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LLNL's CHICOX counter supplies the scattering angle GRETA needs to read nuclear shapes
Lawrence Livermore's CHICOX counter ran nine experiments on 13 nuclei at Argonne's ATLAS facility in its first campaign. Its builders say the data can test predictive models of nuclear shape, though the paper published so far describes the instrument itself.
The Scientist · Science desk
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What happened
- During a run, CHICOX tracks each scattered beam particle while the gamma-ray array records the gamma rays the excited nucleus gives off as it settles.
- Signals that reach both instruments at about the same time count as one event, and the scattering data sharpen the blurry shape outline the gamma rays give on their own.
- Daniel Rhodes and colleagues describe the detector in a 2026 paper in Nuclear Instruments and Methods in Physics Research Section A.
- CHICOX and GRETA will be hosted together at the Facility for Rare Isotope Beams, the site that will supply beams of unstable nuclei.
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Why it matters
- constraint Until CHICOX runs with GRETA itself, its only record alongside a gamma-ray array is with GRETINA, the precursor, so performance figures for the intended pairing are still to come.
- precedent Groups from around the world used CHICOX in its debut. If it serves the 15 to 20 years Wu expects, FRIB shape studies will probably be planned around this one detector.
- capability Fission research relies on similar detection techniques and on models of nuclear deformation, so better shape data from CHICOX can also feed national-security work.
CHICOX was built to work with GRETA, the Gamma-Ray Energy Tracking Array [2]. A gamma ray from one of these collisions is only useful if the collision angle comes with it. "If we don't have CHICOX, then we don't know when the nuclei collide. And when they collide, they scatter at different angles. Without that angle, the gamma-ray information is lost," said LLNL scientist Ching-Yen Wu [5]. The shapes being measured are the ones a textbook sphere leaves out. Nuclei can deform toward a pear, a football or a Frisbee [15].
"Nuclear theorists are working toward a comprehensive, predictive model of nuclei and how they behave. The data we measure with CHICOX are a good test of these models," said LLNL scientist Daniel Rhodes [6]. He is describing what the detector was built for. The evidence published so far is about the hardware, in a paper titled "CHICOX, a heavy-ion detection system for GRETA" [8]. The account of the first campaign gives the number of experiments and nuclei [7]. It does not give the detector's angular resolution, its event rates, or any measured deformation with an uncertainty attached.
For a theorist, those are the figures that decide whether a measurement can tell one model from another. Testing models directly stays a design goal until deformation values for the 13 nuclei are published with error bars [7].
I'd expect the sharper tests to come later, at FRIB. Stable, common nuclei behave very differently from rare and exotic ones, and measurements of exotic nuclei have been limited because highly unstable isotopes are hard to produce [12]. "Doing these types of experiments at FRIB will give us access to a lot more exotic isotopes, so we can push our studies further and further away from the stable isotopes that we see around us and into very unusual systems," Rhodes said [11]. Wu explained why those systems draw interest. "Study of unstable nuclei is very interesting because many properties that we know break down," he said [13].
CHICOX stands for Compact Heavy Ion Counter version X [1]. It is the latest and most advanced in a line of similar detectors designed at LLNL, and the Department of Energy Office of Science Nuclear Physics program supported its development [9].
What to watch
- Physics papers from the first campaign that report deformation values, with uncertainties, for the 13 nuclei measured at ATLAS.
- The first CHICOX experiments with GRETA at FRIB, and which unstable isotopes they target.
- Whether theorists publish direct comparisons between CHICOX shape data and predictive nuclear models.