ANALYTICAL METHODS/Fission Track Analysis 51 Figure (A) Two thermal histories and (B) the resulting apatite fission track ages and track length distributions Despite the difference in thermal history before the onset of final cooling, both scenarios produce essentially the same apatite fission track age and track length distribution The difference in age and track length distribution is well within the uncertainty normally associated with fission track data apatite (U-Th)/He data not seem to be influenced by chemical composition Because of the high kinetic energy with which a-particles are emitted from their parent nuclides, they may be lost from grains as a result of a-ejection For apatite, the so-called a-stopping distance is $19–23 mm, depending on the parent nuclide Conditional on the distance of the parent nuclide from the physical grain boundary, a-particles may be either ejected from, or retained within grains (Figure 10A) Because a-particles are emitted in random direction from their parent nuclei, they have a 50% chance to be ejected if their parent nuclei are located on the physical grain boundary itself a-particles emitted from parent nuclei at more than the a-stopping distance from the grain boundary will never be ejected by a-emission In principle, a-particles can also be implanted into grains when they are ejected from neighbouring grains a-implantation can be ignored in most cases however, because the concentration of parent nuclei normally is much higher in minerals that are used for dating compared to the concentration in the host rock they were separated from a-ejection must be corrected for by the so-called a-emission correction to obtain a geologically meaningful ‘a-ejection corrected’ (U-Th)/He age The a-emission correction is dependent on the mineral and the appropriate a-stopping distances, grain size, and grain geometry Smaller grains require bigger corrections because a larger percentage of parent nuclei will be within a-stopping distance from the grain boundary A complicating factor for making the appropriate a-emission correction is that helium loss by ejection is interwoven with diffusional helium loss The helium diffusion domain in some minerals (e.g., apatite) coincides with the physical grain Within the HePRZ, helium will, therefore, be lost by diffusion at the grain boundary Diffusion will tend to smooth the a-retention profile resulting from a-ejection, which is illustrated in a grain cross-section (Figure 10B) Because they are interlinked, the diffusion process and a-ejection cannot be accurately modelled separately and instead must be incorporated simultaneously in a numerical model The diffusion process and its effect on the a-retention profile becomes more important with a longer residence time in the HePRZ, and application of such a model, therefore, is particularly important when working