Control of the detection threshold of CR-39 track detector for the measurement of ultra heavy nuclei in galactic cosmic rays

The chemical compositions of ultra heavy (UH, Z &#179; 30) nuclei in galactic cosmic rays (GCRs) could determine the nucleosynthesis processes leading to cosmic ray matter and governing evolution of the galaxy, and could lead to our understanding of the structure and properties of the interstellar medium.The observation of UH nuclei in GCRs requires a large exposure area of the detector array because of their extremely low fluxes. It is realized by the use of solid-state track detector such as CR-39 track detector with an advantage of easy extension of exposure area. It is important to increase the detection threshold of heavy ions in order to suppress background tracks produced by lighter ions with Z/b < 30. PEW-x solution (15wt%KOH+xwt%C2H5OH+(85-x)wt%H2O) is known to make track registration sensitivity of the CR-39 track detector desensitive. The sensitivity of track registration for CR-39 track detector etched by PEW-x solution was verified using various heavy ions from HIMAC accelerator. It was found that the track registration sensitivity was declined comparing with a normal etching condition (7 mol/l NaOH) and that the Z/b detection threshold has shifted to a large Z/b with concentration (x) of C2H5OH. In addition to the desensitization effect, the etching solution markedly improves the quality of the etched surface of the detector. Therefore, when we use PEW-x solution as an etchant, we expect to see not only improvement of the etched surface condition but also an increase of the Z/b detection threshold of the detector.24th International Conference on Nuclear Tracks in Solid