March 21, 2024• Physics 17, 47A widening hole between the cerium-140 abundance predicted by theories and that measured in observations of sure stars signifies a possible want for up to date fashions of component formation.
Tomasz Zajda/inventory.adobe.com
New experiments point out that cerium-140 is considerably extra more likely to seize an incoming neutron than beforehand thought.
Tomasz Zajda/inventory.adobe.com
New experiments point out that cerium-140 is considerably extra more likely to seize an incoming neutron than beforehand thought.×
Astrophysicists have a cerium downside—fashions predict that sure stars ought to include a lot much less of this heavy component than astrophysical observations discover. Just lately carried out experiments at CERN’s neutron time-of-flight (n_TOF) facility have widened the hole between idea and observations by 20% [1]. The researchers behind the work say that the outcomes spotlight the necessity for high-accuracy measurements of the nuclear properties of atoms, in addition to for up to date nucleosynthesis fashions of component formation. “Our experiment made the issue worse,” says Simone Amaducci of the INFN Laboratori Nationali del Sud, Italy. “That was sudden, however it’s additionally fascinating as a result of it means there’s something we don’t perceive about how nucleosynthesis occurs.”
A lot of the Universe’s heavier parts type in stars by way of one of many so-called neutron-capture processes, through which an atomic nucleus absorbs a number of neutrons. Within the gradual neutron-capture course of, or “s course of,” the absorptions are unfold out in time. As such, every absorption occasion leads to both a steady nucleus with the identical variety of protons however one extra neutron or an unstable nucleus, which then radioactively decays to supply the nucleus of the following component within the periodic desk—the one with one extra proton.
Utilizing at the moment obtainable fashions of the s course of, researchers have accurately predicted the abundances of parts as heavy as barium (56 protons), lanthanum (57 protons), praseodymium (59 protons), and neodymium (60 protons) in stars which are recognized to be enriched by way of the s course of. However the fashions seem to interrupt down for cerium (58 protons), because the abundance predictions for this component in some low-mass, low-metallicity globular cluster stars have disagreed by as much as 30% with observations. “This discrepancy could be very unusual, as the idea works for the neighboring parts,” says Sergio Cristallo, a staff member who works on issues associated to neutron seize on the Nationwide Institute of Astrophysics in Italy. “There may be nothing within the fashions that ought to trigger such a discrepancy only for one component.”
Cerium has one other intriguing property—it could actually type a so-called magic-number nucleus. A lot of the Universe’s cerium (89%) exists as cerium-140, an isotope of the component that incorporates 58 protons and a magic variety of 82 neutrons. Magic-number nuclei are notably steady and so usually exist in larger abundances than different isotopes of the identical component or of neighboring parts within the periodic desk.
Like different magic-number nuclei, the excessive stability of cerium-140 arises from its low neutron-capture cross part, which is the chance {that a} nucleus of the isotope will take in an incoming neutron. It is usually the parameter measured within the new experiments at CERN, which concerned bombarding a cerium-oxide pattern with a high-energy neutron beam after which measuring the merchandise of that interplay. The seize of a neutron by the cerium-140 within the pattern produced cerium-141, an unstable isotope. The next decay of cerium-141 emitted a cascade of gamma rays, which had been detected once they interacted with a liquid scintillator. These detections had been then used to find out the neutron-capture cross part of the unique cerium-140.
Evaluation of the information signifies that the neutron-capture cross part is 40% larger than measured in earlier experiments, which had decrease accuracy. The next cross part makes it extra probably cerium-140 will seize an incoming neutron and fewer probably that it’ll keep in its cerium-140 type. That in flip results in a prediction of a decrease abundance of cerium-140 than earlier than, Amaducci says. The upper cross part additionally has implications for the abundances of the nuclei that type additional alongside within the s-process chain. With cerium-140 being extra more likely to seize a neutron and type a heavier nucleus, the s course of can proceed sooner, creating the next abundance of heavier nuclei, Amaducci says.
The mismatch between the cerium-140 abundance predicted by idea and that measured in observations of the low-metallicity stars the staff thought of suggests {that a} course of aside from the s course of may additionally produce this nucleus in these stars. Amaducci, Cristallo, and one other of their colleagues, Alberto Mengoni of the Italian Nationwide Company for New Applied sciences, Power, and Sustainable Financial Growth (ENEA), say that one chance is that one other nucleosynthesis pathway, the so-called i course of, is concerned within the making of cerium. This proposed neutron-capture course of bypasses some steady nuclei concerned within the s course of, and—if it performed a giant function—might change the relative abundances of the weather. “It’s a chance,” Mengoni says. “However we don’t know whether or not this i course of can clarify the current scenario.”
The staff will not be the one group finding out the neutron-capture cross part of cerium-140. In February, utilizing a distinct approach, Michael Paul of the Hebrew College of Jerusalem and his colleagues discovered a roughly 15% decrease worth of the cross part than beforehand measured with that approach [2]. Neither Paul nor Amaducci know the supply of the discrepancy between the 2 new values.
To tease out precisely what’s going on, the researchers all agree that extra nuclear measurements must be made. For instance, Amaducci notes that there’s at the moment no experimental knowledge for lots of the nuclei concerned within the i course of. “So, the inputs to the mannequin are very unsure,” he says. Many idea and experimental teams are exploring the i course of, and Mengoni expects that the outcomes of these research will result in some fascinating findings. “It might be that present fashions of nucleosynthesis must be tuned or new fashions invented,” he says. “Understanding these issues is likely one of the most energetic areas in nuclear physics.”–Katherine WrightKatherine Wright is the Deputy Editor of Physics Journal.ReferencesS. Amaducci et al. (n_TOF Collaboration), “Measurement of the 140Ce(n, 𝛾) cross part at n_TOF and its astrophysical implications for the chemical evolution of the Universe,” Phys. Rev. Lett. 132, 122701 (2024).R. N. Sahoo et al., “Stellar s-process neutron seize cross part of Ce isotopes,” Phys. Rev. C 109, 025808 (2024).Topic AreasNuclear PhysicsAstrophysicsRecent Articles Extra Articles