Samarium-147 (<sup>147</sup>Sm or Sm-147) is an isotope of samarium, making up 15% of natural samarium. It is an extremely long-lived radioisotope, with a half-life of years, and an alpha emitter, the only significant one outside the heavy decay chains from thorium and uranium.
Samarium-147 is used in samariumâÂÂneodymium dating. The method of isochron dating is used to find the date at which a rock (or group of rocks) are formed. The Sm-Nd isochron plots the ratio of radiogenic <sup>143</sup>Nd to non-radiogenic <sup>144</sup>Nd against the ratio of the parent isotope <sup>147</sup>Sm to the non-radiogenic isotope <sup>144</sup>Nd. <sup>144</sup>Nd is used to normalize the radiogenic isotope in the isochron because it is a slightly radioactive and relatively abundant neodymium isotope.
The Sm-Nd isochron is defined by the following equation:
where:
Alternatively, one can assume that the material formed from mantle material which was following the same path of evolution of these ratios as chondrites, and then again the time of formation can be calculated (see SamariumâÂÂneodymium dating#The CHUR model).