ABSTRACT of the Doctoral Thesis

Investigation of the Structural Behaviour of the CERN CMS Electromagnetic Calorimeter Barrel Mechanical Support: Analysis and Systematic Studies

by Homayoon Rezvani-Naraghi (2001)

The Compact Muon Solenoid Detector (CMS) is a high performance, general-purpose particle detector to be commissioned in 2005 at CERN, the European Organization for Nuclear Research, located in Geneva, Switzerland. In spite of its compactness, CMS, consisting of several sub-detectors and a super conducting magnet arranged as concentric cylinders, will be one of the world's largest general-purpose particle detectors with an overall length of 21.6m, a diameter of 15 m and a total mass of about 12500 tons. Within CMS, the energy of electrons, positrons and photons is precisely measured by an Electromagnetic Calorimeter (ECAL), comprising a Barrel part (EB) and two Endcaps (EE). The active mass of ECAL consists of dense PbWO4 scintillating crystals (density = 8.28 g/cm³), which were chosen for their optimal performance as detector material and further improved in intensive R&D programmes.

While loaded by the total EB weight, the EB support structure must ensure precise spatial positioning of the barrel-shaped arrangement (inner diameter = 2580 mm, maximum outer diameter ˜ 3020 mm, length ˜ 5820 mm) of an array of 61200 crystals (total mass ˜ 67.4 tons). Because of their fragility, the crystals themselves cannot be used as structural components. On the contrary, the support structure must guarantee that no load is transmitted to the crystals. The requirements for hermeticity, compactness, homogeneity, minimised dead masses and minimised gap sizes lead to application of thin-walled, lightweight structural components of integral type. Compared to conventional applications, further criteria for the selection of the materials for these components are set e.g. by the high radiation environment, magnetism, low atomic masses, transparency to certain particles and strict safety regulations for underground sites.

The present thesis includes systematic studies of the structural behaviour of the EB mechanical support in terms of deformation, stress and stability, using combined numerical, analytical and experimental methods. The doctoral work was carried out at CERN, completely integrated in the EB engineering process. Besides developed models for the EB support structure at different length scales, performed analyses, interpretation of results and recommendations for constructive measures, new analysis methods and algorithms of much broader applicability were developed.

In the structural analyses advantage was taken of the modularity of the EB design, which allowed to a large extent separate analyses at different hierarchical levels and for individual modular units with well defined interfaces. Thereby, not whole structures, but only sub-units of them needed to be modelled, which often had to be analysed under many different loading conditions. The novel concepts of the load spaces and load envelopes as well as the methods developed to combine non-summable structural responses to interacting load cases, to identify the worst load case combination and to determine the corresponding extreme response values turned out to be very practical tools, considerable reducing the computational effort for the majority of the performed analyses. These concepts could be used even more efficiently also in the analysis of the response of the structure to earthquake loading, as a pseudo-static method for seismic analyses proved its suitability for the concerned structures. More case-specific methods were applied e.g. in analyses of perforated plates, using their analogy to composite materials, in the analytical beam models for the submodules or in assessing the required strength for joint elements.

This thesis describes the scientific tasks, solution strategies and results achieved in the course of the research work. It is not an analysis or design report as the presented data are neither sufficiently complete nor adequately arranged for such purposes. With this respect it is referred to the corresponding CERN documents.

revised 011008 (hjb)