Studying the effect of nano lead compounds additives on the concrete shielding properties for γ-rays

Nuclear Instruments and Methods in Physics Research B 360 (2015) 81–89

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Studying the effect of nano lead compounds additives on the concrete shielding properties for c-rays H.E. Hassan a,b,⇑, H.M. Badran a,c, A. Aydarous a, T. Sharshar a,d a

Physics Department, Faculty of Science, Taif University, P.O. Box 888, Al-Hawiah, Taif 21974, Saudi Arabia Cyclotron Facility, Nuclear Research Center, Atomic Energy Authority, Cairo 13759, Egypt c Physics Department, Faculty of Science, Tanta University, Tanta 31527, Egypt d Physics Department, Faculty of Science, Kafrelsheikh University, Kafr El-Sheikh, Egypt b

a r t i c l e

i n f o

Article history: Received 5 June 2015 Received in revised form 18 July 2015 Accepted 27 July 2015

Keywords: Gamma-rays shielding Concrete Nano-lead compounds additives Gamma-ray attenuation coefficient Positron annihilation spectroscopy

a b s t r a c t In the present work the effect of concrete incorporation with two types of nano-lead compounds on its c-ray shielding characteristics is investigated. The concrete samples were prepared according to the local standards of building materials and doped by different percentages of PbO and PbTiO3 nano powders which were prepared using co-precipitation and oxalate precursor techniques, respectively. In addition, commercial PbO2 powder additive was used to check the effect of particle size on concrete attenuation properties. The phase composition and particle size of all the lead-oxide additives were confirmed by XRD and TEM imaging. The c-rays attenuation coefficients were measured as a function of the additive percentage of lead compounds for c-ray energies of 662, 1173 and 1332 keV using 137Cs and 60Co sources. The microstructure changes occurred in the concrete samples doped with Pb compounds additives were probed using the positron annihilation spectroscopy (PAS) and the results were compared with that for normal concrete. The obtained data revealed that the overall defect density of the investigated samples, as seen by the positrons, decreases with increasing the nano-PbO contents which is in agreement with the determined values of the samples apparent densities. It was found that the c-ray attenuation coefficient of concrete doped by nano-PbO is improved. The results are explained in the view of the fine structure enhanced modification and its impact on the c-ray interaction probability at different energies. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Radiation protection materials and tools gained much interest due to the wide spread of using ionizing radiations in several fields. Periodic exposure to ionizing radiation occurs for workers or patients in a severe radioenvironments caused by reactors facilities, X-ray tubes, accelerators and radioisotopic sources imposes safety regulations for protection from that radiation. Selection of suitable shielding materials depends on the type, intensity and energy of radiation source, as well as the shielding properties such as mechanical strength, economic preparation and resistance to radiation damage. The interaction probability of c-rays depends on the incident photon energy, the atomic number and the density (q) of the shielding materials [1–4]. The concrete shielding has suitable structure and density for c-rays attenuation beside its lower cost and easy manufacturing in comparison with pure lead ⇑ Corresponding author at: Cyclotron Facility, Nuclear Research Center, Atomic Energy Authority, Cairo 13759, Egypt. E-mail address: [email protected] (H.E. Hassan). http://dx.doi.org/10.1016/j.nimb.2015.07.126 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

blocks or neutron shielding materials. The recent research orientations in radiation shielding is to develop denser concrete by adding suitable percentage of additives [5–12]. As an example, minerals such as magnetite, hematite, goethite and limonite were incorporated with concrete and their effects have been evaluated [5–8]. Effects of barite and lead additives in concrete have been separately investigated [9–12]. Similarly, the effect of lime/silica ratio of concrete specimens on gamma absorption and variation of attenuation coefficient for cement specimens have been verified [13]. The relation between compressive strength of heavy concrete and the attenuation of c-rays were also considered in special study on high performance concrete [14,15] which was found to be linear. It should be mentioned that there is no available study that deals with the size of additive materials on the concrete c-ray attenuation. The theoretical validation of experimental results of linear attenuation coefficients gained some interest for varieties of concretes at different photon energies [16,17]. Photoelectric effect is the major interaction process of c-rays with the shielding material, while Compton scattering and pair production mechanisms

