High-Z metals such as tungsten create large secondary particle showers when
illuminated by gamma rays.  Gamma rays incident on a tungsten plate can create
electron-position pairs that, in turn, can create high-energy delta rays also
known as knock-on electrons.  Experience at SLAC has shown that gamma rays in
the few-GeV range create on the order of one knock-on per radiation length in
tungsten.  Therefore, tungsten makes an ideal radiator for converting gamma
rays into knock-on electrons.  If knock-ons can escape, a net charge on the
tungsten will accumulate, and if connected to ground, a current signal can be
measured.  Simulations have shown that with a beam power of 1.4 W on a
tungsten plate with a circular area of approximately 100.0 cm$^{2}$, a current between 1.0 nA and 0.01 nA will be generated.  
\par
The geometry of a closely-spaced array of pins attached to a base plate is
designed to allow a maximum of knock-ons created in the tungsten to escape.
To make an effective detector, the tungsten must cover an area comparable to
the area of the GlueX primary collimator, about 300 cm$^{2}$.  The SLAC team
discovered that the ideal geometry for the tungsten emitter segments is an
array of evenly-spaced tungsten pins on a solid tungsten plate with an average
pin density of approximately 19.0 pins per cm$^{2}$ and a pin cross-sectional
area of about 0.1 mm$^{2}$.  The tungsten plates should be roughly 2 r.l. in
depth and the pins should be 4 r.l. to ensure a sufficient shower development.
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    The layout of the tungsten pin-cushion detector used in this design
consists of eight tungsten emitter plates, each of which covers an angular
range of 60$^{\circ}$.  Four of the plates cover a radial range of 2.5 mm to
2.5 cm, while the other four cover a range of 3.0 cm to 6.0 cm.  To decrease
electronic cross talk between the tungsten plates, aluminum walls 1.0 mm in
thickness are placed between the plates.  The tungsten plates are housed in an
aluminum cup and covered by an aluminum plate, which creates a Faraday cage to
shield the system from ambient electromagnetic interference.  The TPCD is
housed inside a boron nitride cup to insulate the detector from ground.  Boron
nitride was used in the simulation because of its high resistivity and
durability in high radiation environments.  A picture of the TPCD layout is
shown in fig.~\ref{ColDetLO}.   
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Each of the eight TPCD plates is read out separately by a current-sensitive pre-amplifier.  Once the relation between beam position and current is known, the position of the gamma ray beam can be determined by measuring ratios between the plate currents.  The original SLAC design used four tungsten plates.  The design in this simulation test used eight plates instead of four in order to cover more area.