Electron Beam Induced Microstructural Changes and Electrical ...

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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 62, NO. 1, FEBRUARY 2015

Electron Beam Induced Microstructural Changes and Electrical Conductivity in Bakelite RPC Detector Material Aneesh Kumar K.V, H. B. Ravikumar, S. Ganesh, and C. Ranganathaiah

Abstract—To explore the structural modifications in terms of crosslink density and electrical conductivity in polymer based Bakelite RPC detector material was exposed to 8 MeV electron beam with the irradiation doses of 20 kGy to 100 kGy in steps of 20 kGy. The microstructural changes of Bakelite upon electron beam irradiation have been studied using Positron Annihilation Lifetime Spectroscopy (PALS), X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) Spectroscopy. Positron lifetime parameters viz., o-Ps lifetime and its intensity showed chain scission at lower doses (at 20 kGy and 40 kGy) followed by crosslinking due to the radical reactions. These changes are effectively explained with the help of FTIR and XRD parameters. The reduction in electrical conductivity of Bakelite material beyond 60 kGy is correlated between conducting pathways and crosslinks in the polymer matrix. The appropriate dose of electron beam on Bakelite might reduce the leakage current of Bakelite RPC detector material and hence improves the performance of the detector. Index Terms—Bakelite RPC, chain scission, cross linking, electrical conductivity, electron beam, leakage current.

I. INTRODUCTION

N

EUTRINO Physics and neutrino Astronomy are the fastest growing fields in the area of high energy physics. Neutrino oscillations and the inferred evidence on neutrino mass are likely to have far-reaching consequences in particle physics. The phenomenon of neutrino oscillations can provide a fruitful evidence for Physics beyond the standard model of particle physics. Resistive Plate Chambers (RPCs) [1], [2], [3] are widely used as the detectors in many High Energy Manuscript received June 13, 2014; revised August 29, 2014; accepted November 12, 2014. Date of publication January 29, 2015; date of current version February 06, 2015. This work was supported by India-based Neutrino Observatory (INO), a Project of the Department of Science and Technology, Government of India(INO-DST Sanction No. SR/S9/Z-01/2010 MU dated 16/07/2010). A. Kumar K.V and H. B. Ravikumar are with the Department of Studies in Physics, University of Mysore, Manasagangothri, Mysore-570006, India (e-mail:[email protected]; [email protected]). C. Ranganathaiah is with the Department of Studies in Physics, University of Mysore, Manasagangothri, Mysore-570006, India, and also with the Government Research Center, Sahyadri College of Engineering and Management, Adyar Mangalore 575 007, Inida (e-mail: [email protected]; [email protected]) S. Ganesh is with the Microtron Centre, Mangalore University, Mangalagangothri, Mangalore-574199, India (e-mail: [email protected]). Digital Object Identifier 10.1109/TNS.2014.2375916

Physics experiments like cosmic ray studies. RPCs are designed as the active detectors in ICAL (Iron Calorimeter) for muon detection, aimed to study atmospheric neutrinos in the proposed India based Neutrino Observatory (INO) Project in India [4]. These RPCs are made up of high resistive materials like glass or Bakelite [5] because of their excellent performance and low cost. The first material of importance in making RPC is a polymer viz., Bakelite and it has shown tremendous possibilities for the fabrication of RPCs [6] compared to glass. In high energy physics experiments these RPCs undergo straining due to over exposure, direct mechanical stress, temperature and different radiations. The RPC detectors are continuously exposed to charged particles like cosmic ray muons, having mass 200 times greater than the mass of an electron and of very high energy, it is expected that there will be modification in the microstructure of RPC material in the long run. Another aspect is in the use of these materials at high operating voltages for longer duration resulting in their aging [7]. But the most important problem faced in such experiments is that Bakelite RPCs exhibit undesirable high leakage current compared to glass RPCs [8]. However, Bakelite has several advantages over glass detectors like low cost, easy to handle in the fabrication of RPCs, as such it is very essential to understand the origin of the leakage current. This problem has not been rectified in the past and it is interesting to find whether the high leakage current owes its origin to the microstructural changes of the Bakelite material. Studies on these aspects will help us to find the ways to reduce the leakage current at high operating voltages and hence enhance the performance of RPC in its application. So far, no such study has been reported in the literature with respect to Bakelite material. It is recognized that, some of the desired physical and chemical properties of the material may be modified with exposure to high energetic radiations like electrons [9], gamma rays, ions, UV-radiation [10] and microwave etc. In recent years irradiation is treated as an effective tool for structural modification of polymers and radiation induced microstructural changes play a significant role in the material applications. In polymer, several polymeric properties viz., optical, mechanical, electrical and chemical etc gets modified through irradiation [11]. These radiation induced modifications can be traced back to learn the possible microstructural changes occurring within the polymeric matrix. However, the exact effect depends on the structure of the polymer and the nature of the radiation used [12]. When a polymeric material is exposed to high energy radiations, the