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contributes at medium and higher energy. The linear attenuation coefficient is the simplest absorption coefficient to be measured experimentally using Lamber–Beer law: I = I0exp(lx), where I and I0 are the incident and transmitted intensities, respectively, x is the thickness of the absorbing material and l is the linear attenuation coefficient. The mass attenuation coefficient (lm = l/q) is more convenient to be used as it is related to reaction cross section, r (cm2) for X- and c-rays: lm = N0r, where N0 is the number of atoms (or molecules) per unit volume of material [2]. Several attempts has been made for theoretical calculations of mass attenuation coefficient for different compounds using XCom or Geant4 computer codes [18–20]. The changes in the microstructure of wide variety of materials specially porous media were successfully studied using the positron annihilation spectroscopy (PAS) [21–25]. The injected positron in porous materials is thermalized and diffused to a depth of a few hundred micrometers. There are two possibilities for the thermalized positron to be annihilated as a free particle or to form a positronium atom (Ps) with two bound states; singlet (para-Ps, p-Ps) and triplet (ortho-Ps, o-Ps). For positron annihilation lifetime (PAL) technique, the free positron or Ps is annihilated with a corresponding lifetime (s) and intensity (I) for each state at the annihilation site. In PAL technique, the measured parameters si and Ii of the positron and Ps lifetime in porous materials depend on the physical and chemical properties of the materials. More information about the investigated medium can be obtained using positron annihilation Doppler broadening (PADB) technique which is associated with the PAS [22]. In this technique, the motion of the electron–positron pair causes a Doppler shift in the energy of the emitted radiation due to the annihilation. The PADB measurements give information about the distributions of electron momenta as the positrons reach almost zero energy at the annihilation site. There are two main parameters related to the spectral line–shape of annihilation c ray; S (for shape) and W (for wings), which give the distribution of the longitudinal momentum component of the annihilating pair. The objectives of this work are to study the c-rays attenuation coefficient of concrete doped by prepared nano-lead compounds (lead oxide and lead titanate) with several percentages and to determine if the nano-size additives provide any improvement compared with lead compound additive of normal size. Moreover, this study deals with the correlation between c-ray attenuation coefficient and the morphological structure of the investigated concrete samples using the PAL and PADB techniques. 2. Materials and methods Concrete samples were prepared according to the local manufacturing standards. The large size aggregate contents was not suitable to form small concrete samples of thickness 10–40 mm. Therefore we replaced it with small size aggregate with total percentage 44%. The contents of the prepared concrete samples were: Portland cement (14%), aggregate (44%), sand (34%) and water (8%). The contents were hand mixed in dry form and then distilled water was added slowly with continuous mixing on a flat surface until reaching soft homogenous form of wet concrete. The obtained mixture was inserted in plastic disk-shaped molds with diameters 30– 40 mm and average thicknesses 10, 15, 24 mm for measurements of c-ray attenuation. Samples were then air-dried at room temperature for four weeks before measurements. The lead oxide (PbO) and lead titanate (PbTiO3) nano-powder additives were prepared using co-precipitation and oxalate precursor methods, respectively. The PbO was formed by coprecipitation reaction according to the following equation:

Heating at 90  C

stirring

PbOðNO3 Þ2 þ 2NaOH ƒƒƒƒƒƒ! PbðOHÞ2 þ 2NaNO3 ƒƒƒƒƒƒƒƒ! PbO þ 2NaNO3 þ H2 O The final solution containing the nano-PbO was left for sedimentation by gravity and the upper clear solution was slowly removed. The obtained powder was washed several times with double distilled water until the pH becomes 7.5 indicating the removal of sodium nitrates. The powder was then allowed to dry at room temperature. Organic acid precursor method was used to prepare the PbTiO3 nano-powders according to the procedure given in Ref. [26]. The lead acetate (CH3COO)2Pb and titanium dioxide TiO2 were dissolved in stoichiometric amount of tartaric acids. The solution was stirred and gently evaporated at 80 °C till a clear and viscous resin was obtained and then dried at 110 °C for 24 h. The dry precursors were heated (calcined) at a rate of 10 °C/min in static air atmosphere up to 600 °C. The PbTiO3 is formed according to sequential reactions given in the following equations:

