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Modular multi-wire drift chambers for the TRIUMF QQD-spectrometer Forster, Brigitta Monica 1986

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MODULAR MULTI-WIRE DRIFT CHAMBERS FOR THE TRIUMF QQD-SPECTROMETER  by  BRIGITTA MONICA FORSTER B.Sc,  University  of B r i t i s h Columbia, 1982  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in  THE FACULTY OF GRADUATE STUDIES Department of Physics  We accept  this  t h e s i s as conforming  to the r e q u i r e d  standard  THE UNIVERSITY OF BRITISH COLUMBIA December 1986  © Brigitta  Monica  Forster  In  presenting  degree at  this  the  thesis  in  University of  partial  fulfilment  of  British Columbia, I agree  freely available for reference and study. I further copying  of  department  this or  publication of  thesis for by  his  or  her  DE-6(3/81)  that the  for  an advanced  Library shall make it  It  is  granted  by the  understood  that  head of copying  my or  this thesis for financial gain shall not be allowed without my written  Physics  The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date  representatives.  requirements  agree that permission for extensive  scholarly purposes may be  permission.  Department of  the  Dec. 19,  1986  - i i -  ABSTRACT  A modular, m u l t i w i r e d r i f t  chamber was b u i l t and t e s t e d i n  the M13 a r e a . The r e s o l u t i o n o b t a i n e d w i t h s i x sense w i r e s , was i n t h e horizontal  drift  d i r e c t i o n a(mean) < (79.9± .2)Lim,  w i t h an a n g u l a r  r e s o l u t i o n o f (9.37±.02)mrads. W i t h the d r i f t  chamber mounted i n the f i r s t w i r e chamber  p o s i t i o n o f the QQD-spectrometer, t h i s corresponds 1.97mm FWHM a t the p o s i t i o n o f the s c a t t e r i n g In the v e r t i c a l d i r e c t i o n , u s i n g a charge  to a resolution o f  target.  d i v i s i o n method, the best  a c h i e v e d r e s o l u t i o n was < 2mm FWHM per w i r e , which g i v e s an angular r e s o l u t i o n o f 71mrads o r 15mm FWHM a t t h e t a r g e t The  chamber was run a t r a t e s  i n excess o f 1 0  A multi-chamber system was t e s t e d , four  cell drift  chambers f o r the h o r i z o n t a l  c e l l modular d r i f t  chamber, l o c a t e d  6  position.  particles/sec.  consisting  o f two modular  d i r e c t i o n , and one e i g h t  i n between and r o t a t e d  by 90°, was  used  f o r the v e r t i c a l p o s i t i o n d e t e r m i n a t i o n .  four  c e l l chamber was a(mean) < (108.5± .9)uim, and f o r the e i g h t  chamber a(mean) < (79.9± was  .2)Lim.  The r e s o l u t i o n  The a n g u l a r r e s o l u t i o n f o r t h i s  1.37 mrad h o r i z o n t a l l y and 9.37mrads v e r t i c a l l y ;  the QQD-spectrometer, t h i s corresponds respectively  a t the t a r g e t  spot.  f o r one cell system  i n connection with  t o FWHM o f 0.267mm and 1.12mm  - i i i-  TABLE OF CONTENTS  ABSTRACT  i i  LIST OF TABLES LIST OF FIGURES  vi vii  ACKNOWLEDGEMENTS  CHAPTER I. INTRODUCTION  ix  1  A. Motivation f o r this project  1  CHAPTER I I . GENERAL PRINCIPLES OF A DRIFT CHAMBER  4  A. The single d r i f t c e l l  4  B. Basic phases  6  1.  Thermalization  7  2.  D r i f t phase  7  3.  Avalanche m u l t i p l i c a t i o n  8  C. Basic Processes  9  1.  D r i f t of charged p a r t i c l e s i n gases  9  2.  Electron d i f f u s i o n  10  3.  D r i f t of electrons i n the presence of magnetic f i e l d s ..  12  - iv TABLE OF CONTENTS (continued)  CHAPTER I I I . PRINCIPLES OF OPERATION  14  A. E l e c t r i c F i e l d  14  B. Gas mixtures  15  C. Resolution  18  D. High rates  18  E. Charge d i v i s i o n  20  CHAPTER IV. THE MODULAR MULTI-WIRE DRIFT CHAMBER (MMDC)  23  A. The concept  23  B. Construction of modules  24  C. Assembly  26  CHAPTER V. MMDC VERSIONS BUILT FOR THE TRIUMF QQD SPECTROMETER  29  A. Design considerations  29  B. The 8 wire MMDC  29  C. The 4 wire MMDC  31  D. Electronics  33  - v -  TABLE OF CONTENTS (continued)  CHAPTER VI. CALIBRATION SETUPS A. Bench testing  38 38  1.  Laser v e l o c i t y c a l i b r a t i o n  38  2.  6-source v e l o c i t y c a l i b r a t i o n  45  3.  Laser e f f i c i e n c y tests  45  B. Beam testing  47  1.  Velocity c a l i b r a t i o n s  47  2.  M u l t i p l i c i t y tests for anodes  47  3.  Data taking f o r resolution  50  CHAPTER VII. DATA ANALYSIS & RESULTS  53  CHAPTER VIII  DISCUSSION AND FUTURE CONSIDERATIONS  71  APPENDIX  75  REFERENCES  76  - vi-  LIST OF TABLES  I.  TDC Correction Coefficients  58  II.  Resolution of the Eight Wire Chamber  60  I I I . Resolution with Changing D r i f t F i e l d  61  IV.  Resolution with Changing Gas Gain  62  V.  Resolution of the Four Wire Chamber  63  VI.  The Horizontal Angular Resolution i n the Three Chamber System  64  - vii -  LIST OF FIGURES  CHAPTER I. INTRODUCTION 1. QQD Spectrometer at Ml3 Focus  2  CHAPTER I I . GENERAL PRINCIPLES OF A DRIFT CHAMBER 1. A Single D r i f t C e l l  5  CHAPTER IV. THE MODULAR MULTI-WIRE DRIFT CHAMBER (MMDC) 1. The Two Basic MMDC frames  25  2. Stringing J i g  27  CHAPTER V. MMDC VERSIONS BUILT FOR THE TRIUMF QQD SPECTROMETER 1. Modular 8-Wire D r i f t Chamber  30  2. Mounted 4- and 8-Wire Chambers  32  3. Electronics Block Diagram  34  4. Preamplifier C i r c u i t Diagram  35  5. Eurocard C i r c u i t Diagram  36  - viii LIST OF FIGURES (continued)  CHAPTER VI. CALIBRATION SETUPS 1. Setup for Laser C a l i b r a t i o n  40  2. Laser C a l i b r a t i o n of D r i f t Velocity  41  3. Laser Velocity Calibration  42  4* Comparison of Beta-Source  and Laser C a l i b r a t i o n  44  5. Laser E f f i c i e n c y Calibration f o r 6 Central Wires  46  6. Setup f o r "in-beam" C a l i b r a t i o n  48  7. Velocity Calibration with Wire Chambers  49  8- Wire M u l t i p l i c i t y as Function of Anode Voltage  51  9. Setup f o r Three Chamber System  52  CHAPTER VII. DATA ANALYSIS & RESULTS 1. Track F i t t i n g : Chisquare D i s t r i b u t i o n  55  2. Chisquare D i s t r i b u t i o n for 6 Wires of MMDC8  59  3. Charge D i v i s i o n Method with 5 5  67  F e  source  4- Charge D i v i s i o n Method: Laser C a l i b r a t i o n of y-Resolution .  68  5. Calculated F i e l d Maps f o r MMDC8  69  6« Signs of Deterioration on Anode Wires  70  CHAPTER VIII DISCUSSION AND FUTURE CONSIDERATIONS 1. Calculated F i e l d Map for Proposed Chamber  73  - ix -  ACKNOWLEDGEMENTS  I would l i k e to express my thanks to my supervisors Dr. R.R.  Johnson  and Dr. K. Erdman  as well  as Dr. D. G i l l f o r  encouragement and guidance during this project. Many thanks go to John Stewart and Eddie Knight expert indebted  advice  and continual  technical  to Dr. D. Hutcheon f o r i n c i t i n g  assistance. discussions  I  f o r their am  further  and suggestions  for the data analysis. Thanks are also due to the members of the PISCAT group. F i n a l l y my deep appreciation goes to my husband Rolf, for h i s support and assistance i n typing this manuscript.  - 1 -  CHAPTER I - INTRODUCTION  I.A. MOTIVATION FOR THIS PROJECT  At TRIUMF  are presently  Proportional one  the front end of the low energy pion QQD -spectrometer at  MWPC  provisions  f o r three  standard  type Multi-Wire  Chambers (MWPC). The spectrometer i s usually running with  at the front  of the f i r s t  quadrupole  magnet  i n the  WC1  p o s i t i o n ( F i g . 1.1), and an other MWPC at the WC3 position between the second  quadrupole  and the dipole magnet. The horizontal  (x) and  the  v e r t i c a l (y) readout from each chamber i s used to project the p o s i t i o n of an incoming pion back to the target. The intrinsic  resolution of the spectrometer i s limited partly by the  resolution of these two detectors,  as well as the multiple  scattering induced mainly by t h e i r window f o i l s , and to a lesser degree the  gas-contents  of the chambers  and the spectrometer  i t s e l f . The  i n t r i n s i c resolution of the MWPC's depends on their wire-spacings which are  1mm i n the horizontal  characteristics  and 2mm i n the v e r t i c a l d i r e c t i o n and the  of the d e l a y - l i n e s  which  a r e used  for  the  i d e n t i f i c a t i o n of the Individual wires. A second l i m i t a t i o n i s imposed by the incoming p a r t i c l e flux which can be tolerated by MWPC's. As a consequence,  the QQD spectrometer does not  operate e f f i c i e n t l y at small forward angles.  Fig. QQD  1.1 spectrometer  at  chambers are shown.  M13  focus.  A l l available  positions  for wire  - 3 -  Single rate  problem,  wire  readout  chambers  could  certainly  but c o u l d not s i m u l t a n e o u s l y Improve the  Dr. K. Erdman suggested the replacement  overcome  the  resolution.  of the two MWPCs w i t h  a drift  chamber, which would o f f e r improvements i n both r e s o l u t i o n  and  rates.  I t i s the  and  intention  of t h i s work t o document the b u i l d i n g  t e s t i n g of d r i f t c h a m b e r s based on h i s d e s i g n and t o comment on implementations.  possible  - 4 -  CHAPTER I I - GENERAL PRINCIPLES OF A DRIFT CHAMBER  Drift  chambers are used extensively i n high energy physics.  They are best known for very high s p a t i a l resolutions as well as high rate  capabilities.  