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Abstract.
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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 50-125kVp 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.
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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.
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In designing effective shielding, many other materials have been
considered as substitute to lead and concrete.
Examples are: gypsum wallboard, plate glass and wood.
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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
1.4x103 kg/m3 depending on the grade and thickness.
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2. Materials and Methods
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2.1 Instrumentation
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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) 61526 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.
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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 50-150 kVp, current range of 60-300 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
dimensions 300 mm x 300 mm x 300 mm.
box-like
shapes
of
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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.
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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.
Figure 1: Experimental set up geometry
300 mm
Dosimeter
200mm 100mm
X-ray
source
900mm
Figure 2: Dosimeter setup at 1m from X-ray source [Arrow
indicating the dosimeter]
Figure 3: Complete experimental set-up indicating sample material
at 0.9m from X-ray source [Arrow showing the fibre-cement boxlike sample]
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]
1
 β 
β γ
K  K o  1   exp αγx  - 
α
 α 
(1)
where;
K is the relative transmission within the shield.
Ko 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.
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.
HVLn =

 γ

2
1


 ln 
 αγ 



 β 
β  β 
1   exp αγx  -    
γ  γ 
 γ 
 - x
β
1

α
 
(2)
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4. Results and Discussions
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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
Ko 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.
Table 1: Experimental readings obtained as transmitted
exposures.
Tube
Voltage
kVp
Ko
mSv
(mAmin)-1
at 1m
K = Dose Rate mSv (mA min)-1 at 1m
3mm
5mm
8mm
10mm
13mm
15mm
50
0.89
0.81
0.76
0.68
0.62
0.54
0.49
70
0.94
0.86
0.81
0.73
0.67
0.59
0.54
100
0.96
0.88
0.83
0.75
0.69
0.61
0.56
125
0.99
0.91
0.86
0.78
0.72
0.64
0.59
X-ray source Setting: mA = 100, Time = 0.06 sec and mAs = 6
Figure 4: Relative transmissions of X-rays through the fibre-cement
for broad beam geometry conditions, produced at tube potentials in the
range 50 -125 kVp.
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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.
Table 2: Fitting parameters for single phase Xray generator [Simpkin, 1995]
Tube
voltage
kVp
Ko
mSv (mA min)- 1
at 1m
α(mm-1) β(mm-1)
γ
50
0.89
0.0388
0.0873
0.5105
70
0.94
0.0230
0.0716
0.7299
100
0.96
0.0147
0.0417
0.8939
125
0.99
0.0192
0.0286
0.9684
Table 3: Computed relative transmitted values using Archer et al. model [1994]
Tube
voltage
kVp
Ko
mSv
(mA
min)-1
at 1m
K mSv (mA min)-1 at 1m
3mm
5mm
8mm
10mm
13mm
15mm
50
0.89
0.6238
0.5029
0.3734
0.3105
0.2395
0.2034
70
0.94
0.7219
0.6167
0.4975
0.4360
0.3629
0.3236
100
0.96
0.8177
0.7409
0.6458
0.5928
0.5251
0.4865
125
0.99
0.8809
0.8188
0.7382
0.6914
0.6296
0.5932
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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.
Table 4: HVL for the fibre-cement building material
Tube
HVL (mm)
voltage 3mm
5mm
kVp
8mm
10mm
13mm
15mm
50
70
100
125
8.09
11.98
19.20
25.44
8.50
12.68
20.07
26.34
9.09
13.66
21.32
27.64
9.46
14.29
22.11
28.47
6.94
10.09
16.87
23.04
7.42
10.87
17.83
24.02
(In2)/α
17.86
30.14
47.15
58.25
Figure 5: HVL variation with penetration thickness
50kVp
70kVp
30
100kVp
125kVp
25
HVL (mm)
20
15
10
5
0
0
5
10
Material thickness (mm)
15
20
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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.
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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 18.33 mm of concrete is
required, while 47.28 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.
Table 5: Comparison with previous studies of HVL at high attenuation
Material
Gypsum
Wallboard
Concrete
Steel
Plateglass
Lead
Ytong
FibreCement
HVL (mm)
50kVp
14.45
16.86
11.00
70kVp
26.15
23.42
22.00
100kVp
39.29
33.58
46.00
125kVp
42.65
48.63
54.00
9.32
5.08
0.34
0.46
0.30
5.75
8.47
5.30
0.06
0.06
12.2
17.86
10.36
12.70
0.78
0.83
1.00
9.41
10.66
8.90
0.17
0.12
0.13
22.1
30.14
12.90
15.31
17.78
1.42
1.80
2.70
15.83
15.06
13.50
0.27
0.27
0.25
33.7
47.15
15.66
18.33
20.32
2.15
3.27
3.60
18.05
17.54
16.00
0.28
0.28
40.0
58.25
Reference
Rossi et al., 1991
Simpkin, 1989
Glaze et al., 1979
Rossi et al., 1991
Simpkin, 1989
Trout et al., 1959
Rossi et al., 1991
Simpkin, 1989
Trout et al., 1975
Rossi et al., 1991
Simpkin, 1989
Trout et al., 1959
NCRP, 1976
Archer et al., 1994
Simpkin, 1989
Tsalafoutas et al., 1998
Current study
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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.
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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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REFERENCES
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