Beta spectrum of 140 La

The β spectrum of140La has been analysed with a thoroughly tested Slatis-Siegbahn β-ray spectrometer equipped with a well-type plastic whose counting efficiency is unity down to 50 keV. The F.K. analysis in which the outer group is properly shape-corrected yields β groups: (2164±2) (8%), (1680±7) (18%), (1365±10) (46%), (1150±5) (19%), (857±15) (4%) and (510±20) (5%) keV. The outer β group with end point 2164 keV has « modifiedBij shape » withD =2.3 ± 2, indicative of complete cancellation in the matrix element combinationY. The experimental shape supports the (g7/2) J =2/2 proton configuration to the 2+ first excited level of140Ce, but a quantitative agreement is possible only when the CVC condition is relaxed.