\chapter{Silicon and Gas Detectors}
\section{PSD Printed Circuit Board}

The PSD PCB allows for the mounting of eight silicon detectors for the centre module and nine for the outer modules (see Figure 4.1). All signals produced by the PSD's are routed to the pre-amplifiers via three fifteen line ribbon cables. The ribbon cables are soldered to the sides of the PSD PCB. The ribbon cables push onto the vertical fifteen pin (male) connectors on the baseplate PCB (see Figure 2.9).

\begin{figure}[ht]
\vspace{14cm}
\special{psfile = /home/oed/Modprotocol2/bob.ps
 hscale=45 vscale=45 hoffset=80 voffset =140}
\special{psfile = /home/oed/Modprotocol2/psd2.ps
 hscale=45 vscale=45 hoffset=120 voffset =-130}
\caption{PSD PCB (top). PSD (bottom).}
\end{figure}


The PSD PCB is suspended from the top of the CsI scintillator mount by four 4mm insulators, which sit on the four threaded rods already in place (see Figure 4.2). The orientation of the PSD PCB with respect to the baseplate is governed by the baseplate PCB's. Hence there is only one correct orientation. The top of the PCB has a series of soldered connectors (female). The six pins on the back of the PSD fit into these connectors, hence securing the PSD in place. These connectors also relay signals produced by the PSD to the ribbon cables and hence to the baseplate and pre-amplifiers.


\begin{figure}[ht]
\vspace{14cm}
\special{psfile = /home/oed/Modprotocol2/mod2.ps
 hscale=100 vscale=100 hoffset=-100 voffset =-230}
\caption{Side view of a MEGHA module.}
\end{figure}

Before installing the PSD's it is good practice to check the continuity between the soldered connectors on the PSD PCB and the continuation of the channel to the back of the baseplate via the glass feedthrough. Each connector on the PCB will have continuity with one glass feedthrough pin at the back of the baseplate.
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N.B. \em The PSD's are very delicate and gloves should be worn when handling them. 
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\rm
The PSD's should be placed such that their six pins (on the back surface of the PSD) line up with the six connectors on the PSD PCB. Apply even pressure to the corners of the PSD until it is securely in place.
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N.B. \em Care must be taken not to touch the delicate corner bonds of the PSD while placing it onto the PCB. A similar bond also exists at the back of the PSD. Do not touch these bonds.
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\rm The actual position sensitive detector can be mounted onto the PSD PCB incorrectly. Only one of the middle pins on the PSD (the one bonded to the back face) actually carry the back face energy signal. This pin must be mounted onto the middle connector of the PSD PCB which has a soldered track. The other middle connector on the PCB will have no track. Figure 4.1 only shows the front view of the PSD PCB. The soldered tracks are easily seen in this figure. It is also evident that some of the PSD mounting points do not have a soldered track running to either of the middle pins. This is because both the back and front of the PCB has soldered tracks. And so to align the PSD correctly, the front of the PCB as well as the back must be viewed.


\section{Anode and Cathode Grid}

Both the anode and cathode grid have extremely fine wire ($\sim$20$\mu$m) soldered across their faces, at a pitch of 1mm. 
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N.B. \em The grids must be handled with great care, and in such a way that the hands do not come in contact with the wires at all.
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\rm Four 6mm spacing insulators separates the PSD PCB from the cathode grid (see Figure 4.2). The cathode grid sits on top of these insulators, and is mounted such that the grid wires are at the top. The anode grid is separated from the cathode grid by 24mm insulators (as shown in Figure 4.2). 

The cathode grid (Figure 4.3) unlike the anode grid (Figure 4.3) does not require resistors or connectors, and hence is easily distinguishable from the anode grid. The cathode grid does however have an ear-thing wire. This wire should be screwed onto a scintillator mount leg.

The anode and cathode grid must be orientated with respect to the baseplate such that the grid wires for both grids lie parallel, and that the gas bias pin on the anode grid is orientated with respect to the `gas in' and `gas out' sockets as shown in Figure 4.4. 

\begin{figure}[ht]
\vspace{13cm}
\special{psfile = /home/oed/Modprotocol2/shield2.ps
 hscale=35 vscale=35 hoffset=112 voffset =190}
\special{psfile = /home/oed/Modprotocol2/cathode2.ps
 hscale=35 vscale=35 hoffset=110 voffset =0}
\caption{Anode grid (top). Cathode grid (bottom).}
\end{figure}


\begin{figure}[ht]
\vspace{17cm}
\special{psfile = /home/oed/Modprotocol2/anode.ps
 hscale=100 vscale=100 hoffset=-75 voffset =-60}
\special{psfile = /home/oed/Modprotocol2/cathode.ps
 hscale=110 vscale=110 hoffset=-95 voffset =-350}
\caption{Anode grid (top). Cathode grid (bottom).}
\end{figure}

Connect the gas bias cable (from the conhex connector) to the bias connector on the anode grid (see Figure 4.4). Making sure that the bias lead travels around the inside of the CsI scintillator mount legs. Also connect the nine coloured wires ( eight for the inner module) from the baseplate PCB's to the anode grid connectors shown in Figure 4.4.  Page seventeen (Figure 2.6) of ``The MEGHA Manual by Steve Chappell et al.'', shows the correct way of attaching these wires to the gas anode grid.

Check the continuity of the signals from the anode grid to the back of the baseplate. 

\section{Module Vessel}

At this stage we must turn our attention to the module vessel. The vessel sits on top of the baseplate, and covers the detectors. The flange of the vessel sits against the baseplate, whereas the other end of the vessel accommodates the mylar window. At this latter end of the vessel a groove has been machined to accommodate the vessel O-ring. The groove is 174mm x 174mm, and the O-ring, like the baseplate O-ring, is made out of one length of rubber. Please ensure the integrity of this O-ring prior to placement in the vessel.
Due to the necessity of testing the detector signals prior to placing the module into the scattering chamber, it is safer to attach a blanking plate to the top of the vessel. The blanking plate is just a sheet of metal with screw holes drilled around its perimeter. To attach the blanking plate, first introduce the O-ring into the groove. Using sixteen M4 16mm allen head screws (vacuum relieved). Bolt the blanking plate to the vessel.
When fixing the blanking plate down,  use a similar method as was adopted for the glass feedthroughs, i.e. opposite corners. This will ensure correct seating of the O-ring.

At this stage the vessel can be lowered onto the baseplate and detectors. While lowering it into the vessel, make sure that no wires become trapped between the baseplate and vessel flange. Sixteen M4 23mm, (non-modified), screws are required to bolt the vessel and baseplate together.
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N.B. \em The length of the screw used is critical as longer screws will protrude through the vessel and damage the aluminum end flange.\rm
