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Inside a hydrogen-filled chamber at Michigan State University\u2019s Facility for Rare Isotope Beams (FRIB<\/a>), a rare form of arsenic met the conditions needed to transform into a rare form of selenium.<\/p>\n Artemis Tsantiri and colleagues documented the transformation as arsenic-73 captured a proton and became selenium-74.<\/p>\n For years, the same step had remained one of the least certain parts of the story, because scientists had no direct measurement to pin it down.<\/p>\n Now that the reaction has been measured, the mystery has narrowed, and the remaining mismatch demands a broader explanation.<\/p>\n Why selenium stands out<\/p>\n Selenium-74 belongs to the small family of p-nuclei<\/a>, proton-rich versions of elements that usual neutron-building routes skip.<\/p>\n Most heavy elements grow when nuclei absorb neutrons and later settle down by radioactive decay into stable forms.<\/p>\n The usual neutron-building script leaves selenium-74 out, making it the lightest member of a rare group scientists have argued over for decades.<\/p>\n Placed oddly on the chart of elements, selenium-74 makes any direct measurement unusually valuable, because the missing steps have been hard to test.<\/p>\n Heat inside supernovae<\/p>\n One favored explanation puts selenium-74 together inside a gamma process, a chain of light-driven nuclear breakups in superheated stars.<\/p>\n In that heat, powerful gamma rays knock neutrons and other particles off older nuclei, pushing matter toward proton-heavy forms.<\/p>\n Afterward, some of those unstable products change a proton into a neutron and survive as the unusual isotopes we observe.<\/p>\n Trouble starts because many of the nuclei along that route vanish quickly, which has kept crucial reaction rates mostly theoretical.<\/p>\n Building the beam<\/p>\n To run the experiment, the team had to make arsenic-73 itself, an unstable material rarely available for direct tests.<\/p>\n Chemists prepared the isotope<\/a>, then engineers ionized it, accelerated it, and sent it toward hydrogen gas at the detector\u2019s center.<\/p>\n FRIB could do that because its secondary accelerator can work on its own, not only as part of the main machine.<\/p>\n Working independently matters far beyond selenium, since other short-lived nuclei can now be prepared for experiments once written off.<\/p>\n Energy released in a flash<\/p>\n After proton capture, the new selenium-74 carried excess energy and quickly shed it as gamma light.<\/p>\n The gamma burst let the detector count how often the reaction happened, which gave the team a handle on the rate.<\/p>\n Astrophysicists care especially about the reverse destruction step, because blazing stellar photons can break selenium-74 apart during an explosion.<\/p>\n By measuring the forward reaction<\/a> in the lab, the researchers could pin down that harder-to-catch reverse process in stars.<\/p>\n Narrowing down the strength<\/p>\n Before this result, standard calculations let the reaction\u2019s strength wander across a very wide range of possible values.<\/p>\n One higher-energy measurement strongly anchored that spread, even though a lower-energy point still remained noisy.<\/p>\n When the team fed the constrained rate into supernova simulations, the uncertainty in selenium-74 abundance dropped by about half.<\/p>\n Cleaner numbers did not settle everything, but they removed a favorite excuse for why models had been disagreeing.<\/p>\n The mismatch remains<\/p>\n Even with cleaner data, Type II supernova models still make too much selenium-74 compared with the solar system record.<\/p>\n Instead, the mismatch probably sits partly in the stellar setup, such as temperature, density, or the starting mix of nuclei.<\/p>\n According to the researchers, nuclear physics alone could not erase that excess, a conclusion that forces attention back onto the explosion itself.<\/p>\n Removing one uncertainty so clearly made the remaining problems in supernova models harder to ignore.<\/p>\n Testing additional stellar reactions<\/p>\n More than 45 scientists from 20 institutions joined the effort, reflecting how difficult measurements with short-lived atoms still are.<\/p>\n \u201cEven though the origin of the p-nuclei has been a topic of study for over 60 years, measurements of important reactions on short-lived isotopes are almost non-existent,\u201d said Tsantiri.<\/p>\n He noted that experiments like this have only recently become feasible, thanks to advanced facilities such as FRIB.<\/p>\n The implications reach beyond this isotope, because the same tools can now test other stellar reactions once left to theory.<\/p>\n Future research directions <\/p>\n With this result in hand, researchers can attack neighboring reactions that shape other rare<\/a> proton-rich elements.<\/p>\n Each direct measurement replaces a theoretical placeholder with data, which makes stellar origin stories less guesswork and more testable physics.<\/p>\n FRIB was built for exactly that kind of work, using short-lived nuclei that almost never sit around long enough to study.<\/p>\n As more of those reactions are measured, astronomers should get a firmer read on which explosions actually forged the rarest isotopes.<\/p>\n The experiment showed that one missing nuclear step could finally be measured on Earth, and that precision changed what models can claim.<\/p>\n Yet the leftover mismatch keeps the bigger mystery alive, pointing scientists toward better supernova conditions and more direct tests.The study is published in Physical Review Letters<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read?\u00a0Subscribe to our newsletter<\/a>\u00a0for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on\u00a0EarthSnap<\/a>, a free app brought to you by\u00a0Eric Ralls<\/a>\u00a0and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"Scientists have directly measured the reaction that produces a rare, hard-to-explain form of selenium, capturing for the 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