Theoretical Study Of X-Ray Beams Transmitted Through Aerated Concrete Used In Shieldings

 

 

FAYEZ H. Al-GHORABIE, SAUD H. Al-LEHYANI AND

SAMEER S. NATTO

 

 

Department of Physics, Faculty of Applied Sciences,

Umm Al-Qura University,

Makkah, P.O. Box (10130), Kingdom of Saudi Arabia

 

دراسة نظرية لانتقال حزم الأشعة السينية خلال الخرسانة المشبعة بالهواء المستخدمة في الحماية الإشعاعية

 

يهدف هذا البحث إلى دراسة انتقال الأشعة السينية خلال مادة تستخدم لأغراض الحماية الإشعاعية في غرف التشخيص بالأشعة السينية . هذه المادة تعرف باسم الخرسانة المشبعة بالهواء، والتي تختلف عن الخرسانة المسلحة العادية المستخدمة في البناء . ولتحقيق هذا الهدف فقد تم استخدام برنامج المونت كارلو EGS4  وذلك لمحاكاة انتقال الأشعة السينية خلال الخرسانة المشبعة بالهواء . عند استخدام البرنامج تم اختيار حقول تشعيع ذات اتساع كبير وجهود تشغيل تراوحت ما بين 50–140 kVp . وقد أظهرت نتائج الدراسة أن الجدران المبنية بمادة الخرسانة المشبعة بالهواء ذات السمك 15-20 cm تكون جيدة كحاجز واق رئيسي من الأشعة الناتجة عن أجهزة الأشعة السينية المستخدمة في عيادات طب الأسنان وعيادات تشخيص أورام الثدي لدى النساء ، وكذلك كحاجز واق ثانوي في غرف التشخيص الأخرى.

 

X-ray transmission through a building material used in diagnostic x-ray rooms for radiation protection purposes was investigated. This material is commonly known as ‘Aerated Concrete’ which is different than normal concrete. EGS4 Monte Carlo code was used for the simulation applying broad beam geometry conditions and using tube potentials in the 50-140 kVp range. The results show that walls of aerated concrete of 15-20 cm thick may offer good protection in dental and mammography rooms as well as in low workload diagnostic rooms as a secondary barrier.

 

Keywords: Monte Carlo simulation, EGS4, aerated concrete, x-ray transmission.

 

 

 


INTRODUCTION

 

        With the increasing use of radiation in many fields, such as industry, medicine  and agriculture, several scientists are trying to study in depth various x-ray interactions in composite materials such as bones, plastics, alloys, soil and water. Most of the work has been done extensively in pure metals, both experimentally as well as theoretically. But in the case of composite materials, the study on the behavior of photon interactions processes such as total, photo, coherent, Compton, pair production, etc., is quite scarce and is available in limited energy range for one or another interaction only (Bhandal & Singh 1993).

Aerated concrete is a building material with a large number of air-pores. Its main constituents are quartz sand, lime and cement which represent approximately 70%, 20% and 10% respectively of its dry weight. The composition of the quartz sand which comprises the basis of aerated concrete is, according to the manufacturer, as follows:

70% SiO2, 10% CaO, 7% Al2O3, 4% Fe2O3, 3% MgO, 2% SO3, 2% K2O and 2% Na2O. Aerated concrete is very light as compared with ordinary concrete. It is easy to build with and presents favorable thermal insulation properties. It is used for wall and roof construction and it comes in various quality grades, shapes and sizes. Aerated concrete products have a dry density between 0.4 and 0.7 g/cm3, depending on the quality grade.

 

        Shielding calculations for radiology x-ray rooms are usually made in terms of the lead thickness that should be added to the walls in order to reduce the exposure outside the room to acceptable levels. Use of lead is not cheap and in some cases may be redundant; for example, when the workload is very low or low x-ray energies are used and only scattered and leakage radiation need to be considered. The National Council on Radiation Protection and Measurements (NCRP), in Report 49 (NCRP 1976), suggests that mineral-based building materials with a composition similar to that of concrete may be used for shielding. Calculations should be made according to concrete attenuation characteristics with appropriate corrections to account for differing densities. However, density corrections are adequate only when the Compton effect is considered. When the photoelectric effect is dominant, as happens at diagnostic energies, it is preferable to use the attenuation characteristics of the specific building material. This has also been pointed out in previous studies (Glase et al. 1979; Christensen & Sayeg 1979; Wohni 1981). The aim of this paper is to study the attenuation properties of aerated concrete and its suitability as a primary shielding material in diagnostic x-ray facilities. This study was performed using the well-known Monte Carlo code EGS4.

