5.5.3.6  Construction

Cerenkov box

 Each Cerenkov is a box with only two walls and which is closed by two windows of different material. These windows are fixed by a set of two frames. There are two identical boxes for the two detectors and as the Cerenkov for the electron arm is bigger there is a suplementary peace, call the raising, which is fixed to the main box with a hermitic gasket.

  

Figure: Each parts, windows raising and main box are fixed together with hermitic joins.

The figure show how the different elements, frames, windows, boxes, are fixed together. The material of these elements are stell for all elements expect the windows which are made of tedlar. The thiknesses are the following:

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the walls of the boxes have a thikness of 2.5 mm

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the walls of the raising have a thikness of 1.5 mm

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the frames have a dimensions of and a thikness of 2 mm.

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the windows are made of two leaves of tedlar of 37.5 of thikness for a leaf (the density of the tedlar is 1.49 g/cm³).

The figure is a detailed description of the raising, and in the figure the dimenson of the boxes are given.

Mirrors

The gas Cerenkov counter is positionned between other plane detectors, one of the requirement is , therefore, to influence as less as possible the properties of the incoming particle. Multiple scattering and energy loss are thus minimized with the choice of materials with low atomic numbers and minimizing the thickness, while keeping the requires rigidity of the whole struture. Since a paper where the details of the employed technique of construction is in preparation, we will only summarize their main features.

The spherical mirrors have a radius of curvature of 90 cm. Their shape is such as that their ``shadow'' onto a planar surface, is a rectangle of (see figure ). The mirrors have been constructed with a rigid backing made of a sandwich of phenolic honeycomb between two triple layers of about of carbon fiber mat ( on each side) glued with epoxy resin. As a reflecting surface, 1mm thick aluminium plexiglas sheets have been used.
The final mirror has a total average thickness of about 230 mg/cm² corresponding to about radiation lengths.

The mirrors are placed on two parallel rows and are suitably tilted to reflect the light towards the PMTs. As they have a spherical shape with radius of curvature R=90 cm, the PMTs are placed at a distance of R/2=45 cm from the mirrors, where the parallel rays of incident light on the mirrors are approximatively focussed. To avoid ``dark zones'' between two adjacent mirrors, they are independent one with respect to the other and overlap partially. Moreover, to avoid the possibility that inclined light rays could hit the mirror along its side edge (loosing the photon which is absorbed before reaching the mirror below), the cut along the edges of the mirror is suitably tilted. In appendix A, detailed geometry of these mirrors are given.

Figure shows a typical curve of reflectivity of the mirrors. This value is around 90% in the UV region which is the most important region to cover because the number of photoelectrons produced by Cerenkov effect is proportionnal to .

  

Figure: Typical curve of reflectivity of the mirrors.