British Journal of Radiology 74 (2001),351-357 © 2001 The British Institute of Radiology
The diagnostic X-ray protection characteristics of PanelcreteTM, AquapanelTM, BetopanTM and Gypsoplak SuperboardTM
I A Tsalafoutas, MSc
1
E Yakoumakis, PhD
2
P Sandilos, PhD
3
L Vlahos, MD, PhD
3 and
Ch Proukakis, MD, PhD
2
1 Medical Physics Unit, Konstantopoulio-Agia Olga Hospital, 3–5 Agias Olgas, Nea Ionia, 142 33 Athens
2 Medical Physics Department, University of Athens, 75 Mikras Asias, 115 27 Athens
3 Radiology Department, Areteion Hospital, 76 Vasilissis Sophias, 115 28 Athens, Greece
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Abstract
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Panelcrete, Aquapanel and Betopan are cement-based building materials with uses similar to those of gypsum wallboard, whose properties as a diagnostic X-ray shielding material have been extensively studied. The X-ray attenuation characteristics of these cement-based boards as well as those of a gypsum wallboard, Gypsoplak Superboard, are investigated for broad beam geometry conditions and for tube potentials of 50 kVp, 70 kVp, 100 kVp, 125 kVp and 140 kVp. Comparisons between these materials as well as with published data for gypsum wallboard are made. An example of their use as secondary barriers is given. Furthermore, it is confirmed that when building materials are considered for diagnostic X-ray shielding, calculations based on data for similar materials and corrected for density differences can be used only as an approximation.
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Introduction
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Shielding of diagnostic X-ray facilities is traditionally made using lead and concrete. Many other conventional building materials have been considered as alternatives, and data on their diagnostic X-ray attenuation characteristics can be found in the literature [1–9].
Conventional building materials are not the material of choice for shielding high workload, high tube potential facilities, especially for use as primary barriers, as calculations result in impracticably large wall thicknesses. In special circumstances, for instance when the workload and the peak tube potential are low and only secondary radiation is of concern, building materials that either already exist or might be used could ultimately lead to dispensing with the use of lead shielding. Even if this is not the case, shielding provided by building materials should not be ignored. These materials can reduce the thickness of lead required, thus reducing the cost of shielding with no compromise of personnel and public protection.
The National Council on Radiation Protection and Measurements (NCRP), in Report 49 [10], has acknowledged that mineral-based building materials with composition similar to that of concrete may be used for shielding. It has proposed that calculations should be made according to concrete attenuation characteristics, with appropriate corrections to account for differing densities. However, as has been noted in other studies [1–4, 9], density corrections are adequate only when the Compton effect is considered. When the photoelectric effect is important, as at diagnostic energies, it is preferable to use the attenuation characteristics of the specific building material.
It has been reported that primary transmission curves and high attenuation half-value layers (HVLs) from different authors do not generally agree well [6, 7]. As stated by Rossi et al [6], these differences can be explained to some extent, because a number of factors may influence measured or calculated results. These factors include generator waveform, X-ray beam HVL, irradiation field used as well as the segment of the attenuation curve used to calculate the high attenuation HVLs. For the same reason, comparison of the attenuation properties of different materials reported by different authors could result in misleading conclusions.
In this study, the diagnostic X-ray attenuation characteristics of Panelcrete, Aquapanel, Betopan and Gypsoplak Superboard are investigated for broad beam geometry conditions and for tube potentials in the range 50–140 kVp. The first three materials are cement-based boards. No reference to their X-ray attenuation characteristics can be found in literature. The last material is widely known as gypsum wallboard and has been studied by many authors [1, 2, 5–8]. In this study, all measurements were made using the same X-ray unit, geometrical set-up and dosemeter, thus a straightforward intercomparison of the attenuation characteristics of these materials is possible.
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Materials and methods
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To determine the X-ray attenuation characteristics of Panelcrete, Aquapanel and Betopan, sheets with an area of approximately 60x60 cm2 and nominal thicknesses of 0.9 cm, 1.2 cm and 1.2 cm, respectively, were used. Gypsoplak Superboard sheets were 60x83 cm2 with a nominal thickness of 1.25 cm.
Panelcrete and Aquapanel are produced in UK and are made of cement and glass fibres. Panelcrete consists of a monolithic core of high density cement, reinforced in the centre of its core with glass fibres. It is available in four thicknesses (0.45 cm, 0.6 cm, 0.9 cm and 1.2 cm) with a nominal dry density of 1550 kg m-3. Aquapanel consists of a monolithic core of high density cement reinforced with glass fibre mesh in both surfaces. It is available in three thicknesses (0.6 cm, 0.9 cm and 1.2 cm) with a nominal dry density of 1100 kg m-3. Betopan is produced in Turkey and is referred to as cement-bonded particle board. The basic difference from Panelcrete and Aquapanel is that it contains wood instead of glass fibres. It is available in eight thicknesses from 0.8–3 cm, with a nominal dry density of 1250 kg m-3. No other information on the composition of these materials was available from the manufactures.
