25-299 Design improvement and characterization of a Floating Gate Dosimeter

25-299 Design improvement and characterization of a Floating Gate Dosimeter

Mission

Both in space and particle accelerators, if for different radiation environments, TID (Total Ionising Dose) effects are responsible for electronic components degradation. For this reason, it is needed, in both environments, to monitor dose levels, thus to perform dosimetry in a mixed radiation field. At particles accelerators at CERN, (the European Organization for Nuclear Research), in particular, multiple dosimetry systems are already in place. The general aim is both to ascertain the life expectancy of electronics that is already deployed in facilities, and to assess TID levels in future deployment spots. Similar needs are witnessed within the space industry. Radiation Hardness Assurance has been part of the Product Assurance procedures since the Eighties, and it is bound to evolve just as interestingly as it has until now, with the general shift of the industry towards COTS components. However, evolution comes at a price, since it requires an increasing monitoring of the space radiation environment, both for modelling and real time purposes. This makes for an increasing need for integration of dosimeters in space systems, thus for an equally increasing demand to shrink dosimetry functionalities into a single component. It is in this niche that the needs of particle accelerators meet the needs of New Space, and it is there that the subject of this thesis proposal is meant to advance research.

In mixed-field environments, the dosimetry technologies used are quite varied, depending on the aim of the measurement. For instance, at CERN, fibre based dosimetry is widely used, but also solid state. The latter version is notably implemented in the form of a Floating Gate dosimeter, the FGDOS, designed and produced by Sealicon, a company based in Màlaga. This device is based on a gate stack, with features substantially bigger than those of a floating gate based memory, but in principle the same functional parts. The stack is composed of two silicon dioxide layers, with a floating polysilicon gate in between. The floating gate is positively pre-charged by tunnel effect and it thus generates an electric field in the two SiO2 volumes on its top and bottom. Those two are the active volumes. There, radiation interacts with the oxide, generating electron-hole pairs, whose carriers are then drifted by the electric field in the gate. The collected negative charge discharges the polysilicon gate, which is read-out by a front end readout transistor. An appropriate characterization makes for an accurate measurement of the charge generated in the silicon dioxide volume, thus of the energy deposited by radiation in the detector, which is TID. Now, this sensor is already characterized and integrated into an IC form, meaning it could be easily integrated in satellites – it was on the ISS. However, it surely has its drawbacks and margins for improvement, which could be the object of further research.

The main issues and improvement margins are in charge collection efficiency. Charge collection efficiency, translatable in “the percentage of collected charge with respect to the generated one”, is necessarily excellent in optical detectors, but is comparatively quite bad in SiO2. This means a consistent amount of the charge generated in the gate stack recombines before detection. The aim of this research proposal is to investigate for improvements both in the material, geometry of the gate stack, and in the general architecture of the FGDOS. New oxides can now be included in the fabrication of electronic components, and some of them might have better characteristics for dosimetry rather than silicon dioxide. Moreover, the geometry of the device still has margin for improvement.

The work associated with this thesis will be divided into three main blocks. The first one will include literature research on the topic, and development of a Geant4 simulation model of the detector. Simulations will be performed of particle-matter interactions for the device, and different geometries and materials will be numerically tested.

The second lot of activities will include the preparation of test setups for the prototypes – which for time and industrial reasons might differ from the optimal solutions found in simulations. The prototypes will be tested both at the Co-60 facility at CERN, and in heavy ions and proton test campaigns in other facilities. In this phase, a possible opportunity for inclusion in payloads might be discussed with CNES.

Finally, the third lot of activities will include data analysis of data collected during the previous test campaigns, further Geant4 simulations, and interpretation of the results. Ideally, the work will be concluded by a summary of optimized design solutions and use recommendations.

The research associated with this thesis will be performed partially at CERN and partially in CNES laboratories, while some of the test campaigns associated with data collection will be performed in particle accelerators across Europe.

Application Process

For more information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse –

Then, prepare a resume, a recent transcript, and a reference letter from your M2 supervisor/engineering school director and you will be ready to apply online before March 14th, 2025 Midnight Paris time!

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