%The Hybrid Array                                 Steve Chappell

\chapter{The Hybrid Detector Array}

\section{Overview}

The basic module of the array consists of a self-contained vacuum-sealed
unit of nine telescopes, each containing three detection elements; an
ionization chamber, a solid state position sensitive detector (PSD) and
a scintillating CsI(Tl) crystal, operating within the same volume of
propane gas. An exploded view of these components is shown in Figure
\ref{mod1}. The front of the module is sealed by a single sheet of
aluminized mylar (typically a few $\mu$m thick) glued to a frame and
bolted, against an `O'-ring seal, to the main body. This forms nine
windows with aluminized surface facing inward, effectively earthed via
the module casing, to provide the gas section cathode. Signals, power,
bias and gas flow are passed to the relevant components through the
stainless steel module base plate. Five modules arranged in a Maltese
cross (Fig. \ref{labels}), with PSD centres tangent to a 600 mm radius
sphere centered on the target, comprise the full array of 44 telescopes.
Un-scattered beam passes through a narrow beam pipe, which replaces the
central telescope in the zero degree module, to the downstream beamline
and onto a Faraday cup for monitoring current.

The characteristics of the individual detection elements are discussed
in the following three sections.

\subsection{Gas Ionization Chamber}

The axial field gas ionization chamber has evolved as a natural
extension of designs developed at the Nuclear Structure Facility,
Daresbury. We use a central gridded anode and shield grid constructed
from Au-W wires (typically 20 $\mu$m diameter at a pitch
of 1 mm) soldered onto a PCB frame in nine 50 mm $\times$ 50 mm
sections .  A common bias is applied to the anode grids (typically
using the relation 2.5 V torr$^{-1}$ cm$^{-1}$) so that, depending upon
experimental application, operation is typically in the range +500-1500
V for gas pressures of 50-150 torr.  Individual signals from each anode
section allow for the coincident detection of ions in different volumes
of the gas.

\subsection{Position Sensitive Detector}

The PSD's used in MEGHA are of the large area resistive sheet variety
and deliver both energy and position information through just five
contacts (Fig. \ref{psd}). These detectors are manufactured by
implanting boron into $n$-type silicon and forming a thin
resistive layer for charge division that also serves as the front anode
of the detector. Additional ion-implanted strip lines border this electrode
in a ``low resistance ring'' to compensate for the ``pin cushion''
distortion that is usually a characteristic of such devices.  Signals
derived from each of the front face corners of the (50 mm $\times$ 50 mm
sensitive area) PSD's used in MEGHA localize event position with a
remarkable degree of linearity (better than 1\%) and show little
evidence of distortion.  Reverse bias is maintained across the PSD by
application of a positive voltage to the rear face cathode or ``energy''
contact. The present stock of PSD's include similar designs manufactured
by Hamamatsu Photonics and Eurysis. The bulk of detectors are of 380
$\mu$m wafer thickness with a nominal operating bias of +80--100 V. See section \ref{psdsection}.

\subsection{CsI(Tl) Scintillator and Photodiode}

The detector (supplied by Quartz \& Silice) consists of a solid piece of
CsI(Tl) crystal, wrapped in 5 $\mu$m thick aluminized mylar, with a 50
mm $\times$ 50 mm square front face and an active depth of 10 mm. The
crystal is tapered, acting as a light guide, over a further 15 mm to a
18 mm $\times$ 18 mm square and glued onto a photodiode (Hamamatsu
S3204-03). The two pins of the photodiode are soldered onto a PCB, where
a high-gain hybrid FET preamplifier is mounted together with other
passive components. Signal connections, bias and the $\pm$12 V power
supply, are via a 14-pin ribbon cable socket (Fig. \ref{csi}).

Operation of these scintillators at 2 $\mu$s shaping time and +30 V bias
gives the best resolution for charged particle detection. The detector
provides a gain of $\sim$ 6 mV/MeV and a full face energy resolution of
300 keV for 5.5 MeV $\alpha$-particles.  An investigation of the point
resolution of the CsI gave resolution values between 150 and 250 keV
with a variation of pulse height across the detector face. Experiments
requiring precise energy calibration may necessitate spline correction,
for example, of this behaviour. See the document {\itshape N.M.Clarke,
Charissa Report CH/Tech/92/08} for more details.