The actual mean densities of the samples of Panelcrete, Aquapanel and Betopan used in this study were estimated by measuring the dimensions and weight of five sheets of each material. The mean densities were 1660±20 kg m-3, 1000±20 kg m-3 and 1320±10 kg m-3, respectively, and the corresponding measured thicknesses were 0.93±0.01 cm, 1.18±0.02 cm and 1.15±0.02 cm.
Gypsoplak Superboard is produced in Greece and is of the same composition as gypsum wallboard [2]. No density specifications were supplied by the manufacturers but measurements on the available sample gave a mean density of 910±10 kg m-3, and the mean measured thickness of five sheets was 1.21±0.01 cm (including the 
0.1 mm thick cardboard compressed on each surface).
The structure of all four boards was investigated by fluoroscopy and radiography. Spots of higher optical density than those reported for gypsum wallboard [2], Leca [3] and Ytong [9] weredetected in the X-ray images of all the materials studied, but no case of extremely high transmission ("hot spot") was observed.
X-ray transmission measurements were made at tube potentials of 50 kVp, 70 kVp, 100 kVp, 125 kVp and 140 kVp using a three-phase, high frequency X-ray unit (MPG-50; CGR, Buc, France). The nominal peak tube potential values were accurate to ±2%, checked with a tube potential meter (QA Test-o-meter; Unfors Instruments, Billdal, Sweden), and the output was reproducible to better than ±1%. Further information on the output and the X-ray beam quality is given in Table 1
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Measurements of the transmitted intensity were made using a Radcal Corporation dosemeter (Model 9010 Radiation Monitor Controller, 90X6-6 ionization chamber; Monrovia, CA) with nominal accuracy of ±4% and reproducibility of ±1%. Response to photons from 20 keV to 1.33 MeV (with build-up cap) is flat to within ±5%, whereas in the 20–140 keV energy range itdoes not exceed ±2%. The dose rate dependence is within ±5% from 4 µGy s-1 to 0.8 Gy s-1, and the specified maximum resolution is 0.01 µGy.
The focus-to-dosemeter distance was set to 160 cm and the first sheet of each material was placed 10 cm from the dosemeter. The material thickness was increased by placing additional sheets towards the X-ray unit. The irradiation field was 55x55 cm2 at 150 cm, to approximate broad beam geometry conditions.
The relative transmission measurement data were fitted using the following equation, first proposed by Archer et al [12] and previously discussed by Simpkin [8]:
where B is the relative transmission, x is the material thickness, and 
, ß and 
are fitting parameters. The fitting parameters , ß and were found with the aid of a suitable commercial computer program (Eureka: The Solver, version 1.0, Borland International).
The nth HVL (HVLn) can be found using Equations (2) and (3) described below, setting therelative transmission values B to 1/2n (for n=1 to 5):
Moreover, as noted by Simpkin [8], it is possible to calculate the HVL as a function of penetrated material thickness using:
At large values of x, this expression will tend toward (ln2)/ and provides an estimate of the HVLs at high attenuation, as required when shielding of leakage radiation is considered [6].
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Results
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Measured data and fitted curves of relative X-ray transmission through the four materials studied are shown in Figure 1. The fitting parameters , ß and are tabulated in Table 2 along with the high attenuation HVLs of these materials, expressed in both cm and g cm-2.
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Table 2. Fitting parameters to  Equation (1) and high attenuation half-value layers [=1n(2)/] for Panelcrete, Aquapanel, Betopan and Gypsoplak Superboard
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Intercomparison of the attenuation data is made in terms of g cm-2 to account for density differences. For this reason, the curves of Figure 1 are redrawn in Figures 2 and 3 as a function of material thickness expressed in g cm-2. The same applies in Figure 4, where the primary transmission curves for 70 kVp and 100 kVp for Gypsoplak Superboard are compared with curves for gypsum wallboard using data from other studies [5–7].
Finally, an example of the use of the materials as secondary radiation barriers is presented in Table 3. The barrier thickness (expressed in g cm-2) is compared with the corresponding thickness that results when using data from the literature for gypsum wallboard [5–7]. The shielding requirements were calculated using the mathematical model developed by Simpkin [13], which is well suited for computerized calculations.
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Discussion
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It can be seen from Figure 1 and Table 3 that in terms of thickness in cm required to reduce the exposure to a certain level, Panelcrete is by far the most effective, followed by Betopan, Aquapanel and Gypsoplak Superboard, respectively. The thickness of Gypsoplak Superboard necessary to produce a certain attenuation is approximately double that of Panelcrete.