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KUMAR K.V et al.: ELECTRON BEAM INDUCED MICROSTRUCTURAL CHANGES AND ELECTRICAL CONDUCTIVITY

drastic changes in their microstructural and electrical properties occur through crosslinking [11], chain scission, chain aggregation and molecular emission due to radical reactions in the polymer matrix. Literature survey reveals that there are extensive studies on these aspects including epoxy resins [13]. Some polymers like Poly methyl methacrylate (PMMA), Poly carbonate (PC) undergo chain scission on exposure to radiations, while crosslinking is also observed in some polymers like Polyethylene (PE), Polystyrene (PS) etc [11]. In certain cases like polycaprolactone, irradiation results in both cross linking and chain scission [12]. The general observations in these studies are that when chain scission occurs upon irradiation, the free volume size increases for certainty. However the o-Ps intensity which depends on several factors like inhibition of Ps formation, type of radicals formed upon irradiation and cross linking process may lead to decrease or increase. Therefore, it is very difficult to generalize the variation of o-Ps intensity and its variation is specific to system types. Also it depends on the type and dose level of irradiation. In the present study, authors carried out experimental investigations on the effects of electron beam irradiation on the microstructural changes in the polymer based Bakelite RPC detector material by making use of one of the well established techniques viz., Positron Annihilation Lifetime Spectroscopy (PALS) [14], and attempts have been made to correlate the effect of electron beam irradiation on the free volume parameters with electrical conductivity. In addition, FTIR and XRD studies are used to understand the structural modifications brought out by electron beam irradiation in Bakelite sample. Authors perspective is that electron beam irradiation induced structural modification due to the cross linking may reduce the leakage current and thereby improves the performance of the polymer based Bakelite RPC detectors. A brief description of the PALS technique is given below: When an energetic positron from a radioactive source enters a condensed medium like polymer, it thermalizes quickly by losing all its energy in a very short time, then annihilates with an electron of the medium. Annihilation usually takes place from different positron states viz., free annihilation, or from a localized state (trapped state) or from a bound state called positronium (Ps). Ps can exist in two spin states, a para-positronium (p-Ps, spin antiparallel), which annihilates with a lifetime of 0.125 ns and ortho-positronium (o-Ps, spin parallel), which annihilates with a lifetime of 140 ns in free space. In condensed matter, the o-Ps annihilates predominantly through a fast channel with an electron of the surrounding medium possessing an opposite spin; a process called pick-off annihilation and the o-Ps lifetime gets reduced to a few nanoseconds. Each of these annihilation processes has a characteristic lifetime. In polymers, the o-Ps lifetime is an important parameter, since positronium is trapped and annihilated in free volume sites, and hence it determines the size of free volume holes in the polymer matrix [15]. II. EXPERIMENTAL A. Sample Preparation Bakelite samples used in making of RPCs in India Based Neutrino Observatory (INO) underground laboratory are used in

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Fig. 1. Chemical Structure of Bakelite.

