Abstract. Fibre-cement products are widely used local building materials for both roofing and wall cladding in Nigeria. The X-ray attenuation

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2 Abstract. Fibre-cement products are widely used local building materials for both roofing and wall cladding in Nigeria. The X-ray attenuation properties of the products for broad beam geometry conditions and for tube potentials in the kVp range were investigated in this work. Comparison with published data for concrete and other building materials was made. The results obtained suggest that the fibre-cement products analyzed in this work are not suitable for primary X-ray shielding in normal diagnostic installations. Nevertheless, walls of the fibre-cement products, typically 8mm thick may offer adequate protection in low workload diagnostic installations as secondary barrier.

3 1. Introduction Generally, medical diagnostic installations are usually shielded with lead, concrete or steel of appropriate thickness. In recent years, the high cost of lead and steel and non-availability of cement locally have necessitated the need for investigation into other materials that could be employed for shielding, particularly in situations where only scattered and leakage radiation need be considered. In this work, one such alternative that is being considered is fibrecement building material. In designing effective shielding, many other materials have been considered as substitute to lead and concrete. Examples are: gypsum wallboard, plate glass and wood.

4 The choice of fibre-cement products in this work is due to their availability locally and their predominant usage as ceiling material in Nigerian hospitals where regular or periodic use of X-rays is made. Also no reference to the attenuation properties of this product can be found in literature. The main constituents of the fibre-cement product investigated in this work are Cement, Polyvinyl alcohol (synthetic fibre), Calcium carbonate and Cellulose constituting approximately 75%, 15%, 5% and 5% respectively of the finished product. They are light weight materials compared with concrete or lead. They are easy to install and present favourably both thermal and acoustic insulation properties. They come in various shapes and sizes and have dry density of 1.4x10 3 kg/m 3 depending on the grade and thickness.

5 2. Materials and Methods 2.1 Instrumentation The measurement of the attenuation properties of the material under investigation was undertaken by means of the PM1621 Polimaster dosimeter. The PM1621 is a professional dosimeter that is designed to measure personal dose equivalent (DE) and personal dose equivalent rate (DER) of both gamma- and X- radiation within the wide energy range from the least values of natural background to up to 1 Sv/h. The dosimeter meets the requirements of the International Electrotechnical Commission (IEC) standard. Its high sensitivity makes it possible to register even the slightest variations of the natural background. The dosimeter stores up to 1000 histories of dose rate measurements and accumulated dose values in its non-volatile memory. All these data can be transmitted to a personal computer (PC) through the infra-red (IR) channel using the special software for further processing and analysis.

6 2.2 X-Ray Source The X-ray source employed for the investigation is the Picker International Roentgen 201 located at the Nigerite Limited Health Centre, in Nigeria. Its main use is for taking the chest X-ray of workers. It is a single-phase frequency X-ray machine with an energy rating of kvp, current range of ma, inherent filter of 2 mm aluminum, and maximum time of 6 seconds. It has a good collimator that allows broad beam geometry. 2.3 Sample Preparation Several sheets of the fibre-cement products were obtained from the factory. The samples were in flat sheets of size 1200 mm x 1200 mm and of different thicknesses (3 mm, 5 mm, 8 mm, 10 mm, 13 mm and 15 mm). The sheets were then constructed into box-like shapes of dimensions 300 mm x 300 mm x 300 mm.

7 2.4 Attenuation Measurement In determining the attenuation properties of the fibre-cement building material, the geometry shown in Fig. 1 was used. The square field size required to approximate broad beam geometry was fixed at 300 mm x 300 mm for a range of tube potentials between 50 and 125 kvp. This size was chosen in order to minimize scattered radiation that is commonly associated with broad beam geometry. The range of tube potentials applied in this work is that recommended for both chest and general diagnostic examinations. Leakage and background radiations were taken care of by placing the dosimeter in a box-like sample (see Figs. 2 and 3) such that its sensor faces the X-ray source. The distance between the barrier (i.e the box-like sample) and the dosimeter, was set at 100 mm in order to achieve maximal relative transmission. Transmission measurements were carried out for increasing thicknesses and varying tube potentials between 50 and 125 kvp.

8 Figure 1: Experimental set up geometry 300 mm Dosimeter X-ray source 200mm 100mm 900mm

9 Figure 2: Dosimeter setup at 1m from X-ray source [Arrow indicating the dosimeter]

10 Figure 3: Complete experimental set-up indicating sample material at 0.9m from X-ray source [Arrow showing the fibre-cement boxlike sample]

11 3. Theoretical Computations The model adopted for computation of the relative beam transmission is as given in equation 1 as described by Archer et al. [1994] K K o 1 β α exp where; K is the relative transmission within the shield. K o the transmission in the absence of the shield. x is the material thickness and α, β, and γ are the fitting parameters which depend on the radiation source and beam quality. The values of the parameters (α, β, and γ) given by Simpkin [1995] were adopted in this study. αγx - β α 1 γ (1)

12 HVL n = x - α β 1 γ β γ β - αγx exp γ β 1 2 ln αγ 1 γ (2) Determination of HVL To further check the attenuation properties of the fibre-cement products, the Half Value Layer (HVL) was calculated as a function of the thickness, using equation 2 as given by Archer et al. [1994]. At large values of the thickness, the expression will tend toward (In2)/α, which provides an estimate of the HVL at high attenuation, as required when shielding of leakage radiation is being considered.

