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LHCb 2001-066 EDR


Report on the Engineering Design Review for the modules of the LHCb Hadronic Calorimeter


Present: R.Dzhelyadin, H.J.Hilke, J.Lefrançois, T.Nakada, M.Nessi (referee), A.Schopper


1.      Introduction


The engineering design of the HCAL modules has been successfully reviewed Monday, March 5, 2001 by an external referee, Marzio Nessi, CERN/EP. Following a number of questions that have been asked by the referee prior to the review meeting (see Appendix), the sub-module and module design and their assembly procedure, the structural integrity of the final system, and the radiation resistance of the optics assembly were presented and discussed in detail. All questions have been answered to the satisfaction of the referee. It has been agreed that a meeting with the Technical Inspection and Safety Commission will take place to discuss the manipulation of the modules and the necessary tooling, before any installation work can start. Details of the discussion are summarized in the following.


2.      Overview


The LHCb hadron calorimeter (HCAL) is a sampling device made out of steel and scintillating tiles, as absorber and active material respectively. The special feature of this sampling structure is the orientation of the scintillating tiles that are running parallel to the beam axis. Wavelength shifting (WLS) fibers are running along the edges of the scintillator tiles that are staggered in depth. Readout cells of different size are defined by grouping together different sets of fibers onto one photomultiplier tube that is fixed to the rear side of the sampling structure. The HCAL is segmented into two sections with square cells of size 131.3 mm and 262.6 mm. The optics is designed such that the two different cell sizes can be realized with an absorber structure that is identical over the whole HCAL. The absorber structure is self-supporting and is made from laminated steel plates of various dimensions that are glued together. The periodic structure of the system allows the construction of a large detector by assembling smaller modules. The overall HCAL structure builds up as a wall with dimensions of 6.8 m in height, 8.4 m in width and 1.65 m in depth. The structure is divided into two symmetric parts that are positioned on movable platforms and that can slide out perpendicular to the beam pipe. Each half is built from 26 modules that are piled up on top of each other in the final installation phase. To facilitate the construction of modules, each module is sub divided into eight sub modules that have a manageable size for being assembled from the individual absorber plates. A total of 416 sub modules have to be produced to form 52 modules that will built up the two halves of the HCAL structure.


A hadron calorimeter of same type is under construction for the ATLAS experiment. Even though the overall LHCb HCAL structure is very different from the ATLAS one, the design of the sub modules of the two systems is very similar. LHCb has greatly profited from the experience of ATLAS for the module design and the prototype construction. The review was focusing on potential problems that could arise from differences in the design of the two systems. Dedicated engineering design reviews on the overall integration of the calorimeter system and on its support structures are foreseen at a later stage and are not part of this review. However, questions related to these items were also addressed as far as they could affect the design of the modules.


3.      Summary


3.1 Sub module assembly on the beam


Following the description in the calorimeter TDR, some more details on the sub module assembly on the beam were requested. The detailed assembly technique and precision requirements were reviewed.


The absorber structure of a sub module is a laminate of steel plates with a repetition of identical periods of 20 mm thickness. One period consists of two 6 mm thick master plates with a length of 1283 mm and a width of 260 mm that are glued in two layers to two sets of eight 4 mm thick spacers of 256.6 mm width and variable length. A sub module has a total thickness of 525.2 mm with a tolerance on the envelope of ±2 mm, which is achieved by requiring a tolerance of ±0.05 mm on the thickness of the master and spacer plates. This tolerance, together with the required flatness of the plates of 0.2 mm, guaranties sufficient space in between the plates for hosting the (3 +0.1 -0.0) mm thick scintillating tiles.


After gluing the master and spacers together, a sub module is completed by welding two bars to both, the front and the back of the structure. From previous experience it is known that this welding can cause the bars to shrink and the sub module to deform. Welding from the center of a bar to the outside in 2 cm steps can minimize this effect. From experience of module-zero production the shrinking is well below 1.5 mm. This is within tolerances and does not cause any problem for the subsequent optics assembly.


The alignment of sub modules within a module was discussed in some detail. A module is assembled from eight sub modules by welding bars to the front and the back. Then a back holder is screwed to the back of a ~4.2 m long module. Whilst the welded bars fix the relative position of the sub modules, the back holder and a transversal strip at the front of each module allow positioning the modules on top of each other. The question was raised on whether it is necessary to weld a bar to the back of a module or whether the back holder could directly hold together the sub modules. It was argued, that the assembly procedure is much simplified when keeping the back holder production independent from the module assembly.


