Geochronology/Argon–argon dating

The age of a sample is given by the age equation:
 * $$t=\frac{1}{\lambda} \ln (J \times R+1)$$

where λ is the radioactive decay constant of 40K (approximately 5.5 x 10−10 year−1, corresponding to a half-life of approximately 1.25 billion years), J is the J-factor (parameter associated with the irradiation process), and R is the 40Ar*/39Ar ratio. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of 39K to 39Ar than a less dense one.

One problem with argon-argon dating has been a slight discrepancy with other methods of dating. A correction of 0.65% is needed.