Nonionizing Radiation Around Linear Accelerators

Abstract

The World Health Organization, the International Radiation Protection Association (IRPA), and the American Conference of Governmental Industrial Hygienists have recommended using time-weighted and time-dependent standards for nonionizing radiation exposure assessment. Such hygienic norms have been developed on the basis of the electromagnetic energy evaluation. Two Varian linear accelerators, Clinac 1800 and Clinac 6X, were examined for different nonionizing radiation hazards. The sources of possible exposures are: radiofrequency driver, klystron (Clinac 1800), waveguide, gun, modulator, magnetron (Clinac 6X), power supply, and control panel with its video display units. Such sources can radiate a wide range of radiation: 50 Hz and harmonics, radiofrequency radiation (very low and low frequencies) up to 100 kHz by short pulses and electric discharges, and microwaves with frequency f ≈ 3000 MHz. The measuring instruments used in our study were gaussmeters (Safe Professional Meters) for extremely low frequency and very low frequency magnetic fields, and RAHAM, model 40 (General Microwave Corporation) for the microwave band. The exposure assessment was made according to the IRPA recommendations (1988) using time-dependent standards. The results show high levels of microwave exposures near the magnetron of Clinac 6X: up to 6 mW/cm² on the surface of the shield. At 20 cm from the shield the exposure duration should not exceed 30 minutes per working day (calculated on the basis of the Bulgarian time-dependent standard), and at 50 cm the power densities are in conformity with the hygienic limits. No excessive exposure levels of microwave radiation were detected around Clinac 1800. The magnetic fields for frequencies below 90 kHz are under the permissible exposure levels assessed on the basis of different international and national standards. Protective measures for the maintenance, measurement, or adjustment of the linear accelerators are suggested.