In the conventional diagram of an atom, the center depicts a nucleus - a spherical cluster of protons and neutrons. But that sphere is a simplification: nuclei can deform into exotic shapes that appear more like a pear, football or frisbee. Understanding why and when nuclei distort is crucial for predicting and modeling how they will behave.
Researchers at Lawrence Livermore National Laboratory (LLNL) have developed a new detector, CHICOX (Compact Heavy Ion Counter version X), that opens up a new era of extremely sensitive studies of nuclear shapes.
"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.
CHICOX was built to work in tandem with another instrument: the Gamma-Ray Energy Tracking Array, or GRETA. During an experiment, a beam of particles strikes a stationary target material. When one of those particles hits a nucleus, they both scatter. The extra energy imparted by this process excites the projectile nucleus, which vibrates, rotates and emits gamma rays as it returns to a more stable state. CHICOX measures the trajectory of the scattered particle, while GRETA detects the gamma rays that are released.
"When two nuclei collide, we detect the particles with CHICOX and then the gamma rays are captured by GRETA," said LLNL scientist Ching-Yen Wu. "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."
Together, the instruments make what is called a coincidence measurement. If they both see a signal at approximately the same time, they mark it as a single event. The pattern of gamma rays captured by GRETA reveals a blurry outline of what the nucleus looks like, while the scattering information from CHICOX sharpens that image.
CHICOX, which was supported by the Department of Energy Office of Science Nuclear Physics program, is the latest and most advanced iteration in a line of similar detectors designed by LLNL researchers.
In the last year, the CHICOX team deployed the device for the first time and successfully carried out nine experiments on 13 different nuclei at Argonne National Laboratory's ATLAS facility with GRETINA, the precursor to GRETA. The LLNL-designed instrument has already captured broad interest from the physics community and was used by researchers from around the world during this first experimental campaign.
The development of this detector technology also has national security applications, as similar detection techniques and models of nuclear deformation are important for understanding nuclear fission.
Going forward, both CHICOX and GRETA will be hosted at the Facility for Rare Isotope Beams (FRIB), which will provide beams of unstable nuclei and therefore even greater opportunities.
"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," said Rhodes.
The behavior of stable and common nuclei is known to be very different from rare and exotic nuclei. So far, measurements of the latter have been limited due to the difficulty of producing highly unstable isotopes. Facilities such as FRIB and ATLAS will help reveal any surprising behavior that may be found in exotic nuclei.
"Study of unstable nuclei is very interesting because many properties that we know break down. So, it's a very important topic to advance our knowledge of nuclear structure more broadly," said Wu. "It is exciting because CHICOX can do this work for the next 15 to 20 years."
