SU‐E‐T‐66: A Particle‐Counting Method to Determine Electron Stopping Powers

Purpose: To evaluate a particle‐counting method to experimentally determine electron stopping powers for application in primary standards and dosimetry protocols for megavoltage reference dosimetryMethod andMaterials:An electron linear accelerator was modified to operate in single‐electron‐per‐pulse operation (i.e., on average, less than one electron per rfpulse). A HPGe detector system was then used to measure the energy of electrons emerging from the accelerator. Thin plates of absorbing material (< 0.5 gcm‐2) were then placed between the exit window and detector andthe emerging electron spectrum was re‐acquired. Initial measurements were made at two energies of 4 MeV and 6 MeV with two different absorbing materials ‐ aluminum and graphite. Up to eight thicknesses of absorber were used for aluminum and four or five for graphite. Results: The electron spectrum emerging from the accelerator was found to have a FWHM of around 70‐100 keV and the detector repeatability in measuring the peak wasaround 5 keV. A peak‐fitting routine was used to determine the peak energy, E, and FWHM of the electron spectrum for each thickness, t, of absorber and thus determine the parameter dE/dt, which is related to the electron stopping power. The standard uncertainty in the determination of dE/dt was in the range 1% to 1.7%. The large uncertainty was due to the limited number of data points, a coarse MCA and low count totals (limited acquisition time and low detector efficiency). Conclusion: The initial measurements demonstrated the possibilities of the approach but highlighted a number of deficiencies in the equipment. A new HPGe system is being commissioned with an optimized detector geometry and high‐resolutionMCA. Combined with increased runtimes it should be possible to determine dE/dt with necessary uncertainty level (< 0.5%) for comparison withcalculated stopping powers.