Characterization of solid state nuclear track detectors to search for heavy ions and strange lets in cosmic rays

Search for rare, exotic particles (e.g. strangelets) in cosmic rays is an active field of research. One way to look for such rare events is to set up large area detector arrays at high mountain altitudes. An ideal choice of detector for such a setup would be solid state nuclear track detectors (SSNTDs), as they do not require power for their operation and also due to their ruggedness and existence of natural thresholds for registration. Use of SSNTDs in charged particle detection is a well established method. But none of widely used standard SSNTDs (e.g. CR-39, Lexan) is suitable as a detector for rare event search in cosmic rays because of their low detection thresholds, which will lead to the recording of a huge low-Z background. So a commercially available polymer, identified as polyethylene terephthalate (PET), was investigated for its suitability as an SSNTD. PET was found to have a higher detection threshold compared to other standard SSNTDs. Systematic studies were carried out to characterize and calibrate PET by determining its ideal etching condition and also its charge response to beams of known energies of 16O, 32S, 56Fe ions from pelletron accelerator at IUAC, New Delhi, India; 129Xe, 78Kr, 49Ti ions from REX-ISOLDE at CERN, Geneva, Switzerland; 238U from GSI, Darmstadt, Germany and also to fission fragments from natural radioactive sources. In addition, pilot studies were carried out in which PET films were given open air exposures at high altitude locations like Darjeeling (2200m), Ooty (2200m) and Hanle (4500m). The results of all these studies firmly establish PET as an SSNTD with a detection threshold higher than other standard SSNTDs and the ability to eliminate the dominant low Z background makes PET particularly suitable as a detector for rare event search in cosmic rays.