Experiments on neutron-rich atomic nuclei could help scientists to understand nuclear reactions in exploding stars

Nov 29, 2013
Figure 1: Gamma rays emitted in the decay of neutron-rich palladium nuclei were detected using the EURICA array of germanium detectors. Credit: © 2013 RIKEN Nishina Center for Accelerator-Based Science

Almost all the mass of an atom is concentrated in its nucleus in the form of protons and neutrons. Some nuclei are intrinsically unstable, existing for only a short time before decaying into stable nuclei. Understanding why some nuclei are stable and others are not could help to explain the presence of matter in the Universe. Hiroshi Watanabe at the RIKEN Nishina Center for Accelerator-Based Science and an international team of scientists have now shown experimentally that nuclei with many neutrons can be more stable than might be expected, but only if the number of neutrons is just right.

The number of in an atom is fixed for each element, but the number of can vary. Palladium have 46 protons, and in naturally occurring nuclei on Earth, the number of neutrons can vary between 56 and 64, resulting in a series of 'isotopes' of palladium. In the lab, scientists can create short-lived nuclei with many more neutrons, known as 'heavy' isotopes.

Using the Radioactive Isotope Beam Factory (RIBF), Watanabe and his colleagues created palladium nuclei with 80 and 82 neutrons—labeled palladium-126 (46 protons plus 80 neutrons) and palladium-128, respectively. "Only palladium isotopes with up to 74 neutrons have been studied to date," explains Watanabe. "Our experiment reached 82 thanks to high-intensity uranium beams and a highly efficient gamma-ray detection system."

The experiment involved accelerating a beam of uranium ions towards a beryllium target. The collision created a wide variety of nuclei with different numbers of protons and neutrons, and the sophisticated ion selection system at the RIBF allowed specific heavy isotope nuclei to be isolated based on their physical and electromagnetic properties to create a pure heavy ion beam. The team then assessed the stability of these artificially generated nuclei by observing gamma rays emitted from nuclear excited states using an array of sensitive detectors known as EURICA (Euroball-RIKEN Cluster Array) (Fig. 1).

Nuclei with 2, 8, 20, 28, 50, 82 or 126 neutrons or protons are empirically known to be stable, representing the 'magic numbers' of nuclear particles. Yet some studies have suggested that these conventional magic numbers disappear if the number of protons and neutrons is highly unbalanced. The results from the RIBF research, however, show that the magic number of 82 neutrons is fairly robust, making palladium-128 relatively stable. "Next, we want to see if 82-neutron

Explore further: 'Magic numbers' disappear and expand area of nuclear deformation

More information: Watanabe, H., Lorusso, G., Nishimura, S., Xu, Z. Y., Sumikama, T., Söderström, P.-A., Doornenbal, P., Browne, F., Gey, G., Jung, H. S. et al. Isomers in 128Pd and 126Pd: Evidence for a robust shell closure at the neutron magic number 82 in exotic palladium isotopes. Physical Review Letters 111, 152501 (2013). dx.doi.org/10.1103/PhysRevLett.111.152501

add to favorites email to friend print save as pdf

Related Stories

Evidence for a new nuclear 'magic number'

Oct 09, 2013

Researchers have come one step closer to understanding unstable atomic nuclei. A team of researchers from RIKEN, the University of Tokyo and other institutions in Japan and Italy has provided evidence for ...

Atomic core, shaken not stirred

Jan 05, 2012

When struck just right, protons and neutrons ring. The sub-atomic particles don’t jingle like when a hammer hits a bell. But they do jiggle in an odd dance where the protons move in one direction and ...

Recommended for you

Backpack physics: Smaller hikers carry heavier loads

21 hours ago

Hikers are generally advised that the weight of the packs they carry should correspond to their own size, with smaller individuals carrying lighter loads. Although petite backpackers might appreciate the ...

Extremely high-resolution magnetic resonance imaging

22 hours ago

For the first time, researchers have succeeded to detect a single hydrogen atom using magnetic resonance imaging, which signifies a huge increase in the technology's spatial resolution. In the future, single-atom ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Graeme
not rated yet Jan 11, 2014
The story has been cut off mid sentence: "Next, we want to see if 82-neutron....