Download Calibration from ten to hundred of meters in an

Optical calibration from ten to
hundreds of meters for the
Neutrino Burst Experiment
(a poor man’s Km3 underwater neutrino telescope)
NESTOR/NOA
Spyridon Koutsoukos
A light beacon can be used to give a short light pulse
that will be used to calibrate in time and synchronise
optical elements in different parts of the telescope.
This light beacon should have:
•Sufficient light intensity to be seen from optical
elements tens or hundreds of meters away
•Light pulse duration in the order of few
nanoseconds.
S. Koutsoukos
VLVnT '08 conference, Toulon
2
The optical properties of water will affect our choice of
source, the number of beacons needed, and their
positioning.
•Attenuation, will affect the light intensity needed,
light wavelength and maximum distance
between beacons and optical elements.
•Scattering will affect the pulse width over long
distances. A photon can be scattered a number
of times before it is detected.
•Dispersion will also affect the pulse width over
long distances. If possible light sources with narrow
linewidth should be used.
S. Koutsoukos
VLVnT '08 conference, Toulon
3
How much light do we need?
For a point source:
AI 0  R / L
I
e
2
4R
where I is the light intensity at distance R, I o is the total
emitted light intensity, A is the target area and L the
transmission length.
For λ=460nm the transmission length L is about 55m
(NIM A 349 1994 242-246)
An 1,2nJ light pulse at 460nm produces ~2,78 * 109
photons.
If these are distributed uniformly on a spherical
surface, only ~2650 photons will be emitted in the solid
angle defined by the area of a 13’’ PMT at 300m, and
about 10 photons will reach the PMT’s surface.
S. Koutsoukos
VLVnT '08 conference, Toulon
4
Potential light sources, advantages and disadvantages
•LEDs
•Solid state lasers
•Semiconductor lasers
•Gas and dye lasers
S. Koutsoukos
VLVnT '08 conference, Toulon
5
Energy
per
pulse
LED
Laser
Diode
SS laser
S. Koutsoukos
50 – 100
pJ
<500 pJ
>1mJ
Pulse
duration
Operational
lifetime
~ 10 ns
up to
100.000
hours
<1ns
>10.000
hours
Few fs
>10.000
hours
Price
Other
~ €0,5
Small size and
weight, good shock
resistance, low
power/heat
~ €1000
Small size and
weight, good shock
resistance, low
power/heat
~ €5000 €50000
SHG needed, can be
very big, shock
sensitive, narrow
linewidth
Can be very big,
shock sensitive, high
power cooling
needed
Gas
laser
100s μJ
<1ns
1.000s of
hours
>€10000
Dye
laser
100s μJ
<1ns
Few hours
>€10000
VLVnT '08 conference, Toulon
6
Engineering aspects
• Power requirements
• Heat dissipation
• Shock resistance
• Operational lifetime
• Geometrical aspects
S. Koutsoukos
VLVnT '08 conference, Toulon
7
NuBE (Neutrino Burst Experiment)
Beacons
S. Koutsoukos
VLVnT '08 conference, Toulon
8
NuBE description
4-floor NESTOR tower (floor diameter = 32m)
Floor to floor distance = 30m
Tower to string distance = 300m
Node to node distance in string = 300m
Each node has two clusters with 8 OMs each
Beacon is situated 60m over topmost floor.
S. Koutsoukos
Indicative
numbers
Photons per
OM (0,2nJ)
Photons per
OM (2nJ)
Floor 1 (top)
2900
29000
Floor 2
800
8000
Floor 3
260
2600
Floor 4 (bottom)
97
970
Strings
0,9
9
VLVnT '08 conference, Toulon
9
Electronics aspects
•Pulse width for LED sources
•Timing and synchronisation of multiple sources
within the beacon
S. Koutsoukos
VLVnT '08 conference, Toulon
10
Conclusions
One can construct a beacon to illuminate and
synchronise optical elements up to hundreds of meters
away.
Careful selection of the position of the beacon is
important. Different light intensities for different areas
of the telescope may be necessary.
A semiconductor laser beacon can provide adequate
power, pulse duration of ~1ns, low price and good
electrical and mechanical properties.
S. Koutsoukos
VLVnT '08 conference, Toulon
11