this experiment. Bakelite samples (P-120, Matt finished NEMA LI-1989, Grade XXX) of density g/cm , manufactured by Bakelite Hylam, India were procured from VECC- Kolkata, India in the form of sheets. The chemical structure of Bakelite is shown in Fig. 1. The rectangular samples of dimension cm cm cm was cut and cleaned in ethyl alcohol. Pairs of these samples were used for electron beam irradiation. B. Electron Beam Irradiation Bakelite samples were irradiated with an electron beam from the Microtron Centre, Mangalore University campus, Mangalore, India. The rectangular samples having dimension cm cm and thickness 0.175 cm were exposed to the electron beam of energy 8 MeV, for different doses up to 100 kGy in the interval of 20 kGy. These samples were used for positron lifetime and FTIR measurements. The specifications of the Microtron are; pulse current 50 mA (max); pulse repetition rate 50 Hz; pulse width s. C. Positron Annihilation Lifetime Measurements Positron annihilation lifetime spectra were recorded for the as received and electron beam irradiated Bakelite samples using positron lifetime spectrometer. The positron lifetime spectrometer consists of a fast-fast coincidence system with BaF scintillators coupled to photomultiplier tubes type XP2020/Q with quartz window as detectors. The BaF scintillators are conical to achieve better time resolution. The two identical pieces of the samples were placed on either side of a Ci Na positron source, deposited on a pure Kapton foil of 0.0127 mm thickness. This sample-source sandwich was placed between the two detectors of PLT to acquire lifetime spectrum. The Co prompt spectrum gave 180 ps as the resolution function. However, to have increased count rate, the spectrometer was operated at 220 ps. All lifetime measurements were performed at room temperature and two to three positron lifetime spectra with more than a million counts under each spectrum were recorded in a time of 1-2 hrs. Consistently reproducible spectra were analyzed into three lifetime components with the help of the computer program PATFIT-88 [16] with proper source and background corrections. Source correction term and resolution function were estimated from the lifetime of well-annealed aluminum using the program RESOLUTION [16]. Since, the single Gaussian resolution function did not fit the entire lifetime spectrum; the resolution function was resolved further into three Gaussian components, which gave quick and good convergence. The net resolution function for this turned out to be 220 ps. The three Gaussian resolution functions so determined were used to estimate the lifetime parameters of some of the well-characterized polymers like polycarbonate, PTFE, etc. Therefore, the three Gaussian resolution functions were used in

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the present analysis of positron lifetime spectra of the Bakelite (Phenol formaldehyde resole resin) samples. The o-Ps lifetime ( ) is related to the free volume hole size by a simple relation given by Nakanishi et al. [17], which was developed on the basis of theoretical models originally proposed by Tao for molecular liquids and later by Eldrup et al. [18]. In this model, Positronium is assumed to be localized in a spherical potential well having an infinite potential barrier of radius with an electron layer in the region The relation between and the radius R of the free volume hole or cavity is,

lite sample of dimension ( cm cm cm) was measured using Keithley 2636A system Source Meter. The sample was sandwiched between two electrodes pasted with silver paste. The computer program Lab Tracer 2.0 Source Meter-Integration software is used to record the voltage and current data. From the recorded voltage and current data, the value of the bulk resistance ( ) is calculated. The electrical conductivity ( ) was obtained for different doses using the relation , where and are the thickness and the area of contact respectively.

P

(1)

Where and is an adjustable parameter. By fitting Eq. (1) with values for known hole sizes in porous materials like zeolites, a value of nm was obtained. With this value of , the free volume radius has been calculated from Eq. (1) and the average size of the free volume holes ( ) is evaluated as (2) The fractional free volume or the free volume content ( then be estimated as

), can (3)

where C is structural constant, Å [19], volume hole size, and is the o-Ps intensity.

is the free

D. X-Ray Diffraction Studies XRD spectra for as received and electron beam irradiated Bakelite samples at various dosages have been recorded by powder X-ray diffractometer. The X-ray diffractometer, RIGAKU-DENKI II miniflex with Ni filtered CuK X-rays of wavelength Å and a graphite monochromator in the diffracted beam was used. Fine powder of as received and electron beam irradiated Bakelite samples were taken in a glass sample holder and X-ray scanning was performed in the range with scanning step of . The crystallinity of the as-received and electron beam irradiated samples at different doses were calculated and also, physical quantities like average crystallite size and average strain have been studied using Williamson-Hall (WH) Plot method [20][21]. E. FTIR Characterization The Fourier Transform Infrared (FTIR) Spectra for the as received and the electron beam irradiated Bakelite samples at different doses were recorded using KBr pellet method in the range of cm with JASCO- 460 Plus, Japan Spectrometer with a resolution of cm . FTIR spectrum was used to identify the variations in the functional groups of the components, based on the position of peak values in the region of infrared radiation. F. Electrical Conductivity Measurements The electrical conductivity of the as received and electron beam irradiated Bakelite samples at different doses were evaluated at room temperature. The electrical conductivity of Bake-

