Measurement of Neutron Activation from a High Energy Varian Linear Accelerator

摘要

Linear accelerators producing photons above 10 MeV may induce photonuclear reactions in high Z components of the accelerator. These liberated neutrons can then activate the structural components of the accelerator and other materials in the beam path through neutron capture reactions. The induced activity within the accelerator may contribute to additional dose to both patients and personnel. This project seeks to determine the total activity and activity per activated isotope following irradiation from a Varian medical linear accelerator at energies above 10 MeV. A Varian 21iX accelerator was used to irradiate a 30 cm x 30 cm x 20 cm solid water phantom with 15 MV x-rays. The phantom was placed at a source-to- surface distance (SSD) of 100 cm and at the center of a 20 cm x 20 cm field. Activation induced gamma spectra were acquired over a 5 minute interval after 1 and 15 minutes from completion of the irradiation. All measurements were made using a CANBERRA Falcon 5000 Portable high purity germanium (HPGe) detector. The majority of measurements were made in scattering geometry with the detector situated at 90° to the incident beam, 30 cm from the side of the phantom and approximately 10 cm from the top. A 5 minute background count was acquired and automatically subtracted from all subsequent measurements. Photon spectra were acquired for both open and MLC fields and activities for each nuclide were estimated from detector efficiencies as determined from Monte Carlo simulations. Based on spectral signatures, the following nuclides were identified: 56 Mn, 62 Cu, 64 Cu, 82 Br, 99 Mo, 122 Sb, 124 Sb, and 187 W. In all cases, estimated activities from the activation products were in the microcurie range.