1 saw   1.1 \documentclass{chowto}
  2           
  3           \title{Design of the SOS Lucite Detector}
  4           \howtotype{reference}
  5           \author{Liguang Tang}

  6 saw   1.2 \category{sos} 

  7 saw   1.1 \date{June 10, 2003}
  8           \begin{document}
  9           
 10           \begin{abstract}
 11           This Howto outlines the purpose of the SOS Lucite detector 
 12           and its design parameters.  General information on mechanical 
 13           installation and electronics setup are also given.
 14           
 15           \end{abstract}
 16           
 17           \section{Purpose}
 18           
 19           This detector was designed and used as part of the particle
 20           identification to suppress protons from lighter particles, 
 21           kaons and pions. It is a total internal reflection type of 
 22           detector.  The Lucite material (C(CH$_3$)CO$_2$CH$_3$ polymer) 
 23           has an index of refraction n = 1.49, so that the threshold 

 24 saw   1.2 $\beta$ for Lucite \v{C}erenkov radiation is 0.672.  

 25 saw   1.1 
 26           The outside medium is air with an index of about 1.  In such 
 27           case, the total internal reflection takes place when the 
 28           incident radiation angle is larger than the so called 
 29           critical angle which is defined by:
 30           
 31           sin($\theta_{\rm critical}$) = 1/n,
 32           
 33           about 42.16 degrees.  Therefore, there is a threshold $\beta$ 
 34           beyond which the total internal reflection will take place and 

 35 saw   1.2 the \v{C}erenkov radiation will in principle remain 

 36 saw   1.1 transmitting inside of the Lucite to the ends, where PMTs are 
 37           attached.  This threshold can be found by
 38           
 39           $\beta_{\rm TIR} \ge 1/\sqrt{n^-1}$,
 40           
 41           corresponding to threshold at $\beta$ = 0.905.  In general with 
 42           normal incident, with a momentum above 1.06 GeV/c radiations from 
 43           kaons will be collectable through TIR.  On the other hand, protons 
 44           need to have a momentum above 2.0 GeV/c in order to have 
 45           collectable TIR radiation.   Thus, proton rejection can be done 

 46 saw   1.2 by Lucite \v{C}erenkov detector up to the maximum SOS momentum,

 47 saw   1.1 while the minimum momentum for the desired lighter particles 
 48           depends on the mass of such particle, as an example of kaons 
 49           mentioned above.
 50           
 51           
 52           \section{Design and Installation}
 53           

 54 saw   1.2 \v{C}erenkov radiation has a continuous energy spectrum, 

 55 saw   1.1 with a stronger intensity in the short wavelength, in the 
 56           ultra-violet region.  In order to minimize the scattering and 
 57           absorption loss, UV transmitting Lucite material was used for 
 58           the SOS Lucite detector.
 59           
 60           The detector consists of eight horizontal Lucite bars, each 
 61           one has a thickness of 2.54 cm and an active area of 40 x 13.7 
 62           cm$^2$.  Tapper light guide with the same material reduced the 
 63           cross section over a 15 cm length to fit a 3 in Phillip PMT 
 64           mounted at each end.  Each bar was individually wrapped by 
 65           black Teflon foil to seal outside light and absorb the escaped 
 66           none - TIR radiation.
 67           
 68           The eight bars were mounted on a common holding frame.  This 
 69           frame was then attached to the supporting frame that holds the 
 70           S2Y and S2X scintillators, in front of the S2Y.  Thus, this 
 71           detector is ahead of S2Y.
 72           
 73           
 74           \section{Electronics setup}
 75           
 76 saw   1.1 There were total of 16 analog signals sent to upstairs counting 
 77           house patch panel.  Due to the low gain of PMTs and attenuation 
 78           in long transmission, the signals are small for split.  Thus,
 79           the signals were first sent to a 10 times linear amplifier.  Then 
 80           it was splitted: 2/3 went to ADC and 1/3 to the 16-channels 
 81           CAMAC discriminator that can provide a sum output signal.
 82           
 83           The PMT HVs were adjusted by single photo-electron peak.  A 
 84           common threshold was used for the discriminator module.  Single 
 85           output from the sum was used in the PID trigger system.  If this 
 86           signal is sent further to another discriminator, by changing the 
 87           threshold level one can select requirement of signal from one-PMT, 
 88           or two-PMTs, or so on.  Due to variation of incident angles, only 
 89           one-PMT requirement was commonly used.
 90           
 91           \section{Performance}
 92           
 93           This detector was mainly used in the kaon experiments in the past 
 94           with SOS as the kaon arm.  It was found that with the above 
 95           mentioned electronics setup, background protons in trigger were 
 96           reduced by a factor of 3.  More detailed analysis in the offline 
 97 saw   1.1 can reduce it further.  However, the angular spread of the 
 98           incident particles was sufficiently large to cause certain level 
 99           of overlap for the summed ADCs bwteen those from protons and kaons.
100           Thus, to maintain no more than 2 % kaon loss, small per centage of 
101           protons remained in the data and were further rejected by other mean, 
102           such as TOF.
103           
104           
105           \end{document}
106           
107           % Revision history:

108 saw   1.2 % $Log: SOS_Lucite.tex,v $
109           % Revision 1.1  2003/06/12 16:15:47  saw
110           % Initial	Checkin
111           %