
^{20}Mg (1978AJ03)(See the Isobar Diagram for ^{20}Mg) ^{20}Mg has been populated in the ^{24}Mg(α, ^{8}He) reaction at E_{α} = 126.9 MeV (1976TR03) and 156 MeV (1974RO17) with differential cross sections (lab) of 3 ± 1 nb/sr (θ = 5°, lab) and ≈ 7 nb/sr (2°), respectively. Assuming the mass of ^{8}He to be 31.601 ± 0.013 MeV, the mass excess of ^{20}Mg is 17.57 ± 0.03 MeV. (1977WA08) adopt a mass excess of 17.568 ± 0.027 MeV, and so do we. ^{20}Mg is then stable with respect to breakup into ^{19}Na + p [see ^{19}Na] and ^{18}Ne + 2p by 2.65 ± 0.03 and 2.33 ± 0.03 MeV, respectively. See (1976WA18) for a display of calculations of the mass of ^{20}Mg and (1972AJ02) for the earlier work. The mass excess of ^{20}Mg appears to involve a deviation from the quadratic IMME prediction (1976TR03). Using the wave function predictions of (1973LA03) and the mass excess of ^{20}Mg (1976TR03) calculate the partial τ_{1/2} for the allowed Fermi transition from the 0^{+}; T = 2 ground state of ^{20}Mg to the 0^{+}; T = 2 state in ^{20}Na [not observed] to be ≈ 3.4 sec, corresponding to a β^{+} branching of ≈ 4%. See also (1976BE1L, 1977SH13).
