Correction factors for the ISO rod phantom, a cylinder phantom, and the ICRU sphere for reference beta radiation fields of the BSS 2

Abstract

The International Organization for Standardization (ISO) requires in its standard ISO 6980 that beta reference radiation fields for radiation protection be calibrated in terms of absorbed dose to tissue at a depth of 0.07 mm in a slab phantom (30 cm x 30 cm x 15 cm). However, many beta dosemeters are ring dosemeters and are, therefore, irradiated on a rod phantom (1.9 cm in diameter and 30 cm long), or they are eye dosemeters possibly irradiated on a cylinder phantom (20 cm in diameter and 20 cm high), or area dosemeters irradiated free in air with the conventional quantity value (true value) being defined in a sphere (30 cm in diameter, made of ICRU tissue (International Commission on Radiation Units and Measurements)). Therefore, the correction factors for the conventional quantity value in the rod, the cylinder, and the sphere instead of the slab (all made of ICRU tissue) were calculated for the radiation fields of 147 Pm, 85 Kr, 90 Sr/ 90 Y, and, 106 Ru/ 106 Rh sources of the beta secondary standard BSS 2 developed at PTB. All correction factors were calculated for 0˚up to 75˚(in steps of 15˚) radiation incidence. The results are ready for implementation in ISO 6980-3 and have recently been (partly) implemented in the software of the BSS 2.

Conversion coefficients from absorbed dose to tissue to the newly proposed ICRU/ICRP operational quantities for radiation protection for beta reference radiation qualities

“…Especially for low energy betas, the scattering and absorption on the longer path, α > 0 • , compared to the normal incidence, α = 0 • , result in smaller doses, especially for h pD (3; α) slab . In contrast, for h pD (0.07; α) slab the values for E mean ≳ 0.5 MeV and α > 0 • are larger than unity due to the dose build-up in the longer path length [16]. This is not the case for α = 75 • as for that large angle the absorption dominates the much longer path length in the phantom compared to the normal incidence.…”

“…at 15 cm in front of the source, by storing the position, direction, energy and type of particle (electron or photon) of millions of particles. Further details of the simulations are contained in the author's earlier work [16].…”

“…no curvature of the surface of the two slabs and an almost negligible curvature of the pillar's surface, see corresponding parts of figure 1. For 45 • and 60 • the ratios for the dose in local skin in the rod are, for E mean ≳ 0.5 MeV, decreasing below unity due to the smaller path length through the rod phantom compared to the slab for H p (0.07;α) resulting in less dose build-up [16]. As for low-energy betas, i.e.…”

“…The geometries are for • H p (0.07; α) slab and H p (3; α) slab : a slab (30 cm × 30 cm × 15 cm) made of ICRU four-element tissue with the scoring volume a cuboid parallel to the surface between the depths of 60 and 80 µm (for H p (0.07)) and 2990 and 3010 µm (for H p (3)), 10 mm high (z-direction, rather long as this is parallel to the rotation axis for α ̸ = 0 • ) and 0.411 mm wide (x-direction, rather small as this direction is perpendicular to the rotation axis for α ̸ = 0 • ); the centre of its front is located at the reference distance of the radiation field surrounded by a cube made of dry air with 1.2 m × 1.2 m × 1.2 m with the surface of the phantom in the geometrical centre of the air volume (this is the same geometry used earlier for that purpose [16]); • D lens (α): a whole-body eye model with scoring volumes the lenses of the eyes and with the maximum value of the absorbed dose in the right and left lens taken for the radiation incident from the left and the right [1,19,20]; the front of the eyes is located at the reference distance of the radiation field surrounded by a cuboid made of dry air (density and composition: see above) with 2.0 m × 2.0 m × 3.0 m (x-, y-, and z-direction) with the phantom in the geometrical centre of the air volume shifted in the z-direction to locate the eyes at the height of the centre of the radiation field; the surrounding air volume is larger here as the phantom is significantly larger than the others; • D local skin (α) slab : a slab with the dimensions of 30 cm × 30 cm × 14.8 cm of ICRU fourelement tissue, the front surface that is covered with a 2 mm layer of skin with the scoring volume a right circular cylinder with its axis perpendicular to the surface between the depths of 50 and 100 µm and a cross-sectional area of 1 cm 2 [1]; the centre of its front is located at the reference distance of the radiation field surrounded by a cube made of dry air with 1.2 m × 1.2 m × 1.2 m with the surface of the phantom in the geometrical centre of the air volume (this is the same geometry used earlier for that purpose [16]); • D local skin (α) pillar : a pillar with the dimensions of 69 mm in diameter and 300 mm in length of ICRU four-element tissue (ρ = 1.11 g cm −3 and composition as above), the cylindrical surface that is covered with a 2 mm layer of skin with the scoring volume at the half-length of the cylindrical surface between the radii 36.4 and 36.45 mm with an area circle of 1 cm 2 projected onto the upper and lower cylindrical surfaces, perpendicular to the radii [1]; the centre of its front is located at the reference distance of the radiation field surrounded by a cube made of dry air with 1.2 m × 1.2 m × 1.2 m with the surface of the phantom in the geometrical centre of the air volume (this is the same geometry used earlier for that purpose [16]) and for • D local skin (α) rod : a rod with the dimensions of 15 mm in diameter and 300 mm in length of ICRU four-element tissue (ρ = 1.11 g cm −3 and composition as above), the cylindrical surface that is covered with a 2 mm layer of skin with the scoring volume at the half-length of the cylindrical surface between the radii 9.4 and 9.45 mm with an area circle of 1 cm 2 projected onto the upper and lower cylindrical surfaces, perpendicular to the radii [1]; the centre of its front is located at the reference distance of the radiation field surrounded by a cube made of dry ai...…”

“…As pointed out above, the same simulation method was used earlier [16] except with different phantoms. Therefore, the validation of the calculations presented earlier [16] is also valid for the calculations performed in this work.…”