  The reaction cross sections of $^{27,28}$P and the corresponding isotones on
Si target were measured at intermediate energies. The measured reaction cross
sections of the N=12 and 13 isotones show an abrupt increase at $% Z=15$. The
experimental results for the isotones with $Z\leq 14$ as well as $% ^{28}$P can
be well described by the modified Glauber theory of the optical limit approach.
The enhancement of the reaction cross section for $^{28}$P could be explained
in the modified Glauber theory with an enlarged core. Theoretical analysis with
the modified Glauber theory of the optical limit and few-body approaches
underpredicted the experimental data of $^{27}$P. Our theoretical analysis
shows that an enlarged core together with proton halo are probably the
mechanism responsible for the enhancement of the cross sections for the
reaction of $^{27}$P+$^{28}$Si.
