NMR population difference from a pulse of known flip angle
immediately after the pulse:

$M_z = M_0cos(\beta )$

$M_x = M_0sin(\beta )$

$M_y = 0$

so for example after a 90 degree pulse, all the magnetization has been rotated into the x-y plane, and after a 180 pulse, the magnetization is in the -z direction.