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2015-8-31

A research group led by Associate Professor TAMII Atsushi, Research Center for Nuclear Physics, Osaka University, systematically measured the spin oscillation which reversed the spin direction of protons and neutrons in the nucleus by using ultra-precise measurement technology employing proton beam technology and found the spin direction slightly aligned in experiment, a world first.

It was thought that, as the spin of a particle like a proton (or a neutron) can be either right-handed or left-handed, and protons are strongly attracted each other, no spin alignment in which two protons spun in the same direction would take place, and this phenomenon had not been observed in experiments.

However, it was predicted that if there was a strong force via the exchange of virtual mesons (pion exchange) between protons and neutrons, spin alignment might take place as theorized by Dr. Hideki Yukawa.

The results of this group's experiment were consistent with a prediction by the latest theoretical calculation based on pion exchange proposed by Dr. Yukawa. This group's achievements are expected to lead to basic physics research for understanding the attractive force between nucleons and the clarification of astronomical phenomenon such as supernova explosion and magnetars.

Abstract

Differential cross sections of isoscalar and isovector spin- M 1 (0+1+) transitions are measured using high-energy-resolution proton inelastic scattering at Ep=295 MeV on Mg24, Si28, S32, and Ar36 at 0°–14°. The squared spin-M1 nuclear transition matrix elements are deduced from the measured differential cross sections by applying empirically determined unit cross sections based on the assumption of isospin symmetry. The ratios of the squared nuclear matrix elements accumulated up to Ex=16 MeV compared to a shell-model prediction are 1.01(9) for isoscalar and 0.61(6) for isovector spin-M1 transitions, respectively. Thus, no quenching is observed for isoscalar spin-M1 transitions, while the matrix elements for isovector spin-M1 transitions are quenched by an amount comparable with the analogous Gamow-Teller transitions on those target nuclei.

Figure 1

To learn more about this research, please view the full research report entitled "Nonquenched Isoscalar Spin-M1 Excitations in sd-Shell Nuclei" at this page of the Advanced Robotics website.


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