The reaction 25Mg (p,[gamma]) 26Al (II conclusions)

Abstract

The measurements described in the preceding paper (I) are used to obtain the position, spin and isobaric spin of energy levels in 26Al.

All the observed intensive high-energy γ-rays can be explained as transitions from resonance levels to the levels found by Browne 7) from the 28Si(d, a) 26Al reaction and to one additional level at Ex = 2.57 ± 0.04 MeV. The low-energy γ-rays fit the same level scheme, plus one more additional level at Ex = 0.235 ± 0.009 MeV. The latter level is actually the first excited state in 26Al, which turns out to be an isomeric state decaying by β+ emission with the long known half life of 6.6 sec.

Spins J, parities and isobaric spins T can be assigned as follows: Ex = 0 (J = 5+, T = 0), Ex = 0.235 MeV (J = 0+, T = 1), Ex = 0.419 MeV (J = 3+, T = 0), Ex = 1.055 MeV (J = 1+, T = 0), Ex = 1.750 MeV (J = 2+, T = 0), Ex = 2.064 MeV (J = 2+, T = 0). The resonance level at Ex = 6.73 MeV has J = 4−, T = 0. Tentative assignments to the other resonance levels will be discussed.

The 6.6 sec β+ decay is remarkable by being one of the few known 0+ → 0+ transitions. The β+ endpoint can best be arrived at by using a cycle involving the 0.820 MeV γ-ray from 25Mg(p, γ)26Al, the Q-value of the 28Si(d, a) 26Al transition to the 1.055 MeV level in 26Al, and a reevaluation of the 28Si-26Mg mass difference. This yields Eβ+ = 3.225 ± 0.015 MeV, and ft = 3200 ± 80 sec. The ft value of this β+ transition can now be used for direct evaluation of the Fermi coupling constant gF. The result is: gF = (1.391 ± 0.017) × 10−49 erg cm3.