UCL
Laser Geiger cell
(update)
Robert L. Flack
March 2008
Cell review meeting, MSSL
1
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, λ = 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).
March 2008
Cell review meeting, MSSL
2
Schematic of the test cell
March 2008
Cell review meeting, MSSL
3
t0
tf
t0
March 2008
Cell review meeting, MSSL
← 45μs →
tf
4
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.
March 2008
Cell review meeting, MSSL
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Investigation of the drift speed of electrons
to the anode after ionisation
March 2008
Cell review meeting, MSSL
6
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
Drift speed ~ 1mm/37.5nS
The tail is due to the
less dense region of
the laser beam.
Time nS
March 2008
Cell review meeting, MSSL
7
Ionisation times v Vertical distance
March 2008
Cell review meeting, MSSL
8
Combined drift times and speeds
3cm in green
5cm in red
March 2008
Cell review meeting, MSSL
9
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.
March 2008
Cell review meeting, MSSL
10
Observation inline with the anode
Tail is constant
across the cell
March 2008
Cell review meeting, MSSL
11
Relative efficiency @ 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
March 2008
Cell review meeting, MSSL
12
Relative efficiency @ 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
March 2008
Cell review meeting, MSSL
13
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.
March 2008
Cell review meeting, MSSL
14
Backup slides
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Cell review meeting, MSSL
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Test cell at Manchester
March 2008
Cell review meeting, MSSL
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Cathode ring + wires
March 2008
Cell review meeting, MSSL
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Test cell at UCL
March 2008
Cell review meeting, MSSL
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Stage for laser
UV-Laser
March 2008
Diverging
lens
Cell review meeting, MSSL
Convex
lens
CCD
camera
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Diverging
lens
Converging
lens
Pulse rate = 20 Hz
Pulse width = 3ns
Power = 5 mW
Laser
light
Focal
point
Pinhole
March 2008
Cell review meeting, MSSL
20
A typical profile
of the laser beam.
Fitted with a 2D polynomial
March 2008
Cell review meeting, MSSL
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