On the basis of simple theoretical considerations it is expected that short-lived positron emitters should be produced in fluorine, sodium, and magnesium under bombardment with 6-Mev protons. These activities have been found and ascribed to ${\mathrm{Ne}}^{19}$, ${\mathrm{Mg}}^{23}$, and ${\mathrm{Al}}^{25,26}$. Half-lives, positron spectra and threshold investigations have been made and found to compare very well with the theory. It is concluded that for isobars of the type $\mathrm{}(n-p)=\pm 1$ the difference in binding energy is due solely to the effect of Coulomb forces up to at least mass number 25. The half-lives are found to depend on the inverse fifth power of the upper limit of the positron spectra in agreement with the theory.

• Received 20 July 1939

DOI:https://doi.org/10.1103/PhysRev.56.512