A re-investigation of the geometry factors for fission-track dating of apatite, sphene and zircon

The geometry factors that apply to fission-track dating of zircon, sphene, apatite using mica external detectors were re-investigated, The ideal geometry factor (2 pi/4 pi) is described by (R+d)/2R, where R is the etchable range of a fission fragment in the mineral and d is an unetchable range related to differences in the registration threshold between the sample and the detector. The geometry factors for zircon, sphene, and apatite, were experimentally determined to amount to 0.68, 0.60, and 0.55 respectively and are consistent with the ideal values for each mineral. Zeta age-calibration factors have been determined for the external detector method using zircon, sphene and apatite age standards and for the population method using apatite age standards. The directly obtained zeta values are quite variable but the geometric corrections yield an identical zeta baseline for the two methods and for the three minerals. Shortening of spontaneous tracks in the apatite standards was also corrected for. These results suggest that fission-track age determinations using the absolute approach may be within reach for both the population method and the external detector method provided that the right correction for the geometry factor is made. It finally follows that the different values that have been found for the decay constant of spontaneous fission of U-238 may be related to technical differences between the conventional external detector and population methods.