The spectral function of the reaction 40Ca(e, e′ p)39K was measured with special emphasis on the knockout of protons from the 1f7/2 and 2p3/2 orbits above the Fermi level. Particularly for the transition to the 3/2− state two-step processes were found to be not negliglible. The spectroscopic strength for the 1f and 2p knockout, integrated up to 10 MeV excitation energy, is much less than the missing strength in the 1d3/2 and 2s1/2 orbits, but it agrees with values expected from calculations involving ground-state correlations.

On the radial distribution of the 1f72 proton orbit

Abstract Cross sections for magnetic elastic electron scattering of high multipolarity from 51 V and 59 Co have been measured. They are interpreted in terms of the rms radius of the 1f 7 2 proton orbit. The results for 51 V and 59 Co 1f 7 2 rms radii are smaller by 3% and 9%, respectively, than predicted by the Hartree-Fock-Bogoliubov theory.

Magnetic elastic electron scattering from 3Be and 13C

Abstract Elastic scattering through 180° from 9Be, 12C and 13C targets has been observed for electrons of 35 to 90 MeV. Magnetic scattering cross sections for 9Be and 13C have been obtained by subtracting charge scattering as deduced from the scattering from the spin-zero nucleus 12C. Theoretical shell model predictions for the magnetic cross sections are derived for (1s)4(1p)A−4 configurations and various coupling schemes. Both harmonic oscillator and Woods-Saxon radial wave functions are used. A comparison of our results with magnetic cross sections calculated in DWBA yields magnetic rms radii 〈r 2 〉 M 1 2 = 3.2 ± 0.3 fm and 〈r 2 〉 M 1 2 = 3.3 ± 0.3 fm for 9Be and 13C. For 13C a combined flt to our low-q data and to earlier high-q data, yields 〈r 2 〉 M 1 2 = 3.4 ± 0.1 fm and a strong preference for intermediate coupling (IC) wave functions. Magnetic dipole scattering from 9Be is also close to the IC prediction, as deduced from a fit to our data and earlier high-q data. The fitted value of the octupole moment Ω = 5±1 μ N fm 1 can only be explained by a deformation of the average nuclear potential. The radial size 〈r 2 〉 1 p 1 2 = 2.85 ± 0.05 fm for the 1p radial wave function is in agreement with both the 9Be and 13C data.

Elastic electron scattering through 180° from the magnetization distribution of the 1f72-shell nuclei Sc, V and Co☆

Abstract Cross sections for the elastic scattering of electrons through 180° from the nuclei Sc, V and Co have been measured in the energy range of 40 to 90 MeV. The rms radii for magnetization distributions have been extracted for various phenomenological models.

Search for nucleon-nucleon correlations in the proton spectral function of 208Pb

Abstract Cross sections for the reaction 208Pb(e,e′p) have been measured with the continuous electron beam from the Amsterdam Pulse-Stretcher facility at NIKHEF-K. The spectral function has been extracted for protons with initial momenta of 300 to 500 MeV/c and binding energies up to 26 MeV. The data are compared to calculations with and without inclusion of nucleon-nucleon correlations. Mean-field predictions significantly underestimate the data and the discrepancy increases with binding energy. For transitions to the valence states the discrepancy is removed by introducing long-range correlations. Above the two-nucleon emission threshold long-range and short-range correlations reduce the discrepancy, but are insufficient to fully account for the measured strength.

A three-magnet system for measurement of electron scattering through 180°

Abstract A three-magnet system permitting measurement of electron scattering through 180° is described. An important feature of this system is that the direction of the outgoing electron beam is the same as in “normal” scattering experiments, which facilitates beam handling and current integration. Some of the calculated optical properties of this system have been checked by experiment. Emphasis is given to the optical properties of the circular pre-target magnet in conjunction with the spectrometer. By appropriate beam steering the scattering angle can be varied by up to 6°. The system has so far been used for experiments on the magnetization distributions of several 1 f 7 2 - shell nuclei and of 7Li.

The magnetic form factor of 51V at very high momentum transfer

Abstract The magnetic form factor of the 51 V nucleus has been measured between 3 and 4 fm −1 . These new data show a clear evidence for the diffraction structure of the M7 magnetic multipole. The data are compared to Hartree-Fock results including meson exchange currents and core polarization effects.