Abstract
The NT-02 neutrino physics target made of the isotropic graphite grade produced neutrinos for the MINOS and MINERVA high-energy physics experiments. The segmented, 95-cm-long NT-02 target was bombarded with a 340 kW, Gaussian 1.1 mm sigma beam of 120 GeV protons reaching $6.516\ifmmode\times\else\texttimes\fi{}{10}^{20}$ protons on target and a peak fluence of $8.6\ifmmode\times\else\texttimes\fi{}{10}^{21}\text{ }\text{ }\mathrm{protons}/{\mathrm{cm}}^{2}$. Reductions in detected neutrino events during the experiment were attributed to radiation-induced damage on the target material leading to the NT-02 target replacement. With future neutrino physics targets aiming at the multimegawatt power regime, identifying life expectancy or fluence thresholds of target materials is of paramount importance, and, therefore, pinpointing the exact cause and target failure mode triggering the neutrino yield reduction is critical. To help unravel the effects of the 120 GeV beam on the isotropic graphite structure at the microstructural or lattice level, x-ray beams from National Synchrotron Light Source II were utilized to study failed in-beam as well as intact NT-02 target segments. The primary objective was to arrive at a scientifically sound explanation of the processes responsible for the target failure by correlating macroscopic observations with microstructural analyses. Results from transmission electron microscopy studies were integrated in assessing the microstructural evolution. The x-ray diffraction study revealed (a) the diffused state reached by the graphite microstructure within the $1\ensuremath{\sigma}$ of the beam where the graphite lattice structure transforms into a nanocrystalline structure, a finding supported by electron microscopy examination, thus providing an indication of the fluence threshold, and (b) the dominant role of the irradiation temperature profile exhibiting a high gradient from the beam center to the heat sink and aggravating the damage induced in the microstructure by the high proton fluence. The effects of the 120 GeV protons on the isotropic graphite target structure are corroborated by observed damage induced by 160-MeV protons and by fast neutrons to comparative doses on similar graphite, an assessment that will aid the design of next-generation megawatt-class neutrino targets.
Highlights
The NT-02 neutrino physics target in the NuMI beam line at Fermilab (Fig. 1) produced neutrinos for the MINOS and MINERVA high-energy physics experiments
With future neutrino physics targets aiming at the multimegawatt power regime, identifying life expectancy or fluence thresholds of target materials is of paramount importance, and, pinpointing the exact cause and target failure mode triggering the neutrino yield reduction is critical
The x-ray diffraction study revealed (a) the diffused state reached by the graphite microstructure within the 1σ of the beam where the graphite lattice structure transforms into a nanocrystalline structure, a finding supported by electron microscopy examination, providing an indication of the fluence threshold, and (b) the dominant role of the irradiation temperature profile exhibiting a high gradient from the beam center to the heat sink and aggravating the damage induced in the microstructure by the high proton fluence
Summary
The NT-02 neutrino physics target in the NuMI beam line at Fermilab (Fig. 1) produced neutrinos for the MINOS and MINERVA high-energy physics experiments. Several studies [20,21,22,23,24] focusing on energetic proton irradiation effects on the physiomechanical properties and the changes in the microstructure have been conducted in recent years This is prompted by the fact that graphite, in its various grades including POCO ZXF-5Q of the NuMI target, has been used in several neutrino experiment initiatives (NuMI/MINOS, T2K, and CNGS) as a low-Z material producing a desired pion spectrum. The direct use of graphite damage observed from a few MeV neutrons to similar fluence (∼1022 n=cm2) to assess damage and/or microstructural evolution in the NuMI graphite target exposed to 120 GeV protons may lead to wrong assessments as to the cause of target failure To bridge this gap, postirradiation, postmortem microstructure-focused experiments such as x-ray diffraction and electron microscopy were necessary. V, assessment and conclusive remarks regarding a high-power target design based on the NuMI target performance in beam, the design parameters that are considered to be dominant, and the limitations of the material choice based on the irradiation damage studies to date are presented
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