0017-9078/91 $3.00 + .W 0 1991 Health Physics Society Pergamon Press plc
Heahh Physics Val. 60, No. 5 (May), pp. 661-664, 1991 Printed in the U.S.A.
Paper COMPOSITE MATERIALS FOR X-RAY PROTECTION Martin J. Yaffe and Gordon E. Mawdsley Department of Medical Biophysics, University of Toronto, Ontario Cancer Institute and Sunnybrook Health Science Center, Reichmann Research Building, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 375 and
Martin Lilley Xenoprene Company, 1194 Caledonia Road, Unit C, Toronto, Ontario, Canada M6A 2W5 and
Ray Servant and George Reh Development Division, Dupont Canada Inc., Box 2200, Streetsville, Mississauga, Ontario, Canada L5M 2H3 Abstract-We have developed and tested a radiation protection material that provides similar attenuation for diagnostic x-ray spectra to that of conventional Pb apron materials with approximately 30% reduced weight. By combining a number of elements with different K absorption energies, such as Ba, W, and Pb, energy attenuation for given spectra can be optimized with respect to total cross-sectional mass loading. Alternatively, garments with much higher protective factors at equivalent weight to conventional garments could be produced. The reduction in the amount of Pb used also reduces problems associated with the toxicity of the material during manufacture and disposal. Back strain can be reduced for personnel performing special radiological procedures that require wearing protective garments for long periods of time.
material weighing from 7.1 to 9.25 kg m-’ for nominal 0.5-mm protection. In two of the samples, a Pb content of 5.65 -t 0.25 kg m-2 was confirmed by chemical analysis. As an approach to increasing the efficiency of such protective materials, i.e., to obtain increased protection at the current typical weight, or to provide equivalent protection at reduced weights, we considered the possibility of using mixtures of elements as x-ray attenuators. Our motivation came from observation of the attenuation properties of Pb (Plechaty et al. 1975) shown in Fig. l a (right scale), where there is a “window” over which Pb displays a diminished capability of absorbing radiation between the energies of 50 keV and the K absorption edge at 88 keV. This occurs because in this energy range, the attenuation via photoelectric interactions with the L-shell is decreasing with energy, and interaction by liberation of K photoelectrons is not yet possible. A typical diagnostic x-ray energy fluence spectrum generated at 100 kilovolts peak (kVp) potential (Birch et al. 1979) is also shown in Fig. 1b (left scale). Comparison of the two curves reveals that in the spectrum there is significant energy contained in the 50 to 88 kVp window region, much of which will be preferentially transmitted through the Pb as demonstrated in Fig. lc. Mass attenuation coefficients vs. energy for Pb, W, and Ba whose K edges lie at 88 keV, 69.5 keV, and 37.4
INTRODUCTION IN MANY areas of radiological imaging, protective garments for shielding against x rays must be worn for long time periods. This is especially true in vascular angiography, cardiovascular imaging, and in invasive procedures such as angioplasty and various urological and surgical techniques. To minimize fatigue and damage to the neck and back of the wearer, it is desirable that protective apparel be as light as possible, consistent with the degree of protection required ( Keane and Tikhonov 1975) . Lead has traditionally been used as the attenuating material in garments designed to provide protection from x rays and low-energy y rays. Typically, powdered Pb is incorporated in a flexible binder (either rubber or vinyl) to provide an attenuation equivalent to that of sheet Pb of either 0.25 or 0.5 mm in thickness. Since Pb has a density of 11.3 X l o 3 kg m-3, the mass of the dominant attenuating material in a 0.5-mm-equivalent garment is 5.65 kg m-2. For most conventional garments, binders and cover materials contribute an additional mass of 2.53.5 kg mW2.Six garments fabricated by five different manufacturers were cut apart and found to be constructed of (Manuscript received 2 July 1990 revised manuscript received 5 October 1990, accepted 18 October 1990) 66 I
Health Physics
662
r
I00
0.8
Y 'm N
E
t:
Photon energy (keV)
Fig. 1. a ) Mass attenuation coefficient of Pb (right ordinate). The area below the dotted line shows a region with relatively reduced attenuation. b ) Typical energy fluence spectrum at 100 kVp (left ordinate). c ) Energy fluence spectrum transmitted by 0.5 mm Pb (left ordinate).
