Post-irradiation examination of refractory metal candidates for antiproton targets at the European Laboratory for Particle Physics (CERN)

The AD-target (Antiproton Decelerator Target) is the main antiparticle production element of the CERN’s (European Organization for Nuclear Research) AD (Antiproton Decelerator) complex. Antiprotons are produced by colliding a pulsed 1.5 * 10^13 ppp (protons per pulse) proton beam of 26 GeV/c momentum from CERN PS (Proton Synchrotron) with a fixed target made of a dense and high-Z material.

However, one major surprising issue is observed. A significant drop in the antiproton production takes place after few days of operation of a new target. For the time being, the real causes of this drop have not been clearly identified. In the framework of the ELENA (Extra Low Energy Antiproton Ring) project, a new upgrade of the CERN AD facility was initiated in 2016. In the context of optimization of the full antiproton production system, an important part of the upgrade involves the redesign of the antiproton target itself. In order to address specific requirements for the new designs and operational procedures, a deep understanding of the target system under irradiation of its evolution with time has to be carried out. The aim is to enhance its performance, reliability and ultimately improve the antiproton production yield.

Two main phenomena have been retained as major damaging concerns for target operation: (i) initially, shock waves as a consequence of the sudden increase of temperature in the target material after each pulse and (ii) in the long term, radiation damage (like voids and bubble creation, gas formation, swelling, embrittlement, etc.). The approach adopted in this work consists in investigating the changes of the microstructure due to the thermally induced stress waves and long-term radiation damage by linking them to the consequent changes of mechanical properties of such materials as well as to the antiproton yield reduction. The main studies focus on iridium, tantalum and tungsten-based materials, with an opening to other materials for comparison.

PIEs (Post-Irradiation Examinations) are performed on targets extracted from the so-called HiRadMat-27 and HiRadMat-42 experiments, together with the opening of one spent AD-target. The objective aimed at understanding the phenomena of material damage occurring in such devices. PIEs consisted in the use of classical metallurgical techniques, such as LOM (Light Optical Microscopy), SEM (Scanning Electron Microscopy) and EBSD (Electron Backscatter Diffraction), XRD (X-Ray Diffraction), in order to highlight possible changes induced in the microstructure. The analyses were completed with micro- and nano-indentation tests, which could allow to show the effects of the proton impact induced-changes (thermally induced stress waves and irradiation).

Post-mortem analyses of the targets of the HiRadMat-27 and HiRadMat-42 experiments show that this damage occurs very rapidly after only a few pulses. In the HiRadMat-27 experiment, all materials (except tantalum) exhibited cracks. The HiRadMat-42 experiment has shed light on the process of spalling damage that occurs for ductile materials such as tantalum in dynamic regime. The opening of the AD-target has shown that the state of iridium is fragmented on several levels. The conclusion of this thesis is that, in the current regime of the AD-target, the effects of thermally-stress waves are the dominant factor in the production of damage in the AD-target.