½Pb

2þ

þ ðC2 O4 Þ2 ! PbC2 O4 ; TiO2 þ H2 C2 CO4 ! TiOC2 O4 þ H2 O heating ð80  CÞ

ƒƒƒƒƒƒƒ! PbC2 O4 þ TiC2 O4 ! PbO þ TiO2 þ 2Co þ 2Co2 annealing ð600  CÞ

! dry precursor ƒƒƒƒƒƒƒƒƒ! PbTiO3 þ Co2 The phase composition of the prepared powders was identified using XRD diffractometer type X’Pert Graphics employing CuKa radiation (k = 0.15405 nm). Commercial lead dioxide (PbO2) (BDH Merck Ltd; purity 94%) was also used as an additive to concrete to check the effect of the particle size on the c-ray attenuation. The size and homogeneity of the three additive compounds (nao-size PbO, PbTiO3 and normal PbO2) were analyzed by Transmission Electron Microscopy (TEM) type JEOL–JEM–2100–HR providing 2000–1,500,000 magnification with 0.23 nm resolution. The mean particle size was estimated by averaging a number of measured bars in the obtained images of each sample. Lead compounds were added to concrete with five concentrations 0% (no additives), 10%, 20%, 30% and 50% of the total weight. For every concentration, three disk-shape samples of diameter 30– 40 mm and average thicknesses 10, 15 and 24 mm were maintained inside their plastic molds to be used in c-ray measurements. In addition, two identical samples of 15 mm thickness were also prepared with the above mentioned concentrations for PAL and PADB measurements. After drying the samples, their apparent densities were experimentally determined by applying Archimedes principle. The c-rays attenuation of the investigated concrete samples was measured as a function of their thickness using the experimental set up shown in Fig. 1. The ORTEC 300  300 NaI(Tl) scintillation detector coupled with ORTEC MCA was used to detect the c-rays photopeakes 662 keV (using 100 lCi 137Cs source), 1173 and 1332 keV (using 10 lCi 60Co source). The concrete specimen was placed at the end of the collimator between the radioactive source and the detector at a distance of 2 cm from the source and 5 cm from the detector (see Fig. 1). To reduce the background, the detector was shielded with lead blocks. The counting time was prolonged until the standard deviation in the net peak area was less than 3%. The detected spectra of the direct and attenuated c rays were analyzed using MAESTRO-32 software. The net areas under the spectral lines 662, 1173 and 1332 keV before (N0) and after (N) attenuation were determined. A linear relation of ln(N/N0) as a function of the sample thickness x was plotted from which one can determine the linear attenuation l. The mass attenuation coefficient lm is calculated from the formula l/q, where l is the linear attenuation coefficient and q is the measured apparent density of the sample.

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line were accumulated in each spectrum. The S-parameter is defined as the ratio of the counts over the central part of the annihilation peak (511 ± 0.75 keV) to its total counts [33]. The W-parameter is more sensitive to the annihilation with high momentum core electrons and is defined as the ratio of summed counts over the left wing region (504.95–507.98 keV) and the right wing region (514.02–517.05 keV) of the annihilation peak to its total counts. The equations used to calculate the S- and W-parameters are given elsewhere [34]. The Doppler broadening spectra were analyzed using SP ver.1.0 program [35]. The centroid channel of the 511 keV peak was carefully defined as it is a base for calculations of S- and W-parameters. The input data for this program are fixed for all spectra of the studied samples. 3. Results and discussion 3.1. XRD spectra

The photographs obtained by TEM for the three types of lead-compound additives are shown in Fig. 4. The image for the PbO powder (Fig. 4a) shows homogeneous spherical particles with sizes ranged from 2 to 15 nm. There are some regions of dense medium which are due to the agglomerated particles mixed with traces of the buffer solution. The diffraction patterns of the crystalline structure were achieved using the highest available magnification from which an interplanner space d = 0.6 nm was obtained. The image for the PbO2 particles (Fig. 4b) shows a relatively larger size than that of the PbO (>95 nm). TEM image of the PbTiO3 particles (Fig. 4c) shows particle sizes from 37 to 130 nm. The large particle size in this particular case may be attributed to the sintering process at 600 °C. β−PbO

