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Generation of the additional fluorescence radiation in the elastomeric shields used in computer tomography (CT).

Szajerski P, Zaborski M, Bem H, Baryn W, Kusiak E - J Radioanal Nucl Chem (2013)

Bottom Line: The lead equivalents for the examined composites were within the ranges of 0.046-0.128 and 0.048-0.130 mm for 122.1 and 136.5 keV photons, respectively.The results clearly indicate that among the examined compositions, the highest values DRF have been achieved with preparations containing Bi+W, Bi+W+Gd and Bi+W+Sb mixtures with gradually decreasing content of heavy metal additives in the following order: Bi, W, Gd and Sb.The respective values of DRF obtained for the investigated composites were 21, 28 and 27 % dose reduction for a 1 mm thick shield and 39 and ~50 % for a 2 mm thick layer (M1-M4).

View Article: PubMed Central - PubMed

Affiliation: Institute of Applied Radiation Chemistry, Lodz University of Technology, Wroblewskiego 15, 90-924 Lodz, Poland.

ABSTRACT

Two commercially available (EP, Z) and eight new elastomeric composites (M1-M4, G1-G4, of thickness ≈1 mm) containing mixtures of differing proportions of heavy metal additives (Bi, W, Gd and Sb) have been synthesised and examined as protective shields. The intensity of the X-ray fluorescence radiation generated in the typical elastomeric shields for CT, containing Bi and other heavy metal additives influence on the practical shielding properties. A method for assessing the radiation shielding properties of elastomeric composites used in CT examination procedures via X-ray spectrometry has been proposed. To measure the radiation reduction ability of the protective shields, the dose reduction factor (DRF) has been determined. The lead equivalents for the examined composites were within the ranges of 0.046-0.128 and 0.048-0.130 mm for 122.1 and 136.5 keV photons, respectively. The proposed method, unlike to the common approach, includes a dose contribution from the induced X-ray fluorescence radiation of the heavy metal elements in the protective shields. The results clearly indicate that among the examined compositions, the highest values DRF have been achieved with preparations containing Bi+W, Bi+W+Gd and Bi+W+Sb mixtures with gradually decreasing content of heavy metal additives in the following order: Bi, W, Gd and Sb. The respective values of DRF obtained for the investigated composites were 21, 28 and 27 % dose reduction for a 1 mm thick shield and 39 and ~50 % for a 2 mm thick layer (M1-M4).

No MeSH data available.


(a) Influence of the shield thickness on the intensities of the two different Bi concentrations (89.9 and 77.1 keV lines); (b) Effect of the shield thickness on the intensity of two W emission lines (for 58.0 and 59.3 keV photons) for composites containing Gd
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Fig3: (a) Influence of the shield thickness on the intensities of the two different Bi concentrations (89.9 and 77.1 keV lines); (b) Effect of the shield thickness on the intensity of two W emission lines (for 58.0 and 59.3 keV photons) for composites containing Gd