keV, respectively, are shown in Fig. 2. Note that because of the different energies of K absorption edges. the attenuation properties of these materials are somewhat complementary over the energy range. Since the mass attenuation curve is related to the probability of interaction per kilogram of material, using one or more other ma-
May 1991, Volume 60, Number 5
terials and "filling in" the Pb window, an improved ratio of attenuation to mass of the material might be achieved for typical diagnostic spectra, especially those from 80 to 130 kVp. Three elements ( Pb, W, and Ba) were considered because of their commercial availability, relatively moderate cost, low toxicity, and location of K edges. X-ray beams are comprised of photons distributed over a spectrum of energies, and, therefore, in designing a composite material it is necessary to optimize for spectra rather than for individual x-ray energies. Because many fluoroscopic, angiographic, and interventional procedures are performed at peak tube potentials of 100 kVp or less, we initially considered the 100-kVp spectrum of Fig. lb. We developed an optimization algorithm to determine, for that spectrum, the mass loadings and proportion by weight of Pb, W, and Ba that would provide the same attenuation factor as a 0.5-mm thickness of Pb while minimizing the total weight. The goal in radiation protection is to minimize the risk, which in this case might be most appropriately achieved by minimizing the effective dose equivalent (EDE) to the wearer. Since this would require knowledge not only of the x-ray beam properties but also the location of radiation-sensitive sites of the wearer, it was decided instead to minimize the integral transmitted energy fluence of the material. For most of the spectra considered, we found that the conditions required to minimize energy fluence also yielded approximately the minimum transmitted exposure. This result is useful in that it enables experimental verification and/or routine testing of material performance with the use of standard dosimeters. RESULTS Theoretical Typical results of the optimization process are illustrated in Fig. 3 where, for a total mass loading of 5.65 kg m-', the percent energy transmission of a mixture with
-w - Ba Composite mixture
0.0
1
"
"
"
"
'
30
"
"
60
90
120
100 kVp energy transmission
Photon energy (keV)
Fig. 2. Mass attenuation coefficients of Pb, W, Ba, and a composite, XL, described in the text.
Fig. 3 . Percent energy transmission for a 100-kVp spectrum through mixtures composed of differing amounts of Pb, W, and Ba with total mass loading of 5.65 kg m-*.
Composite materials for x-ray protection 0 M. J. YAFFEet al.
663
Table 1. Calculated transmission of various protective materials. Energy fluence transmission (YO)
Composition (kg m2)
Exposure transmission (%)
Element
Pure Pb o p t 100" o p t 120 Opt 7oex, Opt 70," XRb XL' a
Pb
W
Ba
70
100
120
70
100
120
5.65 0 0.45 1.34 2.04 2.04 1.65
0 2.04 2.72 0 0 2.04 1.65
0 3.61 2.48 4.30 3.61 1.57 1.20
I .3 0.47 0.69 0.3 I 0.34 0.88 1.82
6.9 2.1 2.4 3.0 3.3 3.1 5.7
8.2 4.6 4.3 5.7 5.6 4.6 8.0
0.36 0.17 0.2 I 0.12 0.11 0.25 0.54
3.2 1.o 1.2 1.4 1.5 1.5 2.8
5.1 2.7 2.6 3.4 3.3 2.9 5.0
Optimized mixture at 100 kVp. Selected composition-5.65 kg m-*. Selected composition, reduced weight-4.5
kg m-*,
particular loadings of Pb, Ba, and W is given. For a given point on the graph, the loading of Pb (not explicitly shown) is simply the difference between 5.65 kg m-' and the sum of the loadings of the other two materials. The graph indicates that there is a minimum energy transmission for a mixture requiring no Pb and consisting of 3.6 kg m-2 of Ba and 2.05 kg m-' of W. The same calculation was performed for a spectrum at 120 kVp where it was found that minimum energy transmission would occur for 2.5 kg m-2 of Ba, 2.7 kg m-2 of W, and 0.45 kg mP2of Pb. The range of composition that provides similar values of transmission is relatively broad, allowing reasonable tolerance in the relative proportions of attenuating elements in a manufactured material. Results of the calculation are also summarized in Table 1. In the first row, the transmission of pure Pb is shown and can be compared to that of mixtures of equal mass loading (5.65 kg m-') in the following five rows. These mixtures, called "Opt," have been optimized in composition to provide minimum transmission of energy (and exposure) at 100 kVp and 120 kVp. At 70 kVp, the conditions to minimize transmission of energy (Opt 70,") and exposure (Opt 70,,,) are different so that two different compositions are shown. Practicalities of fabrication and the desire to allow some flexibilityin the use of different x-ray spectra resulted in a composition slightly different from the optimum configurations shown in Table 1. For example, it was not possible to use Ba in pure elemental form, and a Ba compound was employed instead. The result was a composition, XR, * containing some Pb and reduced loading of Ba from the optimum mixture for a total loading of 5.65 kg m-2. A composition, XL, with the same proportions of the three elements but with reduced total mass loading
