Observations of short-range, high-LET recoil tracks in CR-39 plastic nuclear track detector by visible light microscopy

Using standard visible light microscopy, we are able to observe particle tracks produced by <10 μm range target fragment recoils in CR-39 plastic nuclear track detector (PNTD) following short chemical etching (bulk etch B ≤1 μm). In accelerator irradiations, targets of varying composition, including a number of elemental targets of high Z, were exposed in contact with layers of CR-39 PNTD to beams of 60 MeV, 230 MeV, and 1 GeV protons at doses of 10-50 Gy. Chemical etching of CR-39 under standard conditions (50 oC, 6.25 N NaOH) for 2-4 h (removed layer B = 0.5-1.0 μm) yielded secondary track densities of 105-106 cm-2 observable under a standard optical microscope with 500x-800x magnification. Ordinarily such a short duration etch would not be expected to enlarge the tracks sufficiently for them to be resolved by visible light optics. However, due to the short-range of the particles, a longer chemical processing would have over-etched the tracks until they were no longer recognizable. The tracks we observe in CR-39 PNTD irradiated in these experiments are the result of residual heavy recoil fragments returning to equilibrium via evaporation processes following proton-induced knock out of light particles via preequilibrium processes. Because the heavy recoil particles are very near the end of their ranges (i.e. in the Bragg peak), their LET is extremely high and changes rapidly. Consequently, the tracks they produce in CR-39 PNTD often take the form of long tubes rather than the conical etch pits produced by higher energy particles.