Argon–argon dating, J factor, age spectrum, plateau age কী?

$^{40}\mathrm{Ar}/^{39}\mathrm{Ar}$ dating dates a potassium-bearing mineral by the ratio $R = {}^{40}\mathrm{Ar}^*/{}^{39}\mathrm{Ar}_{\mathrm K}$ of its radiogenic argon-40 to the argon-39 made from its potassium by neutron irradiation. The J factor of an irradiation is $J = (e^{\lambda t_m} - 1)/R_m$, found from a monitor of known age $t_m$ and measured ratio $R_m$, with $\lambda$ the total decay constant of potassium-40. When the sample is degassed in steps of increasing temperature, its age spectrum plots the age of each step against the cumulative fraction of argon-39 released. A plateau age is the weighted mean age of consecutive steps that agree within their uncertainties and release a large part of the argon-39.