\subsection{Preamplifiers}

The pre-amplification of signals from the gas anodes and PSD's is
performed using Charissa designed ``surface mount'' charge sensitive
preamps. These are housed in groups of 18 in ``preamp cans'' mounted on
the rear of each module, three cans per module. The ``low gain'' preamps
are nominally $\times$1 and used for the PSD's. The modified ``high
gain'' variety are approximately $\times$5 and used for the gas anode
signals.

\begin{figure}
\vspace{6cm}
\centerline{\psfig{file=./figs/mod1.ps,height=8cm}}
\vspace{2cm}
\centerline{\psfig{file=./figs/tel.ps,height=6cm}}
\vspace{-4.0cm}
\caption{Components of the nine 3-element telescope MEGHA Module.}
\label{mod1}
\end{figure}

\begin{figure}
\centerline{\psfig{file=./figs/megarr.ps,height=11cm}}
\vspace{1.0cm}
\caption{MEGHA Array: Convention used for telescope and signal labelling.}
\label{labels}
\end{figure}

\section{Module Construction}

Notes on the construction of the MEGHA modules from their component
parts (as shown in figure \ref{mod2}) and the routing of signals can be
found in this section. A complete inventory of module parts is given in
appendix A.\\
{\bfseries\itshape When building the modules please remember to:\\
1) Check and include the many O-rings.\\
2) Use high vacuum techniques (gloves and a clean
bench!) to help minimise pump down times.\\
3) Test for signal continuity BEFORE installing the module.\\
4) Cover any exposed 16-pin feedthroughs. They are transparent and
allow a significant amount of light to strike the detectors.\\
5) Protect the feedthroughs from damage. They are fragile and the most
common source of leaks.\\
6) The length of screws is critical. Washers must be used on all
screws. Refer to ``Screw details for Module Assembly'' in the technical
drawings.}

\begin{figure}
\centerline{\psfig{file=./figs/mod2.ps,height=15cm}}
\vspace{1.0cm}
\caption{MEGHA Module: Side view showing module components and dimensions.}
\label{mod2}
\end{figure}

\subsection{The CsI Array}

Each scintillator unit is cradled by a support, which is attached to the
scintillator mount by two springs. {\itshape\bfseries Note that the
relative orientation of the mount and scintillators is important and
must follow that shown in Fig. \ref{csi}.} A clamp screwed to the
scintillator frame at the four corners then keeps these springs and the
scintillators in position. A single ribbon cable, attached as shown in
Fig. \ref{csi}, delivers power and bias to the scintillator unit and
passes signals to the baseplate feedthrough shown in Fig.
\ref{baseplate}. {\itshape\bfseries Note that this ribbon cable has been
modified to route signals from the same signal output pin along
different lines.} A common test input line takes pulses to the test
circuit of all the photodiode preamps in the module. A summary of the pin
allocations for the CsI array is given in the figure.
\begin{figure}
\centerline{\psfig{file=./figs/csiarr.ps,height=8cm}}
\vspace{2.0cm}
\centerline{\psfig{file=./figs/csi.ps,height=9cm}}
\vspace{1.0cm}
\caption{CsI Array: Orientation of detectors, \& Signal routing.}
\label{csi}
\end{figure}

\subsection{PSD Array}
\label{psdsection}

Four 4 mm PTFE spacers separate the scintillator clamp from the PSD PCB
board. The PSD's simply plug into this board which must be oriented as
shown in Fig. \ref{psd}. {\itshape\bfseries Note that two possible
orientations connect each PSD to the PCB board. However, only one
orientation connects the rear face energy signal (e)! Look for the
central pin with a connection to the Si wafer.} Three ribbon cables
take the PSD signals a to e and pass them to the baseplate PCB inserts.
These then route the signals to the baseplate feedthrough pins as
described in Fig. \ref{baseplate}.
\begin{figure}
\centerline{\psfig{file=./figs/psdarr.ps,height=8cm}}
\vspace{2.0cm}
\centerline{\psfig{file=./figs/psd.ps,height=6cm}}
\vspace{1.0cm}
\caption{PSD Array: Orientation of detectors, \& Signal routing.}
\label{psd}
\end{figure}

The ``offset'' and ``pull-down'' resistors connected to the PSD position
contacts (p in Fig. \ref{psd}) are in the form of plug-in packs so that
these resistances may be varied according to the experimental
application. The resistor packs are located on the PCB board that mounts
directly onto the module baseplate inside the preamp can.  {\bfseries\itshape
Note that the orientation of these resistor packs on the PCB board is
important!}

