Measurement of β−-decay continuum spectrum of 138La

The LaBr₃:Ce scintillator offers the unique opportunity to study the β^- radioactive decay of ¹³⁸La, that is a 2^nd order unique forbidden transition. The ¹³⁸La isotope is one of the rarest isotopes on earth (it is present as 0.09% in natural Lanthanum) and because of its extremely long lifetime, of the order of 10¹¹ years, large amounts of ¹³⁸La are needed for the measurement of the β^- decay spectrum. In literature, only one experimental measurement is present and the results are not reproduced by the standard nuclear theory. A second measurement of the β^- continuum is presented in this work. For this measurement, two LaBr₃:Ce scintillators (3" × 3") and two different experimental techniques were used. The shape of the β^- continuum spectrum, measured down to the energy of 2-3 keV, is found to be identical to the one previously measured and published. On the other side, it is different from the theoretical published spectrum in particular below 75 keV.