Solid state nuclear track detectors (SSNTD) are useful for studying heavy ion abundances in cosmic rays at mountain altitudes because of their light weight and certain Z/Beta detection thresholds which automatically eliminates low Z background. In our work we are using a particular brand of overhead projector (OHP) films which has a very high detection threshold of Z/Beta > 140 and so are particularly suitable for studying rare events in cosmic rays viz. Strangelet detection. The detector material is identified to be polyethylene terepthalate (PET). A sensitive charge response characteristic is a prerequisite for any detector. Previously we have studied the charge response of the PET detector by impinging on it 238U - ion and 16 O- ion beams of known energies from particle accelerators. Results of further studies along this line using S and Ni – ions and a much improved calibration curve will be presented.