What is the mass difference between 7Be and 7Li?
From Table 21-8
Mass [7Be - 7Li] = 7.016930 - 7.016005 = 0.000925 amu
To decay by positron emission the mass difference of the two neutral atoms must be at least twice the mass of the electron (2 X 0.000549 = 1.098X10-3 amu). One accounts for the creation of the positron and the other for one less orbital electron in the daughter atom. As there is not enough mass here, positron emission is not possible.

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