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20Mg (1978AJ03)
(See the Isobar Diagram for 20Mg)
20Mg has been populated in the 24Mg(α,
8He) reaction at Eα = 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 8He to be 31.601 ± 0.013 MeV, the
mass excess of 20Mg is 17.57 ± 0.03 MeV. (1977WA08) adopt a mass excess of 17.568 ± 0.027
MeV, and so do we. 20Mg is then stable with respect to breakup into
19Na + p [see 19Na] and 18Ne + 2p by
2.65 ± 0.03 and 2.33 ± 0.03 MeV, respectively. See (1976WA18) for a display of calculations of the
mass of 20Mg and (1972AJ02) for
the earlier work. The mass excess of 20Mg appears to involve a deviation from the
quadratic IMME prediction (1976TR03). Using
the wave function predictions of (1973LA03)
and the mass excess of 20Mg (1976TR03) calculate the partial τ1/2 for the allowed Fermi transition from the 0+; T = 2 ground
state of 20Mg to the 0+; T = 2 state in 20Na [not observed] to be
≈ 3.4 sec, corresponding to a β+ branching of
≈ 4%. See also (1976BE1L, 1977SH13).