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Frequency response characteristics of electrochemically etched tracks in Makrofol-E polycarbonate track detectors
Nuclear
194
Instruments
and Methods
in Physics Research
B61 (1991) 194-196 North-Holland
Frequency response characteristics of electrochemically in Makrofol-E polycarbonate track detectors
etched tracks
G. Hussain and H.A. Khan SSNTD-Laboratory, Received
NED, PINSTECH,
P. 0. Nilore, Islamaba4 Pakistan
5 April 1991
Characteristics of electrochemically etched tracks in Makrofol-E polycarbonate track detector, exposed to *‘*Cf in a 2n geometry have been studied. These studies were done by carrying out the electrochemical etching in 6N NaOH (kept at room temperature) at a field strength of 27 kV/cm with varying frequencies. The results indicate that the fission fragment tracks are elongated with increasing frequency of the electric field. A decline in the density as well as the size of the discharge spots occurs at high frequencies. The ratio of the number densities of the elliptic to the round electrochemically etched discharge spots is found to increase from 1.8 + 0.4 to 4.6 f 0.7, on increasing the frequency of the oscillating field from 0.1 to 10.0 kHz. One of the plausible explanations of the results observed is based upon the fact that since in an electric field, the dipole molecules are polarized and orient themselves in the direction of the field, the molecules of the side chains, being less rigid than those in the main chain of the polymer, are stretched in one direction.
1. Introduction Experience shows that the conventional chemical track etching technique has certain practical limitations, particularly when one deals with detectors containing low track densities [l]. The difficulty can be partially overcome by the electrochemical etching process originally developed by Tommasino [2,3]. Through electrochemical etching, tracks can be magnified to a suitably enlarged size visible with an unaided eye. It is an elegant aspect of track revelation especially used in the evaluation of low density tracks in fields like neutron dosimetry and low yield nuclear reactions [2-41. Some of the factors dominating the effectiveness of electrochemical etching are etchant parameters and electric field parameters such as field strength and frequency. A detailed study of these parameters has been done by various authors [4-71. Very little work has been done regarding the influence of frequency on the growth of treeing, resulting from the electrical prebreakdown. In the present studies attempts have been made to investigate the role of high frequency (0.1-10.0 kHz) electric field in the treeing phenomenon.
2. Experimental procedure A set of ten Makrofol-E polycarbonate track detectors was exposed to fission fragments from a 252Cf spontaneous fission source in a 2~ geometry for 5 min 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
each. Electrochemical etching was carried out in a 6N NaOH solution, kept at room temperature at 800 V (27 kV/cm) in a range of frequencies 0.1-10.0 kHz. Discharge spot density was determined in each detector under a magnification of 300 x using a Zeiss optical microscope. Diameters of round discharge spots and long elliptical ones were measured at a magnification of 600 X . The discharge spots possessed a branching microstructure. It was considered reasonable to measure the maximum diameter of the discharge spots. Number densities of the elliptic spots and of the round ones were obtained, and the ratios of the elliptic to the round ones were determined. In order to decide about the size of the spots (thinning down effect), the major axis (M) and the minor axis (m) of the long elliptic spots were measured.
3. Results Growth of electric trees around fission fragment tracks was studied at different frequencies at room temperature. Fig. 1 represents change in spot density with increasing frequency of the oscillating electric field. It is found to increase with increase in the frequency upto about 1 kHz, attains a peak value at frequency value of about 2 kHz, followed by a decline at higher frequencies. Variation of the diameter of the discharge spots as a function of the frequency has been presented in fig. 2. Round spots reveal a decrease after a peak at
B.V. (North-Holland)
G. Hussain, H.A. Khan / Makrofol-E polycarbonate
0
4
1.0x104t 0
’ I
’ 2
’ 4
3
5
’
’
6
7
’ 6
of discharge spot density the oscillating field.
2
3
IO
with frequency
5
4
6
7
6
IO
9
II
12
FREQUENCY (k Hz)
II
Fig. 3. Plot of the ratio long elliptic spots/round field of view against frequency.
