The α-particle energies of U234, U238, and Th232 have been measured with a grid ionization chamber. The electronic equipment was designed to provide low noise, high resolution, and good long term stability. Ra226, Em222, Po218, Po214, and Po210 were used as energy standards. An investigation was made of the corrections to the measurements due to variation in pulse rise-time, source thickness, and imperfect shielding of the collector by the grid. It was found that the experimentally observed resolution was satisfactorily accounted for by the combination of these factors and the noise and ionization straggling.Alpha particle pulse heights were measured in terms of the output of a precision pulse generator. The corrected pulse heights were related to the energies of the standard sources by a two-parameter least squares calculation; the standard deviation was never greater than 5 kev. On extrapolating to zero ionization the line so obtained intercepted the energy axis at 83 kev. The α-particle energies deduced using this line were: U234 4.768 ± 0.003, U238 4.195 ± 0.005, Th232 4.007 ± 0.005 Mev.
EP-1497: High resolution air-vented ionization chamber array for QA of VMAT and stereotactic treatments