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\documentclass{chowto}

\title{Design of the SOS Aerogel Detector}
\howtotype{reference}
\author{Liguang Tang}
\category{sos} 
\date{June 10, 2003}
\begin{document}

\begin{abstract}
This Howto outlines the purpose of the SOS Aerogel detector 
and its design parameters.  General information on mechanical 
installation and electronics setup are also given.

\end{abstract}

\section{Purpose}

This detector was designed and used as part of the particle
identification for particles with close masses, such as pions
(139 MeV) and kaons (497 MeV), up to a momentum of 1.873 GeV/c.
The beta separation using the SOS TOF is insufficient to 
distinguish these two types of particles with very close values 
of beta.  

The SOS Aerogel is a $\breve C$erekov light emission threshold type of 
detector.  The index of refraction of the chosen radiator, 
aerogel, is 1.034$\pm$0.001.  The radiation eission threshold 
is 0.967.  In case of pion and kaon separation, kaons under the 
above mentioned maximum momentum will not emit or have sufficient 
emission of $\breve C$erekov radiation.  Thus, the signals from this 
detector are from pions or even lighter particles, 
such as electrons or positrons.

\section{Design}

The detector is shaped like a large rectangular aluminum box with
14 5" photomultipliers (PMTs) attached on the two long sides, 
7 on each side.  The wall thickness of the box was 1.59 mm.  
It was divided into two parts: (1) aerogel tray and (2) 
diffusion box. The two parts were held together by thin steel 
brackets.

The overall thickness of aerogel radiator was 9 cm and the 
active area was $100\times 40~{\rm cm}^2$.  This designed volume was filled 
by individual aerogel tile with a size of $25\times 25\times 3~{\rm cm}^3$.
Three layers of tiles were used.  The tiles were cut in a way 
of making the joints in each layer shifted with respect to the 
joints in the other layers.  Thin wires were strung cross the 
openning surface of the aerogel volume facing the diffusion box 
to hold the tiles in place in the tray.  Two layers of thin 
aluminum mylar were wrapped on the walls of the tray to reflect 
radiation back to the diffusion box through the aerogel tiles.  
The surface facing diffusion box was open.

The diffusion box is an empty rectangular box with 7 5" Burle 
8854 photomultipliers mounted uniformly on each of the two long 
sides.  The walls of the diffusion box were wrapped by 
reflective Millipore paper, with a grade of 450 nm.  The 
Cerenkov radiation was collected in a diffusive way by the 14 
PMTs.  

The detector was designed and built in early 1990s.  The 
obtained aerogel material at that time was still extremely 
hygroscopic and easy to absorb moisture due to humidity. Thus, 
there were two gas feedthroughs on the diffusion box, allowing 
the circulation of dry nitrogen gas.  This is to maintain the 
detector in a clean and dry environment and extend its life.  
Baking the aerogel after long time storage was proven to be 
an effective way to restore the performance of the detector.  
However, extra care must be taken in handling the material 
during the disassembly and reassembly processes.

\section{Installation}

Typically, this detector is installed in the space between 
the last two planes of TOF scintillation hodoscope, S2Y and
S2X.  It is fixed in position by the mounting bruckets on the 
box using "C" clamps to the hodoscope supporting frame.


\section{Electronics setup}

The 14 analog signals from the 14 PMTs were first led to the 
patch panel behind the SOS hut.  Since the signals were slower 
due to diffusion processes, 100 ns cable delay in attempt to 
match the HMS flight path length difference was not used to 
these signals.  The signals were splitted after the upstairs 
patch panel.  1/2 of the signal strength from each individual 
PMT was recorded in the data stream as ADC signal.  Signals  
from the remaining 1/2 were amplied and divided into two groups 
according to a ``shoelace'' configuration, in order to reduce 
any y-dependent inefficiency.  Signals from 7 PMTs in each 
group were then summed, as Sum-A and Sum-B.  The summed signals 
were sent to two separated discriminators.  The threshold of 
each discriminator was set to correspond to a few 
photoelectrons (p.e.), depending on the experimental 
requirement.  The logical AND of the two discriminator output 
then forms the SOS aerogel pretrigger, AERO.  This signal 
was used further in the trigger scheme or logic, depending on 
the specific need of the experiments.

The HVs of the PMTs were gain matched so that the one-photo-
electron (OPE) peaks were aligned at about the same number of 
ADC channels after pedestal subtraction in the ADC spectra.  
Good hardware gain matching could allow a good threshold 
setting for the discriminators in terms of number of p.e.,
which is sometime critical in determining the trigger loss 
and inefficiency for detection or rejection.

\end{document}

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