The University of Mainz and University of Basel groups of Karl-Ludwig Kratz and Friedrich-Karl Thielemann performed a comparison between the observed abundances for the stable element europium (Z=63) and the radioactive elements thorium (Z=90) and uranium (Z=92) to the calculated abundances of an r-process in a Type II supernova explosion.
This isotope is produced in a nuclear reactor from europium or enriched gadolinium.
Comparing the observed abundances for a stable element such as Europium (Z=63) and the radioactive element Thorium (Z=90) to calculated abundances of an r-process in a type II supernova explosion (as from the universities at Mainz and Basel groups of Karl-Ludwig Kratz and Friedrich-Karl Thielemann) allowed observers to determine the age of this star to about 13 billion years.