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Научни трудове на Съюза на учените в България-Пловдив. Серия В. Техника и технологии, естествен ии хуманитарни науки, том XVI., Съюз на учените сесия "Международна конференция на младите учени" 13-15 юни 2013. Scientific research of the Union of Scientists in Bulgaria-Plovdiv, series C. Natural Sciences and Humanities, Vol. XVI, ISSN 1311-9192, Union of Scientists, International Conference of Young Scientists, 13 - 15 June 2013, Plovdiv.
INVESTIGATION OF RADIATION PROTECTIVE PROPERTIES OF
LEADED RUBBER
Yana N. Gluhcheva(1), Diana Adliene(2) and Todorka L.Dimitrova(1)
(1): University of Plovdiv "Paisii Hilendarski", Tzar Assen Str.24, BG-4000
Plovdiv
(2): Kaunas University of Technology, Physics Department, Studentq g. 50,
LT-51368 Kaunas, Lithuania
e-mail(1): yana_gl@abv.bg e-mail(2): diana.adliene@ktu.lt e-mail(1): doradimitrova@uniplovdiv.bg
Abstract
Lead is used in almost all medical applications where radiation shielding is needed. However due to the high toxicity, its application should be restricted and the Pb amount in the shielding should be reduced. X-ray therapy is one of the cancer treatment techniques where application of radiation protection measures is required due to the possible radiation damage of the irradiated organs.
The results of the investigation of the radiation protective properties of leaded rubber are presented in this paper. Leaded rubber taken from radiation protective apron was suggested for shielding of ocular structures (such as cornea, lens, conjunctiva and retina) during superficial treatment (kilovoltage range) of skin cancers in the periorbital region. The chemical content of the rubber was evaluated and used for modelling of the photon interaction processes with biological tissue in the superficial energy range (80 -180 keV). The attenuation coefficients for photoelectric absorption, the coherent and the incoherent scattering were evaluated. The evaluated lead thickness equivalent of the rubber is 1.16 mm. However, it's higher than lead equivalent of Pb plackets that are usually used in superficial therapy.
1. Introduction
Superficial X-ray therapy is offen used to treat superficial lesions, neck nodes, superficial bone metastases and lymphomas at the head and at the neck regions. Despite of the fact that the treatment doses in kilovoltage X-ray therapy are lower in comparison with the external radiation therapy performed by using linear accelerator, they can be harmful to radiation sensitive organs
since the risk for radiation damage of irradiated organs is still present [1, 2].
During superficial head and neck X-ray therapy it worth to protect ocular structures (such as cornea, lens, conjunctiva and retina) and other critical organs located close to the treatment field. The implementation of radiation protection measures for the periorbital region is very specific. Usually Pb plackets are used for eye shielding. However, there are many discussions whether covering of eye or skin in facial area with lead plackets is effective enough and does not enhance the dose beneath the shielding because of scattered photons originating from the applicator, lead shielding and patient/phantom [3, 4].
The aim of this work was to investigate radiation protective properties of leaded rubber which is used for production of protective clothes in radiography and has less lead content as compared to Pb plackets with a purpose to assess its applicability as protective shielding in superficial X-ray therapy.
2. Brief theoretical notes
Photon beam attenuation: When passing through material the photon beam intensity is reduced due to the photon- matter interaction processes. If the original intensity of a narrow monoenergetic photon beam is I0, then:
Ix= l0e-^I = Ir,e (1)
where I is the resulting intensity, m is the linear attenuation coefficient, which depends on the photon energy hn and on the atomic number of the attenuator Z, and x is the attenuator thickness. Very often to present attenuating property of the material the mass attenuation coefficient
¡m = - / p is used instead of the linear attenuation coefficient, where p is the mass density of the matter.
Photon interactions with matter: A photon may interact with matter in several different ways. The probability for that depends of the photon energy and of the atomic number of the attenuator. A photons generated at kilovoltage range participate mainly in three different interaction processes: fotoelectric absorption, Coherent (Rayleigh) scattering and incoherent (Compton) scattering.