12000

10000

8000

β−PbO2

α−PbO

β−PbO

2000

α−PbO

4000

α−PbO

6000

β−PbO

ing a spherical shape for the holes, the mean free volume Vf (nm3) could be calculated by a simple relation [29,30]: Vf = 4/3pR3, where R (nm) is the average radius of the free volume hole. R was determined using a semiempirical equation includes the values of s2 [31,32]. In the case of PADB measurements, the 22Na source-sample 4p configuration was used. The Doppler broadening (DB) line–shape parameters (S and W) were measured using a 25% p-type HPGe detector (Ortec, GEM series) with an energy resolution (FWHM) of 1.6 keV for 1.33 MeV gamma line of 60Co. The amplified signals from an Ortec 570 amplifier were acquired with an Ortec 919 multichannel analyzer (MCA). The 133Ba source was used for energy calibration (68 eV/channel). The DB spectra were measured in air at room temperature. About two million counts in the annihilation

3.2. TEM imaging

β−PbO

The PAL measurements were carried out using a fast–fast coincidence spectrometer with a time resolution (FWHM), measured with 60Co source at 22Na energy window settings, of 320 ps [27]. This PAL spectrometer consists of two Bicron BC-418 plastic scintillation detectors and Ortec electronic modules. A 10 lCi 22 Na source was prepared using a droplet of 22NaCl solution dried onto two identical Kapton foils (7.5 lm thick). The source was sandwiched between two identical concrete samples. The PAL spectra were measured in air at room temperature. Each sample was measured three times with at least one million counts accumulated in each spectrum. The PAL is measured as the time interval between the detection of 1274.5 keV c-ray emitted by the 22Na radioisotope and the detection of one of the annihilation radiations. The resulting PAL spectra were analyzed using LT computer program with a suitable correction for positrons annihilated in the Kapton [28]. The PAL spectra were decomposed into two components, s1 and s2 with the associated formation probabilities I1 and I2, respectively. s1 (150–500 ps) represents the combined effect of p-Ps (with lifetime = 125 ps) in addition to free and trapped positrons, while s2 (>700 ps) represents the o-Ps. All exponential components of the measured PAL spectra were determined with the best-fitting parameters ranged from 1.02 to 1.13. The range of the experimental errors for PAL parameters s1, s2, I1 and I2, determined over multiple measurements, were found to be <1.7 ps, <50 ps, 0.3% and 0.2%, respectively. The mean lifetime of positron is directly proportional to the average defect density s2 I 2 and is calculated using the following relation, sm ¼ s1 II11 þ . AssumþI2

Intensity (A.U.)

Fig. 1. Experimental set up for the measurements of c-ray attenuation coefficient.

The X-ray diffraction spectrum of the prepared PbO is shown in Fig. 2. The intense peaks at several diffraction angles indicate the high degree of crystallinity of the oxide. The spectrum contains two different types of diffraction patterns [36,37] corresponding to a-PbO (tetragonal) and b-PbO (orthorhombic) which matches the JCPDS card number 05-0561 and 88-1589, respectively [36]. The presence of the low intense peak (about 1.7% of the highest peak) corresponds to PbO2 phase [37] that may be generated due to higher oxidation state of lead oxide. The pure well crystalline tetragonal PbTiO3 phase matching with the JCPDS number 75-438 [26] is obviously recognizable in the measured XRD spectrum shown in Fig. 3.

0 10

20

30

40

50

60

70

80

2 theta (degree) Fig. 2. XRD spectrum of PbO nano particles prepared by coprecipitation method.

