Beta-gamma directional correlation in La¹⁴⁰

Abstract

The directional correlation between the nonunique first-forbidden outer beta group of end-point energy 2.175 MeV in La¹⁴⁰ and the 1.6-MeV cascade gamma ray in Ce¹⁴⁰ has been measured at nine beta energies. The measured beta-gamma correlation coefficient ε ranges from (0.080±0.010) to (0.111±0.020) in the beta-energy interval 1.6 to 2 MeV. Taking account of the existing data on shape correction factor and also the beta-gamma circular polarization coefficient ω for θ=160° and W=4.2, the relevant first-forbidden beta-decay matrix-element ratios have been extracted; the ratios of the matrix elements are (in Kotani's notation) x=-0.33±0.15, u=-0.06±0.06, and Y=-1.50±0.20. Absolute values of the matrix elements were obtained from the observed ft value as |∫Bij|/R=(3.3±0.5)×10^-2, |∫r|/R=(1.3±0.6)×10^-2, |∫iδ×_r|/R=(0.2±0.2)×10^-2, |∫iα|=(4.3±1.5)×10^-3. A cancellation among the vector-type matrix elements is apparently the cause of the deviation from the ξ approximation in this case.