FREQUENCY (k Hz)
Fig. 1. Variation
I
’
9
195
track detectors
spots
per
of
^
”
I
^Z
_ J
_
4
_ 5
_
6
_ 7
6
9
IO
J I,
FREQUENCY (k Hz)
Fig. 4. Variation 0
I
2
3
4
5
6
7
6
9
response
as a function
axis against
frequency.
IO
FREQUENCY (k Hz)
Fig. 2. Diametric
of major axis/minor
of frequency.
about 4 kHz. On the other hand the elliptic ones have a tendency to keep on being elongated with the frequency in the presently available range of frequencies. Variation of the ratio, elliptic spots/round spots per field of view with frequency is shown in fig. 3. This ratio increases from 1.8 f 0.4 at 0.1 kHz to 4.6 f 0.7 upto a frequency of about 5 kHz and afterwards attains a sort of plateau. Fig. 4 shows a variation in the ratio major axis/minor axis of elliptical discharge spots as a function of the frequency of the oscillating electric field. The graph, at first exhibits an increase in the ratio upto 1 kHz, then almost a constant value up to 6 kHz; followed by a substantial increase afterwards.
4. Discussion Tommasino et al. proposed that during electrochemical etching in polymeric detectors treeing occurs because the electric field strength at the track tips can become several MV/cm giving rise to an electrical breakdown, which combined with conventional chemical etching produces tree- or bush-like discharge patterns at the pointed ends of the damage trail [8]. However, the process may be simply periodic partial discharge [9] in which complete breakdown of the polymer may or may not take place. The track registration
efficiency in electrochemical etching strongly depends on the frequency and the applied voltage. In the present investigations, electric field strength was kept constant at 27 kV/cm, while the effect of variation in frequency was observed. Discharge spot density increases initially, attains a peak at 2 kHz and then declines. A similar trend has been observed [lo] where the sensitivity (spots/neutron) of 14.7 MeV neutrons induced recoil tracks in Lexan polycarbonate etched electrochemically at 22.5 kV/cm in PEW solution at 70 o C decreases at an applied frequency > 5 kHz. Growth of round treeing shows an increase in the beginning, reaching a maximum position at 4-6 kHz followed by a gradual decrease. The long elliptic spots keep on increasing in length and are thinned down at high frequencies. The number of electrochemically etched elliptic discharge spots increases (as compared to the round ones) with increase in frequency up to 5 kI-Iz followed by a saturation tendency. All of these frequency dependent characteristics predict polarization effect. Polarization W (dipole moment per unit volume) and the electric field intensity E follow the relation: W= XE,
(1)
where X = susceptibility of the material on which the electric field E acts and is related to the dielectric constant K by K=l+4aX.
(2)
Electric field produces polarization by two different processes. First, in para-electric polarization permanent moments may be associated with the molecules of the
196
G. Hussain, H.A. Khan / Makrofol-E polycarbonate
dielectric, making them polar molecules. The applied electric field turns these molecules along its own direction. Secondly, in dielectric polarization the electric field may induce dipole moment in the non polar molecules. A molecule is made up of protons and electrons. In the absence of an electric field, the centre of gravity of the positive charge coincides with that of the negative charge, resulting in a zero dipole moment of the molecule. On applying an electric field, the centres of gravity of the positive and the negative charges experience displacement (relative to each other) which produces the dipole moment. In an electric field, the dipole molecules are polarized in the direction of the field. Therefore, the flexible molecules of the side chains are stretched unidirectionally resulting in the elongation of the treeing structure. Peaks observed in the frequency response curves may be attributed to a resonance effect where the output impedance of the frequency generator matches the input impedance of the etching circuit [ll], resulting in an increase in the differential current. Internal heating during electrochemical etching at the damage sites might contribute partially in mobilising the molecules towards faster orientation. Dielectric loss (i.e. part of the energy of the electric field dissipated as heat) depends on frequency by the relation (ref. [ll] and the references therein): P = 0.5555fE’e’
tan 8,
(3)
where P = electric energy dissipated as heat per unit volume, f= frequency of the applied field, E = field strength, f’ = dielectric constant, tan 8 = loss tangent or the dissipation factor, and c’ tan 6 = loss index. At high frequencies, energy dissipation as heat increases rapidly, raising the temperature at the damage sites of the track. Elevated temperature at the reactive sites of the damage trails, accelerates movement of the molecules with consequent rapid orientation in the direction of the electric field applied.
