Optical calibration from ten to hundreds of meters for the Neutrino Burst Experiment (a poor man’s Km 3 underwater neutrino telescope) NESTOR/NOA Spyridon Koutsoukos

S. KoutsoukosVLVnT '08 conference, Toulon2 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. KoutsoukosVLVnT '08 conference, Toulon3 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. KoutsoukosVLVnT '08 conference, Toulon4 How much light do we need? For a point source: 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 ) An 1,2nJ light pulse at 460nm produces ~2,78 * 10 9 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. KoutsoukosVLVnT '08 conference, Toulon5 Potential light sources, advantages and disadvantages LEDs Solid state lasers Semiconductor lasers Gas and dye lasers

S. KoutsoukosVLVnT '08 conference, Toulon6 Energy per pulse Pulse duration Operational lifetime PriceOther LED 50 – 100 pJ ~ 10 ns up to hours ~ €0,5 Small size and weight, good shock resistance, low power/heat Laser Diode <500 pJ<1ns > hours ~ €1000 Small size and weight, good shock resistance, low power/heat SS laser>1mJFew fs > hours ~ € €50000 SHG needed, can be very big, shock sensitive, narrow linewidth Gas laser 100s μJ<1ns 1.000s of hours >€10000 Can be very big, shock sensitive, high power cooling needed Dye laser 100s μJ<1nsFew hours >€10000

S. KoutsoukosVLVnT '08 conference, Toulon7 Engineering aspects Power requirements Heat dissipation Shock resistance Operational lifetime Geometrical aspects

S. KoutsoukosVLVnT '08 conference, Toulon8 NuBE (Neutrino Burst Experiment) Beacons

S. KoutsoukosVLVnT '08 conference, Toulon9 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. Indicative numbers Photons per OM (0,2nJ) Photons per OM (2nJ) Floor 1 (top) Floor Floor Floor 4 (bottom)97970 Strings0,99

S. KoutsoukosVLVnT '08 conference, Toulon10 Electronics aspects Pulse width for LED sources Timing and synchronisation of multiple sources within the beacon

S. KoutsoukosVLVnT '08 conference, Toulon11 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.