of 4.5 kg mp2was also manufactured, and calculated results are shown in the last row of Table 1. For all materials, calculated transmission values are given for 70, 100, and 120 kVp. The mass attenuation coefficient for XR or XL is shown as the dashed curve in Fig. 2. Due to the inclusion of Pb in the mixture, at 100 kVp the calculated spectralaveraged energy fluence transmission for XR is 3.1% rather than 2.1% for the ideal mixture at that energy. It should be noted that the transmission of the Pb is 6.9%. It can be seen that no one mixture is optimum for all spectra and that a wide range of compositions are possible, each of which gives better performance than pure Pb. In Fig. 4, energy fluence spectra transmitted by either 0.5 mm Pb (5.65 kg m-2) or XL (4.5 kg mp2)are given for both 120-kVp (a) and 100-kVp ( b ) spectra.
* Available commercially as Xenolitem from Dupont Canada Inc., Box 2200, Streetsville, Mississauga, Ontario, Canada L5M 2H3. Model CX60, High Voltage Engineering Corp. (Now manufactured by Halmar Electronics Inc., 900 North Hague Avenue, Columbus, OH 43204.) Model 502-CLl50, Machlett LaboratoriesInc., (acquired by Varian
Canada-EIMAC X-Ray, 50 Galaxy Blvd., Unit 9, Rexdale, Ontario M9W 4Y5). Model 96035, Keithley Instruments Inc., 28775 Aurora Road, Cleveland, OH 44139. Model 356 I7 EBS, Keithley Instruments Inc.
*
Experimental A set of test samples of varying thicknesses of XL and of Ba and W at the individual component loadings was produced, and performance was tested by comparison of measured broad-beam exposure transmission with that of known thicknesses of pure sheet Pb. X rays were produced using a constant potential generatort and a W-anode diagnostic x-ray tube.* Exposures were measured with a 15-cc pancake-type ionization chamberBcalibrated for diagnostic energies and a digital electrometer! A 10-cm X 10-cm area was irradiated at 85 cm from the anode with the chamber placed 5 cm behind the attenuator. Both kVp and beam filtration were changed to provide a wide range of half-value layers (HVL). In Fig. 5, the transmission is plotted vs. HVL of the x-ray beam for both XL and 0.50 mm Pb. The peak kilovoltages used to obtain
Health Physics
664
o'2r
a'
- - -Xenolite
-
May 199 I , Volume 60, Number 5
TM
0.5 mm Pb
(120 kVp)
Exposure Transmitted Exposure Incident
Photon energy (keV)
Fig. 4. Transmitted energy fluence spectra at 120 kVp ( a ) and 100 kVp ( b ) through 0.5 mm Pb (solid curve) and XL (dashed curve), relative to the energy fluence for the K " lines of the incident spectrum.
each data point are also shown. Note that transmission is equal for our reference spectrum ( 100 kVp, 4.3 mm A1 HVL), while for lower and higher kilovoltages, Pb is slightly more attenuating. There are discrepancies between the measured percentage transmission and the calculated values shown in Table 1, probably due to differences between the tabulated spectra used for the calculation and the spectra available in the laboratory. Both scattering and transmitted x-ray fluorescence from the absorber were neglected in the calculations, which might contribute to the differences in the results. Experiments were done to evaluate the effect of layering of elements and of the effect of different means of dispersing the elements within the support matrix of the material. No significant difference (less than 1 % ) was found in the transmission of filters made using solid metals or the same amount (kg m-2) of powdered metal suspended in a plastic matrix. At higher energies (250 kVp), attenuation of fluorescence plays a role in the design of multi-element primary beam "Thoraeus" filters (Johns and Cunningham 1983) requiring the placement of the highest Z material closest to the source of radiation. For multiple-element attenuators, the order of placement of sheets made with the different elements had no effect on the transmitted exposure. When incorporated in an apron material, this mixture provided an overall weight reduction of 25-30% compared to a 0.5-mm Pb/vinyl apron.