Mentions: The X-ray excitation spectra for the four rubber shields containing Bi at practically the same concentration as the other metal additives (0.30–0.35 mass fraction), Bi+W (0.176 W mass), Bi+W+Gd (0.160 and 0.080 mass fractions of W and Gd, respectively) and Bi+W+Gd+Sb (0.154, 0.077 and 0.031 mass fractions of W, Gd and Sb, respectively) are presented in Fig. 2. As shown in the figure, the X-ray photon intensities of 74.8, 77.1, 87.3 and 89.9 keV are observed for shield containing Bi only [25]. Additionally two energy lines that do not originate from Bi excitation, 72.8 and 84.7 keV [25], are also observed and should be assigned to Pb excitation, which is present despite strong shielding by the detector housing. A natural explanation for this fact is that lead housing excitation can arise from cosmic radiation as confirmed during the background measurements and the partial excitation of lead via the Co-57 γ photons, which although strong attenuated, were not fully absorbed by the shield. After analysis of the recorded spectra, one can conclude, that although much weaker than the photon intensity of the Co-57 source, the secondary X-ray fluorescence radiation arising from the protective shield, should be considered as a dose contributor to the irradiated tissue. The dose aspect and the quality of the registered CT scans suffer because of the additional induced X-ray fluorescence radiation. The addition of a second metallic component with a lower atomic number Z could absorb the Bi X-ray fluorescence photons. For this purpose, the addition of tungsten was examined. The presence of tungsten in the elastomeric bismuth shield leads to a substantial reduction of the all Bi line intensities as evidenced by the presented spectra. However, as a result, four new W photon emission lines are generated in the lower portion of the spectrum with energies characteristic of W excitation, 58.0, 59.3, 67.2 and 69.1 keV (M2 composite) [25]. These photons can be absorbed by the addition of another additive with a lower Z and X-ray excitation levels slightly below the energy of tungsten-emitted photons, for example, gadolinium. The addition of this element results in a reduction in the W emission intensity; however, two group of photons of 42.7 and 48.7 keV energy, corresponding to gadolinium emissions (M3) [25], are generated with lower intensities. These photons can be absorbed more easily than those emitted by Bi or W with energies of approximately 60–90 keV. A similar effect of reduced Gd X-ray emission photons could be expected after the addition of Sb (composite M4); however, as shown in Fig. 2, this result is not clear. From the other side, the low-energy Gd X-ray emission is not as important as the Bi and W fluorescence radiation emission together with the 120–140 keV Co-57 γ radiation. More detailed observations can be seen in Figs. 3 and 4 in which the data collected for particular photon emission intensities for Bi, W and Gd are presented for each composite. Given that the lower limit of photon energy visible to our REGe detector is ~40 keV, no γ-ray or X-ray photons below this value were observed, which is of concern to the Sb emission. This fact renders the measurement of photon emissions below 40 keV for Bi, W and Gd impossible. However, due to strong absorption of photons from this energy range and the reduced fluorescence yields for the excitation from shells other than K, the contributions photons at energies below 40 keV could, in practice, be neglected from the total doses outside the shields.Fig. 2


Generation of the additional fluorescence radiation in the elastomeric shields used in computer tomography (CT).

Szajerski P, Zaborski M, Bem H, Baryn W, Kusiak E - J Radioanal Nucl Chem (2013)

