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HK: Hadronen und Kerne
HK 61: Electromagnetic Probes VIII, Properties of Nucleon, Pion
HK 61.3: Group Report
Thursday, March 19, 1998, 15:00–15:30, B
Measurement of the neutron electric form factor at Q2 = 0.67 (GeV)2 via 3He→(e→,e′n) at MAMI — •D. Rohe — Institut f"ur Physik, Universit"at Mainz, Staudingerweg 7, D-55099 Mainz
The electric form factor of the neutron Gen represents a fundamental quantity for calculating form factors of complex nuclei and for checking predictions of N-N potentials. Nevertheless it remains poorly known over a wide range of Q2. One important reason is that the determination of Gen is dominated by the neutron magnetic form factor as well as the form factor of the proton, because no free neutron target of sufficient density exists. In experiments where both the electron beam and the target are polarized, an interference term between electric and magnetic scattering amplitude appears in the asymmetry with regard to the helicity of the electron. Thus Gen is enhanced by Gmn in quasi-elastic kinematics. As an effective polarized neutron target, polarized 3He is used. By metastable optical pumping a starting polarization of 45% is obtained at 6 bar pressure [1]. This method was successfully applied by the A3-collaboration to measure Gen at Q2=0.35(GeV/c)2. To extend this measurement to higher Q2 the use of a high resolution magnetic spectrometer for electrons is necessary instead of a segmented lead glass matrix with an energy resolution of 20 %. The neutron is detected in coincidence in a segmented plastic scintillator. A guiding field, effectively shielded against magnetic field gradients of the spectrometer, permits the preservation and rotation of the 3He-spin in any particular direction. From the ratio of the asymmetries perpendicular and parallel to the direction of Q2, Gen is extracted, independent of beam and target polarization in first order. However for controlling purposes their product is measured in an other spectrometer using elastic scattering at 3He. A pilot experiment of 100 h was successfully performed in summer 1997 and is presently analyzed. The statistics permit a determination of Gen with an accuracy of 25 %.
[1] Becker et al., Nucl. Instr. Meth. A 346 45 (1994)
[2] K.I. Blomqvist et al., accepted for publication in Nucl. Instr.
and Meth. A
[3] W. Heil (contact person), MAMI Proposal A1/4-95
supported by the Deutsche Forschungsgemeinschaft (SFB 201)