The nuclear halo is a unique phenomenon occurring at the limit of nuclear stability. Previous studies have established an enhanced low-energy electric dipole strength as a characteristic feature of halo nuclei. Despite such extensive work on the electric response, there is no experimental evidence on the magnetic response of halos. A gamma-ray lifetime measurement has been performed on the one-neutron halo nucleus 19C, which represents the first measurement of a magnetic transition between bound states in a halo nucleus. This measurement also serves as a means to constrain the spin-parities of the states in 19C. The lifetime of the first excited state in 19C has been measured using both the line-shape method and the Recoil Distance Method. The deduced B(M1;3/2+→1/2+) transition strength represents one of the most hindered M1 transitions among light nuclei. The result is compared to large-scale shell model calculations, which predict a strong hindrance due to the degeneracy of the 1s1/2 and 0d5/2 neutron orbitals. The result establishes the M1 hindrance as another feature of halo nuclei which are dominated by s-wave configurations.The one-proton knockout reaction of 20N is used to study the structure of the bound states in 19C as well as the ground state in 20N. Eikonal reaction model calculations are compared to the measured inclusive cross section. The small inclusive cross section indicates the significant difference between the wave functions of the low-lying states of 20N and 19C. The results support the spin-parity assignment of the excited state in 19C obtained from the lifetime measurement. The coupling of the proton 0p1/2 orbital to a 5/2+ core within 20N is suggested from large-scale shell-model calculations performed for the 2− ground state in 20N, indicating that the degeneracy of the 1s1/2 and 0d5/2 orbitals that occurs in 19C does not persist in 20N. Both the lifetime and knockout reaction analyses support the exclusion of a bound 5/2+ state in 19C. The combined results present a consistent picture of the structure of 19C and provide important data to establish trend of the 1s1/2 and 0d5/2 single particle energies for the N=13 isotones.
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