(a) Influence of the shield thickness on the intensities of the two different Bi concentrations (89.9 and 77.1 keV lines); (b) Effect of the shield thickness on the intensity of two W emission lines (for 58.0 and 59.3 keV photons) for composites containing Gd
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
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Fig3: (a) Influence of the shield thickness on the intensities of the two different Bi concentrations (89.9 and 77.1 keV lines); (b) Effect of the shield thickness on the intensity of two W emission lines (for 58.0 and 59.3 keV photons) for composites containing Gd
Mentions: The X-ray excitation spectra for the four rubber shields containing Bi at practically the same concentration as the other metal additives (0.30–0.35 mass fraction), Bi+W (0.176 W mass), Bi+W+Gd (0.160 and 0.080 mass fractions of W and Gd, respectively) and Bi+W+Gd+Sb (0.154, 0.077 and 0.031 mass fractions of W, Gd and Sb, respectively) are presented in Fig. 2. As shown in the figure, the X-ray photon intensities of 74.8, 77.1, 87.3 and 89.9 keV are observed for shield containing Bi only [25]. Additionally two energy lines that do not originate from Bi excitation, 72.8 and 84.7 keV [25], are also observed and should be assigned to Pb excitation, which is present despite strong shielding by the detector housing. A natural explanation for this fact is that lead housing excitation can arise from cosmic radiation as confirmed during the background measurements and the partial excitation of lead via the Co-57 γ photons, which although strong attenuated, were not fully absorbed by the shield. After analysis of the recorded spectra, one can conclude, that although much weaker than the photon intensity of the Co-57 source, the secondary X-ray fluorescence radiation arising from the protective shield, should be considered as a dose contributor to the irradiated tissue. The dose aspect and the quality of the registered CT scans suffer because of the additional induced X-ray fluorescence radiation. The addition of a second metallic component with a lower atomic number Z could absorb the Bi X-ray fluorescence photons. For this purpose, the addition of tungsten was examined. The presence of tungsten in the elastomeric bismuth shield leads to a substantial reduction of the all Bi line intensities as evidenced by the presented spectra. However, as a result, four new W photon emission lines are generated in the lower portion of the spectrum with energies characteristic of W excitation, 58.0, 59.3, 67.2 and 69.1 keV (M2 composite) [25]. These photons can be absorbed by the addition of another additive with a lower Z and X-ray excitation levels slightly below the energy of tungsten-emitted photons, for example, gadolinium. The addition of this element results in a reduction in the W emission intensity; however, two group of photons of 42.7 and 48.7 keV energy, corresponding to gadolinium emissions (M3) [25], are generated with lower intensities. These photons can be absorbed more easily than those emitted by Bi or W with energies of approximately 60–90 keV. A similar effect of reduced Gd X-ray emission photons could be expected after the addition of Sb (composite M4); however, as shown in Fig. 2, this result is not clear. From the other side, the low-energy Gd X-ray emission is not as important as the Bi and W fluorescence radiation emission together with the 120–140 keV Co-57 γ radiation. More detailed observations can be seen in Figs. 3 and 4 in which the data collected for particular photon emission intensities for Bi, W and Gd are presented for each composite. Given that the lower limit of photon energy visible to our REGe detector is ~40 keV, no γ-ray or X-ray photons below this value were observed, which is of concern to the Sb emission. This fact renders the measurement of photon emissions below 40 keV for Bi, W and Gd impossible. However, due to strong absorption of photons from this energy range and the reduced fluorescence yields for the excitation from shells other than K, the contributions photons at energies below 40 keV could, in practice, be neglected from the total doses outside the shields.Fig. 2

Bottom Line: The lead equivalents for the examined composites were within the ranges of 0.046-0.128 and 0.048-0.130 mm for 122.1 and 136.5 keV photons, respectively.The results clearly indicate that among the examined compositions, the highest values DRF have been achieved with preparations containing Bi+W, Bi+W+Gd and Bi+W+Sb mixtures with gradually decreasing content of heavy metal additives in the following order: Bi, W, Gd and Sb.The respective values of DRF obtained for the investigated composites were 21, 28 and 27 % dose reduction for a 1 mm thick shield and 39 and ~50 % for a 2 mm thick layer (M1-M4).

View Article: PubMed Central - PubMed

Affiliation: Institute of Applied Radiation Chemistry, Lodz University of Technology, Wroblewskiego 15, 90-924 Lodz, Poland.

ABSTRACT

Two commercially available (EP, Z) and eight new elastomeric composites (M1-M4, G1-G4, of thickness ≈1 mm) containing mixtures of differing proportions of heavy metal additives (Bi, W, Gd and Sb) have been synthesised and examined as protective shields. The intensity of the X-ray fluorescence radiation generated in the typical elastomeric shields for CT, containing Bi and other heavy metal additives influence on the practical shielding properties. A method for assessing the radiation shielding properties of elastomeric composites used in CT examination procedures via X-ray spectrometry has been proposed. To measure the radiation reduction ability of the protective shields, the dose reduction factor (DRF) has been determined. The lead equivalents for the examined composites were within the ranges of 0.046-0.128 and 0.048-0.130 mm for 122.1 and 136.5 keV photons, respectively. The proposed method, unlike to the common approach, includes a dose contribution from the induced X-ray fluorescence radiation of the heavy metal elements in the protective shields. The results clearly indicate that among the examined compositions, the highest values DRF have been achieved with preparations containing Bi+W, Bi+W+Gd and Bi+W+Sb mixtures with gradually decreasing content of heavy metal additives in the following order: Bi, W, Gd and Sb. The respective values of DRF obtained for the investigated composites were 21, 28 and 27 % dose reduction for a 1 mm thick shield and 39 and ~50 % for a 2 mm thick layer (M1-M4).

No MeSH data available.