Nuclear matrix elements in the 2.14 MeV beta group in ¹³⁹Ba

Abstract

The spectral shape and beta-gamma directional correlation of the 2.14 MeV(7/2^-→5/2⁺) beta-group in the decay of ¹³⁹Ba have been studied. The spectral shape measured in coincidence with the 166 keV γ-ray shows a large deviation from a statistical shape. The beta-gamma directional correlation coefficient A₂ varies from 0.01 to 0.06 in the energy region 0.5 to 2.0 MeV. The ratios of relevant nuclear matrix elements have been obtained from these two measurements. The absolute values of the matrix elements involved have been calculated from the known ft value. The matrix element ∫ir has been found to be 3.9×10^-3 (in natural units) as compared to a value of 7.1×10^-3 calculated on the basis of the single-particle shell model. The structure of the 5/2⁺ state in ¹³⁹La is discussed on the basis of these results.