Existence of a proton halo in ²³Al and its significance

Reaction cross section σ_R of proton-rich isotones (N=10) near ²³Al and Al isotopes (^23-28Al) on C target have been measured at intermediate energies around 30 MeV/nucleon. An abnormal increase of the experimental σ_R is observed for ²³Al and it suggests that there is an anomalously large matter rms radius in ²³Al. Together with the very weakly binding of the last proton (S_p=0.125 MeV), it indicates that there is a proton halo in ²³Al. This conclusion is also supported by the difference factor d which is deduced from the measured and theoretical σ_R in the Glauber or Boltzmann-Uehling-Uhlenbeck model and is used for the manifest of halo phenomena. The theoretical calculation based on the relativistic density dependent Hartree model shows that there is a proton halo when the last proton is in the 2s_1/2 orbit in ²³Al. The significance of the proton halo in ²³Al is discussed.