The $2_1^+ \to 3_1^+$ gamma width in $^{22}$Na and second class currents

Abstract

Background: A previous measurement of the β − γ directional coefficient in 22 Na β decay was used to extract recoil-order form factors. The data indicate the requirement of a significant induced-tensor matrix element for the decay. This conclusion largely relies on a Standard-Model-allowed weak magnetism form factor which was determined using an unpublished value of the analog 2 + 1 → 3 + 1 γ branch in 22 Na, with the further assumption that the transition is dominated by its isovector M 1 component.Purpose: To determine the 2 + 1 → 3 + 1 width in 22 Na in order to obtain an independent measurement of the weak magnetism form factor for the β decay.Methods: A 21 Ne(p, γ) resonance reaction on an implanted target was used to produce the first 2 + state in 22 Na at Ex = 1952 keV. Deexcitation γ-rays were registered with two 100% relative efficiency high purity germanium detectors.Results: We obtain for the first time an unambiguous determination of the 2 + 1 → 3 + 1 branch in 22 Na to be 0.45(8)%.Conclusions: Using the Conserved Vector Current (CVC) hypothesis, our branch determines the weak magnetism form factor for 22 Na β decay to be |b/Ac1| = 8.7(1.1). Together with the β − γ angular correlation coefficient, we obtain a large induced-tensor form factor for the decay that continues to disagree with theoretical predictions. Two plausible explanations are suggested.