  The proton-proton momentum correlation functions ($C_{pp}(q)$) for
kinematically complete decay channels of $^{23}$Al $\rightarrow$ p + p +
$^{21}$Na and $^{22}$Mg $\rightarrow$ p + p + $^{20}$Ne have been measured at
the RIKEN RI Beam Factory. From the very different correlation strength of
$C_{pp}(q)$ for $^{23}$Al and $^{22}$Mg, the source size and emission time
information were extracted from the $C_{pp}(q)$ data by assuming a Gaussian
source profile in the correlation function calculation code (CRAB). The results
indicated that the mechanism of two-proton emission from $^{23}$Al was mainly
sequential emission, while that of $^{22}$Mg was mainly three-body simultaneous
emission. By combining our earlier results of the two-proton relative momentum
and the opening angle, it is pointed out that the mechanism of two-proton
emission could be distinguished clearly.
