The particles scattered or produced in the target region are collected in a pair of identical magnetic spectrometers, which can rotate around the centre of the target. Each spectrometer consists of a quadrupole followed by two dipole magnets which focus the particles of different momenta incoming from the interaction point to different positions in the focal plane.
Depending on the reaction of interest, the spectrometers are used to detect the scattered electrons or the recoil particles (e.g. e+e- pairs), and to precisely reconstruct their momenta, polar and azimuthal angles, and the interaction vertex. The particle species (electron or positron) can be selected by inversion of the polarity of the magnets. Due to the well-known magneto-optics of the spectrometer, the coordinates at the interaction point can be accurately calculated from the measured coordinates (impact position and impact angles) in the focal plane using a tracking detector. A set of high resolution gas detectors is used for that purpose.
The magnetic design of the spectrometers is realized using a quadrupole magnet (Q) followed by two 45° dipole magnets (D1 and D2). Due to a special shape of the dipole magnets, this results in a horizontal focal plane, where for instance particles with the same momentum hit the same position (dispersive direction), regardless of the angles of the particle momentum.
The two spectrometers are located on opposite sides of the beam line. Both spectrometers can rotate independently around a common pivot point in a wide angular range from 15° to 165° with respect to the beam line. The spectrometers are directly connected to the scattering chamber and therefore share the same vacuum as the target. This minimizes the material for the scattered particles to be crossed before being detected.