Spectrometers

The Hall A spectrometers and associated instrumentation are designed to perform high resolution and high accuracy experiments. The goal is to achieve a missing mass resolution of $\sim$ 200-500 keV to clearly identify the nuclear final state. An absolute accuracy of $\sim$ 1% is also required by the physics program planned in the Hall, which implies $\sim$ 10$^{-4}$ accuracy in the determination of particle momenta and $\sim$ 0.1 mr in the knowledge of the scattering angle.

The instruments needed are a high resolution electron spectrometer (HRES) and a high resolution hadron spectrometer (HRHS), both with a maximum momentum capability matching the TJNAF beam energy, and large angular and momentum acceptance.

Figure 4.1: A side view of the Hall A HRS spectrometer.

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Figure 4.2: A bird's eye view of the Hall A end-station at TJNAF.

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A layout of the 4 GeV/c High Resolution Electron Spectrometer is shown on Figures 4.2 and 4.1. Its main design characteristics are given in the attached table. The spectrometer has a vertical bending plane and 45$^{\circ}$ bending angle. The QQDQ design includes four independent superconducting magnets, three current-dominated cos2$\theta$ quadrupoles and one iron-dominated dipole with superconducting racetrack coils. The second and third quadrupoles of each spectrometer have sufficiently similar field requirements that they are of identical design and construction. The overall optical length, from target to focal plane, is 23.4 m. Optically, the HRHS is essentially identical to HRES. In fact the two spectrometers can be used interchangeably to detect either positively or negatively charged particles as needed by any particular experiment.

The support structure includes all system elements which bear the weight of the various spectrometer components and preserve their spatial relationship as required for 45$^{\circ}$ vertical bending optics.

The alignment and positioning system includes all the elements which measure and adjust the spatial relationship. The support structure consists of the fabricated steel components which support the magnets, detector, shield house and associated equipment. It is composed of the box beam, which supports the outer elements in fixed relative position atop the dipole; the dipole support bracket, upon which the dipole rests on the jacks; the cradle, upon which the dipole rests through the vertical positioning system, VPS; and a portion of the shield house load through the inboard legs of the gantry; the gantry, which supports the shield house and the magnet power supplies; and the bogies, which support the cradle-gantry assembly and slide on the floor plates and provide the driving power to move the two spectrometer arms.

The detector package is supported on the box beam and is surrounded by the shield house. It must perform two functions, tracking and particle identification, PID. The most important capability of focusing spectrometers is measuring precisely the momenta and entrance orientations of the tracks. Momenta resolution of 10$^{-4}$ is obtainable, consistent with the resolution of the incident beam.

A particle traversing the detector stack (Figure 4.3) encounters two sets of horizontal drift chambers (x,y) with two planes of 368 wires in each chamber. The track resolution is $\sim$ 100 $\mu$m. From the chamber information both positions and angles in the dispersive and transverse directions can be determined. The information from these chambers is the principal input of the tracking algorithms.

The chambers are followed by a scintillator hodoscope plane designated S1. This plastic scintillator array provides the timing reference for the drift chambers, and is also used in trigger formation and in combination with a second hodoscope pair it can provide time of flight particle identification. These scintillators can also be used to perform crude tracking.

The next element encountered by a particle is a gas threshold Cerenkov detector. This is used for particle identification. In the hadron spectrometer this gas threshold Cerenkov detector can be swapped against an Aerogel detector, with a similar function.

The second hodoscope plane, S2, is located directly behind the gas Cerenkov. Its function is essentially the same as that of S1. In the hadron spectrometer an option exists to have this hodoscope pair be preceded by a third chamber, to improve tracking. Each of the two spectrometers have gas and Aerogel Cerenkov detectors which can be used when they are in electron detection mode.

The final elements in the detector stack on HRSE are the pre-shower and the lead glass shower calorimeter. This is used for energy determination and PID.

Figure 4.3: The electron spectrometer detector stack.

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Figure 4.4: The hadron spectrometer detector stack.

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The hadron detector is shown schematically in Figure 4.4. It consists of two sets of (x,y) vertical drift chambers identical to those of the electron arm. The remaining part of the detection system is used to define the level 1 trigger, as well as for particle identification and timing. It consists of three minimally segmented planes of scintillation counters equipped with photomultipliers at both ends, and it includes Cerenkov counters (gas CO$_2$ and Aerogel).

In addition, a proton polarimeter is installed in the back of the detector package to measure the polarization of the proton using a segmented carbon analyzer up to 60 cm in thickness to allow measurements over a wide range of proton energies. A pair of front and a pair of rear straw chambers determine the incident and scattered angles, respectively. The third scintillation counter, located at the rear end, provides the trigger for the polarimeter. The polarimeter detectors are dimensioned to accept a 20$^{\circ}$ cone of scattered protons.

Several support systems are necessary in addition to the basic components mentioned above. They include gas supply systems for the wire chambers, high voltage supplies, readout electronics, a second level trigger, software for data analysis and testing, and a remotely controllable mechanical system.

As for the electron spectrometer, all detectors are mounted on a single rigid carriage along with their associated electronics. The FPP components are mounted on an FPP subframe for installation and removal as a unit. The trigger electronics are located next to the detectors, as for the electron arm.

To reduce the resolution degrading effects of multiple scattering, the entire interior of the spectrometer from the pivot to the detector hut is a vacuum vessel. The ends of this evacuated volume are capped by relatively thin vacuum windows.

As mentioned, subsystems will be discussed in more detail in the next three sections. The remainder of this section will describe some features common to the two spectrometers, then the following major sections will be devoted to the specifics that are not common.