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Table 3. Minimum secondary radiation shielding requirements for a non-controlled area (limit=1 mSv year-1) according to Simpkin's model [13]
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The situation is different when thickness is expressed in g cm-2. The effect of density is now removed and the effectiveness of each material as an X-ray absorber depends only on its composition. It can be seen from Figures 2 and 3, and also from Table 3, that whereas Panelcrete remains the most effective, Betopan now appears to be the least absorbing of the four materials. Aquapanel and Gypsoplak present very similar X-ray attenuation properties, between those of Panelcrete and Betopan.
From Figures 2 and 3 and from Table 3 it can be also understood that if data for Panelcrete were used to infer the required thickness of Betopan, the barrier thickness would be underestimated by about 10–20% depending on the tube potential and the attenuation required. To give an example from Table 3, at 70 kVp for 1.5 m distance to the barrier from the focus and scatterer, the calculated Panelcrete barrier thickness is 6.2 cm. Multiplying by the density ratio 1.66/1.32=1.258 results in a 7.8 cm thickness of Betopan, which is only 84% of the thickness actually required (9.3 cm). The same can be seen from the comparison of the corresponding values in g cm-2; the Panelcrete value is 84% of the Betopan value. In the above example, the transmitted dose through the underestimated Betopan barrier would be increased by a factor of 1.89. For the same distance and for 50 kVp, 100 kVp and 125 kVp, the transmitted dose would be increased by a factor of 2.16, 1.71 and 1.52, respectively.
However, these errors are not much larger than the errors that would arise if shielding calculations for Gypsoplak Superboard were made using data for gypsum wallboard from other authors, as can be appreciated by Figure 4 and Table 3. In general, using data from Archer et al [7] would overestimate the barrier thickness required, whereas the opposite is true with data from Simpkin [5] and Rossi et al [6]. It is therefore understandable that when the shielding requirements of an "unknown" material are determined using the available data on similar materials that can be used as calculation standards, the resulting values will be dependent on the data set used, which in turn is much dependent on a number of aforementioned factors.
As far as the usefulness of Panelcrete, Aquapanel, Betopan and Gypsoplak Superboard in shielding is concerned, it is limited to dental and mammography applications and to secondary barrier construction in common diagnostic X-ray facilities. However, the usefulness of these materials in shielding can be further extended if recent studies [14, 15] are taken into account, which report that the workload, tube potential and primary beam intensity assumed by NCRP Report 49 for shielding calculations are in most cases overestimated and result in unnecessarily large barrier thicknesses.
Finally, it should be noted that as density deviations of up to approximately ±10% from the nominal values were observed in the samples studied, and larger deviations cannot be excluded, the density and thickness of the sheets of the specific batch designated for shielding should be carefully determined.
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Conclusion
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In this study, the diagnostic X-ray attenuation characteristics of three cement-based materials and one gypsum-based material were investigated. The parameter values that fit the measured data to the well known three-parameter equation were determined, allowing for accurate reproduction of transmission curves and easy computation of shielding requirements.
Intercomparison of the materials studied verified the perception that density correction in the diagnostic X-ray energy range will not produce accurate results, as the different attenuation properties of the materials alone can give errors in barrier thickness determination of about ±20% (without excluding the possibility of even larger errors). Furthermore, it was shown that the dependence of the available standard data on the aforementioned factors (generator waveform, X-ray beam HVL etc.) can introduce additional errors of the same magnitude.
Therefore, calculations for a building material based on density-corrected data for a similar material can only be used to calculate an approximate estimate of the shielding requirements. Even if we assume that the available data have been derived with an X-ray tube representative of the majority of modern X-ray tubes and under proper broad beam conditions, the errors that may arise from the different attenuation properties create doubts concerning the adequacy of the estimated barrier thickness for shielding.
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Acknowledgments
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The authors would like to thank Mr Dimitris Dontas from Knauf Gypsopiia A.B.E.E. (Hellas), Mr Dimitris Patsakis, and Michael and Takis Korasidis from Gypsoplak (Hellas) for providing the information and the samples used in this project (Panelcrete, Aquapanel, and Betopan as well as Gypsoplak Superboard, respectively).
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Footnotes
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Panelcrete and Aquapanel are trademarks for Knauf. Betopan is a trademark of Tepe Group & Tepe Betopan Co. Inc. Gypsoplak Superboard is a trademark of Gypsoplak, Greece. 
Received for publication July 3, 2000.
Revision received October 27, 2000.
Accepted for publication November 27, 2000.
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References
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Abstract
Introduction
), 100 kVp (
), 125 kVp (
) and 140 kVp (–). —, fitted.
) and Gypsoplak Superboard (
) at tube potentials of 50 kVp, 100 kVp and 140 kVp, with thickness expressed in g cm-2. —, fitted (cement-based boards); -----, fitted (Gypsoplak Superboard).