A drift  chamber consists of one  or several  drift  c e l l s . An i n d i v i d u a l d r i f t c e l l measures a single s p a t i a l coordinate of a charged p a r t i c l e which traverses a suitable medium. This i s achieved by  exploiting  the  physical message provided  by  the  electromagnetic  i n t e r a c t i o n of the p a r t i c l e with the medium. This chapter  gives a b r i e f review  of the general p r i n c i p l e s  which make d r i f t chambers possible. It i s based mainly on three review a r t i c l e s [1-3].  I I . A . THE SINGLE DRIFT CELL  The  most basic c e l l consists of an anode wire and a cathode  plane which are kept at high voltage i n the gas f i l l e d d r i f t space. The electrons produced through i o n i z a t i o n by a charged p a r t i c l e traversing the d r i f t signal. signal  space d r i f t The d r i f t  and  a  towards the anode wire and  produce an e l e c t r i c a l  time, that i s the time difference between the anode  reference  time  signal  (given  for  example  by  plastic  s c i n t i l l a t i o n counters outside the d r i f t c e l l ) i s then proportional to the d r i f t  distance Ax.  given i n F i g . II.1.  A schematic  picture of a single d r i f t  cell is  - 5 -  Fig.  II.1  A single d r i f t  cell  - 6 -  For particle,  the r e c o n s t r u c t i o n of the a c t u a l t r a j e c t o r y  two o r more  drift  cells  A l t e r n a t i v e l y , a multi-anode d r i f t  II.B.  of a charged  can be s t a c k e d behind  each  chamber can be used.  BASIC PHASES  A  charged  particle  traversing  the gas i n a  drift  i o n i z e s a l o n g i t s t r a c k . The primary e l e c t r o n s c r e a t e d i n t h i s lose  their  and/or field  energy  molecules  electric  gas  atoms  until  they  field  and/or  towards  become s e n s i t i v e  anode  the d r i f t  wire,  along  avalanche  the way.  i s superimposed  displacement  multiplication  field  which  A  global  electric  rises  collisions motion  with  i n the  on the random thermal  of the e l e c t r o n  velocity. Finally,  the e l e c t r i c  to the a p p l i e d  the anode, h a v i n g m u l t i p l e  molecules  The average  called  process  T h i s means the e l e c t r o n s a r e a c c e l e r a t e d by  d i r e c t i o n o f the e l e c t r i c f i e l d motion.  chamber  by f u r t h e r i o n i z a t i o n and e x c i t a t i o n o f gas atoms  and s t a r t t o " d r i f t " .  the  is  other.  swarm per u n i t  i n the immediate rapidly.  This  vicinity results  i s n e c e s s a r y f o r a measurable  time of an  i n an electric  signal.  There  are therefore  three d i s t i n c t  phases  which  taken i n t o c o n s i d e r a t i o n when d e s i g n i n g o r o p e r a t i n g a d r i f t  have  to be  chamber:  - 7 -  II.B.l.  Thermalization  The  first  electrons along  energy  and  to emission  accuracy  of  of  i s the  thermalization  cell.  This  ionizing particle,  pressure of the d r i f t  of  limit  phase i s dominated and  the  by  the  It i s t h i s  non-uniform density of the ionized t r a i l -  of delta electrons - which can  a drift  the  encountered  the path of the incident charged p a r t i c l e .  physical extension mostly  phase  due  the p o s i t i o n the  original  composition as well as  the  gas.  II.B.2. D r i f t Phase  After  the  thermalization  process the electrons d r i f t  d i r e c t i o n of the applied e l e c t r i c f i e l d . A constant velocity  i n the  value of the d r i f t  makes i t possible to translate the measured time difference  into a s p a t i a l distance. The  value  and  stability  of  the d r i f t  v e l o c i t y have d i r e c t bearing  on  the resolving power of the d r i f t c e l l and on the maximum rate at which the  cell  can  homogeneity  be and  operated. strength  components and the gas  The of  drift the  pressure.  v e l o c i t y depends mainly on  electric  field,  and  on  the  the gas  - 8 -  II.B.3. Avalanche M u l t i p l i c a t i o n  Avalanche electric limits the  field  for  around  the  extent  on  measurements moderate spread  of  the  depends  -  the have  the  but  also  charge  d i v i s i o n method  Another the  drift  kind  of  with  the  drift  of  details  path  gas  is  drift  respective  anode  that and  acquire  chapter  taking  pressure,  and  the  short  sections  of  chapter  performance  and  be  determines  the  -  to  electron a  smaller  [1].  Detailed  rather  gases. A  narrow  narrow  detector  to  space coordinate  basic i n the  resolution III.  throughout  d i f f u s i o n . It field  strength  introduction next  considerations,  and  can  place to  p a r t i c l e s follows  rates  of  rising  at  angular  be  run  through  at the  III.E).  due  field  the  diameter  the  extra  dispersion  A  spread  quenched  allows  an  rapidly  ultimately  gain  spread  only  process  the  electric of  this in well  not  distance.  charged  optimization  and  (See  the  mixture,  on  mixture  to  basic  angular counter  avalanches  rates,  The the  gains  the  e f f e c t sets  on  shown  higher  This  by  e l e c t r o n i c s , which  detector.  gas  induced  anode w i r e .  strongly  avalanche  of  the  time-measuring  efficiency  avalanches  multiplication is  two  be  length  depends and  on  gas  diffusion  mixtures  discussed  of the  increases  sections, while  specific will  to  the  in  and more and the  - 9-  I I . C . BASIC PROCESSES  I I . C l . D r i f t o f Charged P a r t i c l e s i n Gases  For ions the d r i f t v e l o c i t i e s  w  +  increase l i n e a r l y  with the  applied e l e c t r i c f i e l d up to values of several kV/cm.atm as [ 4 ] :  w  +  (II. 1)  = u"hE/p  where p i s the gas pressure and u specific  +  i s the ion mobility, which i s  f o r each ion i n a given gas. The average ion energy  In this  region remains equal to the thermal value kT [ 4 ] . The  electrons, on the other hand, can much more easily  be  accelerated by the applied f i e l d between c o l l i s i o n s with gas molecules. They reach average energies f a r exceeding  the thermal energy  even at  moderate f i e l d s . Electron theory gives a simple formula f o r the electron d r i f t v e l o c i t y w~ [ 4 ] :  w~ = eET/2m  where T i s the mean found  time  (II.2)  between c o l l i s i o n s .  Unfortunately  i t was  that f o r some gases the c o l l i s i o n cross-section and therefore T  varies strongly with E, going through maxima and minima. This anomaly  - 10 -  (Ramsauer  Effect)  popular d r i f t  was  gas  first  observed  component) and  later  i n Argon also  (incidentally a  i n Krypton and  very  Xenon as  well as several organic compounds l i k e Methane, but not i n Helium and Neon. In addition i t was small and  found  f r a c t i o n of another gas  [5] can  so dramatically change the d r i f t  that even the addition of a very modify the  average electron energy  properties.  II.C.2. E l e c t r o n D i f f u s i o n  A group of electrons  in a drift  gas  will  suffer dispersion  due  to d i f f u s i o n . A charge d i s t r i b u t i o n which at time t=0  by  a  delta  direction  x)  function, by  a  will  be  described  at  time  Gaussian d i s t r i b u t i o n with the  t  i s described  (along  following  a  given  standard  deviation [3] :  a  x  = /2Dt"  where D i s a f i e l d dependent d i f f u s i o n c o e f f i c i e n t and  (II.3)  t i s the time of  drift. It  i s customary to define a c h a r a c t e r i s t i c energy  r a t i o between the d i f f u s i o n c o e f f i c i e n t and the mobility [6]:  as  the  -  ek  where w  11  -  = eE - 5 -  i s the d r i f t  (II.4)  velocity.  In the i d e a l case, where the electron energy i s not modified with increasing values of E, the d r i f t v e l o c i t y increases l i n e a r l y with the f i e l d . Then  equals i t s c l a s s i c value kT and the space d i f f u s i o n  for a d r i f t length x Is given by [ 6 ] :  a  x  =  /  / 2kTx —iE"  / T T  c x  <n'5>  This quantity i s often called the thermal l i m i t to electron d i f f u s i o n . For any gas at a given temperature T, the average electron d r i f t v e l o c i t y i s given by [7]:  e E w" = - ^ - — — 3 mN P 4TT  °° v df / a (v) dv o m 2  3  (II.6)  where N Is the number of gas molecules per cm  3  momentum t r a n s f e r cross section, f  at 1 torr,  i s the  i s the spherically symmetric term  in the expansion of the electron v e l o c i t y d i s t r i b u t i o n function. If one combines II.3 and II.4 one obtains:  - 12 -  2e, x  ax  It not  1  can be seen that  At  (II.7)  7T"  E  only on E/P (since  for  P  the d i f f u s i o n i n any given d r i f t  gas depends  does) but also on ^= .  a given gas pressure, the d i f f u s i o n c o e f f i c i e n t Is smaller  " c o o l " gases, i . e . gases i n which the c h a r a c t e r i s t i c energy  s m a l l even a t high E/P v a l u e s .  is  It should be noted that the given a x  represents the width of the charge d i s t r i b u t i o n , and not the error on its localization. Furthermore, especially  the  diffusion  i s not  always  symmetric,  at high f i e l d s . The longitudinal d i f f u s i o n c o e f f i c i e n t D Li  (with  respect  to the d r i f t  velocity)  tends  to be smaller than the  transverse d i f f u s i o n c o e f f i c i e n t D^. This c l a s s i c a l approach does not work that well when applied to a gas mixture. Only calculations based on rigorous transport theory have accurately reproduced the experimental data. II.C.3. D r i f t of Electrons i n Presence of Magnetic F i e l d s  The drift the  presence of a magnetic f i e l d  other than p a r a l l e l to the  d i r e c t i o n modifies both the d r i f t v e l o c i t y and the d r i f t path of  electron swarm. In noble gases the d r i f t i n g electrons show a broad  spectrum of v e l o c i t i e s even at moderate f i e l d s , since most of their  - 13 -  collisions  are  elastic.  