1. Status of the T2K TPC Laser Calibration
Christian Hansen
UVic
Mike Roney
Mark Lenckowski
Fabrice Retiere
Triumf
March 6, 2006, TPC Canada Meeting, UVic & Triumf

2. Introduction
UV Laser pulses will shine a strip pattern on the cathode to simulate tracks by extracting e- at a given time
Straight laser tracks will be used for readout alignment and distortion calibration
For prototype; alumium strips on copper and carbon loaded kapton to study difference in photo efficiency contrast (3*10^(-8) for al, 1.5*10^(-9) for cu and 1*10^(-10) for carbon)
Two lenses spread the laser to shine the whole cathode

3. Test Setup Laser UV Lens Fiber UV Lenses Lens and fiber holder
Innova 90C FreD Ion Laser
UV Lens
to focus light on fiber
Fiber
5 m, UVM 600 µm fiber
Fabricated cuts
UV Lenses
to spread the light
Lens and fiber holder
fits in the TPC prototype
Power Meter
For efficiency study
Paper
For aperture and homogenous study

4. Solarization
The UVM fiber is the best for long term purposes according to Teodor Tichindelean at
This might need to be confirmed with a long term study

5. Pattern
With the UVM fiber (without cutting the edges) the pattern was homogenous and stable
With a fiber with non fabricated cuts the pattern was inhomogeneous and unstable

6. Efficiency for Fiber and Lenses
With the 5 m UVM fiber the efficiency for the fiber and lenses is about 35 %
Between the three runs the focusing of the laser into the fiber was changed, but the pattern did not change much
With a 2 m fiber with non fabricated cuts we had about 60 % efficiency, but with inhomogeneous and unstable pattern

7. Aperture
The half opening angle of the light cone is 19 to 25 degrees depending on the distance between the fiber and the lens
The wanted distance between fiber and lens is 10 mm in order to not spread the light unnecessarily much (more later)

8. Distribution
An attempt to measure the power distribution after the spreading lenses was made
Do not think the power meter is good enough
Measurement area is position dependent and results fluctuating
Might need a better setup

9. Power Requirement Calculation
For λ = 266 nm the aluminum photo efficiency is 3 * 10-8 (Given by Alexey Lebedev)
Use 100 electrons per cm for MIP (Should use Magboltz dEdx results once gas has been decided)
Minimum aluminum strip width is 2 mm
λ = 266 nm
alPhotoEfficiency = 3 * 10-8
nbrElectrons = 100 cm-1
alStripWidth = 2 mm
minPhotonDensity = nbrElectrons / (alPhotoEfficiency * alStripWidth)
minEnergyDensity = minPhotonDensity * h * c / λ = 12.5 nJ/cm2
 Minimum energy density at the cathode is 12.5 nJ/cm2
If distance between fiber and lens is 10 mm, then the disk radius is 440 mm (page 7)
Assume light from lens will spread homogenously on that disk (approximation)
radius = 440 mm
minEnergyAfterLens = minEnergyDensity * pi * radius2 = mJ
 Minimum energy directly after the two spreading lenses is mJ

10. Power Requirement Calculation (cont)
For 10 mm between fiber and lens; worst case experimental result says the fiber and lens efficiency is 27 % (see page 6)
efficiencyFor5mFiberAndLenses = 27 %
minEnergyPulse = minEnergyAfterDiff / efficiencyFor5mFiberAndLenses = 0.28 mJ
 Minimum pulse energy is 0.28 mJ
Plot shows 10 mm requires less power than other fiber lens distances
Half a mJ per pulse need 10 mW from a laser that gives pulses in 20 Hz
Have asked Tom at continuumlaser.com for specifications for the Minilite laser, wating for reply

11. Conclusions and Future Work
Fiber
For prototype use 5 m of UVM 600 µm fiber
Less efficiency than 2 m fiber with non fabricated cuts
Gain homogenous and stable pattern
10 mm between fiber and lens
Laser
A Minilite laser (model 2, 266 nm) will be installed and demonstrated at UVic March 27
2nd holder
A second holder have to be constructed
to focus laser light into fiber (buy UV lens or use Fabrice’s)
to prevent spreading of UV light
have ed Tom at continuumlaser.com about laser specs
Web
More info here: