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When a massive star dies in a supernova, the blast forges radioactive titanium-44 in quantities that space telescopes can still detect decades later \u2014 but until now, no laboratory had directly measured the nuclear reaction rate that controls how much of it forms. In a pair of papers published in Physical Review Letters, the University of Surrey’s Nuclear Physics Group has delivered the first experimental measurements<\/a> of two reactions that stellar explosion models have long been forced to treat as theoretical guesses.<\/p>\n The findings replace estimates at two of the most consequential nodes in nuclear astrophysics: one governing the production of radioactive titanium in core-collapse supernovae, the other governing the energetics of the most frequently occurring stellar explosions in the galaxy. Both measurements were made at world-leading US accelerator facilities \u2014 Argonne National Laboratory outside Chicago and the Facility for Rare Isotope Beams (FRIB) at Michigan State University in East Lansing, Michigan.<\/p>\n Surrey Got the First Data on a Reaction That Controls Titanium-44 Yields in Supernovae<\/p>\n When a massive star collapses and the resulting shock wave rips outward through its layers, conditions in the innermost ejecta reach temperatures and pressures that trigger a nuclear process called the alpha-rich freeze-out: a cascade of helium-4 capture reactions building elements from calcium upward. One step in that chain \u2014 the pathway that ultimately produces titanium-44 \u2014 is controlled by the rate of a competing reaction that had never been measured in a laboratory.<\/p>\n At Argonne’s ATLAS (Argonne Tandem Linac Accelerator System) facility, Surrey researchers obtained the first experimental data<\/a> needed to constrain that rate. They found the competing reaction proceeds far more slowly than theoretical models had assumed. Counterintuitively, a slower competing reaction channels more material into the titanium-44-producing pathway \u2014 raising predicted titanium-44 production by 20 to 35 percent<\/a> compared to the prior theoretical expectation.<\/p>\n Titanium-44 is among the most scientifically useful isotopes produced in a stellar explosion. With a half-life of roughly 60 years<\/a>, it persists long enough after a supernova that space-based gamma-ray and hard X-ray observatories can still detect its decay signature in relatively young remnants. NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) has previously detected and mapped titanium-44<\/a> emissions in the Cassiopeia A supernova remnant, finding the isotope concentrated in asymmetric clumps near the explosion center \u2014 direct evidence for the turbulent, off-axis nature of the explosion itself. ESA’s INTEGRAL satellite has detected the same titanium-44 gamma-ray line emission<\/a> in SN 1987A, the nearest supernova observed in modern times. Both observatories have long struggled to reconcile what they detect with what models predict \u2014 the observed titanium-44 yield in Cassiopeia A consistently exceeds standard model predictions by a significant margin.<\/p>\n The Surrey measurement hands that modeling enterprise a sharper tool. With an experimentally grounded rate replacing a theoretical placeholder, astronomers running supernova simulations can now make predictions that can be compared with confidence against what NuSTAR and INTEGRAL actually see.<\/p>\n Dr. Christopher Cousins, the study’s lead author and a postdoctoral researcher in Surrey’s Nuclear Physics Group, said the achievement reflects how dramatically the field’s experimental reach has expanded: “It’s exciting to see just how far the field has come. A measurement like this would have been considered out of reach<\/a> only a couple of decades ago, but it now gives us new insight into one of the biggest unanswered questions in astrophysics.”<\/p>\n A Second Measurement Resolves a Standing Controversy About X-Ray Burst Physics<\/p>\n The second study tackled a different stellar explosion entirely \u2014 and a different open question that had divided nuclear astrophysicists for years.<\/p>\n Type-I X-ray bursts are the galaxy’s most frequent explosions<\/a>. They occur when a neutron star in a close binary system pulls hydrogen and helium from a companion star onto its surface. The accumulated fuel eventually ignites in a thermonuclear runaway powered by the rapid proton capture process, or rp-process \u2014 a nuclear assembly line that builds heavier, proton-rich elements by successively adding protons to seed nuclei. These outbursts can repeat every few hours, making them observable targets for X-ray space telescopes that monitor the galaxy continuously.<\/p>\n What happens inside the rp-process at the nickel-56 “waiting point” has been a persistent source of uncertainty. After the nuclear flow reaches nickel-56, two possible pathways compete. In one, material is captured upward toward heavier elements. In the other, a sequence of reactions involving copper-59 can form what nuclear astrophysicists call the nickel-copper (NiCu) cycle: a temporary loop that traps a fraction of the nuclear material between nickel-56 and zinc-60, cycling it back rather than allowing it to proceed upward through the rp-process chain. The properties of this cycle<\/a> are central to how accurately models can reproduce observed X-ray burst light curves.<\/p>\n Whether that cycle operates significantly \u2014 and whether it’s strong enough to alter the shape of the observable X-ray burst light curve \u2014 has been contested. A measurement published earlier in 2026 in The Astrophysical Journal constrained one branch of the cycle (the copper-59 to nickel-56 return path) and found the cycle negligibly small<\/a> for one specific X-ray burst model, appearing to rule out a strong cycle. The Surrey study measured the other branch \u2014 the copper-59 to zinc-60 capture rate \u2014 and found a more nuanced picture.<\/p>\n Working at FRIB, the team used a technique called single-nucleon transfer spectroscopy to probe the structure of an unstable nucleus that cannot be studied with conventional targets. Because copper-59 is radioactive and exists for only a fraction of a second, it cannot simply be held in a target and bombarded. Instead, FRIB produced a beam of copper-59 nuclei accelerated to a fraction of the speed of light and directed into a deuterium target. The copper-59 nuclei in the beam picked up a neutron from each deuteron (which consists of one proton and one neutron), releasing the proton \u2014 a \u2075\u2079Cu(d,n)\u2076\u2070Zn transfer reaction. This process populated excited states in zinc-60<\/a> that are the same states relevant to the astrophysical \u2075\u2079Cu(p,\u03b3)\u2076\u2070Zn reaction inside a bursting neutron star.<\/p>\n The team identified 15 proton-unbound levels in zinc-60, measured their excitation energies from the observed gamma-ray decays and spectroscopic factors<\/a> from angle-integrated cross sections. Incorporating these experimentally constrained results into stellar model calculations reduced the uncertainty in the copper-59 proton-capture rate by more than tenfold<\/a>.<\/p>\n The result: a NiCu cycle in X-ray bursts is indeed possible \u2014 real, not an artifact \u2014 but its branching limited below 38 percent<\/a>. Even a modest branching at that level has a significant impact on the burst light curve, the paper finds, motivating follow-up measurements of adjacent reaction rates to complete the picture.<\/p>\n Connor O’Shea, the study’s lead author and a postdoctoral researcher within Surrey’s Nuclear Physics Group, described what the team settled: “One of the biggest unknowns was whether material becomes trapped in the nickel-copper cycle during an X-ray burst. We’ve shown that it does<\/a>, but likely only a small proportion, giving us a much more realistic picture of these explosions.”<\/p>\n What These Explosions Reveal About Neutron Stars<\/p>\n The payoff of constraining the NiCu cycle extends beyond understanding stellar explosions. X-ray burst light curves are used by astrophysicists as tools for probing the properties of neutron stars themselves \u2014 specifically the neutron star’s radius, mass, and the equation of state of matter at nuclear densities, which is one of the deepest unsolved problems in all of physics.<\/p>\n When a neutron star’s surface erupts in a thermonuclear burst, the detailed shape of the light curve \u2014 how quickly it rises, how it peaks, how it decays \u2014 carries information about the neutron star’s compactness. Extracting that information requires an accurate model of the nuclear burning itself. Prior to this measurement, reaction rate uncertainties in the NiCu cycle introduced a contaminating error into any attempt to read neutron star properties from burst observations. The tenfold reduction in uncertainty achieved at FRIB shrinks that error significantly.<\/p>\n The PRL paper notes an additional downstream consequence: a significant NiCu cycle leads to an increase in the amount of odd-mass nuclei in the burst ashes that settle onto the neutron star’s surface. These odd-mass nuclei can affect Urca cooling in neutron star crusts<\/a> \u2014 a heat-transport mechanism that governs how quickly the crust loses thermal energy between bursts. Better-constrained NiCu rates make it possible to model these crustal processes with greater fidelity.<\/p>\n What Comes Next in Astrophysical Nuclear Measurements<\/p>\n Both measurements are part of a larger program at FRIB and Argonne to systematically replace theoretical placeholders in stellar explosion models with experimentally measured rates. Both facilities are specifically designed<\/a> for access to short-lived, exotic isotopes that only exist under extreme astrophysical conditions \u2014 making them the only places on Earth where measurements like these are currently possible.<\/p>\n The titanium-44 supernova result will feed immediately into nucleosynthesis models used to interpret current and future gamma-ray telescope data. Remaining uncertainties in the same nuclear network mean further measurements will be needed \u2014 but the most consequential rate node has now been experimentally constrained for the first time.<\/p>\n For X-ray burst science, the NiCu cycle involves multiple reaction rates, and the Surrey paper explicitly notes that additional measurements of adjacent rates are needed to fully characterize the cycle’s influence on burst light curves. FRIB’s ability to produce intense beams of proton-rich short-lived isotopes positions it as the natural home for that follow-up work.<\/p>\n Prof. Gavin Lotay, Professor of Nuclear Physics at the University of Surrey and principal investigator on both studies, framed the broader significance of doing both at once: “Despite decades of research, we still don’t fully understand the nuclear reactions that power some of the Universe’s most spectacular stellar explosions. These two studies answer important questions about what happens inside both X-ray bursts and supernovae, providing experimental evidence where scientists previously had to rely on theory and estimates. Together, they give us a much clearer picture<\/a> of how these explosions happen, allowing us to compare our models more closely with astronomical observations and bringing us closer to understanding how the chemical elements are created and spread throughout the Universe.”<\/p>\n Both studies were published in Physical Review Letters<\/a> \u2014 the titanium-44 supernova paper (Phys. Rev. Lett. 136, 252701) and the X-ray burst NiCu paper<\/a> (Phys. Rev. Lett. 137, 022701).<\/p>\n Frequently Asked QuestionsWhy does titanium-44 matter so much to supernova scientists?<\/p>\n Titanium-44 is radioactive with a 60-year half-life, making it long-lived enough that space telescopes can still detect its gamma-ray glow in the remnants of comparatively recent stellar explosions. Because it forms in the innermost layers of a core-collapse supernova \u2014 directly above the newly formed neutron star \u2014 its distribution and yield act as a direct probe of how the explosion unfolded. Asymmetric clumps of titanium-44 detected by NASA’s NuSTAR in Cassiopeia A, for example, revealed that the explosion was not spherical but turbulent and off-axis. Models that cannot accurately predict how much titanium-44 forms lose their ability to be tested against these observations \u2014 which is exactly what the Argonne measurement now makes possible.<\/p>\n What is the rp-process, and why does it produce X-ray bursts?<\/p>\n The rapid proton capture process (rp-process) is a nuclear assembly line that operates on the surface of accreting neutron stars at temperatures near one billion kelvin (1,800,000,000\u00b0F \/ 1,000,000,000\u00b0C). As a neutron star pulls hydrogen and helium from a companion star, the accumulated fuel ignites in a thermonuclear runaway, and the rp-process begins building heavier elements by rapidly adding protons to successive nuclei. This releases an enormous burst of X-ray energy \u2014 observable to space telescopes as a brief, bright flare that can repeat every few hours. The nickel-copper (NiCu) cycle is a branching point within this process where a fraction of the nuclear flow can be temporarily recycled back rather than continuing to build heavier elements, affecting both the energy release profile and the eventual composition of the material that settles onto the neutron star surface<\/a>.<\/p>\n What does constraining these nuclear rates mean for neutron star physics?<\/p>\n X-ray burst light curves carry encoded information about the neutron star producing them \u2014 specifically its radius, mass, and the equation of state of matter at nuclear density. Extracting that information requires accurate nuclear models of the burst itself. When reaction rates like the NiCu cycle are uncertain, that uncertainty transfers directly into uncertainty about the neutron star’s properties. By reducing the NiCu cycle rate uncertainty by more than tenfold, the Surrey measurement allows astrophysicists to be significantly more confident in what they read from burst observations about neutron star interiors<\/a> \u2014 matter at densities far beyond what any laboratory on Earth can replicate.<\/p>\n Why are measurements like this only possible at FRIB and Argonne?<\/p>\n