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and PbO2 are relatively close, this result demonstrate the advantage of nano-size particles. A significant increase of about 14% in the Z-score of concrete with 50% nano-size PbTiO3 is observed. The same trend is also found at 1173 keV (Fig. 7). In the case of 1332 keV c-rays (Fig. 8), the situation differs for concrete mixed with PbO2 and nano-PbTiO3. Overall, at the PbO2 concentrations of 20% and 30% the attenuation coefficient reach the highest values of 56% improvement. The attenuation coefficients for concrete mixes with PbO2 was found to be higher than that with nano-Pb compounds (PbO and PbTiO3). The differ-

700

(101) 600

(102)

100

(112)

(200)

200

(100)

300

(210)

(111)

400

(001)

Intensity (A.U.)

500

(a)

0 20

30

40

50

60

70

80

2.66 nm

2 Theta (degree) Fig. 3. XRD spectrum of PbTiO3 nano particles prepared by precursor technique.

1.61 nm

3.3. c-Rays mass attenuation coefficient The measured values of c-ray mass attenuation coefficients for the prepared samples of normal and doped concrete at different additive percentage of commercial PbO2, nano-PbO and PbTiO3 are listed in Table 1. As a comparison with other previously published data, the values of lm for normal concrete samples were found to be slightly different from that of Refs. [38,39] as given in Table 1. These variations in the mass attenuation coefficients for normal concrete can be accounted for the differences in the type and size of its constituents. The determined values of lm for Pb sheets are also given for comparison. From the listed results in Table 1 it was found that as the lead-compounds additives concentration increases, the density of the doped concretes gradually increases due to the relatively high density of the additive compounds; 9.38, 9.53 and 7.52 g cm3 for PbO2, PbO and PbTiO3, respectively. Incorporating PbO, PbO2 or PbTiO3 into concrete mixes attributed in part to an increase in packing density of the dry concrete. The obtained results of mass attenuation coefficients is represented in Fig. 5 together with the available published data for comparison [38–44]. However, these data are corresponding to different preparation conditions of concrete and there are no theoretical data included in Fig. 5. Most of these data suggest an increase in the mass attenuation coefficient by 14.4% per g cm3. To evaluate the performance of adding various concentrations of lead compounds on concrete, the relative variation of mass attenuation coefficient of doped concrete with respect to the normal one (Z-score) was calculated as:

Z-score ¼

14.7 nm

3.06 nm

(b)

95.2 nm

108.3 nm

(c)

lmðconcþaddÞ  lmðconcÞ lmðconcÞ

57.45 nm

where lmðconcÞ and lmðconcþaddÞ are the mass attenuation coefficient of the normal and doped concrete, respectively. Figs. 6–8 show the column representation of Z-scores for the mass attenuation coefficients at c-ray energies of 662, 1173 and 1332 keV, respectively. For 662 keV c-ray (Fig. 6), a significant increase in attenuation coefficient of concrete mixed with nano-PbO with respect to the normal concrete is observed. It reaches a maximum of 16% at 50% concentration which is about 33% of the shielding performance of pure lead. However, the same improvement did not appear in the case of the commercial PbO2. According to the present results, the c-ray attenuations of concrete mixtures which include 30% and 50% PbO2 were nearly equal and lower than that of corresponding PbO mixtures. Taking into consideration that the densities of PbO

130.4 nm

37.76 nm

Fig. 4. TEM images of the lead compounds additives: (a) Prepared PbO nano particles, (b) Commercial PbO2 (c) Prepared PbTiO3 nano particles.

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Table 1 The measured values of apparent density (q) and c-ray mass attenuation coefficient (lm) for the investigated concrete samples with different composition at 662, 1173 and 1332 keV c-energies.

lm (cm2 g1)

Sample

Add. conc. (%)

q (g cm3)

(Ec = 662 keV)

(Ec = 1173 keV)

(Ec = 1332 keV)

Pb (sheets) Normal concrete Ref. [38] Ref. [39]

– – – –

11.340 2.210 2.460 2.250

0.093 0.063 0.104 0.075

0.070 0.047 0.071 0.055

0.072 0.027 0.067 0.051

Concrete + PbO2

10 20 30 50

2.490 2.500 2.538 2.537

0.057 0.063 0.067 0.067

0.043 0.050 0.051 0.049

0.038 0.042 0.042 0.040

Concrete + PbO

10 20 30 50

2.230 2.270 2.290 2.300

0.065 0.066 0.071 0.073

0.051 0.055 0.055 0.057

0.026 0.036 0.041 0.040

Concrete + PbTiO3

30 50

2.410 2.512

0.055 0.072

0.044 0.055

0.037 0.039

0.16 0.14

2

-1

μ m (cm g )

