Physical analysis of the radiation shielding for the medical accelerators

Journal Article · Sun Feb 01 04:00:00 EST 2009 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.3078029· OSTI ID:21186022

Li, Q F; Xing, Q Z; Kong, C C ^[1]

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1. Department of Engineering Physics, Accelerator Laboratory, Tsinghua University, Beijing 100084 (China) and Key Laboratory of Particle and Radiation Imaging of Ministry of Education, Tsinghua University, Beijing 100084 (China)

Radiation safety standards today require comprehensive shielding protection schemes for all particle accelerators. The original shielding system of BJ-20 (BeiJing-20 MeV), the high-energy medical electron linac, was designed only for the 18 MeV level. And the dose caused by the lost electrons in the 270 deg. bending magnet system was neglected. In this paper, the leakage dose of BJ-20 is carefully analyzed. The radiation leakage dose distribution of the photons coming from the accelerator head is obtained for energy levels of 6, 12, 14, and 18 MeV. The dose of the photoneutrons is especially analyzed for the 18 MeV level. The result gives that even neglecting the dose from the 270 deg. bending magnet system, the shielding system is still not enough for the energy levels lower than 18 MeV. The radiation leakage produced by electrons that are lost in the 270 deg. bending magnet system has been particularly studied. Using beam transport theory and Monte Carlo sampling methods, which have been combined in calculations, we have obtained the distribution of the energy, position, and direction of the lost electrons. These data were then further processed by the Monte Carlo N-particle (MCNP) code as input data. The results show that when the electron loss rate in the 270 deg. bending magnet system is 13.5%, the radiation leakage dose of the photons generated by the lost electrons is 0.1% higher than that at the isocenter, and the corresponding relative leakage dose of the photoneutrons reaches 0.045% around an angle of 170 deg. at 18 MeV level. Both of these parameters exceed radioprotection safety standards for medical accelerators. The original shielding design is therefore not suitable and is also incomplete since the radiation produced by the electrons being lost in the 270 deg. bending magnet system was neglected and the leakage dose for the low-energy levels was not considered in the original design. Our calculations provide a very useful tool for further optimization and design improvement that will enable this radiation shielding to conform to present day safety standards.

• OSTI ID: 21186022
• Journal Information: Journal of Applied Physics, Journal Name: Journal of Applied Physics Journal Issue: 3 Vol. 105; ISSN JAPIAU; ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
BEAM TRANSPORT
DOSIMETRY
ELECTRON LOSS
ELECTRONS
ENERGY LEVELS
LINEAR ACCELERATORS
MAGNETS
MEV RANGE 10-100
MONTE CARLO METHOD
PARTICLE BEAMS
PHOTONEUTRONS
PHOTONS
RADIATION DOSE DISTRIBUTIONS
RADIATION PROTECTION
SAFETY
SAFETY STANDARDS
SHIELDING