B. Reitz

Direct evidence for an orbital magnetic quadrupole twist mode in nuclei

The reactions Ni-58(e, e) and M-58(p, p) have been studied at kinematics favorable for the excitation of J(pi) = 2(-) states by isovector spin-flip transitions with DeltaL = 1. There are states at an excitation energy E-X approximate to 10 MeV which are strongly excited in electron scattering but not in proton scattering, suggesting a predominantly orbital character. This is taken as direct evidence for the so-called twist mode in nuclei in which different layers of nuclear fluid in the upper and lower hemisphere counterrotate against each other. Microscopic quasiparticle-phonon model calculations which predict sizable orbital M2 strength at this excitation energy yield indeed a current flow pattern of the strongest transitions consistent with a twist-like motion

Performance of an ASD-8 based wire chamber readout system

For the medium-energy proton polarimeter mounted at the focal-plane of the Big-Bite Spectrometer at Kernfysisch Versneller Instituut Groningen, a new wire-chamber readout has been developed. The charge-sensitive preamplifier is based on the ASD-8 B chip which has an input impedance of 115 /spl Omega/. This low impedance and the short integration time of 6 ns at a usable sensitivity of 5 fC allow high readout rates and low gas amplifications. This front-end circuit is mounted on all four multi-wire proportional chambers and on two vertical-drift chambers with 3872 wires in total. Measurements have been made using sources and intermediate energy protons. Special attention was given to determine the time-over-threshold properties of the circuit. The time-over-threshold capabilities of the readout system extend future applications to particle discrimination, or, in connection with other detector types, to energy-resolving readout. The operational performance of the readout system is presented.

The Q{sup 2}-Dependence of the Neutron Spin Structure Function g{sub 2}{sup n} at Low Q{sup 2}

We present the first measurement of the Q2 dependence of the neutron spin structure function g2(n) at five kinematic points covering 0.57 (GeV/c)2 < or = Q2 < or = 1.34 (GeV/c)2 at x approximately = 0.2. Though the naive quark-parton model predicts g2 = 0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g2(n) can be predicted using next-to-leading order fits to world data for g1(n). Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q2, indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g1(n) are consistent with next-to-leading order fits to world data.

Electric and magnetic giant resonances studied with electron scattering at the S-DALINAC*

Recent examples of giant resonance spectroscopy with electron scattering at the S-DALINAC are presented. Strength distributions and decay properties of electric resonances in 48 Ca are investigated with the (e,e′n) reaction. First results of a systematic search for spin and orbital magnetic quadrupole modes in 180° scattering are presented for 48 Ca and 90 Zr.