0.1
i'
I I
I
I
2 2.6 3
I
I
I
I
4
5
7
10
HVL (mm Al)
Fig. 5 . Experimental measurement of exposure transmission through 0.5 mm Pb and XL for a variety of x-ray spectra used in diagnostic radiology. Two spectra at 100 kVp are shown; one has been filtered with an additional 2 mm of Al, giving an elevated HVL.
CONCLUSIONS By making use of their different K absorption energies, multiple-attenuating elements can be combined to create protective materials with either improved attenuation of x rays at current weights or equivalent attenuation at reduced weight. Materials can be designed that are optimized for special use, such as angiography where a well-defined range of x-ray spectra is employed. Since such application-specific materials may contain reduced loading of Pb or no Pb at all, the concept of Pb equivalence must be reexplored. Possibly a more useful definition of protective factor would be in terms of the integral transmission of energy fluence over a spectrum or range of spectra for which the material is designed to be used. This would more properly relate to radiation safety and provide a more realistic rating for lighter garments with reduced back strain to the wearer.
REFERENCES Birch, R.; Marshall, M.; Ardran, G. M. Catalog of spectral data for diagnostic x-rays. London: Hospital Physicists' Association: 1979. Johns, H. E.: Cunningham, J. R. The physics of radiology. Fourth edition. Springfield. IL: Charles C. Thomas; 1983:271. Keane, B. E.: Tikhonov, K. B. Manual on radiation protection
in hospitals and general practice. Vol. 3. X-ray diagnosis. Geneva: World Health Organization; 1975:69-7 I . Plechaty. E. F.: Cullen, D. E.: Howerton, R. J. Tables and graphs of photon interaction cross sections from 1.0 keV to 100 MeV derived from LLL evaluated nuclear data library. Springfield, VA: National Technical Information Service; UCRL-50040, Vol. 6, revision 1; 1975.
Heahh Physics Val. 60, No. 5 (May), pp. 661-664, 1991 Printed in the U.S.A.
Paper COMPOSITE MATERIALS FOR X-RAY PROTECTION Martin J. Yaffe and Gordon E. Mawdsley Department of Medical Biophysics, University of Toronto, Ontario Cancer Institute and Sunnybrook Health Science Center, Reichmann Research Building, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 375 and
Martin Lilley Xenoprene Company, 1194 Caledonia Road, Unit C, Toronto, Ontario, Canada M6A 2W5 and
Ray Servant and George Reh Development Division, Dupont Canada Inc., Box 2200, Streetsville, Mississauga, Ontario, Canada L5M 2H3 Abstract-We have developed and tested a radiation protection material that provides similar attenuation for diagnostic x-ray spectra to that of conventional Pb apron materials with approximately 30% reduced weight. By combining a number of elements with different K absorption energies, such as Ba, W, and Pb, energy attenuation for given spectra can be optimized with respect to total cross-sectional mass loading. Alternatively, garments with much higher protective factors at equivalent weight to conventional garments could be produced. The reduction in the amount of Pb used also reduces problems associated with the toxicity of the material during manufacture and disposal. Back strain can be reduced for personnel performing special radiological procedures that require wearing protective garments for long periods of time.