\subsection{Ionisation Chamber}

The shield (cathode) grid of the ionisation chamber is mounted
immediately above the PSD's, separated from them by four 6 mm PTFE
spacers. The grid wires go on the top side of the PCB frame and are
oriented to lie horizontally.  Four 24 mm spacers then separate the
shield from the anode grid. The whole assembly is then secured by
nuts.  Individual anode signals are extracted along cables (bundled as
three sets of three) which plug into each of the baseplate PCB's. A
single common bias is applied to the anode grid via direct connection
through the conhex feedthrough on the module baseplate (Fig.
\ref{baseplate})  The bias cable is fed from the baseplate around CsI
No. 1 to the anode grid. This grid is oriented with wires running
horizontally and on the underside of the PCB frame.  Connections to the
individual telescope grids are shown in Fig. \ref{gasanode}.
\begin{figure}
\centerline{\psfig{file=./figs/gasanode.ps,height=8cm}}
\vspace{1.0cm}
\caption{Orientation of the gas anode grid.}
\label{gasanode}
\end{figure}

\subsection{Windows \& Calibration Grids}

The front face of the module is sealed by a thin mylar window. This is
secured between two frames (the window clamp) then bolted to the module
casing. A calibration grid may be attached to the front of the module
using four screws. Location pins determine the orientation of the
grid.  {\bfseries\itshape Nb. The mylar windows are usually attached
when the modules are in place (bolted to the rear flange) via the
access ports on either side of the chamber. Before sealing the modules
ensure that all of the gas flow pipework is connected and the gas
system valves set to equalize the pressure to atmospheric throughout
the module and chamber volumes. The required valve settings are
described in the evacuation procedure given in section \ref{evac}.}

\subsection{Connections to the Module Baseplate}

\subsubsection{Preamp Cans}

Three preamp cans attach to each module. The front of each of these cans
plugs into the three baseplate feedthroughs serviced by the preamps.
Four screw thread rods support the corners of each preamp-can, screwing
directly into the baseplate. Four long screwthreads are provided for
attaching the module handles. The preamp cans deliver signals on conhex
connectors as shown in Fig. \ref{baseplate}. Each preamp can also has 3
SHV connectors for PSD bias and a plug for the preamp power supply. The
three inputs eT, GT and pT allow a pulser to be connected to the test
circuit of the preamps. eT drives the three PSD ``energy'' preamps, GT
the three ``gas'' preamps and pT the 12 PSD ``position'' preamps.  The
telescope numbering convention is given in Fig.  \ref{labels}.

\subsubsection{CsI Connector}

Connections to the CsI array are made directly to the module baseplate
feedthrough shown in Fig. \ref{baseplate} via the BNC to 16 pin
conversion box.

\subsubsection{Gas Bias}

Bias is applied to the anode grid via connection to the conhex
feedthrough indicated in figure \ref{baseplate}. The remaining feedthroughs are
unused.

\subsection{The Zero Degree Module}

A beampipe replaces the central telescope of the zero degree module.
Apart from the notes below the construction of this module is identical
to the others.\\
When constructing the zero degree module:\\
{\bfseries 1)} Use the zero degree mylar window/calibration grid/window clamp.\\
{\bfseries 2)} Remove the plug on the baseplate to allow passage of beam.\\
{\bfseries 3)} Use the zero degree baseplate PCB's.\\
{\bfseries 4)} Use the zero degree preamp can.\\
{\bfseries 5)} Remember that telescope number 5 doesn't exist!\\

\subsection{Summary of Pin Allocations for each Module}

Figure \ref{baseplate} summarises the pin allocations on the rear of
the module baseplate. The telescope numbering convention is given in
Fig. \ref{labels}.
\begin{figure}
\centerline{\psfig{file=./figs/base.ps,height=8cm}}
\vspace{2.0cm}
\centerline{\psfig{file=./figs/preamp.ps,height=8cm}}
\vspace{1.0cm}
\caption{Summary of pin allocations on the baseplate and preamp cans of
each Module.}
\label{baseplate}
\end{figure}

\subsection{MEGHA Patch Panel}

Most of the cabling for each module goes via a patch panel mounted above
and behind the target chamber. The patch panel layout mimicks the
{\itshape front view} of the 44 telescope array shown in Fig. \ref{labels}.
The complete patch panel layout is included here for reference
(Fig. \ref{patch}).
\begin{figure}
\centerline{\psfig{file=./figs/patch.ps,height=15cm}}
\vspace{1.0cm}
\caption{Layout of the MEGHA patch panel.}
\label{patch}
\end{figure}