track detectors
5. Conclusions
From the present investigations one can conclude that at high frequencies, the electric field polarizes the polymer molecules. Molecules of the side chain being less rigid than those of the main chain, are stretched in the direction of the electric field. Consequently, long discharge spots are developed at high frequencies. Dissipation of electric energy in the form of heat raises the internal temperature at the damage sites. Elevation in temperature being proportional to the frequency, results in an accelerated movement of the molecules which are stretched along the direction of the electric field.
References
VI R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear
Tracks in Solids-Principles and Applications (University of California Press, CA, 1975). VI L. Tonunasino, CNEN Report RT/PROT 71 (1970). [31 L. Tommasino and C. Armellini, Rad. Eff. 20 (1973) 253. [41 M. Sohrabi, Health Phys. 27 (1974) 598. [51 S.A.R. Al-Najjar, R.K. Bull and S.A. Durrani, Nucl. Tracks 3 (1979) 169. WI L. Tommasino, G. Zapparoli and R.V. Griffith, Proc. 10th Int. Conf. on SSNTD (1979) 413. 171 R.M. Eichhom, IEEE Trans. EI-12 (1977) 2-18. 181M.A. El-F&i, M.A. Fadel, MA. Sharaf, N. Rabie and G.M. Hassib, Nucl. Tracks 12(1-6) (1986) 189. 191 A. Chowdhury and R.B. Gammage, Abstract P/82, 23 Annual Meet. (Health Phys. Society, Minneapolis, MN, 1978). WI G. Somogyi, G. Dajko, K. Turek and F. Spumy, Nucl. Tracks 3 (1979) 125. 1111R.C. Singh and H.S. Virk, Nucl. Instr. and Meth. B29 (1987) 579.
194
Instruments
and Methods
in Physics Research
B61 (1991) 194-196 North-Holland
Frequency response characteristics of electrochemically in Makrofol-E polycarbonate track detectors
etched tracks
G. Hussain and H.A. Khan SSNTD-Laboratory, Received
NED, PINSTECH,
P. 0. Nilore, Islamaba4 Pakistan
5 April 1991
Characteristics of electrochemically etched tracks in Makrofol-E polycarbonate track detector, exposed to *‘*Cf in a 2n geometry have been studied. These studies were done by carrying out the electrochemical etching in 6N NaOH (kept at room temperature) at a field strength of 27 kV/cm with varying frequencies. The results indicate that the fission fragment tracks are elongated with increasing frequency of the electric field. A decline in the density as well as the size of the discharge spots occurs at high frequencies. The ratio of the number densities of the elliptic to the round electrochemically etched discharge spots is found to increase from 1.8 + 0.4 to 4.6 f 0.7, on increasing the frequency of the oscillating field from 0.1 to 10.0 kHz. One of the plausible explanations of the results observed is based upon the fact that since in an electric field, the dipole molecules are polarized and orient themselves in the direction of the field, the molecules of the side chains, being less rigid than those in the main chain of the polymer, are stretched in one direction.
1. Introduction Experience shows that the conventional chemical track etching technique has certain practical limitations, particularly when one deals with detectors containing low track densities [l]. The difficulty can be partially overcome by the electrochemical etching process originally developed by Tommasino [2,3]. Through electrochemical etching, tracks can be magnified to a suitably enlarged size visible with an unaided eye. It is an elegant aspect of track revelation especially used in the evaluation of low density tracks in fields like neutron dosimetry and low yield nuclear reactions [2-41. Some of the factors dominating the effectiveness of electrochemical etching are etchant parameters and electric field parameters such as field strength and frequency. A detailed study of these parameters has been done by various authors [4-71. Very little work has been done regarding the influence of frequency on the growth of treeing, resulting from the electrical prebreakdown. In the present studies attempts have been made to investigate the role of high frequency (0.1-10.0 kHz) electric field in the treeing phenomenon.
