Ion Radiation Detection Using Implanted Ultrahigh Molecular Weight Polyethylene Structures (UHMWPE) ()
1. Introduction
Ion implantation is a technology in which one type of external action leads to various defects in materials. The ion beam irradiation affects the polymer structure by cross-linking and degradation [1] . The bombardment of polymers by energetic ions produces dramatic changes due to disruption of original chemical bonding [2] . This eventually results in cross-linking and chain scission, which can produce metastable layers in materials via non-equilibrium processes [3] . The interest in the irradiation of polymers with high energy ions originated from their ability to register nuclear tracks and the applications of polymers as particle detectors or as membranes, which are widely described in the literature [4] [5] [6] . Radiation damage can be measured by a number of different methods, including the solid state nuclear track detector as gamma-ray dosimeter [7] . Generally, the radiation interactions with plastic detectors causes a reduction in the average molecular weight of the latter, which subsequently produces an enhancement in the bulk due to high energy radiation as the γ-dose increases [8] . Ion beam treatment provides a unique way to modify the chemical, structural, optical, mechanical and electrical properties of the polymers by causing irreversible changes in their macromolecular structure. It can be used to change, in a controlled way, the physical properties of the films or to modify the near-surface characteristics of a bulk polymer [9] . Koizumi et al. [10] found that the radical yield is constant at a lower dose and decreases with increasing ion influence. Furthermore, Abdul-Kader et al. [11] studied the changes in the surface layer composition produced by different ion bombardment of polyethylene and isotactic polypropylene. They have observed important hydrogen release with increasing ion dose and they have correlated it with linear energy transfer (LET). Ion implantation technology has been successfully applied to the modification of polymers for improving their surface properties, such as electrical resistance, and optical properties [12] [13] . The increase in the wear resistance of UHMWPE has been achieved by ion implantation [2] [14] [15] . The irradiation effects of ion beams were studied with respect to their structural and electrical properties by Wu et al. [16] . It is well known that irradiation enhances the electrical conductivity in insulating polymers. This increase in conductivity was attributed to the amplifications of the conjugated structure, which indicates relatively great electron freedom. The irregularity in the polymer chains may also give rise to a hopping mechanism, which will enhance the conductivity. Moreover, the effects of radiation on dielectric properties are of particular interest to science and technology and they have many applications in modern engineering [17] [18] [19] .
The extent and the type of defects depend on the nature of radiation as well as its energy and dose. As a result of irradiation, the chemical bonds are damaged and various types of radicals are created. Chen et al. [14] and Allen et al. [15] studied the effects of irradiation by various energetic ions on the physical properties of UHMWPE. The large doses of irradiation caused by the presence of oxygen and UV irradiation were found to influence the response and sensitivity of detectors [5] [20] , but the data available for ultrahigh molecular weight polyethylene and the effect of ion radiation on the physical properties are not sufficient. However, there has not been detailed examination of the changes in the electrical and optical properties due to ion irradiation on UHMWPE. Therefore, in the present work, the author reports the effects of 160 keV Ar ion irradiation on both the electrical and optical properties of UHMWPE structures at different influences. By gaining sufficient knowledge about the ion radiation, the induced effect indicates that this is suitable for use as sensitive ion beam dosimeters.
2. Experimental Techniques
2.1. Sample Preparation
The studied UHMWPE was supplied by Goodfellow Ltd. (Cambridge, UK). The processing characteristics included Mw = 120,000 g/mol, Mw/Mn = 3.4, Tm = 132˚C, Tc = 112˚C and density d = 0.95 g/cm3. The polymer samples used were flat, rectangular wafers with a thickness of 1 mm.
Ion bombardment was carried out in a vacuum at room temperature by means of commercial blazers MPB 202 RP ion implanter at the institute of Electronic Material Technology (ITME), Poland. The beam density was maintained below 0.1 A/cm in order prevent any increases in the sample temperature. The 160 and 300 keV Argon influences ranged from 1 × 1013 to 1 × 1015 ions/cm2, which were originally applied by Turos et al. [21] .
2.2. Optical Measurements
The absorption spectra of the virgin and treated samples were recorded in the wavelength range of 200 - 1100 nm using a lambda 950 Perkin Elmer UV-Vis spectrometer. A perfectly flat piece of the UHMWPE thin sheet was placed vertically in the path of the sample beam, while the reference beam directly reached the detection point. Therefore, the spectral data obtained are absolute values.
2.3. Dielectric Measurements
The electrical properties of all samples were studied before and after ion implantation. The electrical contacts were made to the samples using silver paste. After this, the impedance, resistance, dielectric loss (tanδ) and capacitance measurements were carried out using a Hewlett Packard 4284A LCR meter, which was operated at 0.8 V over the frequency range of 100 Hz - 1 MHz at room temperature. The AC conductivity was calculated using the relation σ(ω) = t/AR. The dielectric constant was calculated using the relationships of ε = Cp/Co, where Cp is the capacitance measured using the LCR meter; and Co = εoA/t, where εo is the permittivity of vacuum and A and t are the cross-sectional area and thickness of the sample, respectively. The exposed area was 0.083 cm2 and the measurements were performed in air.