13 4. Results and Discussions The data obtained as transmitted exposures are presented in Table 1. The quantity K is the transmitted exposure through the sample barrier at 1 m from the X-ray source K o is the exposure without sample barrier. From this data, it was observed that the unattenuated and transmitted exposures increased with increase in tube potential. The transmitted exposures however decreased with increase in sample thickness. The relative transmission curves are shown in Fig. 4.

14 Table 1: Experimental readings obtained as transmitted exposures. Tube Voltage kvp K o msv (mamin) -1 at 1m K = Dose Rate msv (ma min) -1 at 1m 3mm 5mm 8mm 10mm 13mm 15mm X-ray source Setting: ma = 100, Time = 0.06 sec and mas = 6

15 Figure 4: Relative transmissions of X-rays through the fibre-cement for broad beam geometry conditions, produced at tube potentials in the range kvp.

16 Table 2 presents the fitting parameters for the single phase X-ray generator. Table 3 presents the computed relative transmitted values using equation 1 which describes the relative transmission of X-rays through thickness, x of the fibre-cement building material.

17 Table 2: Fitting parameters for single phase X- ray generator [Simpkin, 1995] Tube voltage kvp K o msv (ma min)- 1 at 1m α(mm -1 ) β(mm -1 ) γ

18 Table 3: Computed relative transmitted values using Archer et al. model [1994] Tube voltage kvp K o msv (ma min) -1 at 1m K msv (ma min) -1 at 1m 3mm 5mm 8mm 10mm 13mm 15mm

19 The Half Value Layer (HVL) is another means of determining attenuation properties of shielding materials. It can be calculated as a function of penetrated material thickness, this variation is expressed in equation 2. The HVL values obtained in this work are presented in Table 4. From these data, it is observed that the HVL for the fibre-cement investigated in this work increases with tube potential as shown in Fig. 5.

20 Table 4: HVL for the fibre-cement building material Tube voltage kvp HVL (mm) 3mm 5mm 8mm 10mm 13mm 15mm (In2)/α

21 HVL (mm) Figure 5: HVL variation with penetration thickness kVp 70kVp 100kVp 125kVp Material thickness (mm)

22 In shielding design, HVL is a faster means of determining an approximate estimate of shielding requirement. Comparison between transmission data for the same or different materials are usually made in terms of high attenuation HVL, because of the differences in filtration of the primary spectra which are of minor importance at high attenuation levels. Nevertheless, some authors noted that, any difference observed is expected due to a number of other factors, such as high voltage waveform of the X-ray source, the HVL of the incident X-ray beam, the size of the irradiated field at the attenuating material, the physical density of the material, and the segment of the transmission curve used to calculate the HVL at high attenuation.

23 In Table 5, the high attenuation HVLs of the fibre-cement and other building materials found in literature are tabulated for comparison. As can be seen, the HVLs for the fibre-cement are in the same range as those of Gypsum wallboard at all energies. However, when compared with data for concrete, especially at high energies of 100 kvp and above (where the Compton effect is predominant), an average thickness of mm of concrete is required, while mm of the fibre-cement is required for the same energy. This shows that when considering the total cost of construction of any diagnostic X-ray facility, application of the fibre cement as shielding material would be more expensive when compared with concrete, but cheaper when using lead as basis for comparison.

24 Table 5: Comparison with previous studies of HVL at high attenuation Material HVL (mm) Reference 50kVp 70kVp 100kVp 125kVp Rossi et al., 1991 Gypsum Simpkin, 1989 Wallboard Glaze et al., Rossi et al., 1991 Concrete Simpkin, Trout et al., Rossi et al., 1991 Steel Simpkin, Trout et al., Rossi et al., 1991 Plateglass Simpkin, Trout et al., NCRP, 1976 Lead Archer et al., Simpkin, 1989 Ytong Tsalafoutas et al., 1998 Fibre- Cement Current study

25 A typical sheet of 1200 mm x 2400 mm and 8 mm thick was found to allow a primary transmission of 17.11% at 50 kvp and 15.12% at 125 kvp. This appears to be a good attenuator in that as the peak kilovoltage increases, the energy lost through the absorber equally increases. Also, the fibre-cement can act as a structural wall unlike lead that requires plywood support which necessitates additional cost to the already expensive material.

26 5. Conclusions An investigation into the attenuation properties of fibre-cement building material produced in Nigeria has been undertaken using a single phase X-ray source. Transmission measurements were carried out using a PM1621 Polimaster dosimeter. The results obtained showed that with average primary transmissions of 17.11% and 15.12% at 50 kvp and 125 kvp respectively, walls and ceiling claddings made of the fibre-cement product may offer adequate protection as secondary shielding material in a normal workload environment. However, in a large workload environment, the required wall thickness may be impractical; especially when the cost involved is put into consideration.

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