III. RESULTS AND DISCUSSION A. Positron Annihilation Lifetime Spectroscopy In general, irradiation induced structural changes in polymers are mainly crosslinking and chain scission or degradation. Initially the radiation energy is deposited on a molecule in a random fashion due to the process of inter and intra molecular transfer of energy and soon it gets localized. This causes activation of long lived excited sites and formation of ions and free radicals, resulting in either the breakdown of the molecule or in the formation of crosslinks [22]. These changes are normally reflected in the values of the positron lifetime parameters. The variation of o-Ps lifetime ( ), and free volume size ( ) for different electron irradiation doses in Bakelite is shown in Fig. 2(a). It is clear that the free volume size increases from Å to Å on e-beam irradiation up to 40 kGy and Å at 100 kGy. There then decreases continuously to is about 54 ps increase in o-Ps lifetime ( ) from 1.633 ns to 1.687 ns and Å increase in free volume size ( ) as compared to as received sample up to 40 kGy electron beam irradiation. The increase in free volume size can be explained as follows; Positronium (Ps) formation takes place preferentially in the regions of low electron density, which are the free volume cavities exist mainly in the amorphous regions. Upon irradiation there may be chains scission resulting in increase of the size of the free volume cavities and hence the o-Ps lifetime ( ) [23]. Therefore, the increase in o-Ps lifetime ( ) is attributed to the chain scission of Bakelite polymer up to 40 kGy. The cleavage of OH bonds in Bakelite yields phenyl free radicals and these free radicals may undergo fast reactions with the electrons of the spur, created during slowing down of positrons thus reducing the number of electrons available for the formation of positronium (Ps),thereby positronium formation is inhibited [24]. Remarkable decrease of about 71 ps o-Ps lifetime ( ) and Å free volume size ( ) starting from 40 to 100 kGy irradiation in comparison to as-received sample. This can be attributed to cross linking of Bakelite polymer chains to the radicals formed due to chain scission in the initial stages of irradiation [25]. This can be justified by the increase in the crosslink density ( ) at 100 kGy irradiated sample. This is generally observed in polymers upon irradiation if chain scission is followed by cross linking. On the other hand, the o-Ps intensity ( ) decreases up to 80 kGy and then a small increase (but only about 0.5% only) and then decreases up to 100 kGy dose as seen from Fig. 2(b). Similar kind of behavior was observed in Fig. 2(c), which depicts the fractional free volume ( ) as a function of electron dose. The decrease in can be attributed to the reduction in


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o-Ps, an interaction that may cause ionization or oxidation. All such interactions inhibit Ps formation. Consequently, the value of will be reduced depending on the concentration of such free radicals. Hence the changes in o-Ps intensity are influenced by several factors including inhibition of Ps formation, the probability of Ps formation decreases and hence ( ) [26]. It is also an indication that upon irradiation at higher doses there might be a small number of scissions at some places and cross links at other places leading to decrease in o-Ps intensity. The crosslink density is known to affect the physical and mechanical properties of the polymers. Therefore, accurate experimental measurement of this quantity is important in many applications. Most of the currently available methods for obtaining crosslink density require the polymer system to be in equilibrium in a liquid solution. A simple relation of the kind of equation (6) shows that by measuring the free volume size by a well established technique such as positron lifetime spectroscopy, crosslink density of the sample can be readily obtained without dissolving the sample in an appropriate solvent. The crosslink density ( ) has been calculated according to the kinetic theory of rubber elasticity [27], [28], which provides an empirical relation between and free volume size ( ) [29]. The crosslink density ( ) and the molecular weight C of the polymer are inversely related according to the kinetic theory of rubber elasticity (Ferry 1970), which is given by C

(4)

where is the density of the polymer. The free volume size of the polymer and molecular weight are related by a similar expression, C

(5)

where A and B are constants. Equation (4) estimates the molecular weight of the solid polymers reasonably well in the mass range of 1,500 to 21,000 g/mol and even still higher molecular weights. Since, sample density variations are rather small and the crosslink density is inversely proportional to molecular weight, the correlation between free volume and crosslink density is given by a similar relation (6)

Fig. 2. (a) Variation of o-Ps lifetime ( ) and free volume size ( ) as a function of electron dose. (b) Variation of o-Ps intensity ( ) as a function of electron dose. (c) Variation of fractional free volume ( ) as a function of electron dose.

number of free volume cavities. There is a possibility that the change in the intensity is due to interaction of free radicals with

where and are constants. These constants are obtained from the plots of ln ( ) as a function of ln ( ) for few of the known polymers, whose free volume size and molecular weights are reported in the literature, which is shown in Fig. 3. The values of and so obtained are 97.125 and 0.143 respectively. The calculated crosslink density of as received and e-beam irradiated Bakelite samples at different doses are reported in the Table I. Fig. 4 represents the variation of crosslink density as a function of electron doses. The crosslink density decreases in the early stages of electron doses. This reduction in crosslink density is attributed to the increase in free volume size due to the scission of polymer chains. The increase in crosslink density is observed above 60 kGy up to 100 kGy. This is reflected in the reduction of free volume size which is due to cross linking of polymer chains by the radical reactions. These results can

