A SIMPLE MAGNETIC SPECTROMETER FOR RADIOTHERAPY ELECTRON-BEAMS

A small, lightweight, single-focusing magnetic spectrometer was design ed, assembled, and tested for analysis of electron beams from radiothe rapy electron linacs. The objective was to develop a low cost, simple device that could be easily replicated in other medical centers, and t o demonstrate the practicality of individual electron counting for pre cise analysis of electron spectra. Two methods of spectroscopy have be en developed. One method consists of counting electrons individually a s a function of magnetic field setting. Electrons are deflected throug h 90 degrees in the magnetic spectrometer, through an exit slit, and i nto a scintillation detector. A second method consists of recording th e complete spectrum of electron energies from the accelerator on a str ip of film at a single magnetic field setting. A critical design eleme nt is the 10-cm long collimator for electrons entering the magnet gap, with defining apertures and scraper slits. The spectrometer's cleanli ness of transmission, energy calibration, and resolution were all test ed at 10 and 16 MeV using the nearly monoenergetic electron beam of th e accelerator at the National Research Council of Canada (NRCC). These accelerator tests, and also Monte Carlo trajectory simulations, both show that contamination of the transmitted spectrum due to scattered o r knock-on electrons is negligible. Low-energy characteristics were te sted using a Sr-90+Y-90 beta-particle source. The energy calibration o f the 90 degrees spectrometer mode was based on mapping the magnetic f ield and also electron trajectory computer simulations. That calibrati on agrees with the NRCC's own calibrated scale to 0.8% for the single- particle counting method and to 1.3% for the film method. The energy r esolution was measured to be 2% at 10 MeV, which is adequate for radio therapy linac measurements. The acceptance half angle is 0.5 degrees o r less, depending on the aperture size, which is adequate for electron angular distribution measurements within the forward cone of the elec tron beam. Used with film, the spectrometer is a simple, accurate, and highly transportable device for measuring radiotherapy electron energ y spectra.