0.12 0.10

Present work Ref. [38] Ref. [39] Ref. [40] Ref. [41] Ref. [42] Ref. [43] Ref. [44] ----- Trend line

Normal concrete Eγ = 662 keV

0.08 0.06

0.04 1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

-3

ρ (g cm ) Fig. 5. Comparison between mass attenuation coefficient measured for normal concrete sample in the present work with those given in Refs. [38–44] at 662 keV cray energy.

ences between them become smaller as the Pb-oxides contents increase. The intensity of 60Co source and the experimental conditions of the c-ray spectrometer used make the interference of 1332.5 keV c-ray of 60Co with 1460.8 keV c-ray of 40K affects the reliability of the results obtained from 1332.5 keV c-ray photopeak. The lm values corresponding to 1173 and 1332 keV c-ray energies are found to be relatively small for all the samples. This can be attributed to the low contribution from the photoelectric interaction mechanism while Compton scattering and pair production at these c-rays energies is effective. In addition, coherent scattering which varies as Z2–3, Z is the atomic number of the target material, is also effective in the case of heavy elements [45,46]. One can explain the increasing of lm with the increasing the concentration of the doped lead compounds by considering the increasing in the photoelectric absorption and Compton scattering with increasing the Pb contents in concrete [1,47,48]. On the other hand, the small particle size of the lead compound additive allows more interaction probability of the incident c-ray due to high values of the target surface/volume ratio [49]. However, it should be mentioned that there is a contribution of the effective atomic numbers of the additive materials Zeff = rt,a/rt,el, where rt,a and rt,el are the total atomic and electronic cross sections respectively. It was reported that the ratio of density/Zeff have much changes with changing the additive concentration [50–52]. Furthermore, when

60 Concrete + PbO2 50

Concrete + PbO

60 Concrete + PbO2

Eγ = 662 keV

50

Concrete + PbO

Eγ = 1173 keV

Concrete + PbTiO3

(μ m(conc+add)-μm(conc))/μm(conc) (%)

(μ m(conc+add)-μ m(conc))/μ m(conc) (%)

Concrete + PbTiO3 40 30 20 10 0 -10

40 30 20 10 0 -10

-20

-20

10%

20%

30%

50%

Lead

Fig. 6. Variation of mass attenuation coefficient of concrete doped by different percentages of PbO, PbTiO3 nano particles and commercial PbO2 relative to those of normal concrete and lead sheet at 662 keV c-ray energy.

10%

20%

30%

50%

Lead

Fig. 7. Variation of mass attenuation coefficient of concrete doped by different percentages of PbO, PbTiO3 nano particles and commercial PbO2 relative to those of normal concrete and lead sheet at 1173 keV c-ray energy.

H.E. Hassan et al. / Nuclear Instruments and Methods in Physics Research B 360 (2015) 81–89

(μm(conc+add)-μ m(conc))/μ m(conc) (%)

Concrete + PbO2 160

Concrete + PbO

140

Concrete + PbTiO3

Eγ = 1332 keV

120 100 80 60 40 20 0 -20

10%

20%

30%

50%

Lead

Fig. 8. Variation of mass attenuation coefficient of concrete doped by different percentages of PbO, PbTiO3 nano particles and commercial PbO2 relative to those of normal concrete and lead sheet at 1332 keV c-ray energy.

the radius of the target approach the order of the incident photon wavelength, the Rayleigh Scattering process may contributes to the increasing in lm [53].

