Track detector of CR-39-DAP-copolymer with variable threshold to detect trans-iron nuclei in galactic cosmic rays

Observation of trans-iron nuclei in galactic cosmic rays (Z⩾30Z⩾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/βZ/β detection threshold in order to suppress background tracks produced by galactic cosmic rays with Z/β<30Z/β<30. The track registration sensitivity and detection threshold for heavy ions with various Z*/βZ*/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*/βZ*/β detection threshold of copolymers by changing DAP concentration. In order to detect heavy ions with Z*/β⩾30Z*/β⩾30, the optimum DAP concentration was found to be approximately 50%.