High-Energy Beta Decay of Light Elements

The beta decay of the radioactive members of the mass-eight and -twelve triads has been studied with a spiral-orbit spectrometer having a 1.3% resolution. The end point, half-life and log-ft values respectively are: for N{sup 12}, 16.37 {+-} 0.06 MeV, 11.43 {+-} 0.05 milliseconds, and 4.17; for B{sup 12}, 13.40 {+-} 0.05 MeV, 20.6 {+-} 0.2 msec and 4.11; for B{sup 8}, 14 MeV (broad), 0.75 {+-} 0.02 sec, and 5.72; and for Li{sup 8}, 13 Mev (broad), 0.87 {+-} 0.01 sec, and 5.67. The mass excesses in millimass units are: for N{sup 12}, 22.48 {+-} 0.06; for B{sup 12}, 18.19 {+-} 0.06; for B{sup 8}, 27.08 {+-} 0.13; and for Li{sup 8}, 24.97 {+-} 0.09. The shapes of the Kurie function for the mirror pairs indicate positron and electron transitions to the same levels of the daughter nuclide, with greater percentages in the positron branches relative to the ground-state transitions because of the higher energy available. The Li{sup 8} and B{sup 8} shapes are consistent with the shape of the alpha spectrum following the decay of the Be{sup 8} daughter. Less than 1% of the Li{sup 8} transition and less than 5% of the B{sup 8} transition go to the Be{sup 8} ground state. Besides the main transition to the broad 2.9-Mev level, a broad level near 11 Mev is probably involved. The ft values and the assumption of J = 0, 2, and 4 levels in Be{sup 8} favor J = 3 for Li{sup 8} and B{sup 8}. The N{sup 12} and B{sup 12} spectra indicate transitions of a few percent to the 4.43- and 7.65-Mev levels in C{sup 12}, with higher levels not excluded. For these nuclides J = 1 is favored. The agreement of the measured end points and resulting Q values with published values shows that the spiral-orbit spectrometer can be accurately calibrated with only the theory as a guide. Another evaluation of the spectrometer is that the measured shape of the B{sup 12}. Kurie function is the same within statistics as that obtained elsewhere with a ring-focus magnetic-lens spectrometer