[en] Precision and accuracy in the determination of alpha activity ratios of ²³⁹Pu/²³³U, ²⁴¹Am/²³³U and ²⁴⁴Cm/²³³U was evaluated by preparing synthetic mixtures from solutions of enriched isotopes of ²³⁹Pu, ²⁴¹Am, ²⁴⁴Cm and ²³³U. Alpha spectra were recorded by employing solid state silicon surface barrier detectors coupled to a 4K analyzer and the alpha spectra were evaluated by a method based on the geometric progression decrease for the far tail of the spectrum. Large area detector (i.e. 450 mm²) was observed to reduce the effect of nonhomogeneous distribution, if any, of the two elements present in the source. Precision and accuracy of about 1% is demonstrated. (author)