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International and intersectoral mobility to develop advanced scintillating fibres and Cerenkov fibres for new hadron and jet calorimeters for future colliders

Periodic Reporting for period 2 - INTELUM (International and intersectoral mobility to develop advanced scintillating fibres and Cerenkov fibres for new hadron and jet calorimeters for future colliders)

Reporting period: 2017-03-01 to 2019-02-28

The main objectives of the INTELUM project - in the context of the requirements for future high energy experiments - were to:

• Demonstrate the feasibility of producing 20 to 200km of fibres with constant quality and well understood production costs
• Demonstrate sufficient radiation hardness of the fibres in terms of a degradation of their optical properties of less than 10% at 1 MGy of total ionizing dose
• Assess the feasibility of the new fibre technology concepts
WP1 Heavy Crystal Fibre Production
Initially studies began on LuAG and YAG materials and later on the novel material GAGG:Ce. Optimization based on Mg2+ or Li+ codoping of Ce-doped garnets (LuAG, YAG, GAGG) was introduced and provided a new way to obtain a faster scintillation response and higher light yield whilst keeping a good radiation tolerance. Another approach to have a faster scintillation response was obtained by simultaneous Ga and Ca doping of YAG:Ce. During the project, three different mass production approaches have been identified for achieving a sufficient volume of scintillation fibres based on single crystal aluminium garnets: multiple growth of fibres using micro-pulling down technology, multiple cutting and processing of large size single crystal ingot grown from melt, and multiple growth of single crystal plates using EFG technology. Several fibres have been produced by these methods and characterized. Investigation of the possibilities for multiple pulling for mass production has been performed and crucibles developed allowing 10 fibres to be produced concurrently.

WP2 Light Fibre Production
The research demonstrated the capability to produce, in short time at low cost, several km’s of bright scintillating fibres with different doping levels, good geometrical uniformity and engineered with a radiation resistant cladding. New methods of preform assembly have been developed and tested. The work identified a procedure for coupling a Ce-doped sol gel preform with a low refraction index fluorinated cladding to obtain a satisfactory matching between core and cladding. Pr3+ doping was investigated as an alternative to cerium, showing a shorter decay time. Several metres of SiO2 fibres undoped, Ce and Pr doped were produced and tested for optical performance and radiation damage and to assess their performance in high energy particle beams. Improvement of the radiation hardness needs to be further investigated.

Also, R&D activities were performed on glass material with 9 different compositions using two technologies: solid state, involving a mixture of quartz glass powder with СeF3 or Ce2Si2O7, and sol-gel, by the co-precipitation of water solutions of Ce(NO3)3 and Ca(NO3)2 into NH4HCO3. Several approximately 20cm long fibres were produced from selected compositions and characterized in terms of optical performance and radiation damage. Further progress on optical properties and radiation tolerance are still required.

WP3 Material Studies
Work was performed to optimise the composition and the fibre crystal pulling from the melt using the µ-PD technique. Effort has been made to develop high-purity alumina raw material to be used as starting raw materials for garnet crystal growth for the various applications (e.g. scintillation, laser). For garnet fibre growth, the main target was to produce single crystal fibres with the required length and good optical properties and to surmount dopant segregation problems, the major cause of fibre performance degradation. In addition, the target was to optimize the reproducibility process for mass production. Investigation of the impact of seed orientation and fibres shape on optical and scintillation properties was performed. Several fibres with the various diameters (1 or 2mm) or 1x1x or 2x2 mm2 cross-sections were grown. Optimal condition were found for growing LuAG:Ce and YAG:Ce. Our results showed that growing with a low pulling rate is essential for good optical quality fibres with good light propagation. Based on the results on bulk material, the technology of pulling long codoped (Ce, Mg) garnet YAG fibres by µ-PD technique were investigated in order to get faster decay time. The ratio between the codopants is crucial for improving scintillation properties. Mg2+ codoping in long YAG:Ce fibres reduces the decay time but decreases the attenuation length. Work is ongoing to improve the attenuation length with codoping.

WP4 Optical Characterisation & Radiation Damage Studies
Activities were focussed on defining specifications for fibres in view of their use in detectors at future accelerators, developing characterization benches for the evaluation of the properties of the developed materials (bulk and fibres), and evaluating optical properties and radiation hardness. Simulations for sampling calorimeters were carried out to evaluate the most appropriate fibre dimensions and the minimum attenuation length required to achieve the required detector performance. Several characterization benches were developed and upgraded at different institutes to be able to characterize the optical properties (attenuation length, absorption, photo and radio luminescence, light yield and decay time). The performance studies (in term of optical and radiation damage properties) of YAG and LuAG:Ce doped and codoped Ce, Mg fibres - grown by different producers using different techniques, different raw materials and doping profiles – have been investigated as well as the evaluation of SiO2:Ce/Pr and DSB fibres. Promising results in terms of timing properties have been obtained with garnet material codoped (Ce, (Mg2+, Ca2+)) and work is ongoing to optimise the production of such fibres. To assess the performances of both light and heavy fibres in high energy particle beams, different prototype calorimeter detectors have been built and tested. For heavy fibres, garnet crystal fibres (LuAG, YAG and GAGG) doped with Ce, Mg are considered the most suitable material. For light fibres Silica doped with Ce has demonstrated to be the most suitable material.
The project achieved substantial progress in understanding of the production of heavy and light crystal fibres. Several groups are now able to grow garnet crystal fibres with micropulling down technology and two other technologies for mass producing fibres have been identified. Innovation has been generated in the development of crucibles with multiple dyes, bringing within reach multiple pulling for mass production. Development of high-purity alumina materials for use as starting raw materials for garnet (YAG, LuAG) crystal growth for the most demanding applications has been done.

The results obtained on crystal fibres have triggered an increasing interest in using crystal fibre geometries for a future calorimeter detector in one of the High Luminosity-LHC experiments at CERN and R&D will continue.

It was demonstrated that several kms of light fibres (Si02 Ce or Pr doped fibres) can be produced with acceptable attenuation length in a short time and cost-effective way. The developed technology of silica-based scintillating fibres is being applied in a device for controlling the position of a proton beam in a radiotracer production system.

The partners produced 39 publications in peer review journals, made 50+ presentations at international conferences, and submitted numerous new R&D proposals (e.g. H2020 Twinning and ATTRACT).