Physicists test titanium target windows for particle beam
When Fermilab scientists set out to produce neutrinos for DUNE, they have to be incredibly precise. The PIP-II accelerator will use superconducting structures and powerful magnets to accelerate rapid microsecond bursts of protons that are focused and steered in the right direction, aimed at the DUNE detectors in South Dakota, before they smash into the neutrino-producing target on the Fermilab site.The target—which consists of graphite rods roughly 1.5 meters in total length—is separated from the rest of the accelerator in a vessel filled with helium to help keep temperatures down.
The protons, traveling at their maximum energy, enter the vessel through a window, then hit the bull's eye to produce a cascade of rapidly decaying pions—short-lived subatomic particles—that exit through a second window in the back. In less than a second, the pions will not only have decayed into neutrinos, but those neutrinos—which have almost no mass and travel close to the speed of light—will have reached their destination in South Dakota, a journey of 800 miles.
Designing the target array is no easy task, which is especially true of the windows. They need to have the stamina to withstand the high-power proton beam and temperatures in excess of 200 degrees Celsius, all while maintaining enough structural integrity to hold up against pressure differences across the window. Not only that, but they need to be made as thin as possible to minimize the interaction with the proton beam. Because of these extreme conditions, accelerator windows are made not of glass but of metal.
While metallic windows wouldn't let much light into your home, they don't pose much of a barrier to particle beams. Atoms are mostly made up of empty space, and high-energy protons travel through the interstices within and between the window's atoms with relatively little interaction.
Many particle accelerators use target windows made of beryllium, a rare type of lightweight metal that, up until now, has shown the best results thanks to its exceptional durability. But physicists and engineers are constantly looking for ways to innovate, and those developing target windows for DUNE are investigating titanium alloys, which may have properties that allow them to hold up better than their beryllium counterparts."Titanium has a high specific strength as well as a high resistance to fatigue stress and corrosion," said Kavin Ammigan, a senior engineer at Fermilab. "We're testing to see how these critical properties change when titanium is exposed to proton beams."Titanium alloys have been used at the Japan Proton Accelerator Research Complex – known as J-PARC—for over a decade with promising results. With Fermilab's PIP-II upgrade, the laboratory accelerator complex will accelerate a much higher-intensity beam than it does currently. In order to predict how long titanium windows will last at Fermilab, researchers needed to test samples using similar beam energies.Titanium fatigue samples provided by researchers at J-PARC were sent to Fermilab, where their mechanical properties were tested. The samples were then pummeled by an intense beam of protons at Brookhaven National Laboratory over the course of eight weeks, after which they were returned to Fermilab for another round of testing to determine exactly how the properties of the alloy had changed and degraded over time. By testing both before and after being bombarded by proton beams, researchers can roughly predict how long windows made out of titanium allows can be expected to last in the upgraded accelerator.
The data generated by the project will be useful not only for Fermilab and the PIP-II upgrade, but also for other institutions and future accelerators. The J-PARC accelerator facility, for example, has plans to increase the intensity of its particle beam and will be able to use the results from the current study to predict the lifespan of the titanium target window.Ammigan, Bidhar and Fermilab colleagues have completed their first batch of titanium alloy sample measurements and plan to have a second batch completed in a few months' time, after which they plan on publishing their results.