 

MATERIALS AND METHODS

 

THE EGS4 MONTE CARLO CODE

 

        The EGS4 Monte Carlo code was employed for the system modeling work of this study. The code is well established and widely used (Andreo 1991). The EGS4 code system was developed by Ford & Nelson (1978), during the period 1972-1978, based on work by Nagel at the University of Bonn. It was released in 1978 as a tool for understanding electromagnetic cascade showers in high energy physics. In an effort to adapt the code to low energy physics problems, particularly in the field of Medical Physics, Nelson et al. (1985) extended the code to what is known as EGS4. Several hundred EGS4 distribution tapes are handed out yearly, the majority to Medical Physics institutions. The EGS4 system is a flexible and open code, and with some training it is relatively easy to modify programs or extract information. The general structure of the EGS4 code is shown in Fig. 1.

 

 

Fig. 1. The general structure of the EGS4 Monte Carlo code

 

 

It consists of two distinct components, the PEGS4 preprocessor and the EGS4 simulation code. The PEGS4 creates data sets for each element, compound or mixture used in the simulation, which are read in by the HATCH routine of the EGS4 code itself. The user is responsible for writing three routines: MAIN, HOWFAR, and AUSGAB, which form what is known as the user code. MAIN performs any initialization necessary for the simulation, including the media to be used, particle parameters and cut-off energies and geometry of the simulation. Having called HATCH to obtain media data sets, MAIN then repeatedly calls SHOWER, once for each incident particle. SHOWER and its various subroutines simulate the particle and its products until they leave the region of interest, reach the end of their track or are discarded. Although these routines are never called directly by the user, they themselves frequently call two user-written subroutines, HOWFAR and AUSGAB. HOWFAR is used to determine the distance to the next medium boundary along the current path, and AUSGAB is called to score energy and any other parameters of interest.

 

THE SIMULATED GEOMETRY

 

The irradiation geometry used in aerated concrete transmission simulation is shown in Fig. 2. Blocks of aerated concrete, orthogonal in shape, with dimensions 60×25 cm2 and thicknesses of 5 and 7.5 cm were simulated, with nominal density 0.5 g/cm3. The nominal density indicates the approximate weight of the blocks under typical humidity/ambient temperature conditions, as the density is very dependent on the water content. Higher density values could be observed in recently wet blocks, since aerated concrete, because of its porosity, can absorb and retain a significant amount of water.

Transmission simulations for increasing thicknesses of aerated concrete from 5 to 27.5 cm and for tube potentials of 50, 70, 100, 125 and 140 kVp were performed. Broad beam geometry condition was established using a 60×60 cm2 field. The distance between the aerated concrete barrier and the x-ray source was set at 200 cm, while the distance between the barrier and the scoring region (detector) was set at 10 cm. The x-ray source was modeled on an existing experimental superficial x-ray machine (the Pantak-150). The maximum continuous rating of the tube is 150 kVp.

Fig. 2. The simulated experimental geometry

 

A FORTRAN program called XSPEC, written by Neitzel (1985) and distributed by the American Association of Physicists in Medicine (AAPM), was used to model the x-ray tube output as a photon source for the system. It calculates the x-ray tube spectra for a given kVp value at a given anode angle and using tungsten as target material. The theory for the spectrum calculation in the program was taken from Birch et al. (1979). An example output spectrum obtained from XSPEC is shown in Fig. 3. The detector used in the simulation was assumed as ideal.

 

 

 

Fig. 3. X-ray spectrum for a 100 kVp tube potential calculated using XSPEC program

 

 

RESULTS

The x-ray transmission curves are shown in Fig. 4. The data points were fitted using the following equation, first proposed by Archer et al. (1983) and later discussed by Simpkin (1995) :

                                        (1)

 

In the above equation, B is the relative transmission, x is the material thickness and  are fitting parameters.

Figure (4). Simulated relative transmission of x-rays through aerated concrete for broad beam geometry conditions

 

 

The fitting parameters to Equation (1) for the 50-140 kVp transmission curves are given in Table 1. They were found following the suggestions made by Simpkin (1995) with the aid of a suitable commercial computer program (Eureka, the Solver 1.0 by Borland International).

 

 

                Table 1.  Fitting parameters to Equation (1) for aerated concrete

kVp (volts)

 (cm-1)

 (cm-1)

50

0.2881

0.4132

0.79456

70

0.1599

0.3083

0.86604

100

0.1048

0.2060

1.06732

125

0.0881

0.1529

1.19496

140

0.0780

0.1366

0.21935

 

 

Table 2 presents HVLs for aerated concrete in centimeters, for different radiation qualities. Thickness in g/cm2 can be obtained by multiplying by the selected density of 0.5 g/cm3. The nth HVL tabulated values were found using Equations (2) and (3) described below, setting the relative transmission values (B) to 1/2n (for n = 1-5).

