3.3 The study of the calorimeter readout structure
3.3.2 The final readout structure
The search for a solution to the problem of the asymmetry among the semi cells and of the strong dependence of the response from the position of impact, led the collaboration to the realization of a quite different and innovative readout structure.
Instead of the copper-kapton electrodes, we choose to use ribbons of Cu(98%)-Be(1.8%)-Co(0.2%) alloy. Each ribbon is 18 mm wide, 1268 mm long and 40 μm thick. Horizontally the ribbons are positioned at a distance of 1 cm one from each other and a cell is then defined by the space among three consecutive electrodes. Vertically the strips are separate by 2 mm (see figure 3.8).
An important feature of the final NA48 calorimeter structure is that the cells are arranged in a way that photons originated by a decay occurring∼110 m far from the calorimeter (where the most part of the accepted kaon decays occurs) will impinge with the same incident angle relatively to the cell electrodes what-ever their production angle could be. This “projective” behaviour is obtained adopting slight different transverse dimensions (∼ 1% for both sides of the cell square section) between the front and the back of the calorimeter. Thus the resulting axes of the cells are all pointing to the same position on the beam axis.
Such an approach implies that,to a very good approximation the recon-structed angle of a photon with respect to the beam is independent of longitu-dinal shower fluctuations.
The electrodes ends are fixed to two circular plates of an epoxy-fibreglass composite material (STESALIT [40]), and stretched with a strain of about 2.5 N each. At the front face of the calorimeter, the ribbons are fixed to the plates by attaching them to suitable pieces of trapezoidal cross section which slide into grooves machined on the inner side of the plate. On the other plate, at the back of the calorimeter, there are holes through which pass the screws soldered to the end of the ribbons (see figure 3.9). These screws are used to fix the ribbons, to adjust their tension and to assure the electrical connection of the electrodes to the readout electronics.
20 mm
18 mm 2 mm 40 μ m
CuBe2
+HV +HV
Figure 3.8: Cell structure
For a good control of the tension and to restrict possible displacements of the ribbons from the optimal position, due to flaws in the system of support for the ribbons, we use ”accordion” springs, with 2 mm pitch, worked out at the two ends of the ribbons (see figure 3.9). The springs at the entrance side of the calorimeter are 14 mm long for a measured elastic constant of about 160 N/mm while the ones at the opposite side are 27 mm long for an elastic constant of around 100 N/mm.
Beyond that, the ribbons are forced to pass across slits cut into 5 STESALIT plates 5 mm thick each and 20 cm apart the one from the other. The position of the slits at each plane is alternatively moved 10 mm horizontally with respect to the position of the previous one to force the ribbons to follow a ”zig-zag”
3.3. THE STUDY OF THE CALORIMETER READOUT STRUCTURE 51
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Figure 3.9: Copper-Berillium ribbons trajectory with an angle of around 50 mrad (see figure 3.10).
This arrangement is intended first of all to mediate (over the longitudinal development of the shower) the effect of the response dependence from the horizontal position of impact of the particles.
Another, and not less important effect of the zig-zag positioning of the rib-bons is that of reducing the free length of the ribrib-bons from 125 cm to 20 cm, thus allowing more precise geometrical tolerance on the electrodes positioning and assuring their stability in presence of the electric forces.
Figure 3.11 shows the residual impact point dependence observed after the adoption of the new readout structure. It can be seen that the residual depen-dence has been reduced at the 1% level, that the anode drop is significantly smoother and hence easier to correct and that no systematic difference is any-more observed between the two half of the cell.
The most important result anyway is the uniformity (within our calibration
LKr CALORIMETER ELECTRODE STRUCTURE
CuBe ribbons Beam tube
Back plate
Front plate Outer rods
Spacer plates
Figure 3.10: Readout structure (view of a quarter of the calorimeter)
precision) of the response dependence from cell to cell. This allows the deriva-tion of a unique correcderiva-tion funcderiva-tion, for all the calorimeter cells, to be applied using the impact point information deduced by the calorimeter itself.
At this level of accuracy, the dependence on the vertical impact position, that was negligible respect to the change due to the horizontal position in the old structures, becomes now significant. This vertical dependence is mainly due to the distortions of the electric field in the intermediate zones among two cells and to the fact that the drifting electrons don’t induce current only in the cells in which they travel, but also in the vertical neighbours. This effect was not taken into account in the discussion of section 3.1 where it was assumed that the vertical dimension of the cell was larger than the shower transverse dimension.
The vertical dependence is anyway observed to be uniform from cell to cell and is hence corrected like the horizontal dependence.