Improvement of charge resolution for high Z/β particles in CR-39 nuclear track detectors by means of two-step etching technique

We verified that the two-step etching method of the combination of pre-etching with PEW (Potassium hydroxide + Ethanol + Water) solution and post-etching with NaOH solution makes drastic improvement of charge resolution for high Z/β particles (Z: nuclear charge, β: velocity) in CR-39 nuclear track detectors. Detectors were exposed to several heavy ions from Ne to Kr with the energies of <500 MeV/n at HIMAC (Heavy Ion Medical Accelerator in Chiba). The optimum etching conditions were determined to be a pre-etching of PEW solution with concentration of 60 wt% of ethanol for 40 min and a post-etching of 7N NaOH for 12 h. This resulted in a charge resolution for Kr (Z/β ∼ 60) of 0.18 c.u. (charge units). The Z/β detection threshold was increased as a function of ethanol concentration in PEW pre-etching solutions. We found that the two-step etching method would be useful technique for the precise charge identification by a single track measurement. The 60 wt% ethanol concentration in PEW would be adequate selection for the purpose of trans-iron nuclei measurement discriminating abundant light nuclei in galactic cosmic rays.