 

 

Table 2.  Half-value layer for aerated concrete for different kVp x-rays

 

kVp

HVL (cm)

 

(ln 2)/a: High attenuation HVL estimate (cm)

First

Second

Third

Fourth

Fifth

50

1.15

1.48

1.76

1.99

2.15

2.4

70

1.79

2.43

3.03

3.50

3.83

4.3

100

2.81

4.01

5.04

5.75

6.17

6.6

125

3.67

5.22

6.43

7.17

7.54

7.9

140

4.14

5.93

7.31

8.13

8.54

8.9

 

 

 

                                                 (2)

                         

                                                             (3)

 

The above equations can be used to acquire the tenth-value layer (TVL) values, by setting the relative transmission values to 1/10n. Moreover, as noted by Simpkin (1995), it is possible to calculate the HVL as a function of penetrated material thickness, using the following equation:

 

 

                  (4)

           

For large values of x, this expression will tend toward (ln 2)/for broad x-ray beam, and provide an estimate of the HVLs at high attenuation, as is required when shielding of leakage radiation is considered. These values are also given in Table 2, where it can be seen that the fifth HVL approximates the high attenuation HVL estimate. The variation of HVL with penetrated material thickness is shown in Fig. 5.

 

 

 

 

 

 

Fig. 5. Variation of HVL with thickness of aerated concrete as predicted by Equation (4)

 

 

The high attenuation HVLs of aerated concrete (in g/cm2) and other building materials found in the literature, are tabulated for comparison in Table 3. Values for lead are also given for reference.

 

 

Table 3. The high attenuation HVL of aerated concrete, lead, concrete and

other building materials at different kVp x-rays

Material

Study

Density (g/cm3)

HVL (g/cm2)

 

50 kVp

70 kVp

100 kVp

125 kVp

140 kVp

Aerated concrete

Current study

0.500

1.22

2.21

3.37

4.00

4.51

Leca

(Wohni 1981)

0.85

1.51

2.17

3.31

4.21

4.34

Gypsum

 

 

 

 

 

(Glase et al. 1979)

(Christensen and Sayeg 1979)

(Rossi et al. 1991)

(Simpkin 1989)

(Archer et al. 1994)

0.75

0.74

 

0.70

0.73

0.75

0.83

0.96

 

1.01

1.23

1.25

1.65

1.71

 

1.83

1.71

2.20

3.45

2.74

 

2.75

2.45

3.52

4.05

----

 

2.98

3.55

3.34

----

---

---

---

---

Concrete

 

 

 

(NCRP49 1976)

(Simpkin 1989)

(Rossi et al. 1991)

(Simpkin 1995)

2.35

2.35

2.20

2.37

1.01

2.19

----

1.82

1.97

2.43

----

3.23

3.76

3.60

2.83

4.19

4.70

4.30

3.45

4.69

----

----

----

4.76

Lead

(NCRP49 1976)

(Simpkin 1989)

(Archer et al. 1994)

11.35

11.35

11.35

0.068

----

0.062

0.170

0.120

0.134

0.307

0.278

0.302

0.318

----

0.321

----

0.284

----

 

 

DISCUSSION

 

            Comparisons between transmission data for the same or different materials are usually made in terms of high attenuation HVL (in cm or g/cm2) since the differences in filtration of the primary spectra are of minor importance at high levels of attenuation (Wohni 1981). Nevertheless, it can be seen that significant differences are still observed in the HVL values reported by different authors. As commented by Rossi et al. (1991), to some extent these differences are expected because x-ray beam HVL may influence measured or calculated results.

HVLs of aerated concrete (in g/cm2) are similar to those of Leca and Gypsum Wallboard (Archer et al. 1994) at all energies. When comparing aerated concrete with data for concrete from Simpkin (1995, 1989), it appears that aerated concrete is a more effective attenuator than concrete at all energies. However, when comparing aerated concrete with data for concrete from the NCRP (1976), aerated concrete is less effective attenuator than concrete at 50 and 70 kVp. The concrete to aerated concrete HVL ratio, for the 50-125 kVp range, is given in Table 4. It can be seen that for high energies (where the Compton effect is predominant) this ratio tends to 1.