f i e l d s , the Lorentz force angles  and  thus  impair  Consequently,  In  high  transverse  magnetic  v x B w i l l cause a wide range of d e f l e c t i o n the  performance  of  the  detector.  Organic  admixtures to the noble gases allow the d r i f t i n g electrons to transfer energy quickly to rotations and vibrations of complex molecules. leads  to the  desired narrowing of the v e l o c i t y  distribution  This  for  the  electrons. D r i f t chamber operation In strong magnetic f i e l d s w i l l not be discussed any further here. Although a l l the d r i f t chambers constructed i n this thesis were tested and intended  to be used i n the fringe f i e l d  of the TRIUMF QQD  been shown that similar  spectrometer,  i t has  drift  c e l l s f i l l e d with various gas mixtures could tolerate f i e l d s as high as 0.13  Tesla  before  corrections  to  the  data  were  necessary  [8].  - 14 -  CHAPTER I I I - PRINCIPLES OF OPERATION  III.A. ELECTRIC FIELD  A  homogeneous  v e l o c i t y throughout  E-field  is  essential  for  a  constant  drift  a d r i f t c e l l . However, i n the immediate v i c i n i t y of  an anode, the homogeneity w i l l cease since the E - f i e l d varies rapidly as 1/r, where r i s the distance from the wire center. This leads to an avalanche The  m u l t i p l i c a t i o n which i s necessary  amplification  threshold.  for a measurable  signal.  increases exponentially with applied voltage above  Threshold  occurs  when an  electron can  between c o l l i s i o n s to ionize a molecule i n the next  gain enough energy collision.  Since the v e l o c i t y d i s t r i b u t i o n i s a function of the e l e c t r i c f i e l d and the pressure through the r a t i o E/P,  i t i s extremely  to  saturates  use  a  gas  threshold. field,  i n which  the  drift  Saturation should  making  the  imperfections and  chamber  mechanical  velocity  extend  response  to  the  almost  highest  at  important a moderate  values  independent  of  from  the local  tolerances i n the placing of the cathode  and the f i e l d defining wires. In multi-wire ground function  drift  potential  chamber designs, alternating  i s to help  focus incoming  to  with  separate  there are often f i e l d  at  anodes  (see  Fig.  V . l ) . Their  individual  drift  cells  and  electrons. Those f i e l d wires also prevent  coupling between anodes.  wires  thus  to  electrostatic  - 15 -  III.B. GAS MIXTURES  The will  composition  of  the  gas  mixture  used  in a  driftchamber  depend very much on the s p e c i f i c experimental requirements;  such  as high rate c a p a b i l i t i e s , low working voltage, long l i f e t i m e , e t c .  Noble avalanche  gases  are  often chosen  as  m u l t i p l i c a t i o n occurs at much lower f i e l d s than with complex  molecules.  However, during the avalanche  atoms are  formed. The  state  only  between  the main component since  through  the  first  excited  a  noble  radiative  excited  gases can  process,  state  process  and  the  and  excited and  ionized  return to the ground the  ground  energy state,  difference (11.6eV f o r  Argon, 16.6eV for Neon) i s well above the i o n i z a t i o n potential of any metal used  for wires  or cathode  planes (7.7eV f o r copper,  6.0eV f o r  aluminium). This puts l i m i t s on the allowed gain for the chamber before i t enters a regime of permanent discharge.  On  the  other  hand,  most  organic  compounds  alcohols) have many non-radiative excited states due v i b r a t i o n s . Because this energy  range,  allows  the quenching  (hydrocarbons,  to rotations  and  the absorption of photons i n a wide  efficiency  of a polyatomic  gas increases  with the number of atoms i n the molecule. But  large,  complex  molecules  with  very  high  quenching  a b i l i t i e s w i l l at the same time reduce dramatically the gas-gain at the  - 16 -  anode  wire.  The  problem  with  dioxide, i s that secondary  very  simple  molecules,  emission which may  like  carbon  lead to a discharge, has  occasionally been observed. The counting  polymerization  of  hydrocarbons,  rates, makes i t mandatory to operate  gas flow configuration. Non-polymerizing  especially  at  the chamber i n an open  quenchers  like  methylal  [ (OCH.J )^CYi.^ ] are not e f f e c t i v e against photo-ionization and emission,  but,  i f added  hydrocarbon-ions It  into  then  field gas  small  quantities w i l l  secondary neutralize  a non-polymerizing species. i n which the  drift  an extended plateau at a s p e c i f i c e l e c t r i c f i e l d . It  extremely  important  that the  threshold at a l l times. mixtures  in  i s advisable to choose a gas mixture  v e l o c i t y reaches is  even  high  as  well  as  chamber be  operated  above  this  Characteristics of d i f f e r e n t gases and  their  associated  velocities,  are  readily  a v a i l a b l e i n the l i t e r a t u r e . A compilation of many works i s found i n Ref.  [9]. The  chamber  general  depend  on  requirements  the  detector  for  the  design.  characterized by the applied e l e c t r i c f i e l d , and  A  single  of  drift  a  drift  cell  is  i t s length to depth r a t i o  the potential on the anodes. As far as the d r i f t gas i s concerned,  a very high d r i f t is  performance  v e l o c i t y i s required i f the emphasis of the chamber  towards handling high rates. Better s p a t i a l resolution i s achieved,  i f f i r s t of a l l the d r i f t gas i s kept at high pressure and by chosing a  - 17 -  slow  velocity  "cool"  gas, where  the c h a r a c t e r i s t i c  electrons i s close to the thermal l i m i t  Unfortunately, many of  energy  e of  the  e ~ kT  those very  low  velocity  gases,  like  Di-methyl-ether, do not have a saturation v e l o c i t y . Although very good resolution can be attained with i t ,  the chamber has to be constantly  and c a r e f u l l y monitored [10]. While  testing  the chambers b u i l t  for this project  they worked well with either Argonlong as the noble gas content was  i t was  found, that  or Neon-hydrocarbon mixtures, as  not dropped below 50% i n volume. In  mixtures with more then 50% hydrocarbons, the signal to noise r a t i o i n the  electronics  no  longer  allowed  the  chambers  to  be  operated  at  maximum e f f i c i e n c i e s . Except when stated, a l l d r i f t gases were mixed on s i t e using the TRIUMF mobile mixing units. A l l tests were done at 1 atmosphere pressure with continuous flow. A l l flow meters have been i n d i v i d u a l l y calibrated  f o r the pertinent gases.  Methylal was  only needed for the  high rate tests. At these times the noble gas was bubbled through the liquid  methylal  which  was  kept  at  zero  degrees  Celsius.  This  corresponds to a (3 - 5)% methylal content In the noble gas. The  gas  i n section VI.A.l.  doping  for the laser c a l i b r a t i o n runs i s discussed  - 18 -  III.C. RESOLUTION  The  s p a t i a l resolution l n a d r i f t chamber Is limited mainly by: -  the physical  width  and  the s t a t i s t i c a l  spread  of the  o r i g i n a l i o n i z a t i o n track - the value and s t a b i l i t y -  of the electron d r i f t v e l o c i t y  the dispersion due to d i f f u s i o n , which depends on the gas mixture as well as the gas pressure and the e l e c t r i c  field  strength.  Dispersion physical reduced actual  always increases  with d r i f t  extension of the i o n i z a t i o n track by increasing  depth  distance.  and the d i f f u s i o n can be  the gas pressure to several  of the d r i f t  cell  can further  Both the  limit  atmospheres.  The  the resolution i f  there i s a time difference between i o n i z a t i o n clusters belonging to the same track a r r i v i n g at the same wire.  III.D. HIGH RATES  The  count  rate  i s limited  most  severely  by space  accumulation i n the d r i f t region, and an e f f i c i e n c y loss caused  charge by the  f i n i t e duration of the signals, usually referred to as dead time l o s s .  - 19 -  I I I . D . l . Space charge  The amplification  positive will  lower  ions  which  the e l e c t r i c  are field  Due to their greater mass, ions w i l l have a electrons.  generated  in  the  gas  strength near the anode. slower d r i f t v e l o c i t y than  This results i n a pulse height drop and could consequently  a f f e c t the chambers e f f i c i e n c y . Since this effect depends on the extension of the  avalanche  near the anode wire, the most important design parameters w i l l be the radius of the sense wire, gas gain and gas composition.  III.D.2. Dead time  At therefore  high  rates,  the  signals  occasionally  overlap  and can  not be counted i n d i v i d u a l l y .  