material weighing from 7.1 to 9.25 kg m-’ for nominal 0.5-mm protection. In two of the samples, a Pb content of 5.65 -t 0.25 kg m-2 was confirmed by chemical analysis. As an approach to increasing the efficiency of such protective materials, i.e., to obtain increased protection at the current typical weight, or to provide equivalent protection at reduced weights, we considered the possibility of using mixtures of elements as x-ray attenuators. Our motivation came from observation of the attenuation properties of Pb (Plechaty et al. 1975) shown in Fig. l a (right scale), where there is a “window” over which Pb displays a diminished capability of absorbing radiation between the energies of 50 keV and the K absorption edge at 88 keV. This occurs because in this energy range, the attenuation via photoelectric interactions with the L-shell is decreasing with energy, and interaction by liberation of K photoelectrons is not yet possible. A typical diagnostic x-ray energy fluence spectrum generated at 100 kilovolts peak (kVp) potential (Birch et al. 1979) is also shown in Fig. 1b (left scale). Comparison of the two curves reveals that in the spectrum there is significant energy contained in the 50 to 88 kVp window region, much of which will be preferentially transmitted through the Pb as demonstrated in Fig. lc. Mass attenuation coefficients vs. energy for Pb, W, and Ba whose K edges lie at 88 keV, 69.5 keV, and 37.4
INTRODUCTION IN MANY areas of radiological imaging, protective garments for shielding against x rays must be worn for long time periods. This is especially true in vascular angiography, cardiovascular imaging, and in invasive procedures such as angioplasty and various urological and surgical techniques. To minimize fatigue and damage to the neck and back of the wearer, it is desirable that protective apparel be as light as possible, consistent with the degree of protection required ( Keane and Tikhonov 1975) . Lead has traditionally been used as the attenuating material in garments designed to provide protection from x rays and low-energy y rays. Typically, powdered Pb is incorporated in a flexible binder (either rubber or vinyl) to provide an attenuation equivalent to that of sheet Pb of either 0.25 or 0.5 mm in thickness. Since Pb has a density of 11.3 X l o 3 kg m-3, the mass of the dominant attenuating material in a 0.5-mm-equivalent garment is 5.65 kg m-2. For most conventional garments, binders and cover materials contribute an additional mass of 2.53.5 kg mW2.Six garments fabricated by five different manufacturers were cut apart and found to be constructed of (Manuscript received 2 July 1990 revised manuscript received 5 October 1990, accepted 18 October 1990) 66 I
Health Physics
662
r
I00
0.8
Y 'm N
E
t:
Photon energy (keV)
Fig. 1. a ) Mass attenuation coefficient of Pb (right ordinate). The area below the dotted line shows a region with relatively reduced attenuation. b ) Typical energy fluence spectrum at 100 kVp (left ordinate). c ) Energy fluence spectrum transmitted by 0.5 mm Pb (left ordinate).
keV, respectively, are shown in Fig. 2. Note that because of the different energies of K absorption edges. the attenuation properties of these materials are somewhat complementary over the energy range. Since the mass attenuation curve is related to the probability of interaction per kilogram of material, using one or more other ma-
May 1991, Volume 60, Number 5
terials and "filling in" the Pb window, an improved ratio of attenuation to mass of the material might be achieved for typical diagnostic spectra, especially those from 80 to 130 kVp. Three elements ( Pb, W, and Ba) were considered because of their commercial availability, relatively moderate cost, low toxicity, and location of K edges. X-ray beams are comprised of photons distributed over a spectrum of energies, and, therefore, in designing a composite material it is necessary to optimize for spectra rather than for individual x-ray energies. Because many fluoroscopic, angiographic, and interventional procedures are performed at peak tube potentials of 100 kVp or less, we initially considered the 100-kVp spectrum of Fig. lb. We developed an optimization algorithm to determine, for that spectrum, the mass loadings and proportion by weight of Pb, W, and Ba that would provide the same attenuation factor as a 0.5-mm thickness of Pb while minimizing the total weight. The goal in radiation protection is to minimize the risk, which in this case might be most appropriately achieved by minimizing the effective dose equivalent (EDE) to the wearer. Since this would require knowledge not only of the x-ray beam properties but also the location of radiation-sensitive sites of the wearer, it was decided instead to minimize the integral transmitted energy fluence of the material. For most of the spectra considered, we found that the conditions required to minimize energy fluence also yielded approximately the minimum transmitted exposure. This result is useful in that it enables experimental verification and/or routine testing of material performance with the use of standard dosimeters. RESULTS Theoretical Typical results of the optimization process are illustrated in Fig. 3 where, for a total mass loading of 5.65 kg m-', the percent energy transmission of a mixture with
-w - Ba Composite mixture
0.0
1
"
"
"
"
'
30
"
"
60
90
120
100 kVp energy transmission
Photon energy (keV)
Fig. 2. Mass attenuation coefficients of Pb, W, Ba, and a composite, XL, described in the text.
Fig. 3 . Percent energy transmission for a 100-kVp spectrum through mixtures composed of differing amounts of Pb, W, and Ba with total mass loading of 5.65 kg m-*.