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Fig. 3. ln

as a function of ln

.

deviation in crystalline nature from the as received to electron beam irradiated sample is due to the systematic shift of atoms from their mean position owing to high energy and the strain is because of the presence of point defects (such as vacancies, site disorder) and poor crystallinity [21]. This exhibits the changes in width (FWHM) of the peak in the XRD spectra on irradiation. XRD spectra of as received and electron beam irradiated Bakelite samples are shown in Fig. 5(a), and Fig. 5(b) shows average crystallite size and crystallinity of the samples as a function of electron dose. Bakelite is highly amorphous in nature and exhibits the average crystallite size Å and 19% crystallinity. Both average crystallite size and percentage of crystallinity of the electron beam irradiated samples show a sharp decrease from its original values (i.e. crystallite size from Å to Å and crystallinity from 19% to 16.33%) in the lower electron beam doses (20 and 40 kGy). After 40 kGy, both average crystallite size and percentage of crystallinity show increasing trend and exhibits maximum values at 80 and 100 kGy. The reduced average crystallite size and percentage of crystallinity at the lower electron dose indicate the chain scission and increased average crystallite size and percentage of crystallinity may be due to the cross linking of polymeric chains of Bakelite [31], [32] owing to the radical reaction during irradiation. This is confirmed in the variation of crosslink density as a function of irradiation doses which is shown in Fig. 4. These results are effectively explained with the help of FTIR Spectra. C. FTIR Studies

Fig. 4. Variation of crosslink density as a function of electron dose. TABLE I CALCULATED CROSSLINK DENSITY OF THE AS RECEIVED AND ELECTRON BEAM IRRADIATED BAKELITE

also be explained with the help of FTIR spectra. The decrease in fractional free volume shows that there is an increase in the crosslink density and as such polymer chain mobility hinders [30]. B. X-Ray Diffraction Results W-H plot is the best known method for evaluating the average crystallite size and average strain in the system of study. The

The nature of chemical and structural modifications caused by electron beam irradiation on Bakelite can be effectively explained by FTIR studies as well which goes well with PALS results. This spectroscopic technique provides valuable insights into the chemical and physical nature of the crosslinking process. The extent of crosslinking due to irradiation from the observed shift in wave number and change in infrared transmittance associated with the specific functional groups is quantified [22]. There are certain interesting results found from literature on these aspects [33], [34], [35]. FTIR studies have been conducted to study the structural modification induced in Bakelite RPC detector material during electron beam irradiation and these results are shown in Fig. 6. In the FTIR spectra the absorbance bands are seen at 1053 (single bond stretching vibrations of CH OH), 1457 (C-H aliphatic/-CH -deformation vibration), 1647 (C C aromatic ring), 2923 (in phase stretching vibration of CH alkane), 3394 (Phenol/OH) and cm (OH stretch) [32]. From the figure, it is observed that there is no much change in the functional group corresponding to wave number 1457, 1647, and 2923 cm upon electron beam irradiation. This indicates that the functional groups viz., C-H aliphatic bridge, C C and CH alkane may not undergo scission during irradiation and the contribution of these functional groups for the structural modification of Bakelite is marginal. However, there exist remarkable changes in the wave number 3394 cm (Phenol-OH) and 1053 cm (single bond stretching vibrations of-CH OH). The absorption band at 3394 cm in the as received Bakelite shifted to 3420 cm


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Fig. 6. FTIR spectrum of as received and electron beam irradiated Bakelite samples.[(a) to (f) represents FTIR spectra of as received to 100 kGy electron beam irradiated Bakelite samples in the interval of 20 kGy respectively.

linking of the polymeric chains through the covalent bond formation between carbon and hydrogen at higher dosages by the radicals formed during irradiation at lower dosages. A prominent band at the wave number range 1457 cm , corresponds to methylene bridges in p-p’ (para) and methylene bridges in o-o’ (ortho) positions. The IR transmittance slightly increases at lower doses and decreases in their intensity at higher electron dose rates. This clearly indicates the formation of cross linking on electron irradiation at higher dose rates [34], [38]. Based on the above discussion of the FTIR results, it is inferred that the chain scission is the dominant process at lower doses and cross linking is predominant at higher doses. The chemical changes arise due to the rearrangements of free radicals after the chain cleavage, may contribute much for the microstructural modifications in the Bakelite. Fig. 5. (a) XRD spectra of as received and electron beam irradiated Bakelite samples. (b) Average crystallite size and crystallinity of the Bakelite samples as a function of electron dose.