 

 

Table (4).  Concrete to aerated concrete high attenuation HVL ratios for

different  kVp x-rays

 

Concrete Data

Source

HVL’s ratio

 

50 kVp

 

70 kVp

100 kVp

125 kVp

140 kVp

(NCRP49 1976)

 

0.83

0.89

1.11

1.18

---

(Simpkin 1989)

 

1.80

1.10

1.07

1.08

---

(Simpkin 1995)

 

1.49

1.46

1.24

1.16

1.05

 

 

In typical constructions, aerated concrete walls are 15-20 cm thick. A wall of aerated concrete of 20 cm thickness allows primary transmission of 1.2% at 70 kVp and 8% at 125 kVp. When leakage radiation is considered, 20 cm of aerated concrete is equal to about 1.4 and 0.76 TVL, at 70 and 125 kVp, respectively. Thus, aerated concrete is a good attenuator below 70 kVp (dental and mammography facilities) but a relatively poor attenuator above 100 kVp.

            In common aerated concrete construction, the blocks are surfaced with at least 10 mm of plaster (with a typical density of 1.6 g/cm3) on one or both sides and the effective wall thickness is thus increased. The joints between the aerated concrete blocks should be covered by a generous amount of plaster. It is also good practice, when aerated concrete is used for shielding, to substitute two 10 cm thick blocks for one 20 cm thick block, offset so that the seams do not meet. Local density inhomogeneities do not seem to present a problem. To compensate for such inhomogeneities, a commonly employed practice is to allow one extra half-value thickness (Christensen & Sayeg 1979).

 

CONCLUSION

 

            Aerated concrete is a building material which is not suited for use as a primary shielding material in common diagnostic X-ray facilities. However, in dental and mammography applications it can fulfil the shielding requirements. As a secondary barrier, aerated concrete could be used in low-workload conventional diagnostic X-ray facilities. For large workloads, the required wall thickness may be impractical, especially when the available space is limited. However, if aerated concrete is likely to be employed as a building material anyway, its use should be taken into account when shielding calculations are being made, thus reducing the cost of shielding.

 

REFERENCES

 

Andreo, P. 1991. Monte Carlo techniques in medical radiation physics. Physics in Medicine and Biology 36: 861-921.

 

Archer, B. R., Fewell, T. R., Conway, B. J. & Quinn, P. W. 1994. Attenuation properties of diagnostic x-ray shielding materials. Medical Physics 21: 1499-1507.

 

Archer, B. R., Thomby, J. I. & Bushong, S. C. 1983. Diagnostic x-ray shielding design based on an empirical model of photon attenuation. Health Physics 44: 507-517.

 

Bhandal, G. & Singh, K. 1993. Study of the mass attenuation coefficients and effective atomic numbers in some multielement materials. Applied Radiation and Isotopes 44: 929-939.

 

Birch, R., Marshall, M. & Ardran, A. 1979. Catalogue of spectral data for diagnostic x-rays. HPA Scientific Report Series 30, York, UK.

 

Christensen, R. C. & Sayeg, J. A. 1979. Attenuation characteristics of gypsum wallboard. Health Physics 36: 595-600.

 

Ford, R. & Nelson, W. 1978. The EGS4 code system: Computer programs for the Monte Carlo simulation of electromagnetic cascade showers. Stanford linear accelerator center, Stanford, USA.

 

Glase, S., Schneiders, N. & Bushong, S. 1979. Use of gypsum wallboard for diagnostic x-ray protective barriers. Health Physics 36: 587-593.

 

National council on radiation protection and measurements. 1976. Shielding Materials. In: Structural shielding design and evaluation for medical use of x-rays and gamma rays of energies up to 10 MeV, NCRP 49, Washington, DC, USA.

 

Neitzel, U. 1985. Program XSPEC. American association of physicists in medicine (AAPM), USA.

 

Nelson, W., Hirayama, H. & Rogers, D. 1985. The EGS4 code system. Stanford Linear Accelerator Center, Stanford, USA.

 

Rossi, R. P., Ritenour, R. & Christodoulou, E. 1991. Broad beam transmission properties of some common shielding materials for use in diagnostic radiology. Health Physics 61: 601-608.

 

Simpkin, D. J. 1989. Shielding requirements for constant potential diagnostic x-ray beams determined by Monte Carlo calculation. Health Physics 56: 151-164.

 

Simpkin, D. J. 1995. Transmission data for shielding diagnostic x-ray facilities. Health Physics 68: 704-709.

 

Wohni, T. 1981. Broad beam attenuation in Leca for 50-l40 kVp x-rays. Health Physics 40: 205-209.

 

 

 

(Received 3/12/1419; 20th March 1999, accepted 7/5/1420; 18th August 1999)