Polarized high pressure [sup 3]H⃗e target at MAMI

In the frame of the A1-Collaboration at the Mainz Microtron a test measurement of doubly polarized 3H⃗e(e⃗,e′n) scattering from a high pressure target was performed in July aiming for the determination of the neutron electric form factor Gen at high momentum transfer (Q2=0.7(GeV/c)2). Due to the small value of Gen compared to Gmn a preferred procedure is to determine the asymmetry in the exclusive quasi elastic scattering of polarized electrons (P≈70%, I⩾2μA) from polarized 3H⃗e. The scattered electrons are detected in a high resolution magnetic spectrometer while the scattering angles of the outgoing neutrons are measured in a plastic scintillator. In this reaction the polarized 3H⃗e nucleus serves as an effective polarized neutron target. Because of the large magnetic field gradients caused by the spectrometer and limited space at the target place, the 3H⃗e gas is polarized elsewhere and transported to the target place in specially prepared glass cells. The glass cells are designed for high pressure (up to 10 bar) and with thin windows to prevent background. To reduce the relaxation due to the magnetic field gradients a μ-metal shielded guiding field of 4 Gauss is used. The guiding field is generated by three independent coils which permit the rotation of the target spin in any direction desired, especially perpendicular and parallel with respect to q⃗. So we are able to take the ratio of the asymmetries A⊥/A∥, which in first order depends only on kinematical factors and the ratio Gen/Gmn. The method of metastable optical pumping is used to polarize the 3H⃗e at about 1 mbar with a LNA laser (≈6W). Subsequently the gas is compressed by means of a two-stage-piston compressor up to 6 bar with a polarization of roughly 45%.