Results on detailed ANSYS calculations were presented, showing that adequate safety factors were respected in the design of the back holder and in the cross sections of the bars that are welded to the modules and sub modules. It was agreed that all calculation results would be summarized in a dedicated note by June 2001, which will facilitate discussions in a forthcoming safety meeting. 


3.2 Gap size between modules and space for fibers


The modules are stacked on top of each other to form an 8.4 m high wall. After production, every module is numbered and his top and bottom neighbour is defined. The modules are positioned relative to each other residing on keys that are positioned at the back of the modules. The gap size in between modules is defined to be 2 mm (distance from master to master plate) and is adjusted by machining each key accordingly. The spacer plates are retracted by 2.25 mm as compared to the edges of the master plates, and the scintillating tiles by 2.0 mm respectively This allows hosting the 1.2 mm thick fibers that are collecting the light along the edges of the scintillating tiles, without being damaged while running along the sides of the spacer plates. 


The effect of the gap of 2 mm between modules on the uniformity in response was discussed. Measurements in test beam have shown that no degradation in the energy measurement is seen when scanning over several modules with e.g. 50 GeV pions at various typical incident angles.


3.3 Structural integrity of final assembly


The details of the support structure and the overall integration of the HCAL modules will be subject of dedicated engineering design reviews at a later stage. However, the structural integrity of the final assembly was discussed in some detail, and it was shown that the design of the individual modules is coherent with the design of the overall mechanical structure. In particular the positioning of modules on top of each other, and the insertion of steel tubes that penetrate the modules at the place of the scintillating tiles for calibration purposes, were explained.


The referee questioned the required tolerance of 40 mm on the flatness of the ~4.2 m long back holder and suggested that 100 mm should be sufficient. It was agreed that the final tolerance would be determined from experience of the ongoing module 0 production. It was also agreed that the forces on the keys situated in between modules are to be determined, taking into account the current regulations for possible seismic events. There is still a margin of design that could allow us to make the key system appreciably larger than foreseen at present. Calculations will be available before the engineering design on support structures, to finalize their size.


Furthermore, the manipulation during installation of the ~10 tons heavy modules was discussed. A special tool has been designed for lifting and turning the modules after assembly, and for manipulating them during installation. Details on how the module is attached to the tool were discussed and it was agreed that, prior to any installation work, the existing ANSYS calculations on the mechanical stability of the tooling are going to be discussed with the safety group at CERN.


3.4 Radiation resistance of the optics system


The radiation resistance of the optics system was reviewed. In the technical design report the behaviour under irradiation of all optical components used in the HCAL are described in detail. However since the TDR, polystyrene of type PSM-115 (that was foreseen for the scintillating tile production) is no longer available on the market with the required optical specifications. A candidate for replacement is the BASF-165H polystyrene. Its resistance to radiation has been tested at the Serpukhov 70 GeV proton synchrotron in the range of 0 to 1400 krad, and compared to PSM-115. It is shown that the loss in light yield and in attenuation length versus radiation dose is very similar for both polystyrene types, and that the HCAL performance under irradiation is not expected to be affected by the choice of different scintillating material. 


Referring to the TDR, the radiation hardness of the different fiber types under consideration was also summarized. So far the fibers were tested for total radiation doses of the order of 0.5 Mrad, which corresponds to the expected dose for the cells closest to the beam after an LHCb operation of about ten years. One fiber candidate shows clearly the highest resistance to radiation, but cannot compete with the decay time of another candidate. In order to determine the distance from the beam at which the fast fiber would be sufficiently radiation resistant, a further test at lower radiation doses of the order of 50 to 90 krad is foreseen in the near future.


3.5 Schedule of operations


The expected production procedure and schedule was discussed, presenting the design and status of all tooling and production equipment to the referee. A lot of experience in sub-module production was gained from building a total of six HCAL prototypes. From this experience and since the start of module-0 production at the IHEP institute in Protvino, we are confident to construct one module consisting of 8 sub modules within 2 weeks. A total of 108 weeks is needed to assemble the required 432 sub modules and 54 modules (including two spares), which correspond to about 2.5 years with one production line. If necessary an already existing second production line at IHEP, which is currently still running for the ATLAS HCAL construction, could be used for LHCb at a later stage.



Appendix:    Questions by the referee


1) Give more details about the sub-module assembly on the beam (precision requirements, assembly technique, etc.)


2) Give more details on the gaps size between modules and the space for the fibers.


3) Describe the structural integrity of the final assembly.


4) What is the radiation resistance of the optics system?


5) Describe the schedule of operations.