For  a high rate chamber, the following design c r i t e r i a should  be observed: - small wire spacing to reduce the rate per wire, - small gap width to reduce space charge effects and reduce signal t a i l s caused by late a r r i v i n g i o n i z a t i o n , - low noise preamplifier to operate at low gas amplification, -  small anode radius  to provide fast  reduce l o c a l space charge,  anode signals  and to  - 20 -  large  bandwidth  amplifier  to allow  fast  clipping  of  signals, -  dead  time  of discriminator  matched  to width  at base of  clipped signal [11]•  F i n a l l y , i f the chamber i s run at high rates over an extended period  of time,  additional  care  should  be  taken  condition  of the wires i n the chamber. Only highly  be  The hydrocarbon  used.  reduced  to preserve the p u r i f i e d gases can  content i n the gas mixture may have to be  and a non-polymerizing quencher,  such as Methylal,  must be  added.  III.E. CHARGE DIVISION  In particles  the charge  is  determined  d i v i s i o n method, along  the position  the wire,  through  of impacting the  ratio  of  resistances. If Q and Q, are the amounts of charge collected at the up down up and the down end of the sense wire respectively, then  given by: ^up ^down wire length Q + Qj ' 2 up down  the position i s  - 21 -  In a d r i f t  cell,  the charge d i v i s i o n method can be used to  measure a second space coordinate i n the y d i r e c t i o n . If  applied  gives  to  the  Ion  signal  the y coordinate, but  on highly r e s i s t i v e  due  rate c a p a b i l i t y of the d r i f t  cathode wires, i t  to the low mobility of the ions the  chamber would drop s i g n i f i c a n t l y . A more  promising method would be to place r e s i s t i v e wires close to the anodes and process the e l e c t r o s t a t i c a l l y induced electron s i g n a l .  It  has  been shown by several authors  time measurements and on  the  charge d i v i s i o n can be performed  same r e s i s t i v e  through  anode.  capacitive decoupling  preamplifiers.  The  [12,13,14] that  signals  Both ends of directly  are  then  wire  simultaneously are  connected  to the input of low  Impedance  processed  the  drift  separately f o r charge  d i v i s i o n and f o r timing. The order  to  value  keep  resolution  along  the  of  the  noise  the wire.  anode wire level On  low  resistance should and  the other  to  improve  be  the  high i n position  hand, a small resistance i s  required i n order to have good l i n e a r i t y i n the position determination and high resolution i n the d r i f t for  time measurement. The  resolving  time  the charge d i v i s i o n method i s about one electrode time constant T  for optimum position resolution and l i n e a r i t y [15]. ( T = RC, where R, C are  the  resistance and  capacitance  per  unit  length of the  line).  A  - 22 -  detailed  analysis  [16]  has  shown  that  the  best  compromise  for  simultaneous position and d r i f t time resolution i s achieved by [13]:  R  where  L  stands  for  the  =  2TT(L/C)  inductance  1 / 2  per  unit  length  of  wire.  This  corresponds to the c r i t i c a l damping condition for transmission l i n e s . The of  charge the  measurement  amplifier,  amplifier [16].  the  i s seriously limited input  stage  of  by  which  the  input  should  be  impedance a  current  - 23 -  CHAPTER IV. - THE MODULAR MULTIWIRE DRIFT CHAMBER (MMDC)  IV.A. THE CONCEPT  The f i r s t attempts at the new QQD front end detector were the construction and the testing of a more conventional multi-wire d r i f t chamber. Out of the c o l l e c t e d experiences during that project grew the design of modular d r i f t instrument  which  specifications,  cells.  i s fixed  modular  Rather by  type  than being limited to a single  i t s rigid  drift  cells  dimensions  and  are capable  of providing  v e r s a t i l e detector combinations which can be e a s i l y b u i l t , and  repaired  different of  whenever  the need  arises.  They can also  maintained,  be adapted to  tasks which may require the change of the characteric  a drift  cell,  design  depth  as well as the replacements of wires by new ones of  d i f f e r e n t materials or/and diameters. With boards, and  the exception of the actual  a l l of these d r i f t  calibrated  equipment.  printing  of the c i r c u i t  chambers were strung, assembled,  at TRIUMF, without  requiring  sophisticated  altered, tools or  - 24 -  IV.B. CONSTRUCTION OF MODULES  The modular 1.5 mm  thick printed  drift  chambers were constructed e n t i r e l y out of  circuit  boards.  The G-10 material i s cut into  i d e n t i c a l frames with a 50 mm x 50 mm opening. 12 positioning holes are drilled  symmetrically throughout the border. Only two c i r c u i t designs  are necessary: The type A boards (see F i g . IV.la) contain the f i e l d defining wire cage and are used as the two end boards f o r the chamber.  Field  wires are 100 um Cu-Be, strung at a tension of 120 grams and are 5 mm apart from each other. The voltage drops evenly from the two cathodes on either side of the frame towards the centre wire, which i s grounded. P o t e n t i a l differences between wires are obtained through a chain of 12 MOhm r e s i s t o r s which are soldered d i r e c t l y onto the boards. The inside edges of the c i r c u i t boards, which define the cathode planes, are painted with a conducting graphite solution.  The type B boards (see F i g . IV.lb) have the centre e l e c t r i c a l contact pins o f f s e t towards one side. wires  (anodes)  potential. strung  and the potential  They are used f o r both the sense  wires, which  are kept  Anode wires are 20 um thick gold plated  at ground  tungsten and are  at a tension of 50 grams. The two anodes used f o r the charge  d i v i s i o n measurements are 20 ym thick high resistance (3.5 kfi/m) NiCr wires at a tension of 10 grams.  F i g . IV.1 The two basic MMDC frames  type A  board  type B  board  - 26 -  The 120  grams.  potential wires Care must  be  are 100  taken  that  um  thick CurBe at a tension of  the  contact  pin offset  on  the  boards which are strung with potential wires are a l l towards the same side, while  for a l l the sense wires the boards have to be rotated by  180 degrees.  This gives a maximum distance between the contact pins of  the grounded potential wires and  the pins for the anodes (which carry  h i g h positive p o t e n t i a l ) , thus reducing the chance of sparking between them. As before, the inside edges of the frames which define the cathode planes are painted with a conducting graphite solution.  For stringing the wires, a stringing j i g was used (Fig.IV.2). The  printed board  i s positioned with  four  locating pins onto a sled  which moves l a t e r a l l y . The wire s i t s i n V-grooves located at either end of  the c i r c u i t  board  and  i s brought to the required tension.  A three  p o s i t i o n toggle determines the wire position either at centre or 200 iim to  the right or l e f t of i t . The tolerance of the j i g was measured to be  < 10  um.  IV.C. ASSEMBLY  The  chamber  is  assembled  by  pushing  threaded  nylon  rods  through the positioning holes a l l around the c i r c u i t boards. Distances between anodes and potential wires, which determine the c h a r a c t e r i s t i c s of a single d r i f t c e l l , are set by nylon washers which were  machined to  whl  ch to  e  8gl  s  ,  P°si  ci  ft 6  «t4 s  *t  P O s i  ,  s  ^o  n  [  °cati  at  c  en  Pins t r e  °nto  tne  Three  - 28 -  the desired thickness of 0.5mm, rendering a c e l l depth of  4mm.  A modular type driftchamber i s b u i l t s t a r t i n g with one type A wire  cage  board,  followed  by  as  many sequential d r i f t c e l l s  desired and finished with another wirecage  as  are  board. Nylon bolts are used  on either end of the threaded rods to fasten the frames together. The  finished  chamber  i s very  rigid,  even  i f only  three  individual  d r i f t c e l l s are used, which i s the minimum of c e l l s needed i n order to solve the l e f t - r i g h t  ambiguity.  This method of assembly not only allows f o r an easy exchange of wire planes, but also the addition of extra d r i f t c e l l s for better resolution as well as the removal of c e l l s i f there are, f o r example, space limitatons. Wire planes can be pre-fabricated with any number of d i f f e r e n t wire diameters fast  replacement  and/or wire materials. This permits not only  of a damaged wire, but also the complete exchange of  s p e c i f i c wires with d i f f e r e n t ones which are more suited to a new or another  set of circumstances. The  task  c h a r a c t e r i s t i c depth of some or  a l l of the consecutive d r i f t c e l l s can be changed simply by i n s t a l l i n g a set of washers with appropriate thicknesses. The  assembled  chamber  was  mounted  inside  a  leak  proof  container with 0.5 mil thick Mylar windows. Gas tight feedthroughs have to be i n s t a l l e d for the high voltages, the preamplifier bias voltage, the signals, and the d r i f t gas flow tubes. Pictures of are shown i n F i g V.2.  mounted MMDC's  CHAPTER V. - MMDC VERSIONS BUILT FOR THE TRIDMF QQD SPECTROMETER  V.A. DESIGN CONSIDERATIONS  The  s p e c i f i c a t i o n s f o r the new chambers asked  c a p a b i l i t y , high angular of  the incident  resolution  spatial  particle  resolution i n the x-direction  beam),  i n the y - d i r e c t i o n  Furthermore,  the chambers  atmospheric  pressure.  for high rate  and to a lesser (height  should  degree  angular  of the p a r t i c l e  beam).  