Control of the detection threshold of CR-39 PNTD for measuring ultra heavy nuclei in galactic cosmic rays

It is necessary to develop a track detector with higher detection threshold and high charge resolution for ultra heavy cosmic ray observation. The track registration sensitivity in CR-39 track detector etched by PEW (Potassium–Ethanol–Water) solution was verified using heavy ion beams at the HIMAC accelerator. It was found that PEW solution makes effectively to desensitize the track registration sensitivity in CR-39 detector. Its detection threshold of Z∗/β is successfully raised to be Z∗/β = 39 by using PEW solution with the concentration of ethanol of 45 wt%. However this produces a “fuzzy track”. A two-step process of etching involving a post-etch with 7 mol/l sodium hydroxide solution produced very clear track edges and remarkably improves the charge resolution for heavy ions.