3. Results and Discussions
3.1. Optical Properties for UHMWPE
The study of the optical absorption and particularly the absorption band edge is a useful tool for providing information about the electronic band structure, localized states, type of optical transition and optical energy gap in polymeric materials [22] . Figure 1 shows the typical plots of the absorption spectra before and after ion implantation for UHMWPE samples with a thickness of 1 mm. From the picture, it is evident that the optical absorption spectral distribution is sensitive to the radiation influence. With an increase in the ion dose, a consequent enhancement in the intensity of the absorption was observed. Generally, the radiation interaction with plastic detectors causes a reduction in the average molecular weight of the latter. The implantation induced alteration of the polymer structure and composition changes optical properties. It is believed that the ionizing radiation causes structural defects (called color centers or oxygen vacancies in oxides), leading to their density change upon the exposure to the rays [23] .
Optically transparent polymers acquire some color after implantation. The color changes to deep brown or grey with increasing ion influence, while a metallic luster appears at high influences (1 × 1015 cm−2), which is fairly consistent with a previous study [1] . This phenomenon is consistent with the red shift of the absorption edge, which is shown in Figure 1. This is due to an increase in the average band gap in the conjugated polymer system, which causes lower energy to be received by ion bombardment. According to Planck’s constant formula, E = hc/λ, as the wavelength increases, the energy required for electron transition between the localized states will be less. This results in the position of the fundamental absorption edge shifting towards larger wavelengths. This behavior can be attributed to the fact that the degradation of sample starts at a dose
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Figure 1. UV-Vis absorption spectrum for the UWMHDPE implanted by 160 keV Ar+ ions with different influences.
greater than 1015 ion/cm2. It is clear that for the sharp absorption edges in the curves of the virgin and Ar+-implanted UHMWPE samples 656 - 662 and 482 - 492, there are no significant change in the stability of the polymers up to a influence of 1015 ions cm−2, which shows good recyclability in the irradiation detection when a constant wavelength shifts before and after the detection of the ion beam. This sensor can be useful for the monitoring of ion bombardment and quality control in ion bombardment synthesis in the lab and biomedical industry.
The absorption coefficient α(ω) of the optical absorption near the band edge shows an exponential dependence on the photon energy hω (Figure 2(a)), which obeys the Urbach’s empirical relation (1): [24] .
(1)
where α0 is a constant and ∆E is the width of the band tails of the localized states.
Equation (1) can be written as:
(2)
The variation of the logarithmic of the absorption coefficient (lnα(ω)) with the photon energy after UHMWPE exposure to irradiation is presented in Figure 2(b). The experimental value for the absorption band tail energy, ∆E, of the localized state is obtained from the slope of the linear portion of curves (slope = 1/∆E). Figure 2(c) shows the variation of ∆E with different influences.
The values of the optical energy gap for the UHMWPE before and after implantation were estimated using the Mott and Davis’ model [25] for the directly and indirectly allowed transition using Equation (3):
(3)
where B is a constant, α is the absorption coefficient and Eopt is the optical energy gap of the system and (n) is the index used to determine the nature of the electronic transitions during the absorption process. According to Equation (3), the direct and indirect transitions of (Eopt)d and (Eopt)in can be obtained by extrapolating the linear portions of the curves, which represents (αħω)2 and (αħω)1/2 compared to the photon energy (ħω) for UHMWPE at different ion fluences. The linear fit for this relation is shown in Figure 3(a) and Figure 3(b). The indirect transition in many amorphous materials fit the case for n = 2. For direct transition, a reasonable fit of Equation (3) with n = 1/2 is particularly achieved at the higher absorption values found at the edge [26] [27] . The values of (Eopt)d and (Eopt)in were found to decrease slowly as the ion dose increased. A significant decrease of up to 1014 ion/cm2 was followed by a gradual decrease of up to 1015 ion/cm2. Such behavior is due to the crystalline structure of UHMWPE detector, which leads to the changes in the degree of disorder [28] . Davis and Mott [29] reported that the presence of high density of localized states in the band structure is responsible for the lower values of the optical gap. The variation of the optical energy gap with electronic irradiation energies and doses can be explained
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Figure 2. (a) Variation of absorption coefficient as a function of photon energy (ħω) for the UHMWPE irradiated and unirradiated samples for (1) 0; (2) 1 × 1013; (3) 1 × 1014; (4) 1 × 1015 ions/cm2 influences; (b) Variation of ln α as a function of photon energy (ħω) at different influences α(ω); (c) Variation of ∆E with different influence.
