Download Effective Atomic Numbers for Some Low-Z

Effective Atomic Numbers for Some Low-Z Materials.
R. R. Bhosale1. D. K. Gaikwad1, P. S. Kore1, S. D. Raut1, V. V. Awasarmol1, C.V. More1, P. P. Pawar1 and M. N. Rode*
Department of Physics, Dr. Babasaheb Ambedkar Marathwada Univesity, Aurangabad: 431004, India (M.H)
*
Department of Physics, Vaidyanath College, Parli-Vaijnath, Dist.Beed-431515, India.
E-mail address: [email protected].
___________________________________________________________________________________________________________
Abstract—Effective atomic numbers (Zeff) of some Low-Z materials such as Polyethylene (C2H4), Teflon (C2F4) and Mylar
have been computed using measured mass attenuation coefficient in the incident energy of photons ranging from 10 keV to
1500 keV. Accurate data on the effective atomic number widely use in the radiation therapy for dose calculation and also for
characterization of composites. Mean atomic numbers of chosen samples were found close to Zeff. It was also observed that the
effective atomic Numbers (Zeff) Polyethylene and Teflon smaller than Mylar. The experiment was carried out by using NaI (Tl)
detector in narrow beam good geometry set-up. Experimentally computed results are found to be in good agreement with the
theoretical values calculated based on XCOM data.
K ey Words— Mass attenuation coefficient, effective atomic number, effective electron density.
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INTRODUCTION
F
ROM last few years as the use of radiation increases and
gives immense importance in many fields in nuclear physics, electronic industry, material modification, medical science, coating, paint industry, agriculture industry etc. The interaction process mainly depends upon the intensity and the type of
absorbing material. The gamma rays have greater penetrating
power and obey different absorption laws [1-3]. As the use of
radiation is wider for different applications, it is most important
to study the interaction & absorption of gamma radiations in
materials. For the study of absorption & interaction the basic
quantities are effective atomic number, electron density & mass
attenuation coefficient [4-6]. The study of attenuation coefficient
gives more importance to materials in the energy 10-1500 KeV.
The gamma radiations from the energy region 200 keV to 1500
keV interact with material mainly due to dominance photoelectric and Compton Effect photon interaction processes. Due to the
interaction of gamma ray with matter certain changes may occur
in materials like defects, color centers may changes, so the radiation studies on materials gives the idea about materials properties. The effective atomic number, electron density, total attenuation cross-section are the basic quantities in determining the
penetration of radiations on materials. The scattering and absorption are closely related to the density and atomic number of an
element [7-8].
Atomic number hold good for single element, for composite
material atomic number replaced by effective atomic number.
Composite material composed of different chemical composition
for which single value cannot suitable across entire energy region, the value which represent atomic number in composite
material is effective atomic number. The effective atomic number is not constant value, varies with energy. Mostly effective
atomic number is use to calculate absorbed dose of biomolecules in radiotherapy. The mass attenuation coefficient investigates atomic and molecular level properties of the material.
Moreover, penetration and energy deposition of gamma ray photon in biological important material can be calculated by mass
attenuation coefficient. Data on the effective atomic numbers
have been published for different of materials in view of medical
physics, radiation biology and dosimetry applications [9-14].
The number of investigators calculated mass attenuation coefficient, effective atomic number (Zeff) and electron density for
elements and composite materials separately. Recently, mass
attenuation coefficient, effective atomic number and electron
density, for important bio molecules calculated by [15-19].
In the presentin paper, effective atomic number of Polyethylene,
Teflon and Mylar have been computed experimentally using
good resolution detector. Results of Zeff compared with theoretical values. Close scrutiny of literature shows effective atomic
number of chosen samples have not been reported before in the
well collimated narrow beam good geometry setup.
EXPERIMENTAL
The six radioactive sources 57Co, 133Ba, 137Cs, 54Mn, 60Co
and 22Na were used for irradiation of samples. These sources
emitted gamma energy of photons 122 keV, 360keV, 511keV,
662keV, 840keV, 1170keV, 1275keV and 1330 keV were collimated. A NaI (Tl) scintillation detector having good energy resolution was calibrated using the aforesaid sources. Counted signals from the detector were amplified and analyzed with 13-bit
multichannel analyzer. The efficiency of NaI (Tl) detector is
higher at low source energy. The uncertainty in measured experimental data is estimated to be less 4 %. It was observed that the
attenuation of photons of the empty containers were negligible.
The sample thickness was selected in order to maintain desired
transmission ratio. For measurement of incident and transmitted
photon energies a narrow beam good geometry set up was used.
The values of mass attenuation coefficients were also obtained
using the XCOM program at all photon energies of current interest.