Composite materials for x-ray protection 0 M. J. YAFFEet al.
663
Table 1. Calculated transmission of various protective materials. Energy fluence transmission (YO)
Composition (kg m2)
Exposure transmission (%)
Element
Pure Pb o p t 100" o p t 120 Opt 7oex, Opt 70," XRb XL' a
Pb
W
Ba
70
100
120
70
100
120
5.65 0 0.45 1.34 2.04 2.04 1.65
0 2.04 2.72 0 0 2.04 1.65
0 3.61 2.48 4.30 3.61 1.57 1.20
I .3 0.47 0.69 0.3 I 0.34 0.88 1.82
6.9 2.1 2.4 3.0 3.3 3.1 5.7
8.2 4.6 4.3 5.7 5.6 4.6 8.0
0.36 0.17 0.2 I 0.12 0.11 0.25 0.54
3.2 1.o 1.2 1.4 1.5 1.5 2.8
5.1 2.7 2.6 3.4 3.3 2.9 5.0
Optimized mixture at 100 kVp. Selected composition-5.65 kg m-*. Selected composition, reduced weight-4.5
kg m-*,
particular loadings of Pb, Ba, and W is given. For a given point on the graph, the loading of Pb (not explicitly shown) is simply the difference between 5.65 kg m-' and the sum of the loadings of the other two materials. The graph indicates that there is a minimum energy transmission for a mixture requiring no Pb and consisting of 3.6 kg m-2 of Ba and 2.05 kg m-' of W. The same calculation was performed for a spectrum at 120 kVp where it was found that minimum energy transmission would occur for 2.5 kg m-2 of Ba, 2.7 kg m-2 of W, and 0.45 kg mP2of Pb. The range of composition that provides similar values of transmission is relatively broad, allowing reasonable tolerance in the relative proportions of attenuating elements in a manufactured material. Results of the calculation are also summarized in Table 1. In the first row, the transmission of pure Pb is shown and can be compared to that of mixtures of equal mass loading (5.65 kg m-') in the following five rows. These mixtures, called "Opt," have been optimized in composition to provide minimum transmission of energy (and exposure) at 100 kVp and 120 kVp. At 70 kVp, the conditions to minimize transmission of energy (Opt 70,") and exposure (Opt 70,,,) are different so that two different compositions are shown. Practicalities of fabrication and the desire to allow some flexibilityin the use of different x-ray spectra resulted in a composition slightly different from the optimum configurations shown in Table 1. For example, it was not possible to use Ba in pure elemental form, and a Ba compound was employed instead. The result was a composition, XR, * containing some Pb and reduced loading of Ba from the optimum mixture for a total loading of 5.65 kg m-2. A composition, XL, with the same proportions of the three elements but with reduced total mass loading
of 4.5 kg mp2was also manufactured, and calculated results are shown in the last row of Table 1. For all materials, calculated transmission values are given for 70, 100, and 120 kVp. The mass attenuation coefficient for XR or XL is shown as the dashed curve in Fig. 2. Due to the inclusion of Pb in the mixture, at 100 kVp the calculated spectralaveraged energy fluence transmission for XR is 3.1% rather than 2.1% for the ideal mixture at that energy. It should be noted that the transmission of the Pb is 6.9%. It can be seen that no one mixture is optimum for all spectra and that a wide range of compositions are possible, each of which gives better performance than pure Pb. In Fig. 4, energy fluence spectra transmitted by either 0.5 mm Pb (5.65 kg m-2) or XL (4.5 kg mp2)are given for both 120-kVp (a) and 100-kVp ( b ) spectra.
* Available commercially as Xenolitem from Dupont Canada Inc., Box 2200, Streetsville, Mississauga, Ontario, Canada L5M 2H3. Model CX60, High Voltage Engineering Corp. (Now manufactured by Halmar Electronics Inc., 900 North Hague Avenue, Columbus, OH 43204.) Model 502-CLl50, Machlett LaboratoriesInc., (acquired by Varian
Canada-EIMAC X-Ray, 50 Galaxy Blvd., Unit 9, Rexdale, Ontario M9W 4Y5). Model 96035, Keithley Instruments Inc., 28775 Aurora Road, Cleveland, OH 44139. Model 356 I7 EBS, Keithley Instruments Inc.