2. Experimental procedure A set of ten Makrofol-E polycarbonate track detectors was exposed to fission fragments from a 252Cf spontaneous fission source in a 2~ geometry for 5 min 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
each. Electrochemical etching was carried out in a 6N NaOH solution, kept at room temperature at 800 V (27 kV/cm) in a range of frequencies 0.1-10.0 kHz. Discharge spot density was determined in each detector under a magnification of 300 x using a Zeiss optical microscope. Diameters of round discharge spots and long elliptical ones were measured at a magnification of 600 X . The discharge spots possessed a branching microstructure. It was considered reasonable to measure the maximum diameter of the discharge spots. Number densities of the elliptic spots and of the round ones were obtained, and the ratios of the elliptic to the round ones were determined. In order to decide about the size of the spots (thinning down effect), the major axis (M) and the minor axis (m) of the long elliptic spots were measured.
3. Results Growth of electric trees around fission fragment tracks was studied at different frequencies at room temperature. Fig. 1 represents change in spot density with increasing frequency of the oscillating electric field. It is found to increase with increase in the frequency upto about 1 kHz, attains a peak value at frequency value of about 2 kHz, followed by a decline at higher frequencies. Variation of the diameter of the discharge spots as a function of the frequency has been presented in fig. 2. Round spots reveal a decrease after a peak at
B.V. (North-Holland)
G. Hussain, H.A. Khan / Makrofol-E polycarbonate
0
4
1.0x104t 0
’ I
’ 2
’ 4
3
5
’
’
6
7
’ 6
of discharge spot density the oscillating field.
2
3
IO
with frequency
5
4
6
7
6
IO
9
II
12
FREQUENCY (k Hz)
II
Fig. 3. Plot of the ratio long elliptic spots/round field of view against frequency.
FREQUENCY (k Hz)
Fig. 1. Variation
I
’
9
195
track detectors
spots
per
of
^
”
I
^Z
_ J
_
4
_ 5
_
6
_ 7
6
9
IO
J I,
FREQUENCY (k Hz)
Fig. 4. Variation 0
I
2
3
4
5
6
7
6
9
response
as a function
axis against
frequency.
IO
FREQUENCY (k Hz)
Fig. 2. Diametric
of major axis/minor
of frequency.
about 4 kHz. On the other hand the elliptic ones have a tendency to keep on being elongated with the frequency in the presently available range of frequencies. Variation of the ratio, elliptic spots/round spots per field of view with frequency is shown in fig. 3. This ratio increases from 1.8 f 0.4 at 0.1 kHz to 4.6 f 0.7 upto a frequency of about 5 kHz and afterwards attains a sort of plateau. Fig. 4 shows a variation in the ratio major axis/minor axis of elliptical discharge spots as a function of the frequency of the oscillating electric field. The graph, at first exhibits an increase in the ratio upto 1 kHz, then almost a constant value up to 6 kHz; followed by a substantial increase afterwards.
4. Discussion Tommasino et al. proposed that during electrochemical etching in polymeric detectors treeing occurs because the electric field strength at the track tips can become several MV/cm giving rise to an electrical breakdown, which combined with conventional chemical etching produces tree- or bush-like discharge patterns at the pointed ends of the damage trail [8]. However, the process may be simply periodic partial discharge [9] in which complete breakdown of the polymer may or may not take place. The track registration
efficiency in electrochemical etching strongly depends on the frequency and the applied voltage. In the present investigations, electric field strength was kept constant at 27 kV/cm, while the effect of variation in frequency was observed. Discharge spot density increases initially, attains a peak at 2 kHz and then declines. A similar trend has been observed [lo] where the sensitivity (spots/neutron) of 14.7 MeV neutrons induced recoil tracks in Lexan polycarbonate etched electrochemically at 22.5 kV/cm in PEW solution at 70 o C decreases at an applied frequency > 5 kHz. Growth of round treeing shows an increase in the beginning, reaching a maximum position at 4-6 kHz followed by a gradual decrease. The long elliptic spots keep on increasing in length and are thinned down at high frequencies. The number of electrochemically etched elliptic discharge spots increases (as compared to the round ones) with increase in frequency up to 5 kI-Iz followed by a saturation tendency. All of these frequency dependent characteristics predict polarization effect. Polarization W (dipole moment per unit volume) and the electric field intensity E follow the relation: W= XE,
(1)
where X = susceptibility of the material on which the electric field E acts and is related to the dielectric constant K by K=l+4aX.
