H. R. Froelich

A Variable Energy Racetrack Microtron

A racetrack microtron whose energy may be varied continuously from 4.5 to 18 MeV has been built at the University of Western Ontario, and has been operated to full energy. Electrons from an external electron gun are injected with an electrostatic inflector, and accelerated in a short sidecoupled linac section. The RF power delivered to the linac section can be varied by means of a power splitter, thus allowing the energy gain in the section to be adjusted continuously from 1.5 to 3 MeV. The accelerator can be operated in three different modes: single traversal of the linac section with a final beam energy of 1.5 to 3 MeV, n = 2 with three traversals of the linac covering the final energy range from 4.5 to 9 MeV and n = 1 with six traversals of the linac and a corresponding energy range of 9 to 18 MeV. The magnetic guide field consists of two 180° magnets with alternating high and low field regions. To satisfy the resonance conditions the average induction can be varied up to 0.6 T and the lengths of the drift space between the magnets can be adjusted by moving the pole pieces of the magnets.

Performance of a Multicavity Racetrack Microtron

Measured characteristics of both the extracted beam and the beam within the accelerator are given for a racetrack microtron with a three-cavity accelerating structure. Final beam energies of 9 to 15 MeV with currents up to 30 mA have been achieved by passing the beam six times through the cavities and beam energies of 4.5 to 7.5 MeV with currents up to 60 mA have been obtained after three traversals of the cavities. An improved injection system and magnetic guide field design are described, and the suitability of this type of machine for radiation therapy is considered.

Four-Sector Racetrack Microtrons

Racetrack geometries of the guide field are capable of freeing the microtron type of electron accelerator from its inherent limitations. Two completed racetrack microtrons of 2.5 and 6.3 MeV energy are described. A possible application of the design features of these accelerators to microtrons of up to 50 MeV is discussed. Preliminary results of a design study for a 200 MeV racetrack microtron are presented.

Design of a Shuttle Microtron for Radiation Therapy

A design proposal for a new type of electron accelerator for radiotherapy applications is presented. The three-pass, fixed-magnet shuttle microtron is designed to allow continuous variation of electron beam energy from 4 to 24 MeV, while providing maximum current levels for x-ray treatment at 6, 12, and 20 MeV. The reflector magnets are structured for both axial and radial beam focusing and the entire accelerator fits into the horizontal arm of a rotatable therapy gantry.

Annular‐cathode electron gun for in‐line injection in a racetrack microtron

A compact annular‐cathode electron gun which allows direct, efficient injection into the accelerating structure of a racetrack microtron has been designed, built, and tested. The gun operates under pulsed conditions with applied high voltages of 40 kV or more and delivers an output current in excess of 1 A. Design and construction details are presented for both a basic gun and a gun with built‐in output current monitor. Gun performance in a test chamber and in the multicavity racetrack microtron at the University of Western Ontario is described.

Current‐voltage characteristics of n‐amorphous low‐pressure chemical vapor deposited silicon films on p‐crystalline silicon

Heterojunction devices have been fabricated by a low‐pressure chemical vapor deposition technique whereby n‐type amorphous or microcrystalline silicon films were grown on p‐type crystalline silicon substrates. Heterostructures produced under various conditions of thin film deposition were subjected to detailed I‐V curve analysis. It is seen that for amorphous‐crystalline heterojunctions the current transport is through tunneling in the low bias range and limited by electron‐hole recombination in the high bias range. For the microcrystalline‐crystalline junctions however, recombination current at the interface dominates the current transport process. Illuminated I‐V curves corresponding to films deposited at different substrate temperatures (Ts ) and dopant gas‐to‐silane ratios (R) show that the high values of the short‐circuit current (Isc), open‐circuit voltage (Voc ), and fill factor (FF) are achieved at values close to Ts ∼620 °C and R ∼4×10−3 in conformance with dark I‐V characteristic data.

Energy Spectrum of the Electron Beam in a Racetrack Microtron

The energy spectrum of the beam of a four‐sector racetrack microtron has been measured here, using a high‐resolution double focusing beta‐ray spectrometer. For an energy gain per orbit of 750 keV, it was found that 50% of the integrated current in the eighth orbit (6 MeV) was contained in an energy interval of 34 keV and 90% of the beam current in a 73‐keV interval. This narrow energy spread (±0.3%) is of particular interest in the generation of submillimeter radiation and in the use of the microtron as an injector for high‐energy synchrotrons.