An alternative method for chronometric determinations involving curium

Abstract

A method for quantifying ratios of isotopes of plutonium (Pu), americium (Am), and curium (Cm) using inductively coupled plasma mass spectrometry (ICP-MS) is described that does not require radiochemical separations or a chemical yield monitor. This approach provides more rapid analysis, which is important for chronometric applications related to nuclear forensics analysis. To demonstrate its utility, we used it to quantify the ingrowth ²⁴⁰Pu (t_1/2 = 6563 years) from ²⁴⁴Cm (t_1/2 = 18.10 years) in a solution of unknown “age” (e.g. time since last separation). Results are compared to similar samples for which the time since separation was known. In addition, alpha spectrometry was used to validate the ICP-MS measurements. In this case, ²³⁸Pu and ²⁴¹Am were used as chemical yield monitors for ²⁴⁰Pu and ²⁴⁴Cm, respectively. The relative standard deviation for the isotope ratio method using ICP-MS was slightly greater than the traditional radiometric approach, but sufficient for this application. Measured activity ratios of ²⁴⁰Pu and ²⁴⁴Cm provided an age for the unknown sample that linked it to research activities involving the production of curium isotopes for thermoelectric heat sources during the late 1970's.