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Site studies

 

Many different criteria have to be looked at before choosing a site for the deployment of a large-scale detector. We have started studuing them at a site in the Toulon area (off-shore from La Seyne) at a depth around 2400 m.

  1. Water quality:

    The sensitivity of the experiment will depend on the water transparency and the light scattering at large angles. The knowledge of the dependence of these two parameters as a function of the wavelength is required. These measurements are delicate as they need to be made in-situ with a 30m long measurement system with well characterized optical sources at different wavelengths. Some measurements by DUMAND and NESTOR exist. A sketch of a design of our water transparency measuring device is shown in figure gifa.

  2. Water depth:

    The water above the detector is a natural shield against atmospheric muons. The rate of down-going muons drops by a factor 10 going from 2500 to 4400 m. Figure gif shows the rate of muons induced by atmospheric neutrinos and of direct atmospheric muons for an undersea depth of 3000 m.

  3. Deep sea currents:

    Currents have to be taken into account in the mechanical design of the detector. Currents are changing during the year, so they have to be measured on a year scale over the vertical range covered by a future detector. This will be done in collaboration with sea science specialists.

  4. Optical background:

    K dissolved in salt water decays emitting electrons with an energy spectrum up to 1.3 MeV. Each electron produces 5 Cherenkov photons on average in the wavelength sensitivity window of the PMT. This gives rise to a photon flux of about 100cms for a 50 m water attenuation length. This light emission is very likely to be site-independent in the Mediterranean.

    Figure gif displays the variation of the background frequency of the 8'' Hamamatsu PMT with the threshold on the output signal. Data shown are taken during the immersion (then the background is due to PMT dark counts + the contribution of K) and before the immersion (background due to PMT dark counts only). One notices that above 120 mV (ie 2 p.e. threshold) the background ``during" and ``before" are roughly equal, and hence consists mainly in single p.e.

    On the other hand, bioluminescence (light emitted by a wide range of sea animal species) is time dependent and also site dependent. Bioluminescence in the deep sea is not well known and one should measure the time structure of the emitted light as well as spatial correlations. Many measurements have been made and more are currently under way, see figure gifb. Measurements from April 97 are displayed in Figure gif, showing the correlation between the current velocity and the bioluminescent activity above a 0.3 p.e. threshold. On top : evolution with time of the percentage of time spent above 250 kHz during each run. This frequency cut has been chosen because electronics under construction would be inefficient above it. One run lasts 30 minutes and there are 4 runs per day. Bottom : evolution with time of the current velocity.

  5. Bio-fouling:

    Deep sea bacteria have the tendency to colonize the surface of immersed objects where they gather to form a sticky bio-film and make sediments hold on to it. The speed of formation of this bio-fouling is site- dependent. It may affect the transparency of the optical module in the long term. The rate of accumulation has to be measured at the site and anti-fouling solutions have to be investigated. Sea science physicists are proposing different practical ways. Collaborations with EEC partners of the Bio-film Reduction on Optical Surfaces programme has started and measurements using the line described in figure gifc are under way.

    An LED light source is housed in one glass sphere and PIN diodes at different polar angle ponts inside another sphere measure the light. Two long-term measurements have been done, in each period the apparatus remained many months in deep-water. Figure gif shows the decrease of glass transparency due to biofouling on the different points on the sphere. Biofouling has a very small effect near the horizontal plane, and slightly decreases transparency at higher latitudes.

  6. On shore station:

    Issues such as: lab space, pier availability, ship and submarine availability, as well as general support have to be considered.
    The cost of access to these supports is site dependent and has to be taken into account in the cost evaluation of the project.



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Jean-Philippe Laugier
Fri Oct 31 11:48:44 MET 1997