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A Particle-Counting Method to Determine Electron Stopping Powers

M McEwen

M McEwen1*, A DuSautoy2, G Bass3, (1) National Research Council, Ottawa, ON, (2) Canadian Nuclear Safety Commission, Ottawa, ON, (3) National Physical Laboratory, Teddington, Middlesex

SU-E-T-66 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

To evaluate a particle-counting method to experimentally determine electron stopping powers for application in primary standards and dosimetry protocols for megavoltage reference dosimetry
Method and Materials:
An electron linear accelerator was modified to operate in single-electron-per-pulse operation (i.e., on average, less than one electron per rf pulse). 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 and the 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.
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 was around 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).
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-resolution MCA. Combined with increased runtimes it should be possible to determine dE/dt with necessary uncertainty level (< 0.5%) for comparison with calculated stopping powers.

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