Theory
Effective atomic numbers (Zeff) have been calculated for chosen
samples using the measured values of mass attenuation coefficient and atomic number of the samples by following relation:
90
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Vol. 8 (3) December 2015
Print ISSN 0974-0678, online: 2320-9593, www.bionanofrontier.org
¦ n A P
i
i
Z
U i
i
eff
¦ n A / P
i
i
(1)
U i
4.8
i
4.6
Where, μ/ρ, Zi and Ai are the mass attenuation coefficient, atomic number and atomic weight of the ith constituent element of the
samples, respectively.
4.4
Polyethelene
Teflon
Mylar
4.2
4.0
3.8
3.6
Zeff
R ESULTS AND DISCUSSIONS
The values of mean atomic number calculated from chemical
formula for some Low-Z materials are presented in table1. Experimentally measured results of mass attenuation coefficients
have been used for computation of effective atomic number of
Polyethylene (C2H4), Teflon (C2F4) and Mylar displayed in the
table 2along with the theoretical results calculated using XCOM
program based on the mixture rule for energies122keV, 356keV,
511keV, 662keV, 840keV, 1170keV, 1275keV & 1330 keV. Fig.1
depicts the effective atomic numbers of the chosen samples as a
function of energy. It is also observed that the effective atomic
number of the Polyethylene tends to almost constant as a function of incident energy of gamma photons. Also, effective atomic
number of the Teflon almost independent of energy due to dominance incoherent scattering is shown in the Fig.1. It is also seen
from Fig.1 and table 2 that the effective atomic number of Mylar
initially decreases with energy of incident photons of energy and
tends to be almost constant. The values for these Low-Z materials at energies 122, 356, 511, 662, 840, 1170, 1275 & 1330 keV
calculated experimentally using NaI(Tl) detector and theoretically using XCOM were compared. Errors in the calculated results
due to different uncertiantity were calculated and taken care. The
overall estimated error in computed data is of the order of 2%.
Uncertainties in thickness (<0.6%) preparation, impurity and
non-uniformity of a sample is less than 0.6 %. The photon
buildup, multiple scattering and pulse pile-up effects were reduced to minimum value by choosing optimum thickness (µt<4)
of a sample and optimum count rate and counting time of a detector with a good resolution. The error due to a non-uniformity
of the sample was negligible. The transmission experiment in
narrow beam geometry setup was performed and repeated for
each sample with periods of 300s- 900s in order to minimize
statistical uncertainty (<0.5%), the small deviation in count after
every repetition which confirmed good sensitivity of instrument.
in the present energy range, small-angle scattering corrections
were found to be very very small.good agreement between the
experimental and theoretical values has been observed.
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
0
200
400
600
800
1000
1200
1400
Energy (keV)
Fig.1. Typical plot of Zeff versus Energy.
CONCLUSIONS
Effective atomic numbers of the chosen samples have been computed in view of understand the irradiation effects on the low
atomic number composites. It has been observed that the effective atomic number of the Polyethylene smaller than the Teflon
and Mylar. Investigation of present samples reveals the sufficient
information for shielding of gamma energy of photons by low-Z
composites. Obtained results in the present energy region should
be useful for selection of phanthom materials.
Table 1. The mean atomic numbers calculated from the chemical formula for Low-Z Materials
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Low-Z
Molar mass (g/mol)
Chemical Formula
Mean atomic number, < Z >
1.
Polyethylene
28.05
C 2H5
2.4285
2. Teflon
100.02
C 2F4
8.00
3. Mylar
100.12
C 10H10O2
3.9090
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Table 2. Effective atomic number, Zeff of some Low-Z materials.
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Low-Z
122keV
356keV
511keV
662keV
Meth.B.
840keV 266, 1127-1131
1170keV
1275keV
1330keV
Exp. Theo. Exp. Theo.
Exp. Theo.
Exp. Theo. Exp. Theo.
Exp. Theo. Exp. Theo. Exp. Theo
------------------------------------------------------------------------------------------------------------------------------------------------------------------------Polyethylene 2.093 2.095 2.149 2.148 2.165 2.169 2.182 2.180 2.194 2.196 2.224 2.226 2.217 2.219 2.205 2.210
Teflon
3.820 3.823 3.820 3.822 3.815 3.819 3.818 3.815 3.817 3.818 3.815 3.817 3.815 3.816 3.815 3.816
Mylar
4.754 4.755 4.532 4.533 4.510 4.508 4.672 4.674 4.662 4.663 4.647 4.649 4.642 4.644 4.640 4.638
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A CK NOWLEDGMENT
One of the authors (D.K. Gaikwad) wish to thank UGC New
Delhi for providing RGNF.
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