nanometric voids in concrete samples ranges from 0.164 to 0.202 nm (see Table 2). The calculated values of mean free volume are also listed in Table 2 for the concrete samples with different concentration of PbO additives. It is noticed that the average value of Vf for the concrete samples doped by nano-PbO with concentration 20%, 30% and 50% is larger than that of normal concrete, which is in contrary with the behavior of density measurements. According to the determined values of I2, this unexpected behavior can be explained by decreasing the voids concentration with 75% for the concrete samples of PbO concentration in the range 20–50%. The lifetime component, s1, is ranged from 222.3 to 233.4 ps. The range of the intensity of s1 (I1) is 97.68–98.80%. Fig. 10 shows the determined mean lifetime sm as a function of nano-PbO concentration in the investigated concrete samples. Taking into consideration that the mean lifetime is directly proportional to average defect density, it is noticed from Fig. 10 that sm is slightly decreasing with increasing PbO concentration. This implies that the overall defect density, as seen by the positrons, decreases with

Table 2 Average radius and mean free volume of the larger voids determined for concrete samples with different concentration of nano-PbO additive.

3.4. Positron annihilation studies

R (nm)

Vf (nm3)

Normal concrete Concrete-10% PbO Concrete-20% PbO Concrete-30% PbO Concrete-50% PbO

0.166 ± 0.005 0.164 ± 0.005 0.202 ± 0.004 0.191 ± 0.010 0.193 ± 0.005

0.0190 ± 0.0006 0.0186 ± 0.0005 0.0346 ± 0.0007 0.0291 ± 0.0016 0.0299 ± 0.0007

300 280

Mean Lifetime (ps)

3.4.1. Positron annihilation lifetime parameters Fig. 9 shows the measured lifetime components s1 and s2 and their corresponding intensities I1 and I2 for the concrete samples with different nano-PbO additive concentration. The determined lifetime component s2 was found to be within the range of 983– 1231 ps, which indicates that the concrete samples contain nanometric voids. Furthermore, the Ps was formed inside the nanometric void intensity I2 ranged 1.2–2.32%. The decrease of I2 with the increasing of the PbO additive concentration may be due to the agglomeration of the nanometric voids caused by the backing effect of nano additives in concrete samples. This result is consistence with the obtained results of the c-ray attenuation and porosity measurements (see Table 1). However, the obtained value of s2 is lower than that for pure water (1.7 ns) [54,55] which indicates that the water content in the concrete samples is very low. Also, the increase of s2 for high concentration of PbO (20–50%) is probably due to the above mentioned reason and the decrease of the delocalized electron density. The calculated average radius of the

Sample type

260 240 220 200 180 160 140 0% PbO

30% PbO

τ1 I1

240

99.5 99.0 98.5

230 98.0

220 210

97.5

200

97.0

1300

3.0

τ2

1200

2.5

I2

2.0

1100

1.5 1000

1.0

900

0.5

800

0.0 0% PbO

10% PbO

50% PbO

Fig. 10. The mean lifetime versus the PbO additive concentration for concrete samples.

250

τ1 (ps)

20% PbO

Sample Type

260

τ2 (ps)

10% PbO

I1 (%)

180

I2 (%)

86

20% PbO

30% PbO

50% PbO

Sample Type Fig. 9. The measured values of PAL parameters (s1, s2, I1 and I2) as a function of nano-PbO additive concentration for concrete samples.

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S-parameter W-Parameter

S-Parameter

0.470

0.0250 0.0245 0.0240

0.465

0.0235 0.460

0.0230

0.455

W-Parameter

0.475

0.0225

0.450

0.0220 Concrete-0%

Concrete-10%

Concrete-20%

Concrete-30%

Concrete-50%

Sample Type Fig. 11. The variations of S- and W-parameters for the concrete samples with different PbO concentrations.

0.472

0.027

0.468

W-Parameter

S-Parameter

0.470

0.466 0.464 0.462

0.026 0.025 0.024

0.460 0.458 0.8

0.023 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

0.460

0.462

0.464

I2 (%)

0.466

0.468

0.470

S-Parameter

Fig. 12. The variation of S-parameter as a function of the o-Ps intensity (I2) for concrete samples with different PbO additive concentration. A linear fitting of the experimental data is represented by the solid line.

Fig. 14. The S–W plot of concrete samples with different PbO and PbTiO3 additives concentration. The solid and dashed fitting lines are for samples with PbO and PbTiO3 additives, respectively.

