Analysis of short-range tracks and large track fluences in CR-39 PNTD using atomic force microscopy

The standard method of analysis used with CR-39 plastic nuclear track detector (PNTD)— chemical etching followed by visible light microscope scanning—is limited to fluences less than 105 cm−2 and to particles of range ∼<8 μm (the minimum bulk etch needed to enlarge a track sufficiently to be measured using visible light). At fluences above 105 cm−2 the tracks begin to overlap, making analysis difficult. High-LET heavy nuclear recoil fragments often have ranges of 1–10 μm and bulk etch ≥8 μm results in over-etched tracks that are difficult to interpret. Both of these issues can be resolved by using a short etch (2–4 h 50 °C, 6.25 N NaOH) followed by atomic force microscopy (AFM) analysis. The dimensions of the post-etch tracks are typically a few hundred nanometers, a size within the resolution of an AFM. Because AFM provides a 3-D topographical map of the post-etch PNTD surface, there is more useful information contained in an AFM image than in a standard image of the post-etch CR-39 surface obtained using an optical microscope and CCD camera. We are developing a method based on AFM scanning, followed by matrix analysis (as opposed to image processing), which allows us to directly extract the relevant geometrical parameters of the tracks in an AFM image. This method is also amenable to automation. Progress in developing this method is illustrated with results from AFM analysis of CR-39 PNTD exposed to high fluences of energetic protons at the Loma Linda University Medical Center (LLUMC) Proton Therapy Facility.