The ⁴⁰Ar/³⁹Ar variant of ⁴⁰K-⁴⁰Ar dating relies on creating some ³⁹Ar in each sample by bombarding it with fast neutrons in a nuclear reactor (see figures below).  The ³⁹Ar serves as a proxy for the ⁴⁰K parent isotope; this works because the ³⁹K:⁴⁰K ratio is constant in Earth. The ³⁹Ar atoms are created when these neutrons are absorbed by ³⁹K atoms and the nuclear reaction: ³⁹K(n,p)³⁹Ar occurs.  To gauge the efficiency of the ³⁹K(n,p)³⁹Ar reaction, samples are co-irradiated in the fast neutron fluence with standard minerals whose age is known independently.  Once the samples are returned from the reactor, we measure the isotopic composition of the argon, obtain the ⁴⁰Ar/³⁹Ar ratio and solve the age equation as outlined below.  The neutron fluence efficiency parameter J is obtained by measuring the ⁴⁰Ar/³⁹Ar ratio of the standard crystals and solving the age equation for J.  With this J value, one may then solve for the age t of the co-irradiated samples.

The PowerPoint slides here take you step-by-step through the process of packaging your samples for an irradiation:

• Packaging samples for irradiation [PowerPoint]