be run with  To achieve  this,  (width  the d r i f t  gas  at  a compromise was necessary  between high s p a t i a l resolution and high rate c a p a b i l i t y i n the design of  the d r i f t  cell.  Two d i f f e r e n t  systems were developed  and tested.  They are described i n the following sections.  V.B. THE 8 WIRE MMDC  This comprised  eight consecutive d r i f t c e l l s , which were to measure p a r t i c l e  trajectories determined  system consisted of one single chamber, which i n turn  i n the x-z-plane.  The t h i r d  v i a charge d i v i s i o n read-out  dimension  off the f i r s t  y was to be and last  wire.  The layout for this modular 8-wire d r i f t chamber i s shown i n Fig.  V.l.  The dimension  maximum d r i f t alternatively  from cathode to cathode i s 50 mm, i . e .  distance i s 25 mm. by ±200um  the  The anode wires are set off-centre  i n order  to solve  the l e f t - r i g h t  side  - 30 -  RESISTOR CHAIN XL • ^ ^ o - ^ v//7///////\  -y/,//'/-/•;//A  -\Y to c  < c  V / / ' A Y / / / A Y7/7/7777/7A &7/7/7////A  o  •\////7/?//7A -Y///7/ '7777A •Y7/7/7'7/77A AV////T///7A - Y / / / / A  c  • Y7//7//7777A  Y///;;7y//7Zi  a as <  E-H  Y/777777777A U////7////A  •Y//////7S//A  -\/////?////A • Y//////V/777\  \///7~/7777\ 177/7?/7/7A \///,-77////A \///7/~77/7A V///>>//7//ZA Y7/7//7v7777\ Y77/77T7777A '/////7V///A o-»»»^i7/V ///7//7A LL II  -Y/////-//;;A -Ys 7s 7, ,'7/7/A •Y 7//77.7/777A  •X//.A-/////A  • \7//77/7/777\~-Y////777//Y  /A  25mm  FIELD DEFINING WIRES  UZZZZZZZZZA-  CATHODE PLANE ANODE  -HV1  +HV2  CORRECTION POTENTIAL -HV3 o  POTENTIAL WIRE AT GROUND  Fig. V.l Modular 8-wire d r i f t chamber. Max. d r i f t distance i s 25 mm. Anode wires are  offset  ±200  correction wires.  ym  to resolve  left/right  ambiguity.  Two  field  - 31 -  ambiguity. Two f i e l d correction wires are connected to a separate power supply. This which  provides the option of applying any independent voltage  may be required  special  to correct  field  aberrations  specific  to  the  p o s i t i o n of the f i r s t and l a s t d r i f t c e l l i n the chamber. The  mounted MMDC8 i s shown i n Fig.  V.2a.  The high voltages for the cathodes, the correction wires, and the  anodes are fed through an RF choke. The e l e c t r i c signals are picked  up d i r e c t l y form the pins on the boards and decoupled through 1000 pF capacitors. They  pass through a preamplifier  and then leave  the  gas  container.  V.C. THE 4 WIRE MMDC  The  second system which was b u i l t  for the QQD spectrometer  used three chambers. The t r a j e c t o r i e s i n the xz-plane were given by the first  and t h i r d  (MMDC4).  chamber, each of which consisted  The less  important y trajectory was measured only  second chamber, the previously was  rotated  of four d r i f t  f o r this  cells i n the  tested eight-wire model (MMDC8), which  purpose by 90 degree with respect  to the other  chambers. Since there i s no need f o r a charge d i v i s i o n measurement i n this three-chamber  system, a l l anode wires were made from 20 \im thick  gold-plated tungsten wire. Fig.  V.2b shows  a mounted  MMDC4.  For this  system, the  feedthroughs are the same as described for the MMDC8. The gas container  - 32 -  B F i g . V.2 a) Mounted 8-wire chamber MMDC8 b) Mounted 4-wire chamber MMDC4 Copper  shield  f o r electronics  i s removed  i n both  pictures  - 33 -  shown i n F i g . V.2b was made s p e c i f i c a l l y to hold one MMDC4. The other MMDC4 for this three-chamber  system was simply mounted into the spare  gas container used i n the MMDC8 t e s t i n g .  V.D. ELECTRONICS  The data a c q u i s i t i o n block diagram i s shown i n F i g . V.3. The signals from each wire are fed through a 1000 nF decoupling capacitor into a fast rise-time (< 5 ns) hybrid preamplifier (see F i g . V.4 f o r a circuit  diagram). Regular  anodes used  anodes  are read  out from  one side  only,  f o r charge d i v i s i o n measurements are read out from both  sides. The signals are brought outside the gas container, where they are  further  amplified  and discriminated  i n the "Eureocard" (see F i g .  V.5). Both the "Eurocard" and the hybrid preamplifiers were fabricated i n the TRIUMF electronics shop. Out wire.  These  of the "Eurocard" comes a logic pulses are fed through  timing pulse for each  a twisted  pair  cable  into the  counting room, converted from ECL to NIM and provide the stop signals for  an eight  channel  time  to d i g i t a l  converter (TDC,  LeCroy  Model  2228). For  the charge d i v i s i o n measurements, the analogue  signals  from top and bottom of each wire are transmitted i n a similar way from the Eurocard v i a twisted pair cable d i r e c t l y converter (ADC,  to an analog to d i g i t a l  LeCroy Model 2249A). A separate ADC unit was used for  - 34 -  .0  Z A HKEAMP1JFIER A  F i g . V.3 E l e c t r o n i c s block, diagram  DISCKMINATOB  EVENT GATE  FAN  - 35 -  ->+6V  BFR31  100:  0.1 100 0.1  v  ^T2  -  o.i -> + OUTPUT  BFR93 50  1 BFR93 K -> - OUTPUT 0.1 147  50 -www1/ BFR93  INPUT <-  GROUND BFR31  MMBF4416  BAV99 GROUND < —  -> -6V  F i g . V.4 Preamplifier c i r c u i t diagram A l l capacitors i n units of uF A l l resistors  i n units of fi  - 36 -  9> + 12V<01  r ¥  2N4416  om X + m  450  +  '  t - o 0.1  450  BFR93  2K  £  0.1  -4  +  wl  100  10  CO CO  Hh —I ( -  10  10 0.1  Hf-  0.1  0.1  30  0.1 ECL OUTPUT  INPUT*—  BFR93  BAV99  0.1  2K 2N4416  47  GKOUNW— 0.1 CM  S  Q  5  F i g . V.5 Eurocard c i r c u i t diagram A l l capacitors i n units of uF A l l resistors  i n units of ft  100  100  -  37 -  each wire. One of the two signals was fanned out, discriminated, and used i n coincidence with the "event gate" signal as input gate f o r the ADC. The event gate i s given by one discriminated s c i n t i l l a t o r or for  output  the coincidence of the discriminated outputs of two s c i n t i l l a t o r s the bench testing. For the in-beam tests the "good event" strobe  signal of the QQD spectrometer was used. The ADC's and TDC's are read v i a CAMAC into the memory of a PDP-11 process computer and logged onto magnetic tape. The f i n a l data analysis was done on the TRIUMF VAX-cluster.  - 38 -  CHAPTER VI. - CALIBRATION SETDPS  VI.A.  BENCH TESTING  VI.A.l. Laser v e l o c i t y  The  velocity  calibrations  calibration  of a chamber includes not only the  measurement of the d r i f t v e l o c i t y i t s e l f , but also the determination of the v a r i a t i o n of t h i s v e l o c i t y throughout a l l areas of the detector. Simulating gives  the advantage  position  tracks of i o n i z i n g of having  p a r t i c l e s with a pulsed  a narrow, well  collimated  laser  beam whose  can be defined with great accuracy. Laser calibrations  have  been the subject of much debate. The source of the i o n i z a t i o n i s a two photon  absorption  chamber gas [17].  i n some low i o n i z a t i o n Such impurities  potential  are d i f f i c u l t  impurity  to control  i n the  since they  could come from d i f f e r e n t sources such as the outgassing of materials from  which  the  detector  was  constructed,  pump  o i l vapour,  or  contaminated gas mixtures. This randomness of molecules available f o r ionization  has lead  to inconsistent  data  for different  chambers, or  even f o r the same chamber, tested at d i f f e r e n t times.  A pulsed N^-Laser was used for this c a l i b r a t i o n which has a wavelength of 337nm (hv=3.68 eV).  Ionization can occur whenever there  are molecules present with an i o n i z a t i o n potential  I < 2hv (< 7.36 eV).  - 39 -  In  order  to  obtain  a  controlled  and  reproducible  intensity  i o n i z a t i o n , the chambers were doped with small quantities (~60  of  ppm)  of  N,N-diethylaniline CgH^N(C2H^)^, which has an i o n i z a t i o n potential of 6.99  eV. The gas induced i o n i z a t i o n i n the chamber i s expected to  b u i l d up as the concentration of the dopant increases with time. The build-up  was  exponential  over  the  first  five  to  ten  minutes  and  l e v e l l e d out a f t e r ~ 45 min, which i s consistent with Ref. [17]. The  setup  for  the  laser  calibrations  chambers i s shown i n F i g . VI.1. The laser beam was quartz  lenses  and  the  beamspot  cleaned  up  by  of  the  QQD  drift  collimated with two  a pinhole. A mirror,  mounted on a micrometer transport (± 2jjm) deflects the beam through the chamber. The chamber i t s e l f could be displaced v e r t i c a l l y (±200 mm)  on  i t s stand. An exposed p l a s t i c s c i n t i l l a t o r , tube was lights  used i n the  mounted on a photomultiplier  to trigger the electronic readout. During the tests the laboratory room were dimmed  i n order  photomultiplier which was operating at a low Voltage A  typical  Fig.  VI.2.  time  changed with  raw  TDC  histogram  to protect the  (700V).  for a single wire i s shown i n  Varying peak heights have no significance as the exposure each position.  F i g . VI.3  shows TDC  channel  versus laser beam position f o r the Argon(70%)-Isobutane(30%) mixture.  The  linearity  v e l o c i t y throughout  of  this  the chamber.  