(2)
Electric field produces polarization by two different processes. First, in para-electric polarization permanent moments may be associated with the molecules of the
196
G. Hussain, H.A. Khan / Makrofol-E polycarbonate
dielectric, making them polar molecules. The applied electric field turns these molecules along its own direction. Secondly, in dielectric polarization the electric field may induce dipole moment in the non polar molecules. A molecule is made up of protons and electrons. In the absence of an electric field, the centre of gravity of the positive charge coincides with that of the negative charge, resulting in a zero dipole moment of the molecule. On applying an electric field, the centres of gravity of the positive and the negative charges experience displacement (relative to each other) which produces the dipole moment. In an electric field, the dipole molecules are polarized in the direction of the field. Therefore, the flexible molecules of the side chains are stretched unidirectionally resulting in the elongation of the treeing structure. Peaks observed in the frequency response curves may be attributed to a resonance effect where the output impedance of the frequency generator matches the input impedance of the etching circuit [ll], resulting in an increase in the differential current. Internal heating during electrochemical etching at the damage sites might contribute partially in mobilising the molecules towards faster orientation. Dielectric loss (i.e. part of the energy of the electric field dissipated as heat) depends on frequency by the relation (ref. [ll] and the references therein): P = 0.5555fE’e’
tan 8,
(3)
where P = electric energy dissipated as heat per unit volume, f= frequency of the applied field, E = field strength, f’ = dielectric constant, tan 8 = loss tangent or the dissipation factor, and c’ tan 6 = loss index. At high frequencies, energy dissipation as heat increases rapidly, raising the temperature at the damage sites of the track. Elevated temperature at the reactive sites of the damage trails, accelerates movement of the molecules with consequent rapid orientation in the direction of the electric field applied.
track detectors
5. Conclusions
From the present investigations one can conclude that at high frequencies, the electric field polarizes the polymer molecules. Molecules of the side chain being less rigid than those of the main chain, are stretched in the direction of the electric field. Consequently, long discharge spots are developed at high frequencies. Dissipation of electric energy in the form of heat raises the internal temperature at the damage sites. Elevation in temperature being proportional to the frequency, results in an accelerated movement of the molecules which are stretched along the direction of the electric field.
References
VI R.L. Fleischer, P.B. Price and R.M. Walker, Nuclear
Tracks in Solids-Principles and Applications (University of California Press, CA, 1975). VI L. Tonunasino, CNEN Report RT/PROT 71 (1970). [31 L. Tommasino and C. Armellini, Rad. Eff. 20 (1973) 253. [41 M. Sohrabi, Health Phys. 27 (1974) 598. [51 S.A.R. Al-Najjar, R.K. Bull and S.A. Durrani, Nucl. Tracks 3 (1979) 169. WI L. Tommasino, G. Zapparoli and R.V. Griffith, Proc. 10th Int. Conf. on SSNTD (1979) 413. 171 R.M. Eichhom, IEEE Trans. EI-12 (1977) 2-18. 181M.A. El-F&i, M.A. Fadel, MA. Sharaf, N. Rabie and G.M. Hassib, Nucl. Tracks 12(1-6) (1986) 189. 191 A. Chowdhury and R.B. Gammage, Abstract P/82, 23 Annual Meet. (Health Phys. Society, Minneapolis, MN, 1978). WI G. Somogyi, G. Dajko, K. Turek and F. Spumy, Nucl. Tracks 3 (1979) 125. 1111R.C. Singh and H.S. Virk, Nucl. Instr. and Meth. B29 (1987) 579.