Optical shape analysis based on discrete Fourier transform and second order moment calculation of the brightness distribution for the detection of sub-micron range low energy tracks

To recognize sub-micron range low energy tracks recorded in a super fine grained nuclear emulsion (Nano Imaging Tracker), an elliptical fitting method was devised to analyze anisotropic images taken by an optical microscope. In this paper, we will report on this newly developed method using discrete Fourier transform and second-order moment analysis of the brightness distribution. We succeeded in lowering the ellipticity threshold, thereby improving the detection efficiency and angular resolution. Notably, the success of detecting carbon 30 keV tracks is the first such achievement in the world, where the incident direction of carbon 30 keV ions was determined with an accuracy of 41 degree and an efficiency of 1.7%.