Experiments and theory of ¹³⁸La radioactive decay.

We measured with unprecedented accuracy key features of the (138)La radioactive decays as β particle energy distribution from 0.5 keV to the end-point and ratios of electron capture probabilities PL/PK, PM/PK and PM/PL. This was achieved by making use of LaBr3:Ce and CeBr3 scintillator detectors. The advantage of the presented technique relies on the double role of LaBr3:Ce as source of (138)La and detector medium resulting in a relatively efficient counting statistics and unaltered β energy detection. The experimental results are compared to advanced computational techniques and significant deviation is found below 20 keV with the computational spectrum showing a 5% excess of β particle relative to the experimental spectrum at 10 keV.