Science1 publisher2 min readPublished
Measuring krypton-88's neutron capture cut its uncertainty from a factor of eight to about three
An international team inferred the rate from gamma rays given off by krypton-89 at Argonne and found it below theoretical predictions. Feeding it back into i-process simulations raised how much strontium they make.
The Scientist · Science desk
What happened
- An international team led by former FRIB graduate student Caley Harris, with researchers from 12 institutions in the United States, Canada and Europe, published the first experimental study of the reaction on June 8 in Communications Physics.
- The measurement narrowed the uncertainty on krypton-88's neutron-capture rate from at least a factor of eight to about a factor of three.
- The rate the team extracted sits consistently below theoretical predictions of the same quantity.
- Put into leading i-process models of heavy-element formation, the new rate raised the predicted amount of strontium and brought the simulations closer to astronomical observations.
- FRIB's SuN detector was installed at Argonne's ATLAS accelerator, where the team made krypton-89, krypton-88 plus one neutron, and read its gamma-ray emissions to infer the capture rate.
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Why it matters
- constraint With the rate pinned to a factor of three, a leftover strontium shortfall can no longer be blamed on this reaction; anything remaining has to be argued from other rates or from the astrophysical conditions.
- contradiction Theory sat above experiment for this nucleus, so the calculated rates still feeding the rest of the i-process network carry the same question.
- decision The follow-up Herwig describes is simulation and theory work. The next move is with the modellers, not the accelerator.
- capability A capture rate on a nucleus too short-lived to hold in front of a neutron beam can be obtained from the gamma decay of the nucleus one neutron heavier, and other short-lived isotopes are reachable the same way.
A factor of three is still a wide window on a reaction rate. The narrowing changes the range of values a model may assume. Taken as a one-sided multiplier, the window closed by 8/3, about 2.7 times [1]. Taken as a two-sided band, from one-eighth to eight times the central value down to one-third to three times, the span shrinks by 64/9, roughly sevenfold [2]. Either reading leaves a rate that models are sensitive to, since this reaction had been named the key unknown in the network [15].
Krypton-88 is short-lived and captures on it happen infrequently, so the reaction could not be measured head-on [16]. The route was indirect by design [3], and the rate came out of gamma rays emitted by krypton-89 [10].
The gap being probed goes back three decades. Since the 1950s, three processes have accounted for the elements heavier than iron [12]. In the 1990s, abundances measured in very old stars fit none of them, and strontium was one of the misfits [13]. The proposed fix, the i-process, works at neutron densities and timescales on the order of minutes, in between s- and r-process conditions [14]. It reproduces much of what astronomers see. It has also consistently made too little strontium, which modellers traced to missing or uncertain nuclear data, krypton-88 above all [15].
"Our models had flagged neutron capture on krypton-88 as the key unknown behind the strontium shortfall," said Falk Herwig, professor of physics and astronomy at the University of Victoria and a co-author of the study. "The measurement guides our next simulation and theory steps." [9]
The chemical signatures of very old stars are read through models of how their elements formed, strontium included [18]. The improvement reported when the new rate goes into those models is qualitative, closer agreement without a figure for how much of the shortfall is left [6].
"It turns out that explaining the abundances of elements in the universe is slightly more complicated than previously thought," said Artemis Spyrou, professor of physics at FRIB and in Michigan State University's Department of Physics and Astronomy [8].
What to watch
- Whether the follow-up simulations Herwig points to close the strontium gap or move the shortfall onto another reaction in the network.
- Whether the same indirect technique, applied to other short-lived isotopes, also finds calculated capture rates running above experiment.
- Publication of the network calculations that show how much strontium the lower krypton-88 rate actually adds.