Overview

The High Resolution Spectrometer Vertical Drift Chambers provide a precise ($\pm 125~\mu$m) measurement of the position and angle of incidence of both recoil electrons (in the HRSe) and knockout protons (in the HRSh) at the respective spectrometer focal planes. This information may be combined with the knowledge of the spectrometer optics to determine the position and angle of the particles in the target.

Each Hall A spectrometer boasts its own VDC detector package. These packages are located on permanent rails mounted on the spectrometer decks in the shielding huts above the outrun windows but beneath the space frames. The packages consist of two VDCs, and are identical in all aspects. The VDCs have been constructed without guard wires. Each VDC is composed of two wire planes in a standard UV configuration - the wires of each plane are oriented at 90$^\circ$ to one another, and each plane is oriented at 45$^\circ$ with respect to the nominal particle trajectories (see Figures 5.1,5.2).

Figure 5.1: Relative VDC geometry

Figure 5.2: Relative VDC geometry

Operation of the VDCs requires the application of both High Voltage (HV) across the chambers themselves and Low Voltage (LV) across the preamp/disc cards, which are mounted on the sides of the VDCs, within the confines of the protective aluminum Faraday cage. The chamber gas is a combination of argon (Ar) and flammable ethane (C$_2$H$_6$) which is bubbled through alcohol. Gas is routed from bottles located in the Hall A gas supply shed to gas supply control panels located on the main level of the space frames in the detector huts.

As charged particles pass through the chamber gas in the VDCs, they produce ionization. This ionization drifts along the electric field lines defined by the high voltage planes and the signal wires. Ionization is collected in the form of analog pulses on the signal wires. The pulses are then amplified, discriminated and used to start multihit TDCs, which are subsequently stopped by the overall event trigger. The TDCs are read out by the CODA acquisition software. The data are histogrammed online by the DHIST software. In-depth offline data analysis requires the ESPACE software.