Photoelectric effect: It's a prosees defined as an interaction between a photon and a a tightly bounded orbital electron of an attenuator. During this process the whole photon energy is absorbed. The atomic cross section (attenuation coefficient) for the photoelectric effect at is proportional to Z4/ hn3, where hn is the photon energy and Z is the atomic number of the attenuator.
Coherent (Rayleigh) scattering: Coherent scattering is caused by photon interaction with a bound orbital electron (i.e. with the whole atom). During this process the photon loses none of its energy and is scattered through only a small angle. The atomic cross-section for coherent scattering asR is proportional to (Z/hn)2. In tissue equivalent materials the relative importance of coherent scattering is low compared with the other photon interactions.
compton effect (incoherent) scattering: The Compton effect is interaction between a photon and a free or stationary orbital electron. During this process the photon loses some of its energy to the Compton electron and scatters with reduced energy through certain angle. The atomic Compton attenuation coefficient asC is linear proportional to the atomic number Z of attenuator.
If the photon has energy hn and attenuator has atomic number Z, the attenuation coefficient m is given as a sum of coefficients for individual photon interactions:
= ¡=a T+a^R +a°C .
(2)
The radiation effects in matter caused by photon interactions are evaluated in terms of
absorbed dose, which is equal to mean energy s imparted to matter of mass m in a finite volume by ionizing radiation:
D = ds / A (3)
Dose in air is measured in terms of air kerma K(0) - kinetic energy released in the matter.
3. Experimental setup and measuring methods
Experimental samples: 3 mm thick pieces of leaded rubber used to produce protective clothes for radiographers were investigated. This material contains lead and often other metals (e.g., tin, tungsten, antimony, barium) to shield the person from radiation. Metals are homogeneously mixed with synthetic rubber. From 2 to 5 layers of thin sheets of metal-impregnated rubber are placed together to form the protective material. The manufacturers may vary the number of sheets, the percentage of metal, the grade of rubber and the mixture of metals to affect flexibility, durability, radiation absorption efficiency, and weight of protective material. However this information is not public. Lead equivalent of these materials should be not less than 0.25mmPb.
Experimental methods: In order to perform modeling of attenuation properties of experimental samples, the chemical content of the leaded rubber was evaluated using the method of the X-ray energy dispersion spectroscopy. Scanning electron microscopy with additional functions (SEM ISM 5600 + EDS, Brucker X Flash QUAD 5040) was used to perform these measurements.
Mathematical modeling of the attenuating properties of experimental samples was performed using XCOM data base [5].
Experimental measurements: Lead equivalent of the experimental samples was performed according to the international recommendations [6] on the basis of the measurement results obtained using the experimental set up shown in the Fig. 1. The irradiation of the samples was performed by diagnostic X-ray machine MULTIX PRO. The "Barracuda" (RTI Electronics) multimeter with multi-purpose detector was used for the voltage measurements and in combination with detector R100B - for the dose measurements.
X-ray tube
Rubber Detectar
100 cm
Fig. 1. Experimental set up for the evaluation of the lead equivalent
Firstly, the transmission of the X-rays B(x) was determined by measuring the air kerma K(0) without sample and the dose D(x) beneath the leaded rubber placed onto the detector:
B<X< =
gfr)
K(0)
B( X) =
D( x) K (0)
Then the thickness of the lead equivalent x was calculated:
x= -mi^-Ji x= ± (B~r+P/a)
X ^ ( 1 + p/a ) (5)
where a, b, g are fitting parameters, depending on the applied voltage. For X-ray tube voltage of 100 kV, a = 2.430mm-1, p =14.44mm-1 and y = 0.7422.
Several measurements have been performed for each sample and the average lead equivalent for the leaded rubber was evaluated.
4. Results and discussions
The pprotective features of the rubber shielding depend on the lead content in it. The chemical content and normalized weight fractions of the leaded rubbee components were evaluated using standard energy dispersion spectroscopy (EDS) measurement and aire; presented in Fig. 2.