PbO content. The direct linearity (r = 0.87) with a negative slope of the experimental data shown in Fig. 10 supports our explanation. This result is in agreement with the determined values of densities and lm of c-rays.

lower S-parameters than that with PbTiO3. This may be attributed to the different molecular ratio of Pb atoms in both Pb compounds. This is also in agreement with the behavior of the c-ray mass attenuation of the doped concrete samples. For the concrete samples with PbO concentrations above 20%, the values of W-parameter are higher than those values of pure concrete and of 10% concentrations. This is attributed to increasing number of high momentum core electrons in the case of samples with relatively high PbO concentration. On the other hand, the results show an increase of high momentum core electrons for samples with PbTiO3 additive compared with samples of PbO additive. The correlation between S- and W-parameters are represented in Fig. 14. The linear fitting of data points shows relatively good linearity (r = 0.89 and 0.99 for samples with PbO and PbTiO3 additives, respectively). This indicates that the defects in the studied samples have the same structure and good homogeneity of concrete samples with PbO and PbTiO3 additives. The two regression lines shown in Fig. 14 are referring to two different types of trapping sites in the PbO and PbTiO3 doped concrete samples.

0.468 0.466 0.464 0.462 0.460 0.458 0.456 0.454 0.452 0.450

4. Conclusions The morphological structure and c-ray shielding properties of the nano-size PbO and PbTiO3 doped concretes were studied using

0.028

S-parameter W-Parameter

0.027 0.026 0.025 0.024

W-Parameter

S-Parameter

3.4.2. Positron annihilation Doppler broadening parameters The S- and W-parameters determined from the spectral line shape of the positron annihilation spectra of concrete samples were determined and plotted versus the PbO additive concentrations as shown in Fig. 11. The results indicate that the S-parameter decreases with increasing nano-PbO concentrations. This can be explained by the direct relation between the S-parameter and the probability of positron annihilation with the low momentum valence electrons that are most abundant in defects. This result is consistent with the obtained PAL results, as the concentration of nanometric voids I2 decrease with increasing the concentration of PbO additive. To confirm the above findings the variation of S-parameter with the o-Ps intensity (I2) for the concrete samples with different PbO concentrations were plotted as shown in Fig. 12. It is noticed that good direct linearity of the data points was obtained according to the fitting line (r = 0.92). The calculated S- and W-parameters of the concrete samples with 30% and 50% concentrations of PbO and PbTiO3 additives are shown in Fig. 13. Concrete mixes with incorporated PbO have

0.023 0.022 PbO-30%

PbO-50%

PbTiO3-30%

PbTiO3-50%

Sample Type Fig. 13. The variations of S- and W-parameters for the concrete samples with 30% and 50% concentrations of PbO and PbTiO3 additives.

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positron annihilation and NaI(Tl) spectrometers, respectively. XRD and TEM measurements confirmed the phase purity and particle sizes of the prepared Pb compounds. The two chemical methods used in this work (coprecipitation and oxalate precursor) are simple, economic and productive techniques for preparation of nano-lead compounds additives. The comparative study performed on the determined values of mass attenuation coefficients for the concrete doped by nano-PbO and normal-size PbO2 revealed that improvement was attained for the former at 662 and 1173 keV c-ray energies. The nano-particles are effective in decreasing pores and increasing the electron density of concrete which in turns increases the interaction probability with c-rays. This was successfully confirmed by PAS and PADB measurements. The nano-PbTiO3 additive showed lower performance on concrete shielding due to the low Pb contents and density and the fact that the prepared PbO had smaller particle size. The obtained results depict that addition of nan-PbO to concrete can enhance its c-ray mass attenuation coefficient to reach 33% of pure Pb sheets at 662 keV. The obtained parameters of PAL and PADB indicated that both techniques are highly sensitive to any changes occurred due to addition of nano-Pb compounds.

Acknowledgements This work was supported by Taif University through the project number 1/435/3402. The authors present their appreciations to Prof. M.M. Hessien, Taif University, for his help during lead titanate preparation.

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