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a wire). For wire thickness of ∼ 10 − 3 radiation lengths, this considerably reduces the multiple scattering in the target versus that in a true foil and leads to a much bet- ter mass resolution. The maximum number of wires that a beam electron can pass through per plane is ∼ 10 in the configuration illustrated in Figure 6 or Figure 7 as- suming 10 µ m diameter tungsten wires. Wires as thick as 15 µ m can be used without significantly compromising mass resolution. 

The plane of the target wire mesh will be vertical. The mesh plane will have 4–5 zig-zags, each with length 5 − 10 cm, which result in multiple intersection points allowing an extra factor of 4–5 for rejection of accidental tracks in offline analysis. (Figure 6 illustrates a target with 4 zig-zags.) 

The central angle of the spectrometer varies with the position of the target. In this experiment, such a varia- tion is very useful because it extends the range of invari- ant mass covered with one setting of the spectrometers. For several settings suggested in our run plan, only two planes of wire mesh are needed, one at the front and one at the back of the acceptance region. 

There are two considerations to take into account when selecting the material. The first consideration is to achieve the highest possible ratio of signal to background, while keeping the background rate low enough so as not 

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to overwhelm the triggering and DAQ system. The sec- ond is whether a thin foil or a thin wire of a particular material is available. Large backgrounds come from pions produced in photo-production from nucleons, and from electrons produced in the radiative tail of electron-proton elastic scattering. These backgrounds do not mimic the signal, but if their rate is too large, they can overwhelm the DAQ system. These considerations favor the use of a tungsten target, with a total thickness between 0.5% and 10% radiation length, with thicker targets used in higher- energy runs. Reduction of the thickness at low energies is required to limit the rate in the electron spectrometer and also minimizes the multiple-scattering contribution to the pair mass resolution. 

The heat load of the target is also an important con- sideration. This is mitigated by rastering the beam and using materials like tantalum or tungsten. The tilted tar- get cools from a large area; for example, with the raster size 0 . 5 × 5 mm and proposed geometry (Figure 6), the cooling area is 20 cm $^{2}$. For the parameters of APEX (an electron beam of 80 µ A on a 10% X$_{0}$ tungsten target) the head load is about 140 W (or 7 W/cm $^{2}$), which re- sults in the equilibrium target temperature of 1000 ◦ K. Experimental study has demonstrated that 1 kW/cm 2 is 