plot  demonstrates  a  number  drift  constant  gas  drift  - 40 -  55  55  i-J  o  <  PH  Nl E-"  < PH  PULSED N 2 LASER DRIFTCH AMBER ( mounted on u vertical mobile stage)  SCINTILLATOR  F i g . VI.1 Setup f o r laser c a l i b r a t i o n of d r i f t v e l o c i t y and e f f i c i e n c y  - 41 -  HUH  I N C 0  u N T S  4000 3900 3600 3700 34,00 3500 3400 3300 2 3200 3100 3(>00 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1B00 1700 X X lfaOG IbOO 1400 1300 iroo 1 100 1000 900 * iX BOO X XX 700 X XK 600 4 AX IX 500 400 XX X XX 300 XI XX XX 5XX XX 200 1 XX 7XIX XX6445XXX 100 500  OO  *OG  HIUlOCfcAM  01 01 *l  A-DCT-  WIKE « 6  tax XX XX XX XX  4 X  5  XX  1  XX xx;i  X X  x >:x  X  X  XXX XXX XXX  X X  XXX XXX  XXI X kX  '.iXXX XX XX  XXX  X  X X9 XX4  9  X X.  GX XX XX  3  X X X X X tx X XV  XXX  IX X X  4 X X X X 7X XX9  X X X6 XX X7 XX XX XXX XXX i'l XX XXX XXX7 2XXX7 (XX XX XX 1 tXXB635XK XXX4435XXXXS32 4XXXXX  X X  41 8XXX5 3XXXX2 XlXXXXXXXX744r>XXXXX977XX  4 X  00  F i g . VI.2 Laser c a l i b r a t i o n of d r i f t v e l o c i t y (Example f o r wire #6 of MMDC8) Peak separation corresponds to 2 mm s h i f t of l a s e r beam position.  - 42 -  "  MMDC8 wire #6  1400  •  o  1  1  <  •  •  PI cd  1  1200  •  •  • •  1000 •  •  •  -  800  •  •  CJ  Q E-  -  •  •  600  • • •  • •  <0  F  400 -30  - 2 0 - 1 0  0  i  10  20  30  Distance to Anode Wire (mm)  F i g . VI.3 Laser v e l o c i t y c a l i b r a t i o n . L i n e a r i t y demonstrates constant d r i f t v e l o c i t y throughout the chamber  - 43 -  The  following  observations  were  made  concerning  laser  c a l i b r a t i o n s for d r i f t chambers: - No data should be taken within the f i r s t hour a f t e r doping. - Direct contact  between the laser beam and  be avoided, since the photo-electrons surface, -  For  any wire  should  ejected from the wire  w i l l r e s u l t i n gigantic anode signals.  chambers which need windows thicker than 0.5mil, i t  will  be  necessary  to  replace  the Kapton or Mylar window  with a material which i s transparent  to UV r a d i a t i o n ( i . e .  Aclar). - Chambers which require aluminized for  Due  laser c a l i b r a t i o n s .  to  calibrations  windows are not suitable  the  cannot  intrinsic be  used  to  width  of  find  the  the  detector. They are, however, a clean and  i o n i z i n g beam, laser  true  resolution  of  the  fast method to determine the  magnitude and uniformity of d r i f t v e l o c i t i e s throughout a chamber. The  laser beam was  which  allows  e a s i l y collimated over large distances  several  chambers  to  be  tested  and  (50-100  calibrated  cm), in  coincidence. A  further  advantage of  laser c a l i b r a t i o n s i s important i f  chambers are to be used i n strong magnetic f i e l d s : As the laser l i g h t path, which l i b e r a t e s the d r i f t electrons, i s not affected by magnetic forces, the influence of any  magnetic f i e l d on a d r i f t chamber can  studied d i r e c t l y by using this method.  be  - 44 -  a)  1 0 6  R u Source  b)Laser  F i g . VI.4 Velocity c a l i b r a t i o n f o r MMDC8 (raw TDC histograms for wires #1,4,8) Neighbouring peaks i n the histograms correspond to 5 mm s h i f t of the c a l i b r a t i n g beam. Deterioration of the s p a t i a l d e f i n i t i o n of the electron  trajectory with increasing  clearly visible.  penetration into the chamber i s  - 45 -  VI.A.2. B-Source Velocity  A source testing place  of  further the  (see  velocity  3 emitter  F i g . VI.1)  where  the  chamber. In the  Calibration  mirror  1 0 6  c a l i b r a t i o n method was  Ru  was had  (100  deflected  position  set-up from the  collimated  the  sequence of F i g . VI.4,  the  The  used, with the  i s s t a r t i n g out well collimated. even worse at  uCi).  attempted using a  i t can  Laser  3-source i n  beam  through  be seen that  When i t reaches wire #4,  of wire #8,  laser the the  the 3-beam  however, and  i t s wide dispersion  make i t  unsuitable for the c a l i b r a t i o n of multi-wire d r i f t chambers.  VI.A.3. Laser E f f i c i e n c y Tests f o r Wires  A graph which shows the e f f i c i e n c y of each wire as a of  the  position  of  the  ionizing  made from  the  same data which were collected for the laser v e l o c i t y c a l i b r a t i o n .  The  number of "triggers" received from the s c i n t i l l a t o r was  how  many times each wire had  fired.  (the  stopped  UV-laser  coincidence of triggering, lower l i m i t s .  beam at  was  least  two,  track  (Fig. VI.5)  Since only one in  the  was  function  compared to  s c i n t i l l a t o r was  plastic)  rather  than  used the  which would help to guard against noise  the plotted e f f i c i e n c y results are only to be taken as  - 46 -  MMDC8 efficiency calibration (Laser)  100,  1 O  O  0  o°o°oo°  80. 100.  1  1 •  • •  1  1  1  0  0  1  1  •  •  80. 100.  1  1  • •••••••  1  1  1  O  1  0  80. 100.  1  1 B  B  100.  B  R  BH - B  1  B  1  •  •  1  1  •  •  80. 100.  1  « 1  1  « 1  B  B  1 •  •  1 •  •  1  I  1  1  20  25  B B  B  83.  0,  1 0  , 5  10  15  30  Distance Beam-Anode (mm)  F i g . VI.5 Laser c a l i b r a t i o n : e f f i c i e n c y of the 6 central wires of MMDC8  - 47 -  V L B . BEAM TESTING  VI.B.l. V e l o c i t y  Calibrations  For the pion beam v e l o c i t y c a l i b r a t i o n s , the MMDC8 was set up between two conventional MWPC's (see F i g . VI.6). The location of the pion  i n the d r i f t  straight  chamber was calculated  by projection,  assuming  a  l i n e trajectory through the two MWPC's space coordinates. For  the purpose of testing a l l the d r i f t  space, i . e . the f u l l width of the  chamber, the beam had to be completely defocussed. In the scatterplot (Fig.  VI.7) the traceback position  against each  i n the d r i f t  the TDC channel number. There  wire,  and again  the l i n e a r i t y  chamber  i s a similar of the d r i f t  i s plotted  scatterplot velocity  from  extends  throughout  the chamber. For the purpose of calculating an exact d r i f t  velocity,  very  distance TDC-axis,  narrow  calibration  i n t e r v a l s . The r e s u l t i n g  slits  were  defined  at regular  histograms were projected  onto the  producing a peak for every c a l i b r a t i o n window. A straight  l i n e f i t through a l l the c a l i b r a t i o n peaks was used f o r c a l c u l a t i n g the drift velocities.  VI.B.2. M u l t i p l i c i t y Test f o r Anodes  The MMDC8 was then mounted into the WC1 position at  the front of the QQD spectrometer.  (Fig.  1.1)  In order to determine the gas  -  O  u  48  -  K Ed  K Ed CQ  u  u  Ed  Ed OS  SS  00  1 Flg.  PION  130  mm-  BEAM  215mm-  VI.6  Setup for " i n beam" c a l i b r a t i o n of d r i f t  velocity.  - 49 -  T I « E  • i 1 1 1 1 490 0 1 480 0 1 . 470.0 I . . 4*0 0 1...::.. 490.0 1..::::. 440. 0 1 . . : . . : . . . . . 430.0 1 . . : . . : . 420. 0 1....::.:... 410 0 1 . . i:. 400. 0 : : i: :. . 390.0 1. .::.:... 380 0 1 : 370.0 1 ...•:.... 3*0 0 I. . . . . i: 3*0.0 1 ::•:... . . . 340. 0 1 . . . : aa: 330 0 I :.X: 320 0 1 .a: 310.0 I : . i . 300. 0 .::!:.... 890 0 1 SSO. 0 1 . . . . .X. 270. 0 1. 2*0. 0 1 . . . .8: 290. 0 1 . :. X: : 240. 0 1 X 230. 0 1 . . :. X: :. . . 220. 0 1 . . IX. . . 210. 0 1 : iX 200 0 - . : . . 1*0. 0 1 . : . Xl. 180. 0 1 . . X. : J70. 0 1 . . . .X 1*0. 0 1 . . :. 8. : . . 190. 0 1 140. 0 1 Bi: 130 0 1 . ,S. : . . . 120  0  1  .  -1.90  -1.00  1  • : «< • . : «B. : . . :Xi. :. . . : iX. . . . . XX.. . . . . .8X: . . . .SS: . . . . . . . XX . .Bi: ...:•• .:»•:... XX : . 4X •  • • • • B :  . . . lt«: . . . X6X: «X : .BS: . . . . . . . X#X:  .  • : * • « :  • : XSX: 8*i: ..:iti: ..:.•«:.... : iXX: . . . . :8Si: . : *• • • • :B4i • X*: : . IBI:  .  .  :BS. : XB . . ,»X. . . . . : X«:. . . :6S  ...  . . XX  . : XX. . . . . . XX. . . ... . : XB. : : Bi. . . . : XX: : . . :SS: Bi: . . : XX. : . . . : |XX: . . : X I I : . . . . . B. : S i : : aXi: : : BSX. XXi: . . . . aOM.: . . . : XX,. ..  -0 90 • 10*i  1  .  ...  . .  .  -2.00  1  . . .  . 1 6 . : . .  1.  110 0 1 100. O *0. 0 I . 80. O I 70 0 I *0 0 I 90 0 I 40 0 I 30.0 I. 20. 0 I 10. O I 0 0 -2.90  1  .  .  0.00 KD1STX)  F i g . VI.7 V e l o c i t y c a l i b r a t i o n with wire chambers  • 0.90  1 OO  ...  . . .  ' | 1.90  | 2 00  2  - 50 -  gain  needed  f o r an  efficient  operation  of  the  chamber,  the  m u l t i p l i c i t y , that i s the number of wires which had f i r e d i n response to a "good event" trigger, was monitored. A histogram of m u l t i p l i c i t y versus  the anode voltage i s shown In F i g . VI.8.  VLB.3. Data Taking f o r Resolution  Data mounted  in  has been taken with  the  WC1  position,  as  the MMDC8 chamber, when i t was i t was  intended  to  be  during  experiments. At times the rate through the chamber was i n excess of 10 p a r t i c l e s / s e c . The three chamber system was set up as shown i n Fig.  VI.9 f o r both the v e l o c i t y c a l i b r a t i o n s and the data taking.  s  B cr M  O i-h  09  • <  M OO  8 HIRES FIRED 7 WIRES FIRED 6 HIRES FIRED 5 HIRES FIRED  to 0)  c  3 O  o 3  a  o. <! O rt QJ 00 lt>  4 HIRES FIRED 3 HIRES FIRED 2 HIRES FIRED  •0 O  1 HIRE  <  FIRED  - 52 -  X  o  S i-H  H 55 I—f  O C/2  -I I  K Ed 03  Ed  m  X O  as  as  o  o  Ed  Ed  X  OH  co u O  u o  u  H 55 O CO  Q 2  PION BEAM  —  34 m m  32mm 126mm  F i g . VI.9 Test setup f o r three-chamber system  79mm  - 53  -  CHAPTER VII. - DATA ANALYSIS AND RESULTS  When  the  measured  drift  times  are  to  be  converted  into  distances, the following points have to be considered: Each TDC of  each  channel has a s p e c i f i c delay time and the position  anode wire  i n the  chamber  i s only known within  mechanical  tolerances. While the electronic delays certainly could be calibrated, the exact determination of any wire position i s very d i f f i c u l t without having  a detector available with at least  resolution  than  electronics  delay  the  chamber  times  which  and  the  is  wire  to  similar be  or better  calibrated.  