Abstract
The Telescope Array (TA) cosmic rays detector located in the State of Utah in the United States is the largest ultra high energy cosmic rays detector in the northern hemisphere. The Telescope Array Low Energy Extension (TALE) fluorescence detector (FD) was added to TA in order to lower the detector's energy threshold, and has succeeded in measuring the cosmic rays energy spectrum down to PeV energies. In this contribution we describe a measurement of the cosmic rays composition using TALE FD data collected over a period of $\sim4$ years. TALE FD data is used to measure the $X_{max}$ distributions of showers seen in the energy range of $10^{15.3}$ - $10^{18.3}$~eV. An increase in the $X_{max}$ elongation rate is observed at energies just above $10^{17}$~eV indicating a change in the cosmic rays composition from a heavier to a lighter mix of primaries.
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