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Figure 3. (a) The variation of (αħω)2 compared to ħω and (b) the linear fitting of the variation of (αħω)1/2 compared to ħω for the non-irradiated as well as the irradiated UHMWPE with ion beams at different fluences.
as the change in the degree of disorder. From the density of state model, it is known that Eopt decreases with increasing degree of disorder of the amorphous phase [27] . At this stage, one may expect that a band tail can be possibly created due to irradiation. The decrease in Eopt leads to a shift in the band tail ∆E towards the higher energy region and hence, the values of ∆E calculated by Equation (1) are expected to increase as the radiation dose is increased. Our experimental results are consistent with theoretical consideration [30] . One should point out that the sensitivity to ion irradiation of UHMWPE detector at higher doses is appreciable. Above these higher doses, the sample suffers a crosslinking, at which the values of Eopt decrease slowly.
3.2. Electrical Frequency Response for UHMWPE
The implantation-induced disorder of polymers leads to a change in the conductance due to alteration of the electronic structure. Typically, the conductance increases with ion influence due to the carbonization of the polymer. An exception is the high-fluence implantation of metals, which increases the metal volume fraction and also contributes to the charge carrier transport.
Figure 4 shows the variation of electrical conductivity σac with log frequency for the pristine and irradiated samples. A significant and exponential increase in conductivity has been observed around 300 kHz for pristine and irradiated samples. It is also observed that the variation of σac with log frequency is identical for pristine and irradiated samples at lower frequency, while this was higher for irradiated samples at higher frequencies.
The increase in conductivity at a given frequency due to irradiation may be attributed to the scissioning of polymer chains and of the subsequent increase in the free radicals, unsaturation, etc. [31] . An ac field of sufficiently high frequency applied to a metal-polymer-metal structure may cause a net polarization, which is out of phase with the field. This results in ac conductivity and it appears at frequencies that are greater than that at which traps are filled or emptied [32] . As an example, the decrease in the resistance with an increase in the ion influences is shown in Figure 5(b) for the case of Ar+-implanted UHMWPE. The graphs also demonstrate good correlation of both the direct energy band gap (Eopt)d in Figure 5(a) and the decrease in resistance with the increase in ion influences.
Figure 6 shows the variation of the dielectric constant (ε) with frequency for the pristine and irradiated samples. As evident from the plot, a sharp decrease in the dielectric constant has been observed for the irradiated samples. For these samples, the dielectric constant decreases by a factor of 40 after the implantation compared to non-implantation samples. After implantation, the dielectric constant remains almost constant, except when it decreases at a sufficiently high frequency where only electronic polarization dominates. At low frequencies, the motion of the free charge carriers is constant and thus, the dielectric constant remains unchanged. As the frequency increases, the charge carriers migrating through the dielectric get trapped at defect sites and induce an opposite charge in their vicinity. Due to this, they slow down and the value of the dielectric constant decreases [33] .
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Figure 4. Plot of ac conductivity (σac) versus log frequency for non-irradiated and irradiated UWMHDPE at different influences.
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Figure 5. (a) The variation of optical band gap and (b) Resistivity versus ion influences of 160 keV argon-implanted UWMHDPE.
The plot of the dielectric loss tangents (tanδ) versus log frequency for pristine and irradiated UHMWPE samples in the frequency range of 300 Hz - 1 MHz is shown in Figure 7. It is observed that tanδ decreases with a moderate increase in frequency and no relaxation peaks are observed after the implantation. Tanδ decreases as the frequency increases and becomes negative beyond a frequency of 80 kHz for the non-irradiated and irradiated UHMWPE with ion beams up to the influence of 1014 ion cm−2. This shows the dominance of capacitive contributions [34] . However, by increasing the dose above 1015 ion cm−2, there is an inverse process of the reduction in tanδ, which is considerably more pronounced via degradation and crosslinking. It can be assumed that this last process is connected with the release of hydrogen during the irradiation of polyethylene, which is relatively consistent with a previous study [35] .
4. Conclusion
A simple optical/ electrical sensor is demonstrated as a device to detect the ions. A study of 160 keV energy Ar+ ion-induced effects in UHMWPE has been
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Figure 6. Plot of dielectric constant (ε) versus log frequency for (a) non-irradiated and (b) irradiated UHMWPE with ion beams at different influences.
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Figure 7. Dielectric loss tangent versus log frequency for pristine and Ar+ ion beam exposed UHMWPE.
conducted. The measurements of the optical spectra in the range of 360 - 840 nm have been carried out for the unirradiated and irradiated samples and optical constants have been determined. The analysis of the results of the optical study reveals that the absorption coefficient of the polymer increases up to an influence of 1014 ions cm−2, which is probably due to cross-linking without any degradation effects. Furthermore, these results indicate that UHMWPE gets chemically degraded at the highest Ar+ ion influence used, which was 1015 ions cm−2. There is an exponential increase in the conductivity with the log of frequency and the effect is significant after implantation. However, the tan δ and the dielectric constant ε decrease with an increase in ion influence. As the influence increases, the polymer surface becomes hydrogenated amorphous carbon when the influence exceeds 1 × 1015 ions/cm2 because of the degradation of the polymer from the scissioning of bonds. This is also corroborated by optical absorption spectra of UWMHDPE implanted by 160 keV Ar+ ions with different influences.