>
<D
in
s
s
o O it
£ J
ISl
Ö 1
<D HI
S Ii
¡1
i
u
1
Element Series Normalized Error,
C, [wt.%] [%]
Oxygen K-series 48.99 5.8
Carbon K-series 25.78 4.6
Sodium K-series 0.98 0.1
Lead M-series 22.34 0.9
Magnesium K-series 0.24 0.0
Silicon K-series 0.63 0.1
Aluminium K-series 0.15 0.0
Calcium K-series 0.52 0.0
Potassium K-series 0.37 0.0
Total: 100.0
Fig. 2. Chemical conten t of the leaded silicon rubber.
It was found that there is approximately 22 % o° lead in the elemental composition of the leaded rubber. The data from Fig. 2 were used for modelling o° the photon attenuution erf the experimental samples and for theoretical evaluation of the lead equivalent of the leaded rubber.
Modelling was performed by usrng the data base XCOM [5]. The comgprison oo the photon cross eections for the leaded rubber and for pure lead in the eneegy range up to 200 keV are provided in Fig.3. and Fig.4
Leaded Rubber
1,0E+01 1,0E+00 1,0E-01 1,0E-02
0 0,1 Energy, MeV
0,2
-----Coherent Scatter.
----Incoher. Scatter.
Photoel. Absorb.
Tot.with Coherent
Fig. 3. Kilovoltage photon cross sections (mass attenuation coefficients) for leaded rubber. Mass attenuation coefficients are shown on log scale
Lead
Ç»
?5 £ o
c
ou o
«5 o o c O
ra
3
c
0)
1,0E+02 1,0E+01 1,0E+00 1,0E-01 1,0E-02
0 0,1 Energy,MeV
0,2
-----Coherent Scatter.
----Incoher. Scatter.
.........Photoel. Absorb.
-Tot.with Coherent
Fig. 4. Kilovoltage photon cross sections (mass attenuation coefficients) for pure lead. Mass attenuation coefficients are shown on log scale
From the modeling results the lead equivalent of the leaded rubber was estimated to be 1.13mm at 100 kV X-ray generator voltage.
In order to evaluate the experimental value of the lead equivalent for the rubber, series of dose measurements were performed by using the experimental setup described in Section 3 of this article. During all measurements were kept constant the next parameters: 100 cm distance between the focal spot and the detector; X-ray tube voltage of 100 kV and current of 200 mA. The measured air kerma without shielding K(0) was estimated to be 1659.10 DGy and the dose with shielding D(x) was varying from 7.33 to 9.75DGy. The average lead equivalent was estimated to be 1.16 ± 0.47 mm Pb for leaded rubber. It was comparable with the theoretical value within the range of standard deviation. 248
5. Summary
Investigation of shielding properties of rubber used for designing of radiation protection clothes has been performed. It was found that this rubber consist of ~22% (wt) of lead, which is the most important component related to the photon attenuation properties. The modelling results have shown that the photoelectric absorption property of leaded rubber is lower in comparison with the Pb itself and the contribution of the incoherent scattering to the total attenuation is higher. The scattering effects inside of the shielding material are undesirable since they may enhance the dose beneath the shielding. Nevertheless, the evaluated lead equivalent of 3 mm thick leaded rubber samples was found to be 1.16 mm. This value is more than sufficient to protect eyes and other skin regions from the unnecessary exposure during the superficial X-ray treatment.
References
1. Butson M. J., Cheung T., Yu P., Price S., Bailey M. Measurement of radiotherapy superficial X-ray dose under eye shield with radiochromic film. Physica Medica 2008 (24): p. 29-33.
2. Medvedevas N., Adliene D., Laurikaitiene J., Andrejaitis A. The role of shielding in superficial X-ray theraphy. Radiation Protection Dosimetry (2011), Vol. 147, No. 1-2, pp. 291295.
3. Wolstenholme V, Glees J. The role of kilovoltage X-rays in the treatment of skin cancers. Eur Oncol Dis 2006: p. 32-35.
4. Medvedevas N., Adliene D., Laurikaitiene J. Distribution of scattered radiation in superficial X-ray therapy. Proceedings of the International Conference Medical Physics in the Baltic States 8, 133 - 136 (2010).
5. XCOM: http://www.nist.gov/pml/data/xcom/
6. NCRP Report 147. Structural Shielding Design for Medical X-Ray Imaging Facilities (2004).
