Laser Geiger cell (update)

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Laser Geiger cell(update)
UCL

• Robert L. Flack

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Introduction
Investigated the drift speed of the electrons to
the anode after ionisation in a 3cm and 5cm
diameter cell. The 3cm diameter cell was
constructed with the help of the University of
Manchester. The 5cm cell was constructed at UCL
(thanks to Brian and Derek). A nitrogen laser, ?
337nm, is used to simulate the ionisation of
the gas by a charged particle (double photon
capture by impurities from the pump oil in the
gas).
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Schematic of the test cell
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t0
tf
tf
? 45µs ?
t0
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DAQ(Thanks to Gianfranco Sciacca)

• Use a Camac TDC.
• Event triggered by PMT.
• Anode pulse stops the event, if it appears within
  a pre-specified time window of 340ns.
• If outside of the window it is assumed to be a
  cosmic.

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Investigation of the drift speed of electrons to
the anode after ionisation
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Laser 4 mm under the anode
Interpretation of the distribution is that it is
due to the geometry of the laser beam.
The peak is due to the increase density of
photons at the focal region.
Peak 150nS
The tail is due to the less dense region of the
laser beam.
Drift speed 1mm/37.5nS
Time nS
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Ionisation times v Vertical distance
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Combined drift times and speeds
3cm in green 5cm in red
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Efficiency

• The strategy is as follows. The ionisation
  process, double photon capture, is the same in
  the 3 and 5cm cell.
• Hence an estimate for the relative efficiency can
  be found for this process.
• The absolute efficiency for the totally different
  process, ionisation of the He gas by a charged
  particle, in a 3cm cell is known from NEMO3.
• Assuming the efficiency for both processes scale
  identically then an estimate of the efficiency
  for the 5cm cell can be calculated.
• Specifically the number of ionisations per mm.

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Observation inline with the anode
Tail is constant across the cell
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Relative efficiency _at_ 2mm

• Using data 2mm from the anode
• 3cm 958 events in 13.6mm 71/mm
• 5cm 1198 events in 13.6mm 88/mm
• The time taken to take the data is 4 times faster
  for the 3cm cell.
• Rel eff 4 x 71/88 3.2

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Relative efficiency _at_ 8mm

• Using data 8mm from the anode
• 3cm 1900 events in 17.8mm 107/mm
• 5cm 2083 events in 17.8mm 117/mm
• The time taken to take the data is 2 times faster
  for the 3cm cell.
• Rel eff 2 x 107/117 1.8

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Conclusion

• The 5cm cell appears to work satisfactorily.
• The distribution of speeds in the 3 and 5cm cells
  are consistent with NEMO3 out to 23mm radius.
• The relative efficiency is 3.2 at 2mm and 1.8 at
  8mm.

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Backup slides
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Test cell at Manchester
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Cathode ring wires
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Test cell at UCL
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Stage for laser
CCD camera
Convex lens
UV-Laser
Diverging lens
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Diverging lens
Converging lens
Pulse rate 20 Hz Pulse width 3ns Power 5 mW
Laser light
Focal point
Pinhole
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A typical profile of the laser beam.
Fitted with a 2D polynomial