Saturation of ion yield in 2,2,4,4 tetramethylpentane for nonrelativistic particles

The ion yield of 2,2,4,4 tetramethylpentane has been measured for electric fields between 600 V/cm and 3600 V/cm using 400 MeV/ c protons and 150 MeV/ c pions from the TRIUMF accelerator. In this measurement particle trajectories are selected to be parallel to the electric field of the ion chamber. A comparison of the ion yield for minimum ionizing particles (400 MeV/ c pions) with the ion yield for more highly ionizing particles shows a saturation of the free electron production for highly ionizing radiation. The result can be parametrized using Onsager Theory and a Birks factor, k_B = 0.176±0.025 cm/MeV for values of d E/d x between 1.59 and 7.49 MeV/cm. A calculation of the Birks factor from the free ion yields at each electric field gives a field dependence of k_B = 0.222±0.014 cm/MeV at 600 V/cm dropping to k_B = 0.141±0.021 cm/MeV at 3625 V/cm, where it is consistent with data measured by the UA1 collaboration using higher electric fields.