\chapter{Baseplate Assembly}
\section{The Baseplate}
\rm Ensure that the baseplate (see Figure 2.1) is clean and free from grease and other deposits. This can be achieved by cleaning with an alcohol based solvent as mentioned in the `Introduction'.

\begin{figure}[ht]
\vspace{12cm}
\special{psfile = /home/oed/Modprotocol2/baseplate.ps
hscale=90 vscale=90 hoffset=-30 voffset =-210}
\caption{Module baseplate (front view).}
\end{figure}

Firstly ensure that the beam-pipe O-ring is in place, see Figure 2.2. A groove exists in the central cavity of the baseplate to accommodate the O-ring. Prior to placing the O-ring in the groove, ensure that there are no tears, or dust attached to it. Again clean it with alcohol if necessary. A tear on the O-ring (or dust) can compromise the high vacuum requirements when the chamber and modules are in operation. Therefore it is necessary to ensure the integrity and cleanliness of all O-rings prior to their use.

If the baseplate is to be used for other than a centre module, then after placement of the beam-pipe O-ring, a blanking plug may be fitted through the cavity at the centre of the module. See Figure 2.2. For the centre module, the blanking plug is replaced by the beam-pipe.

\begin{figure}[ht]
\vspace{7cm}
\special{psfile = /home/oed/Modprotocol2/blankplug.ps
 hscale=90 vscale=90 hoffset=-40 voffset =-280}
 \caption{The beam-pipe O-ring and the beam-pipe blanking plug.}
\end{figure}

The fit between the O-ring and blanking plug is quite tight. A delicate turning motion of the plug will aid in it's placement. When placing the blanking plug ensure that the groove machined into the plug head (see Figure 2.2) lines up with the rectangular groove cut into the back of the baseplate. The rectangular grooves are there to accommodate the pre-amplifier cans, and hence correct alignment of the plug is necessary.

Four small counter sunk screws are required to fix the plug in place. These screws have had part of their thread planed off along their axis. See Figure 2.3. This is required to ``vacuum relieve'' screws which sit inside a vacuum. Please make sure that all screws which lie within the vacuum of the module, or chamber, are vacuum relieved.

\begin{figure}[ht]
\vspace{3cm}
\special{psfile = /home/oed/Modprotocol2/vacscrew.ps
 hscale=70 vscale=70 hoffset=30 voffset =-260}
\caption{Example of a vacuum relieved screw.}
\end{figure}

\section{Sixteen Pin Glass Feedthrough}

The baseplate accommodates ten, sixteen pin glass feedthroughs. See Figure 2.4. The feedthroughs are used to supply the detectors with bias and power. They also allow the extraction of the signals that the detectors produce as they are hit by ions. The sixteen pins sit in a fragile glass-like material and hence the vacuum integrity of the module is undermined if a fracture should occur in the glass. For this reason it is very important to handle the glass feedthroughs carefully. 
Before installing any feedthroughs into the baseplate, hold the glass feedthrough up to a light source and check for fractures. Do not use any feedthroughs which have cracks, and have not been repaired using an araldite based glue. Repaired feedthroughs will still have a visible crack, but will have been vacuum tested, and are safe to use.

\begin{figure}[ht]
\vspace{4cm}
\special{psfile = /home/oed/Modprotocol2/feedthrough.ps
 hscale=70 vscale=70 hoffset=-10 voffset =-240}
\caption{Sixteen pin glass feedthrough.}
\end{figure}

The body of the glass feedthrough has a rectangular groove machined into it. The groove is used to accommodate an O-ring. See Figure 2.5. The O-ring is circular when not in place, but can be compressed into the rectangular groove. As with all O-rings please ensure that the glass feedthrough O-ring is clean and has no faults. Four allen screws (M2.5 8mm) and washers are required to attach the feedthrough to the back of the baseplate. A series of screw holes can be located on the back of the baseplate. The screws do not require vacuum relieving and hence non-modified screws should be used.

To ensure that no fracturing of the glass takes place while screwing the feedthroughs down; hand tighten the four screws first. Tighten the screws diagonally not adjacently (i.e. opposite corners). Follow the same procedure with an allen key, after hand tightening. Turn the screws a quarter turn at a time. In this way it is ensured that the feedthrough is tightened evenly, and
no excess pressure is put on any one corner.

This procedure must be performed for all ten glass feedthroughs. After placement of all ten, hold the baseplate up to a light source and check if any of the feedthroughs (other than repaired feedthroughs) have cracked while being placed. Replace any cracked feedthroughs.
Also check that none of the sixteen pins have been bent during this process, as this will restrict the placement of the pre-amplifier packs and baseplate PCB's (printed circuit boards) later. If any are bent, delicately straighten them ensuring no cracks form in the resin.


\begin{figure}[ht]
\vspace{4cm}
\special{psfile = /home/oed/Modprotocol2/feedthroring.ps
 hscale=70 vscale=70 hoffset=-10 voffset =-240}
\caption{Glass feedthrough O-ring.}
\end{figure}

\section{Swagelok Gas Fittings}
The baseplate has two tapped circular holes, a large and small one, either side of twelve conhex connectors. The small hole is for `gas in' and the large hole is for `gas out'. A small and large SwageloK fitting, screws into the holes respectively. See Figure 2.6. Both fittings require O-rings to maintain the vacuum in the modules. These O-rings should already be in place.

