Design of the third-generation neutron spallation target for the CERN's n_TOF facility

The neutron Time-Of-Flight (n_TOF) facility at the European Laboratory for Particle Physics (CERN) is a pulsed white-spectrum neutron spallation source producing neutrons for two experimental areas: EAR1, located 185 m downstream of the spallation target, and EAR2, located 20 m above the target. The facility is based on a lead target impacted by a high-intensity 20 GeV/c proton beam. It is designed to study neutron-nucleus interactions for neutron kinetic energies from a few meV to several GeV, with applications in nuclear astrophysics, nuclear technology, and medical research. The facility is undergoing a major upgrade in 2019-2020, which will include the installation of the new third-generation target. The second-generation target consists in a water-cooled lead cylinder, while the new target will be cooled by nitrogen to avoid erosion-corrosion phenomena and contamination of the cooling water with radioactive lead spallation products. The new design will be optimized also for the vertical flight path. The operation of the new spallation target will start in 2021. This paper presents an overview on the evolution of the design and on the related R&D activities (including beam irradiation tests) carried out to ensure the best performance for both experimental areas and avoid the contamination issues of the previous targets.

Design of the third-generation lead-based neutron spallation target for the neutron time-of-flight facility at CERN

Nicola Colonna, Raffaele Esposito, Roland Logé

The neutron time-of-flight (n_TOF) facility at the European Laboratory for Particle Physics (CERN) is a pulsed white-spectrum neutron spallation source producing neutrons for two experimental areas: the Experimental Area 1 (EAR1), located 185 m horizontally from the target, and the Experimental Area 2 (EAR2), located 20 m above the target. The target, based on pure lead, is impacted by a high-intensity 20-GeV/c pulsed proton beam. The facility was conceived to study neutron-nucleus interactions for neutron kinetic energies between a few meV to several GeV, with applications of interest for nuclear astrophysics, nuclear technology, and medical research. After the second-generation target reached the end of its lifetime, the facility underwent a major upgrade during CERN's Long Shutdown 2 (LS2, 2019-2021), which included the installation of the new third-generation neutron target. The first- and second-generation targets were based on water-cooled massive lead blocks and were designed focusing on EAR1, since EAR2 was built later. The new target is cooled by nitrogen gas to avoid erosion-corrosion and contamination of cooling water with radioactive lead spallation products. Moreover, the new design is optimized also for the vertical flight path and EAR2. This paper presents an overview of the target design focused on both physics and thermomechanical performance, and includes a description of the nitrogen cooling circuit and radiation protection studies.

AMER PHYSICAL SOC2021

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

Elvis Fornasiere

The AD-target (Antiproton Decelerator Target) is the main antiparticle production element of the CERNâs (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.

EPFL2020

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

Elvis Fornasiere

EPFL2020