Samarium-147 (¹⁴⁷Sm 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 1.06×10¹¹ years, although this can range from 1.05×10^11[1] to 1.17×10^11[2] years. It is mainly used in radiometric dating.^[3]

Samarium-147 is used in samarium–neodymium dating. The method of isochron dating is used to find the date at which a rock (or group of rocks) are formed.^[4] The Sm-Nd isochron plots the ratio of radiogenic ¹⁴³Nd to non-radiogenic ¹⁴⁴Nd against the ratio of the parent isotope ¹⁴⁷Sm to the non-radiogenic isotope ¹⁴⁴Nd. ¹⁴⁴Nd 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:

${\displaystyle \left({\frac {{}^{143}\mathrm {Nd} }{{}^{144}\mathrm {Nd} }}\right)_{\mathrm {present} }=\left({\frac {{}^{143}\mathrm {Nd} }{{}^{144}\mathrm {Nd} }}\right)_{\mathrm {initial} }+\left({\frac {{}^{147}\mathrm {Sm} }{{}^{144}\mathrm {Nd} }}\right)\cdot (e^{\lambda t}-1),}$

where:

t is the age of the sample,

λ is the decay constant of ¹⁴⁷Sm,

(e^λt−1) is the slope of the isochron which defines the age of the system.

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 Samarium–neodymium dating#The CHUR model).^[4][5]

• Isotopes of samarium