positions  can  be  spatial  Both  the  calculated,  using a simple model for the chamber response. The TDC data depend on two types of parameters: a) Variables which vary from event to event, which are e s s e n t i a l l y due to track position and angle, and b) Variables which are constant during a run, such as the wire offsets and TDC channel delays.  For "type  each event, corrected times t ^ with respect to the  b)" parameters can be computed:  C  nk  =  'nk  +  T D C  k  e l  +  £  nk k W  f f  where t , i s the raw TDC reading f o r wire number k, nk °  del TDC . allows f o r nk  - 54 -  differences  i n delays i n the e l e c t r o n i c s ,  allows f o r the wire  offset r e l a t i v e to the median plane of the anodes, and  £  n k  S  r+1 ' -1  i f the track i s to the l e f t i f the track i s to the right  This implies that e . , _ = - e , . nk+1 nk v  The correction  following i s a step  by step  description  c o e f f i c i e n t s TDcf ^ and W.°^ were determined k k 6  of how the  f o r the 6 inner  wires of the MMDC8. The data set was r e s t r i c t e d to events where a l l s i x wires  had  fired.  The  left-right  ambiguity  assuming that a l l p a r t i c l e t r a j e c t o r i e s chamber. points offsets, through for  Then a least (raw TDC  values)  squares was  was  resolved  by  were i n the l e f t half  f i t f o r the trajectory  calculated.  Due  the right  half  half.  of the  to the data  to the ±200 um  the chi-square values f o r the tracks which actually the l e f t  first  wire  did go  of the chamber were much lower than the values F i g . VII. 1  separated by this method.  shows that events  can be c l e a r l y  A similar histogram i s obtained by assuming  that a l l tracks went through the right half of the chamber. Assuming no wire o f f s e t , the p a r t i c l e track can be described by p o s i t i o n  and angle parameters  was chosen as reference i n order  ^ ,  T A n  « The centre of the chamber  to make the error  matrix diagonal.  - 55 -  CHI  o 0 I 300 0 I •00 3*0  370 3*0 330 340 330.  0 3 H : H 14-jok WS  I  • 14 O II O II O II  Oiii a n o II 310 O 3O0 0 8*0 0  aao  II -I II  o  II  370 O 8*0 O 340 O 340  II II II II II  e  ax o  830 O i n 810 0 - I I 8O0 i«o o in o in II 170 o i n 1*0 0 I K 0 140 o 130 o i n 180 0 i n 110 o i n 100 0 « 0 . o II • 0 o II 70 o  -••  o o 40 o 30 o 0 II io o o 0 S 00  ao e  CO CO  w• H O  1  CO  ja  I co E-  H E- I X.  o  OS  »  I  I  114*1138 33411 I I I I I I I I 3 4 4 m n IIIIIIIIII7S •34 s i e i n i i III1IK11IIII4I4 1 I 9 447III4XI11IIII IIIIIIIIIIIIIII4144 I 7 4 I 9 I 4 7 I I I I I I I I E I I I I I 1 •IIIlIlllllI»IIIIII*iaS7»4433333l33iai I 1 1  30 00  40 00  •0.00  •O 00  4 I  TO 00  Fig. VII.l Track f i t t i n g : Chisquare d i s t r i b u t i o n obtained assuming a l l t r a j e c t o r i e s i n right half of chamber. Similar histogram i s obtained i f a l l t r a j e c t o r i e s are assumed i n l e f t half of chamber.  100 00  - 56 -  Then the corrected  nk  The  3.5n  time t ^ i s given by  + (k - 3.5)T.  (VII.2)  An  c o r r e c t i o n c o e f f i c i e n t s TDC^ ^" and  can now be determined by  6  the maximum l i k e l i h o o d method (see Appendix). During the f i r s t pass through the data, compute the corrected times t', . T D c f ^ c a n be s e t to zero and the i n i t i a l guesses for W.°^^ nk k k o o can be the same for each wire. A least squares f i t i s made to determine the track p o s i t i o n and angle.  The 2x2 normal matrix i s computed, where  data s e t are the s i x t*. and the variables are T ~ ..and T . , the nk 3.5 An d i a g o n a l terms are the squares of uncertainties i n T ~ and T . . Now J. o An the  c  the  l e a s t square value x 2  the  position resolution ( i n unit of d r i f t times), then  X  The  2  = \Kn~ k=l  T  f°  r  the event can be calculated.  3.5 " < - ' > An> k  i n i t i a l guess f o r a  3  5  T  2/  °l  '  < k»'°k> k=l A  2  can be common to a l l wires, x  a common histogram, as well  as accumulated  separately  If o^ i s  ( V I I  2  i  s  '  3 )  binned into  f o r each wire.  The contribution from this event to the 2x2 error matrix for each wire can  be  calculated.  corrections  The  inverted  error  matrix  i s used  to  find  to TDC^''' and W ° ^ ^ at the end of t h i s pass, i . e . c^ ^and 6  e  - 57 -  c,°^ are determined such that k  A I ( nk ' k d  e  - e c - k k  l  C  n  £  0 f f  C  }  2  (  V  I  I  >  4  )  k  (summed over a l l events), Is a minimum. At the end of the pass O  i s multiplied  q  by /y /N 2  (where N i s the number of degrees of freedom, i n this case N=4n), i n order to get a new estimate f o r the resolution per wire f o r the next pass. Also make the substitutions:  TDc£  e l  „off w. k  It  was  + c£  e l  ->  TDcf  , off ..off + c. ==> w. k k v  found  that  and  1  a  . , f o r use with the next pass,  only 2 to 3 passes were needed to find  the correction c o e f f i c i e n t s . As shown In Table I, only very few events have to be analyzed to give r e l i a b l e c o e f f i c i e n t s . F i g . VII.2 shows the chisquare d i s t r i b u t i o n f o r a l l s i x wires after the f i r s t pass (without any  TDC  corrections) .  The  second  column  shows  the c h i s q u a r e  d i s t r i b u t i o n f o r each wire with the f i n a l TDC c o e f f i c i e n t s .  - 58 -  TABLE I  TDC C o r r e c t i o n  // of Events  Coefficients  c o r r e c t i o n t o i n d i v i d u a l w i r e s ( u n i t s : TDC channels)  analyzed  #1  #2  #3  #4  #5  375  -18.97  14.53  16.63  1.72  -16.60  2.69  3648  -19.00  14.03  17.16  1.73  -16.06  2.14  18547  -18.94  14.36  16.55  1.91  -16.18  2.31  #6  after  first  pass of a n a l y s i s  w i t h f i n a l TDC  F i g . VII.2 C h i s q u a r e d i s t r i b u t i o n f o r a l l s i x w i r e s of MMDC8.  coefficients  -  60 -  TABLE II Resolution  of the Eight  (MMDC8 6 w i r e s  Chamber  considered)  VELOCITY  a(mean)  ANGULAR  TARGET  (um/nsec)  (um)  (mrads)  FWHM(mm)  I  51.8  145+1  17.1±.l  3.59  II  51.9  114±1  13.4±.l  2.81  III  40.9  77.4±.2  9.11±.02  1.91  IV  54.8  151±1  17.8±.l  3.74  V  60.8  143±1  16.8±.l  3.53  VI  64.4  126±1  14.8±.l  3.11  GAS  Key  Wire  t o the gas mixtures: I  ARGON 8 0 % - METHANE 2 0 %  II  ARGON 5 0 % -  ETHANE 5 0 %  III  ARGON 5 0 % -  ETHANE 5 0 %  IV  NEON  80% -  ETHANE 20%  V  NEON  65% -  ETHANE 3 5 %  VI  NEON  50% -  ETHANE 50%  Helium Mylar  from  (PREMIX) (PREMIX) (IN  HELIUM)*  the spectrometer  window i n t o  leaked  t h e chamber.  through  - 61 -  TABLE I I I  Resolution with Changing D r i f t  Field  (MMDC8 6 wires considered)  FIELD  a(mean)  (KV/cm)  (ym)  ANGULAR  TARGET  (ym)  (mrads)  (mm) FWHM  a(wire #2)  1.20  119.Oil.4  160.4±4.4  14.0±.2  2.94  1.40  113.7±0.6  148.8±1.9  13.4±.l  2.81  1.60  115.2±0.9  153.3+2.8  13.5±.2  2.84  (AR 50% - ETH 50%) PREMIX  - 62 -  TABLE IV  Resolution with Changing Gas Gain (MMDC8 6 wires considered)  ANODE (KV)  a(mean)  a(wire #2)  ANGULAR  (um)  (ym)  mrads)  TARGET (mm) FWHM  1.90  113.9±0.5  148.0±1.7  13.41.1  2.81  1.85  114.0H.6  150.815.2  13.41.2  2.81  1.80  113.7±0.6  148.8H.9  13.41.1  2.81  1.75  113.7±1.8  155.715.7  13.41.2  2.81  1.70  116.0±1.6  166.615.6  13.61.2  2.86  (AR 50% - ETH 50%) PREMIX  -  63 -  TABLE V  Resolution of the Four Wire Chamber (MMDC4 4 wires considered)  GAS  VELOCITY  a(mean)  ANGULAR  (um/nsec)  (um)  (mrads)  I  68.9  138.111.2  27.11.2  II  43.2  104.610.8  20.51.2  III  66.8  124.5+1.2  24.41.2  IV  67.6  130.010.9  25.51.2  Key to the gasmixtures: I II  (ARGON + METHYLAL)*  70% - IS0BUTANE 30%  ARGON 80% - ETHANE 20%  III  (ARGON + METHYLAL)*  80% - ETHANE 20%  IV  (ARGON + METHYLAL)*  50% - ETHANE 50%  The Argon was bubbled through l i q u i d Methylal wich was kept at 0° C. Estimated Methylal content ~(3-4)% of Argon content.  - 64 -  TABLE VI  THE HORIZONTAL ANGULAR RESOLUTION IN THE THREE CHAMBER SYSTEM ( 2 MMDC4 positioned 200mm apart)  GAS  ANGULAR  TARGET  (mrads)  FWHM (mm)  I  1.63  0.34  II  1.29  0.26  III  1.47  0.31  IV  1.53  0.32  Key to the gasmixtures: I II  (ARGON + METHYLAL)*  70% - ISOBUTANE 30%  ARGON 80% - ETHANE 20%  III  (ARGON + METHYLAL)  80% - ETHANE 20%  IV  (ARGON + METHYLAL)  50% - ETHANE 50%  The Argon was bubbled through l i q u i d Methylal which was kept at 0° C. Estimated Methylal content i s ~(3-4)% of Argon content  - 65 -  To convert  the TDC channels Into actual d r i f t distances and  resolutions of wires, the calibrated d r i f t v e l o c i t i e s were needed. TDC  measurements were done with the same unit (LeCroy Mod. to 740 nsec  All  2228). I t s  time  range was extended  to allow electrons to d r i f t the  full  25 mm maximum distance even In very slow gas mixtures. This TDC  unit was then calibrated and found to measure 0.38  nsec/channel.  The r e s u l t s of the beam tests, done with 50 MeV/c pions, are summarized i n the following tables: Table  II shows the resolution of the eight wire MMDC with  d i f f e r e n t gas mixtures. Only high  resistance wires  consistent below).  