SwageloK fittings must be tightened to a specific amount. A manual on all SwageloK equipment is available at the A.N.U.
The designs of the large and small fittings are the same. These should be screwed into the tapped holes available, at the back of the baseplate, by hand until tight. Tighten further, using a spanner, for half a turn.


\begin{figure}[ht]
\vspace{7cm}
\special{psfile = /home/oed/Modprotocol2/SwageloK.ps
 hscale=90 vscale=90 hoffset=-65 voffset =-260}
\caption{SwageloK fitting.}
\end{figure}


\section{Conhex Connectors}
The baseplate has twelve holes to accommodate \em vacuum feedthrough \rm conhex connectors. See Figure 2.7. The conhex connector requires a small O-ring (which sits in a groove on the connector) a washer and a nut.

Ensure the the signal pin (which can be seen at both ends of the connector) is not bent. The fixed body of the connector and O-ring sit against the front of the baseplate body. The part of the connector which protrudes through the hole at the back of the baseplate requires the washer and nut, to fix the connector in place. The twelve conhex connectors must be tightened down securely so as to ensure the vacuum integrity of the module. Bias is supplied to the gas anode grid through one of these connectors, the others are redundant.

\begin{figure}[ht]
\vspace{7cm}
\special{psfile = /home/oed/Modprotocol2/conhex.ps
 hscale=90 vscale=90 hoffset=-65 voffset =-280}
\caption{Vacuum feedthrough conhex connector.}
\end{figure}

\section{Baseplate Legs}
The back of the baseplate has a screw hole at each corner. Baseplate support legs can be screwed into these. Attach the legs (see Figure 2.8)  to the back of the baseplate using M4 24mm allen head screws. These screws do not require vacuum relieving.

\begin{figure}[ht]
\vspace{6cm}
\special{psfile = /home/oed/Modprotocol2/baselegs.ps
 hscale=90 vscale=90 hoffset=-55 voffset =-280}
\caption{Baseplate support leg.}
\end{figure}

\section{Baseplate O-Ring}
Around the perimeter of the front of the baseplate is a square groove 210mm x 210mm. This groove is used to accommodate the baseplate O-ring. The O-ring is large, and is made out of one piece of rubber joined together at each end. Ensure that this joining has no faults before installing it into the baseplate. If the O-ring becomes out of shape, boiling it in water for 30 minutes will re-shape it.

\section{Baseplate PCB's}
Prior to installing the CsI scintillators, the baseplate PCB's must be put in place. Three separate PCB's attach to the front of the baseplate (see Figure 2.9). Each PCB has three sixteen pin (female) sockets, which attach to the glass feedthroughs already in place.

\begin{figure}[ht]
\vspace{12cm}
\special{psfile = /home/oed/Modprotocol2/BasePCB.ps
hscale=90 vscale=90 hoffset=-30 voffset =-200}
\caption{Module baseplate, showing the three baseplate PCB's.}
\end{figure}

The PCB's must be placed onto the baseplate in a particular arrangement. This arrangement is shown in Figure 2.9. For the centre module the middle PCB has a notch cut out of it to allow for the beam-pipe.

Each PCB also has a row of fifteen (male) pins at one end. These pins attach to the Position Sensitive Detector (PSD) PCB via a ribbon cable. Each board also has a three pin (male) connector. This takes the gas energy signal, from the gas anode grid, out to the pre-amplifiers.
\newline
\newline
N.B. \em Care must be taken when installing the baseplate PCB's. The PCB connectors attach to the delicate glass feedthroughs.
\newline
\newline
\rm A female socket with three wires connected to it, attaches to the three pin connector on the baseplate PCB. The three wires attach to the three pin connector via a common socket. The wires are of different lengths and colour. Pink is 130mm , purple is 230mm, and yellow is 150mm.

After connection there should be nine wires in all for an outer module, and eight for a centre module. 
These wires relay the signal from the gas anode grid to the back of the baseplate. 
Once the wires are in place, check the continuity between the gold end of the wire and the corresponding output pin on the glass feedthrough, at the back of the baseplate. To find which pin on the feedthrough corresponds to which wire, follow the tracks on the baseplate PCB, from the three pin connector to the sixteen pin connector on the back of the PCB.

At this stage it is also a good idea to attach the gas bias lead. The gas bias lead has a conhex connector on one end  and a gold connector on the other. Screw the conhex connector onto the conhex fitting (of which there are twelve on the baseplate) nearest the `gas in' socket. Check the continuity between the gold pin and the conhex connector which protrudes from the back of the baseplate. The lead runs along the inside of the CsI mounts feet, and pushes into a connector on the gas anode grid. The position of this gas bias connector is shown on page seventeen (Figure 2.6) of ``The MEGHA Manual'', as well as figure 4.4 of this manual.

The baseplate assembly is now complete.

If there is enough time, and a leak checker is available. It is advisable to check the vacuum integrity of the baseplate. This can be done by following the procedure in section 4.3, to built up the module vessel, with a blanking plate. Screw the vessel onto the baseplate. Cap the small SwageloK fitting `gas in', and evacuate the module via the large SwageloK fitting `gas out'. A helium leak checker will register any leaks in the baseplate or module vessel.

Make sure the baseplate is leak tested before continuing.
