Structural shielding design and evaluation for megavoltage x- and gamma-ray radiotherapy facilities

The invention provides an X-ray source having a generator for generating an electron beam, an accelerator for accelerating the generated electron beam in a desired direction, one or more magnetic elements for transporting portions of the electron beam in a more than one desired direction, and a shaped target made from a material having an atomic number lying within a predetermined range of values, the transported parts of the electron beam producing a fan beam of X rays upon striking the shaped target.

System and method for x-ray source weight reduction

There is an increased interest in determining the photon reflection coefficient for layered systems consisting of lead (Pb) and concrete. The generation of accurate reflection coefficient data has implications for many fields, especially radiation protection, industry, and radiotherapy room design. Therefore, this study aims to calculate the reflection coefficients of photons for various lead thicknesses covering the concrete. This new data for lead, layered over concrete, supports various applications, such as an improved design of the mazes used for radiotherapy rooms, which helps to reduce cost and space requirements. The FLUKA Monte Carlo code was used to calculate photon reflection coefficients for a concrete wall with different energies. The reflection coefficient was also calculated for a concrete wall covered by varying thicknesses of lead to study the effect of lining this metal on the concrete wall. The concrete's reflection coefficient data were compared to internationally published data and showed that Monte Carlo calculations differed significantly from some of the extrapolated data. The absorbed dose of backscattered photons for various thicknesses of lead covering the ordinary concrete has been tabulated as a function of the reflection angle. Also, the reflection coefficient as a function of the Pb thicknesses covering the ordinary concrete has been figured to study the dose reduction factor. The generation of accurate data for reflection coefficients is vital for many fields, especially for radiation protection and radiotherapy room design. The new data have been presented for lead layered over concrete in various applications, such as an improvement in the design of the mazes used for radiotherapy rooms, thereby reducing the cost and space requirements. In addition, the Monte Carlo method enables calculating the energy distribution of reflected photons, and these were shown for a range of angles.

/pdf/monte-carlo-simulation-of-photons-backscattering-from-6wsssl5gy2.pdf

Monte Carlo simulation of photons backscattering from various thicknesses of lead layered over concrete for energies 0.25-20 MeV using FLUKA code.

Stainless steel has a robust microstructure that makes it an excellent material for shielding in a range of radiation applications. Thus, knowledge of backscattering photons and photon reflection coefficient is required to minimise radiation risks to passers-by and workers in close proximity to the radiation region. The stainless steel utilised in this research is composed of a blend of iron (74%), chromium (18%), and nickel (8%). The FLUKA Monte Carlo algorithm was used to study the effect of photon reflection coefficients at various energies and for various stainless steel thicknesses. The data was tabulated as a function of the angle of reflection. Moreover, the reflection coefficient was calculated as a function of different energies to determine the dose reduction factor compared to standard shielding concrete. Regarding the reflection coefficient values, the saturation depth of the stainless steel is at a thickness of 2 cm. Therefore, there is no more increase in photon backscattering behind this thickness. Also, stainless steel showed better radiation shielding properties for less than 2.8 MeV incident energies. 

Photon backscattering for various stainless steel thicknesses from 0.25 to 20 MeV using Monte Carlo simulation FLUKA code

The angle of obliquity is used in radiation shielding calculations to account for the longer path length x rays will see when obliquely incident on the protective barrier. According to the National Council on Radiation Protection and Measurements (NCRP), use of the angle of obliquity is explicitly assumed for primary radiation, so that an angle of obliquity for secondary radiation is never addressed. However, in the example section of the latest report, it specifically recommends against using an angle of obliquity for scattered radiation. To check this assumption, the existence or not of an angle of obliquity for scattered radiation has been investigated for bremsstrahlung x-ray beams of 4, 6, 10, 15, and 18 MV and for barriers consisting of concrete, lead, and steel using a Monte Carlo approach. The MCNP Monte Carlo code, v4.2C, has been used to generate scattered radiation at 30 degrees from a water phantom and incident on a secondary barrier at the same angle relative to the normal to the barrier. The barrier thickness was increased from zero to a thickness sufficient to reduce the fluence (f4 tally) to <10(-3). A transmission curve was created for each energy-barrier material combination by normalizing to zero thickness. The results for the first tenth-value layer (TVL) in concrete (5 energies) show an average angle of obliquity of 21.7 degrees +/- 5.6 degrees , and for the first two TVLs averaged 29.7 degrees +/- 3.9 degrees . The results for the first TVL in lead (3 energies) show an average angle of obliquity of 27.7 degrees +/- 4.0 degrees , and for the first two TVLs averaged 20.5 degrees +/- 5.8 degrees . There are no data in the NCRP reports for 30 degrees scattered radiation attenuated by steel with which to make a comparison.

John R. Styczynski

Papers

Do angles of obliquity apply to 30° scattered radiation from megavoltage beams?