*
Experimental A set of test samples of varying thicknesses of XL and of Ba and W at the individual component loadings was produced, and performance was tested by comparison of measured broad-beam exposure transmission with that of known thicknesses of pure sheet Pb. X rays were produced using a constant potential generatort and a W-anode diagnostic x-ray tube.* Exposures were measured with a 15-cc pancake-type ionization chamberBcalibrated for diagnostic energies and a digital electrometer! A 10-cm X 10-cm area was irradiated at 85 cm from the anode with the chamber placed 5 cm behind the attenuator. Both kVp and beam filtration were changed to provide a wide range of half-value layers (HVL). In Fig. 5, the transmission is plotted vs. HVL of the x-ray beam for both XL and 0.50 mm Pb. The peak kilovoltages used to obtain
Health Physics
664
o'2r
a'
- - -Xenolite
-
May 199 I , Volume 60, Number 5
TM
0.5 mm Pb
(120 kVp)
Exposure Transmitted Exposure Incident
Photon energy (keV)
Fig. 4. Transmitted energy fluence spectra at 120 kVp ( a ) and 100 kVp ( b ) through 0.5 mm Pb (solid curve) and XL (dashed curve), relative to the energy fluence for the K " lines of the incident spectrum.
each data point are also shown. Note that transmission is equal for our reference spectrum ( 100 kVp, 4.3 mm A1 HVL), while for lower and higher kilovoltages, Pb is slightly more attenuating. There are discrepancies between the measured percentage transmission and the calculated values shown in Table 1, probably due to differences between the tabulated spectra used for the calculation and the spectra available in the laboratory. Both scattering and transmitted x-ray fluorescence from the absorber were neglected in the calculations, which might contribute to the differences in the results. Experiments were done to evaluate the effect of layering of elements and of the effect of different means of dispersing the elements within the support matrix of the material. No significant difference (less than 1 % ) was found in the transmission of filters made using solid metals or the same amount (kg m-2) of powdered metal suspended in a plastic matrix. At higher energies (250 kVp), attenuation of fluorescence plays a role in the design of multi-element primary beam "Thoraeus" filters (Johns and Cunningham 1983) requiring the placement of the highest Z material closest to the source of radiation. For multiple-element attenuators, the order of placement of sheets made with the different elements had no effect on the transmitted exposure. When incorporated in an apron material, this mixture provided an overall weight reduction of 25-30% compared to a 0.5-mm Pb/vinyl apron.
0.1
i'
I I
I
I
2 2.6 3
I
I
I
I
4
5
7
10
HVL (mm Al)
Fig. 5 . Experimental measurement of exposure transmission through 0.5 mm Pb and XL for a variety of x-ray spectra used in diagnostic radiology. Two spectra at 100 kVp are shown; one has been filtered with an additional 2 mm of Al, giving an elevated HVL.
CONCLUSIONS By making use of their different K absorption energies, multiple-attenuating elements can be combined to create protective materials with either improved attenuation of x rays at current weights or equivalent attenuation at reduced weight. Materials can be designed that are optimized for special use, such as angiography where a well-defined range of x-ray spectra is employed. Since such application-specific materials may contain reduced loading of Pb or no Pb at all, the concept of Pb equivalence must be reexplored. Possibly a more useful definition of protective factor would be in terms of the integral transmission of energy fluence over a spectrum or range of spectra for which the material is designed to be used. This would more properly relate to radiation safety and provide a more realistic rating for lighter garments with reduced back strain to the wearer.
REFERENCES Birch, R.; Marshall, M.; Ardran, G. M. Catalog of spectral data for diagnostic x-rays. London: Hospital Physicists' Association: 1979. Johns, H. E.: Cunningham, J. R. The physics of radiology. Fourth edition. Springfield. IL: Charles C. Thomas; 1983:271. Keane, B. E.: Tikhonov, K. B. Manual on radiation protection
in hospitals and general practice. Vol. 3. X-ray diagnosis. Geneva: World Health Organization; 1975:69-7 I . Plechaty. E. F.: Cullen, D. E.: Howerton, R. J. Tables and graphs of photon interaction cross sections from 1.0 keV to 100 MeV derived from LLL evaluated nuclear data library. Springfield, VA: National Technical Information Service; UCRL-50040, Vol. 6, revision 1; 1975.
