Multi-MW accelerator target material properties under proton irradiation at Brookhaven National Laboratory linear isotope producer

The effects of proton beams irradiating materials considered for targets in high-power accelerator experiments have been studied using the Brookhaven National Laboratory's (BNL) 200 MeV proton linac. A wide array of materials and alloys covering a wide range of the atomic number (Z) are being scoped by the high-power accelerator community prompting the BNL studies to focus on materials representing each distinct range, i.e. low-Z, mid-Z and high-Z. The low range includes materials such as beryllium and graphite, the midrange alloys such as Ti-6Al-4V, gum metal and super-Invar and finally the high-Z range pure tungsten and tantalum. Of interest in assessing proton irradiation effects are (a) changes in physiomechanical properties which are important in maintaining high-power target functionality, (b) identification of possible limits of proton flux or fluence above which certain materials cease to maintain integrity, (c) the role of material operating temperature in inducing or maintaining radiation damage reversal, and (d) phase stability and microstructural changes. The paper presents excerpt results deduced from macroscopic and microscopic post-irradiation evaluation (PIE) following several irradiation campaigns conducted at the BNL 200 MeV linac and specifically at the isotope producer beam-line/target station. The microscopic PIE relied on high energy x-ray diffraction at the BNL NSLS X17B1 and NSLS II XPD beam lines. The studies reveal the dramatic effects of irradiation on phase stability in several of the materials, changes in physical properties and ductility loss as well as thermally induced radiation damage reversal in graphite and alloys such as super-Invar.