Nikolaos Simos (Brookhaven National Laboratory)
Low-Z targets, in particular graphite, have been used extensively in the production of intense neutrino beams for neutrino experiments. The reason is three-fold : (1) the yield of useful pions – parents of the neutrinos of interest – from low-Z materials is well matched to the requirements of most neutrino experiments; (2) peak energy deposition in low-Z targets is lowest; and (3) graphite exhibits superior thermal and mechanical properties that are confirmed by experience performance data from nuclear reactor operations. The need for multi-MW level operations and beams associated with proposed long baseline neutrino experiments (including LBNE), has prompted a more rigorous effort to identify potential limitations of low-Z materials and in particular graphite grades and carbon composites when operating at multi-MW levels. This multi-faceted feasibility study is centered on an experimental effort through which proton irradiation damage of materials at fluence levels that are representative of the conditions in neutrino super-beam initiatives is assessed. Concerns that anticipated material damage due to irradiation with high power proton beams cannot be reliably extrapolated from the extensive data on neutron irradiation helped initiate a series of irradiation damage studies using the 200 MeV proton beam from the BLIP isotope production facility at Brookhaven National Laboratory (BNL). Special attention has been paid to various graphite grades, carbon composites of different structural architecture, Beryllium as well as low-Z alloys such as Albemet (alloy of Beryllium and Aluminum). Preliminary results expressed through proton fluence thresholds confirm the anticipated dissimilarities between the interactions of thermal neutrons in nuclear reactors and energetic protons interacting with the same materials. Recent experience from neutrino experiments using graphite-based targets has provided further confirmation of what has been experimentally observed. As a result, a series of analytical feasibility studies along with DPA (Displacement Per Atom) model developments have been initiated looking at the material damage dependency on proton energy and irradiation rate. In this paper relevant experimental results from the BNL irradiation damage study will be presented along with results on the feasibility studies which, while attempting to quantify the interaction of energetic protons with the array of low-Z target materials, provide the basis for making lifetime predictions and target material choices for the LBNE experiment. Also presented in this paper are target performance results of the NuMI experiment that are put in context with the experimental observations at BNL.