Overview

The detector package of each spectrometer has trigger, tracking, and particle ID components. In addition, the Hadron spectrometer has a unique proton polarimeter. Particles which have passed through the magnetic elements first encounter the tracking detectors to minimize the multiple scattering contribution to the angular and energy resolutions of the spectrometer. The tracking part consists of two identical vertical drift chambers. The trigger detectors include two planes of thin plastic scintillator counters, gas and aerogel Cerenkov counters and a shower counter. On the hadron spectrometer the shower counter or large scintillator counter can be used in the trigger. Particle ID is provided by several techniques. For electron identification the electron arm, EA, has the gas Cerenkov counter and two layers of a segmented lead glass shower counter. Because the hadron arm, HA, also can be used for experiments with electrons it is equipped with a short version of the gas Cerenkov counter and one layer of a segmented lead glass shower counter. Pion identification in both spectrometers relies on an aerogel Cerenkov counters which presently have aerogel radiator with a refraction index, n, of 1.025. Aerogel Cerenkov counter commissioning is not yet completed. For particle momenta below 800 MeV/c, the dE/E in the scintillator and shower counters can be used for separation of pions and protons. The large distance between planes of the trigger scintillator counters (2 -3 $m$) allows a direct measurement of the particle speed with resolution ( sigma ) of 0.07. Measurement of the time of flight on the long path from the target to the spectrometer ( $~ 25 m$ ) provides another powerful particle ID for coincidence experiments. The focal plane polarimeter on the HA operates with proton momenta up to 3 GeV/c with a figure of merit $~ 0.03$.
The detector packages are installed inside of the Shielding Huts (SH). Access to the Shielding Huts is via very heavy swinging front doors. The main structure of the SH is made from 3 $in$ thick steel plates. The side walls and bottom surfaces of the SH are covered inside with 1 $in$ thick lead slabs. Outside of the steel box, concrete is used for neutron protection. The front door has about 34 $in$ of concrete and 3 $in$ of lead. Side walls are covered with 17 $in$ of concrete. The roof of the SH has 10 $in$ of concrete above 3 $in$ of steel. The lower half of the side walls facing the beam dump have an additional cover of 15 $in$ of concrete. Additional ``Line of Sight Shielding'', LSS, is installed at a distance of $~5 m$ from the target. This consists of 2 to 3 $m$ of concrete. High energy pions interact in this concrete before decaying. The LSS reduces the rate of high energy muons, which are produced in pion decay. The overall result from SH and LSS is a reduction factor of 10 to 20 in the counting rate of a single scintillator counter ( according to calculations ). The 2 VDCs provide accurate tracking information. They are mounted on a movable frame which slides along Thompson rails to hard stops. The position of the VDC on the frame and location of the Thompson rails is surveyed relative to the Hall center. The rest of the detectors are mounted on a detector frame which can be moved out of the Shielding Hut for detector maintenance.