Polarization of Positrons from ${\mathrm{Ga}}^{66}$, ${\mathrm{Ga}}^{68}$, ${\mathrm{P}}^{29}$, and ${\mathrm{Al}}^{25}$

The longitudinal polarizations of thermalized positrons from ${\mathrm{Ga}}^{66}$, ${\mathrm{Ga}}^{68}$, ${\mathrm{P}}^{29}$, and ${\mathrm{Al}}^{25}$ have been observed from measurements on annihilation with polarized electrons in a magnetic medium. When allowance is made for the depolarization suffered by the positrons in coming to rest, the observations are in qualitative agreement with the assumption that the initial polarizations are proportional to $+\frac{v}{c}$ with the same proportionality constant as for positrons from ${\mathrm{Cu}}^{64}$.