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FLC Group Test-beam Studies of the
Laser-Wire Detector 13 September
2006 Maximilian Micheler Supervisor Freddy
Poirier
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Introduction
The goal of the Test-Beam Studies is to measure
the performance of the lead-tungstate detector
used at the Laser-Wire experiment at PETRA
II ? Calibration, for the use of the detector
at the Laser-Wire experiment ? Resolution,
mainly for simulation purposes
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Experimental setup

• Detector is placed in line with the electron beam
  as to achieve an EM shower in the centre of the
  lead-tungstate crystal. (achieved by mounting the
  detector on an adjustable table)
• Hamamatsu R6236 photomultiplier tube (54mmx54mm
  active photo cathode surface) for detection of
  the energy dissipation in the crystal

• Two scintillators (one horizontal and one
  vertical scintillator) were placed in front of
  the detector and connected to a coincidence unit
  to only record signals for electrons which
  entered the crystal in the centre
• The electron beam energies were selected by
  changing the currents in the magnets using a
  computer with a provided software in the
  Test-beam 24 control room.

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Electronics

• Scintillator signal processed using
• Amplifier
• Discriminator
• Coincidence unit
• TTL signal converter
• TTL signal used for triggering integration of PMT
  output signal.

• Using a Computer (ADC Break-out box in the
  figure) and an integrator card to integrate the
  PMT signal and to record the area.
• The setup of the test-beam studies was chosen to
  have the same conditions as in the Laser-Wire
  Hut
• Identical electronics for the read-out system,
  i.e. ADC box and integrator card
• Similar timing for read-out system

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Data Acquisition

• Signal from PMT is a negative voltage peak.
• Integrator card integrates this peak and outputs
  the area as a positive voltage which is recorded
  by the computer.
• For every single beam energy and PMT supply
  voltage around 1000 integrator card voltages have
  been measured.

• The 1000 area measurements were plotted on a
  histogram and fitted with a Gaussian function
• The two graphs are taken for a PMT supply voltage
  of 1115V at a beam energy of 3.6GeV
• Mean, Standard deviation, and Errors on the
  Standard deviation from Gaussian function

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Detector Performance

• Calibration plot the mean of the integrated
  signal against the corresponding beam energy for
  the 4 different PMT supply voltages
• This calibration plot was expected to show a
  linear dependence of the integrated signal on the
  beam energy
• Linear fittings of the individual data sets show
  a non-linear behaviour

Calibration plot

• Resolution plot the PMT normalised resolution
  (Gaussian width/integrated signal) against the
  beam energy
• Resolution is expected to decrease with
  increasing beam energy.
• At a PMT supply voltage of 1300V this is not the
  case

Resolution plot
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Control Tests

• Possible reasons why the results differ from
  expectation
• Saturation of PMT
• Cutting the signal
• ? Incorrect delay
• ?Incorrect integration width
• Saturation of the read-out system

• Investigation of the raw PMT output signal
  recorded from the oscilloscope (as shown). The
  following quantities were directly calculated
  from the raw signal
• Signal amplitude (difference between constant
  base line and minimum voltage)
• Signal area (sum of the relative signal w.r.t.
  the base line within the integration range)

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Control Tests PMT saturation effects

• Plot of signal amplitude against signal area
  shows a linear dependence
• Fit y 0.0274x 0.0474
• where y represents the signal amplitude and x the
  signal area.
• Plot of signal amplitude against beam energy
  shows a linear dependence
• Fit y 0.404x 0.076
• where y represents the signal amplitude and x the
  beam energy.
• Therefore the signal area also shows a linear
  dependence on the beam energy
• ? No detector saturation while increasing the
  beam energy

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Control Tests Cut-off effects

• 1µs integration delay w.r.t. the TTL trigger,
  2.4µs integration width
• The two graphs show a integration with an
  asymmetric cut-off of the signal from the tail
  and the front of the PMT raw signal,
  respectively.
• Signal area decreases due to cut-off. However,
  the plots of the signal area against the beam
  energy for the different cut-off points are still
  linear.
• ? Cut-off effects do not seem to be responsible
  for the non-linear characteristics of the
  integrated signal
• ?No cut-off effects due to integration width as
  width is sufficiently long enough to detect
  entire signal (width approx. 110ns)

tail
front
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Control Tests Read-out system saturation
At the present time, the read-out system is under
study to check how it performs with voltages
higher than it is designed for.
Conclusions

• Resolution plot and Calibration plot achieved for
  the Laser-Wire detector
• Results used for calibration of the signal at the
  Laser-Wire experiment
• After the calibration the data from the
  Laser-Wire experiment will be compared with
  previous simulations