The SMC polarized target

The Spin Muon Collaboration (SMC) at CERN uses the largest solid polarized target in operation to measure the spin-dependent structure functions of the nucleons. The target is made of butanol, doped with EHBA-Cr(V), and has a volume of $2 \times 1280$ cm$^{3}$. The target halves are dynamically polarized to opposite directions to minimize many systematic errors, and the spin orientation can be reversed by rotating the magnetic field in the frozen spin mode without losses. Frequency modulation of the microwaves is used to increase the maximum polarization and the growth rate. The polarization is measured with 10 series $Q$-meters with an accuracy of 3–5%.