Reference value of the JNdi-1 isotopic material without normalization†

Abstract

The most used international reference material for neodymium isotope ratios is the JNdi-1 standard. The literature reference values were determined using Thermal Ionization Mass Spectrometry (TIMS) with a conventional internal normalization. In nuclear studies, such normalization is not possible for samples after irradiation, as there is no known isotope ratio that can be considered as a reference ratio. Nd isotopic analysis is essential for calculating the burnup of a reactor. To offer reference values without normalization, 61 measurements of the JNdi-1 material were obtained in three different laboratories on four thermal ionization mass spectrometers using the total evaporation method. Acquired measurements were compared to the exponential mass fractionation law demonstrating that the dominant bias comes from isotope fractionation which can be minimized using the total evaporation method. The suggested reference values and associated uncertainties with a coverage factor of 2, which indicates approximate 95% confidence, were calculated using the DerSimonian–Laird procedure (n = 3): ¹⁴²Nd/¹⁴⁴Nd = 1.13966(23), ¹⁴³Nd/¹⁴⁴Nd = 0.511613(50), ¹⁴⁵Nd/¹⁴⁴Nd = 0.348729(33), ¹⁴⁶Nd/¹⁴⁴Nd = 0.72329(15), ¹⁴⁸Nd/¹⁴⁴Nd = 0.242505(95) and ¹⁵⁰Nd/¹⁴⁴Nd = 0.23780(14). All these ratios are significantly different from those obtained after normalization using ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219. The new values obtained for the JNdi-1 can be used in nuclear laboratories where the Nd isotope ratios differ from the natural isotopic compositions or when the total evaporation method is used without internal normalization.