D. Electrical Conductivity Results

when the electron beam dose is 20 kGy and remains at 3420 cm for higher electron dose viz., 40,60,80 and 100 kGy. The intensity of transmittance increases marginally at this wave number in the lower electron dosage up to 40 kGy (from 41% to 48%) and then decreases to 24% at higher electron dosages (60-100 kGy). This increase in intensity is attributed to the chain scission and the formation of more number of alcoholic groups (Phenol) during irradiation [36]. This may further leads to the formation of OH and free radicals [37]. The decrease in intensity of O-H group at higher electron doses suggesting the cross linking of polymeric chains of Bakelite due to the formation of hydrogen bonds by the free radicals released during the scission of hydrogen bonded phenolic group. The absorbance band at 1053 cm corresponding to single bond C-O stretching vibrations of -CH OH- group shifted to 1036 cm at 20 kGy and then increases to 1055 cm and remains around 1055 cm for higher dosages. The IR transmittance increases up to 40 kGy and then decreases in their intensity at higher electron dose rates. This may indicate the cross

In general, polymers are insulators and they exhibit low electrical conductivity. However, if the ions liberated from the surface of the micelle to the free condition upon heat treatment or exposure to high energy radiations may improve the electrical conductivity in polymers [39]. When a polymeric material is exposed to high energy radiation, the drastic change in the electrical conductivity is due to the crosslinking, chain scission, chain aggregation and radical reactions in the polymer matrix. In general, chain scission results to formation of free radicals upon irradiation and hence increases in free volume size ( ) of the polymer. This increase in free volume size ( ) facilitates the motion of the ionic charge carriers and hence the electrical conductivity [40]. However, cross linking restricts the motion of the ionic charge carriers due to the reduction in free volume, thereby reduces the electrical conductivity. There is also possibility of o-Ps forming composite system with few radicals and hence reduce conductivity. However, both processes namely chain scission and cross linking results in the change of crosslink density. Fig. 7 represents the variation of free volume size and conductivity as a function of electron dose and they exhibit identical

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electrical conductivity for 40 kGy electron irradiated sample is more compared to the as received sample indicative of increased mobility of a large number of OH and free radicals produced from the cleavage of hydrogen bonded phenolic groups. The measured low electrical conductivity m at 100 kGy is indicative of cross linking of polymeric chains through hydrogen bonds. This is also evident from the XRD results from crystallinity increasing from 19.1% to . The increased crosslink density at higher electron dose possibly reduces the leakage current and hence improves the performance of Bakelite RPC detector. Further studies on these lines are required to get a better understanding of the leakage current problem faced in the RPC detectors. Fig. 7. Variation of free volume size ( function of electron dose.

) and electrical conductivity ( ) as a

TABLE II CALCULATED PERCENTAGE OF CRYSTALLINITY, CRYSTALLITE SIZE, ELECTRICAL CONDUCTIVITY AND FREE VOLUME IN BAKELITE POLYMER MATERIAL WITH DIFFERENT ELECTRON BEAM DOSES

behavior upon electron beam irradiation. The electrical conductivity ( ) of the as received sample is m . The calculated crystallinity, crystallite size, electrical conductivity and free volume of the as received and electron beam irradiated Bakelite samples at different doses are reported in the Table II. It was observed that the electrical conductivity increases on irradiation of electron beam dosage up to 40 kGy and then decreases to m at 100 kGy. The increase in conductivity for the low electron doses may be due to the scission of hydrogen bonded phenolic groups lead to the formation of OH and free radicals [32], [41]. At higher doses (above 60 kGy) cross linking is the predominant process and the formation of crosslinks of the network hinders the movement of chains inside the polymer. XRD results show increased average crystallite size and crystallinity at higher electron doses indicate the formation of crosslinks of Bakelite polymeric chains and this may reduces the formation of free radicals and their mobility and hence the reduction in electrical conductivity [42], [43]. This is also evident by the reduced free volume size ( ) obtained from the PALS study. IV. CONCLUSION Based on the above discussion of the results the following conclusions can be arrived at: PALS results revealed that chain scission and cross linking are the predominant processes under 8 MeV e-beam irradiation in polymer based Bakelite RPC detector material. The FTIR spectroscopy indicates the scission of hydrogen bonded phenolic groups leading to the formation of OH and free radicals in the low dosage region. The

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