Observation of trans-iron nuclei in galactic cosmic rays (Z ≥ 30) requires a high performance cosmic ray detector telescope with a large exposure area because of their extremely low fluxes. A solid-state track detector such as CR-39 has the advantage of easy extension of exposure area, but it is necessary to raise the Z / β detection threshold in order to suppress background tracks produced by galactic cosmic rays with Z / β < 30. The track registration sensitivity and detection threshold for heavy ions with various Z β were verified by using track detectors of copolymers of CR-39 and DAP (diallyl phthalate) with various DAP concentration. We found that it is possible to control the Z / β detection threshold of copolymers by changing DAP concentration. In order to detect heavy ions with Z / β ≥ 30, the optimum DAP concentration was found to be approximately 50%.
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