responses,  the six inner wires were considered. The  i n the f i r s t due mostly  Most s i g n i f i c a n t  and last  to e l e c t r i c  positions did not give field  aberrations (see  i s the improvement i n the resolution due to  the addition of Helium to the Argon-Ethane mixture. Table  III shows  the resolution  for the same chamber  with  d i f f e r e n t e l e c t r i c f i e l d s , while Table IV summarizes how the resolution changes with changes i n the gas gain, i . e . d i f f e r e n t anode voltages. The test results for the four wire MMDC are shown i n Table V. while the horizontal resolution of the three chamber system tests are listed  i n Table  VI.  The v e r t i c a l  dimension  was determined  previously tested 8-wire MMDC which was rotated by 90 degrees  by  the  for this  purpose. Therefore the v e r t i c a l resolution for the three chamber system i s also contained i n Table I I .  - 66 -  The r e s u l t s f o r the charge d i v i s i o n measurements are shown i n F i g . VII.4. The resolution i s approximately 2 mm. As seen i n the map of the equipotential lines for  the charge d i v i s i o n read-out (In c e l l #1 and c e l l #8) are not very  efficient On  for the MMDC8 (Fig. VII.5a), both wires used  for p a r t i c l e s passing more than 10 mm  the other  hand,  the applied  correction  away from the anode.  voltage was  not able to  correct the f i e l d near the anode ( F i g . VII.5b). So f o r tracks close to the  centre  of  the chamber,  e f f e c t i v e c e l l depth of 6mm,  the f i r s t  and  last  drift  cell  had an  which i s much more than the 4 mm standard  depth f o r a l l other c e l l s . It follows that the resolution i n the f i r s t and l a s t c e l l i s worse.  All in  the chambers have been running with incoming pion rates  excess of 10  6  particles/sec.  Testing of the chambers has been severely limited by frequent break-downs  of  the  newly  developed  preamplifiers  and  amplifier  -  discriminators. The sense wires were damaged during the testing, when a premixed  drift  gas was found to be heavily contaminated with  In F i g . VII.6 the magnified wire surface i s shown.  propene.  - 67  cl)  y=+20mm  c.2) y=+10mm  a) P u l s e h e i g h t spectrum  of  5 5  c.3) y=  Fe  Omm  c.4) y=-10mm  c.5) y=-20mm  b) O s c i l l o s c o p e s i g n a l s from both  c) D i f f e r e n c e o f s i g n a l s from b)  ends o f anode w i r e ( t o p s i g n a l  for different y-positions of  i n v e r t e d , source a t c e n t r e o f  the  wire)  Vertical:  5 5  Fe  source. lOOmV/div.  Horizontal. lOnsec/div.  Fig.  VII.3  Charge d i v i s i o n method w i t h  5 5  Fe  source  -  68  -  FWHM =  2.0mm  J  I  F i g . VII.4 Laser c a l i b r a t i o n of y-resolution with the charge d i v i s i o n method. Peak s h i f t s correspond to 5 mm.  - 69  -  A  B  F i g . VII.5 Calculated  f i e l d maps f o r MMDC8  a) without correction voltage b) with correction voltage of 300 V.  - 70 -  B F i g . VII.6 a) Signs of deterioration on anode wire surface due to extensive sparking caused by contaminated d r i f t gas. b) Magnified section of wire from a ) . On the r i g h t a new wire f o r comparison  - 71 -  CHAPTER VIII. - DISCUSSION AND FUTURE CONSIDERATIONS  The modular  tested  multi-wire d r i f t  chambers  (MMDC),  due to the  design, have proven to be a capable and f l e x i b l e system. "In  beam" tests, performed under conditions of a " r e a l " experiment, showed the  MMDC's  resolution though  to have  high  rate  capability  and very  good  spatial  for the coordinate perpendicular to the sense wires. Even  the charge-division  read out techniques were not pursued very  far, mainly because of f i e l d deficiencies i n the respective d r i f t  cells  and the lack of suitable amplifiers, the achieved resolution along the sense wires i s comparable  to the second-coordinate resolution of the  presently used MWPC's. Experiments using the QQD spectrometer, which require or  no v e r t i c a l  existing  resolution  at the target  MMDC8 chamber exactly  little  spot, can be run with the  as i t was set up and mounted for the  t e s t i n g i n t h i s thesis. There are big advantages over the 2 front MWPC set-up: much higher rate c a p a b i l i t y , reduced multiple scattering as two window f o i l s  are eliminated  and the spectrometer i t s e l f  can run  in  vacuum. With the existing modules, one can also build a detector for experiments which do need v e r t i c a l resolution by i n s t a l l i n g two 6-wire MMDC's, one of which i s rotated by 90 degrees, at the front end of the spectromenter. This w i l l advantages  provide a system which,  i n addition  to  the  outlined above, offers s i g n i f i c a n t l y improved resolution at  - 72 -  the  target  spot  both  i n the horizontal  and v e r t i c a l  direction.  Considerations would have to be given to the space available between the spectrometer and the target chamber as well as to the decoupling of the e l e c t r i c f i e l d s from the two MMDC's inside the gas container. A l o g i c a l extension of the present MMDC system k i t would be the  construction  planes  make  of larger  i t possible  sized module frames. to build  drift  chambers  suggestion with regard to QQD spectrometer size  suitable  maximum d r i f t remains  to be placed  into  anode wire  of any size.  to the ones  the WC3 position.  with these units w i l l  which  have  A  i s to build modules of a As long as  space i s kept at 25 mm and the depth of each d r i f t  at 4 mm, MMDC's b u i l t  performance  Multiple  now been  the cell  have a similar  tested.  The QQD  spectrometer could then be run with e.g. a four-wire MMDC at the front end  (WC1) and another  high  horizontal  horizontally  four-wire MMDC at the WC3 position.  and v e r t i c a l  a trajectory  resolutions  can be f i t t e d  could through  Extremely  be obtained, eight  data  since  points.  V e r t i c a l l y , four data points would be available over a large baseline if  again only the f i r s t  and l a s t wire of each chamber i s read out v i a  charge d i v i s i o n . The  electric  considerably improved "A"  boards  drift  by f i r s t  field  i n the new modules  can be  changing the wire spacings on the type  from 10 mm to 2 mm, and secondly, by placing  two closely  spaced wires i n front of each anode plane. These wires have to be at the same potential as the anodes (see F i g . V I I I . l ) .  -  73  -  F i g . V I I I . l Calculated f i e l d map f o r the improved proposed modular system  - 74 -  The big improvement i n the resolution due to the addition of Helium  to the Argon-Ethane gas mixture should be further  Helium  i s normally  not used  as a component  in drift  investigated. gas mixtures.  Therefore, systematic measurements of chamber resolution as a function of  Helium  contents of three component  gas mixtures  are necessary to  e s t a b l i s h and optimize the findings of t h i s work. Finally,  the most  s i g n i f i c a n t improvement  for any of those  d r i f t chambers would be the implementation of Charge Coupled Devices (CCD's)  f o r a l l wire  read  outs,  since  this  preamplification of the anode signals obsolete.  would  render  the  - 75 -  APPENDIX - MAXIMUM LIKELIHOOD METHOD  C (a,,a_....,a ) the " t h e o r e t i c a l " expression for x., where i 1 2 n i  C a l l t= i  J  the a are a set of parameters. In t h i s case £. are l i n e a r functions of m 1 the parameters a : m v  2 C  i  =  ^ im m c  a  1  =  1  " "  6  m=l Then build a "data vector" X N c, m  i = l °i  and a "measurement Matrix" M N c . c.. - I ml Then solve f o r M  - M. lm a = M ^"X (M i s now a 2x2 matrix)  - 76 -  REFERENCES  [I]  F. S a u l i , P r i n c i p l e s of Operation of Multi Wire Proportional and D r i f t Chambers, CERN Report 77-09 (1979)  [2]  G.  Charpak,  F.  Sauli,  High  Resolution Electronic  Particle  Detecors CRN-EP (1984) [3]  F. Sauli, Limiting Accuracies i n Multiwire Proportional and D r i f t Chambers, Wire Chamber Conference, Vienna (1978)  [4]  E.W.  McDaniel, E.A. Mason, The Mobility and D i f f u s i o n of Ions  in Gases, Wiley & Sons, New York (1973) [5]  E. Townsend, Electrons i n Gases, Hutchinson, London (1947)  [6]  V. Palladino, B. Sadoulet, Nucl.Inst.Meth. 128, 323 (1979)  [7]  L.G.  Christophorou, D.L.  McCorkle,  D.V.  Maxey, J.G.  Carter,  Nucl.Inst.Meth. 163, 141 (1979) [8]  A. Breskin, G. Charpak, B. Gabioud, F. Sauli, N. Trautner, W. Duniker, W. Schultz, Nucl.Inst.Meth. 119_, 23 (1974)  [9]  A.  Peisert,  F.  Sauli,  Gases: A Compilation,  Drift  and  Diffusion of Electrons  CERN Report 84-08 (1984)  [10]  F. V i l l a , Nucl.Inst.Meth. 217, 273 (1983)  [II]  A.H. Walenta, Nucl.Inst.Meth. 217, 65 (1983)  In  - 77  [12]  CW.  Fabjan, J. Lindsay, F. Piuz, F. Ranjard, E. Rosso,  A. Rudge, S. Serednyakov, F.O. [13]  -  W.J. W i l l i s , H.B.  Petersen, Nucl.Inst.Meth. 156, 267  S. Bartalucci, R.Bertani, S. Berolucci,  Jensen,  (1978) M. C o r d e l l i , R. D i n i ,  P. Giromini, H. P a l l o t t a , A. R u t i l i , A. Sermoneta, M.  Spadoni  Nucl.Inst.Meth. 192, 223 (1982) [14]  D.M.  Binnie, Nucl.Inst.Meth. 192, 231 (1982)  [15]  J.L. A l b e r i , V. Rakeka, IEEE Trans.Nucl.Sci., NS-23, Nol, 251 (1976)  [16]  F. Schneider, CERN Report 82-06 (1982)  [17]  K.W.D. Ledingham, C. Raine, K.M. M. Towrie, C. Trager, CM. (1984)  Smith, A.M.  Campbell,  Houston, Nucl.Inst.Meth. 225,  319  

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