Name of the exercise

Detector and Trigger: Scintillators, trigger logic, input to readout modules (ADC & TDC)

Responsible for the exercise

Jorgen Petersen

Description of the exercise

The aim of the exercise is to get an understanding of the detector and trigger logic used in exercise # 4. The signals from two scintillation counters are analysed and used to build a trigger based on a coincidence between the signals. In addition the inputs to the readout modules (QDC and TDC) are setup.

What will the students learn

  • cabling up a scintillator (HV and signal)
  • using a scope to measure the properties of a signal
  • using typical NIM modules (discriminator, scaler, coincidence unit)
  • setting up the coincidence logic (trigger).
  • prepare analogue and NIM signals to readout modules (QDC and TDC).


2 hours

List of material

  • two complete scintillation counters (scintillator, light guide, PM, base, shielding)
  • NIM Crate + power supply
  • CAEN N470 four channel high voltage power supply
  • CAEN N145 Quad Scaler
  • LeCroy Quad Discriminator 621 L
  • LeCroy Coincidence Unit 465
  • small screwdriver (to adjust threshold & pulse width on the LeCroy units)
  • two delay units
  • oscilloscope
  • voltmeter
  • Lemo cables

Relevant information

Instruction sheet

  • general note: whenever there are two parallel outputs from a (NIM) module make sure that they are both cabled i.e. either terminated with 50 Ohm or connected to another unit. This ensures that the pulses have the correct NIM voltage levels: 0 and -0.8 Volts

  • install the scintillation counters close to each other with maximum overlap between the scintillator areas.
  • check that the scintillator PM bases are connected to the N470 NIM high voltage supply
  • switch ON the NIM crate
  • connect an output from scintilator # 0 to an oscilloscope( 10ns LEMO), terminate the other output with 50 Ohm.
  • set the nomimal high voltage on scintillator # 0 using channel 0 of the N470 HV supply. The voltage is marked on the label glued onto the base. N470_ShortGuide.pdf
  • look at the signal on the scope( volts/div ~ 50 mV, time/div ~ 20ns). What is the maximum voltage of the signal?
  • connect the cable to the input of the first channel of the discriminator.
  • connect an output to the oscilloscope(0.5 Volts, 50 ns) and adjust the pulse width to around 100 ns using a small screwdriver.(terminate the other output with 50 Ohm)
  • connect the output to the first channel of the NIM scaler(N415) using a short LEMO cable(1ns).
  • set the discriminator threshold to around 50 mV: adjust the voltage on the test point using a DC voltmeter and a small screwdriver, see NIMCrate.pdf. The voltage is 10* the threshold value i.e. the voltage should be around 0.5 Volts. You may need three hands ...
  • what is the scaler rate?
  • vary the threshold around 50 mV and check the variations in scaler rate. Set the threshold such that the rate is ~ 200 Hz.
  • repeat all the (nine)points above for scintillator #1 (replace first by second and 0 by 1 ...)
  • Given the scaler rates measured above, what is the probability of random(unphysical) coincidences between pulses from the two scalers?
  • connect an output from each of the two discriminator channels to the oscilloscope and check that they have a timing overlap i.e are coincident
  • connect the cables to the first inputs of the coincidence unit (LeCroy 465) using short LEMO cables(1ns).
  • connect an output from the coincidence unit to a scaler input. What is the rate? Given that the rate of cosmic muons is about 100 per second per m**2, does the rate make sense?
  • connect an output of the coincidence unit to channel 1 of the oscilloscope
  • connect the (other) analogue output from scintillator #0 to a delay unit (LEMO 10ns) and the output of the delay unit to channel 2 of the oscilloscope.
  • using channel 1 as a trigger, observe the analogue signal on channel 2. Assuming that the signal is triangular, what is the charge of the signal, see ChargeOfPulse.pdf
  • adjust the delay unit such that the analogue signal falls within the NIM pulse from the coincidence: inputs to the QDC in exercise # 4 are now ready (analogue + gate)
  • repeat the previous three points for scintillator #1.
  • connect a cable from the first discriminator to channel 2 of the oscilloscope and check the timing wrt the output from the coincidence (channel 1). Similarly for the second discriminator. The inputs to the TDC in exercise 4 are now prepared(trigger + timing signals).
  • the signals from the discriminators are sometimes about twice as long as expected. What could the reason be?



  • I = V/R ~ 250 mV/50 Ohm = 5 mA. Q = I * t = 5 mA*10 ns = 50 pC.
  • the analogue signals from the PM have a shape where they could possibly trigger the discriminator twice

-- JorgenPetersen - 2009-08-24

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf ChargeOfPulse.pdf r2 r1 manage 54.1 K 2009-10-26 - 14:14 JorgenPetersen  
PDFpdf N470_ShortGuide.pdf r3 r2 r1 manage 37.6 K 2009-10-22 - 11:07 JorgenPetersen  
PDFpdf NIMCrate.pdf r2 r1 manage 425.8 K 2009-10-22 - 16:39 JorgenPetersen  
PDFpdf ScintillatorDaqSchool.pdf r8 r7 r6 r5 r4 manage 70.6 K 2009-11-01 - 14:56 JorgenPetersen  
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Topic revision: r19 - 2009-11-01 - JorgenPetersen
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