Cosmic Muons

Scintillation counters

  1. Function:
  2. Counting charged particles directly
  3. Counting neutral particles through secondary interactions
  4. Time of Flight (TOF) information
  5. Energy loss per distance traveled (dE/dx) energy loss information
  6. Start the data recording, Triggers
  7. Scintillating fibers � fancy tracker
  8. Range measurements

 

Fig 1. Scintillation counter schematic (side view)

i.                    scintillator: active part which responds to charged particles (charged particle leads                  to a flash of mostly visible light, l ~ 300 -800 nm, violet-green; intensity ~104                       photons/cm/MeV

ii.                  light guide transparent polished Lucite to collect, guide the light from scintillator to                   PMT

iii.                 photomultiplier: photon � electron-multiplier � electronic pulse

  

 

Fig 2. Signal pulse

Photo:  Signal Pulse

 

 

Fig 3. Discriminator

  1. because pulse heights and widths vary, threshold will cut off signals at different points, leading to timing differences � time jitter
  2. as threshold increases, time jitter increases
  3. threshold too high: time jitter (from timing POV, low threshold is good)
  4. threshold too low: trigger on noise (to protect from spurious electronic noise, a high threshold is good)

 

Photo: Discriminator

Photo: Input and output signal.

 

  

Fig 4. Solution: 50-50 splitter and RTC Discriminator

Photo: Electronic Devices

 

 

Signal out of disc has amplitude (National Instruments and Methods � NIM -- standard amplitude is �0.75 v ) and width (can control); good width ~ 5-50 nsec

 

NIM standard �0.75 v; TTL standard +5 v

 

Fig 5. NIM pulse

Photo: NIM Pulse

Fig 6. Scintillation counter

i.                    Charged particle traverses the medium

ii.                  Coulomb interaction with atomic electron � particle suffers dE/dx, loss of energy in                   the medium per unit length, of the charged track

iii.                 Energy loss is converted into photons (~104 photons/MeV deposited)

iv.                Photon has a frequency and time spectrum

v.                  Attenuation length   NL = N0 e-L/L � fast scint have short attenuation length                          L~1m, avg scint have long att length ~ 3m

vi.                Try to maximize no. photons / cm in the scint (more and more fast photons needed)

vii.               sin qcrit = 1/n; if n=1.5 then qcrit ~ 45 deg; need well-polished edges and light                        guide with relatively small angles

 

  

Fig 7. Behavior of scintillation output signals

  

Fig 8. Total Internal Reflection in a scintillator

  1. direct light is fastest
  2. smaller angle of reflection � faster transmission
  3. fraction of light at TIR is ~ 10-20%

 

 

 

 

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