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Study of current drain during electrochemical etching of polycarbonate detectors
Radiation Measurements 40 (2005) 337 – 342 www.elsevier.com/locate/radmeas
Study of current drain during electrochemical etching of polycarbonate detectors Gh. Zainali∗ , A. Afkar National Radiation Protection Department, Atomic Energy Organization of Iran, P.O. Box 14155-4494, Tehran, Iran Received 27 August 2004; received in revised form 27 February 2005; accepted 23 March 2005
Abstract Polycarbonate (PC) detector is one of the common detectors for neutron and radon gas detection. Using this detector it is possible to measure the dose in mSv, by counting tracks/cm2 on an etched surface. In this paper, a special procedure has been suggested to determine the dose based on current drain during the etching process. In these experiments the effects of voltage, frequency, effective etched area, PC detector’s thickness, etched area (one side or two sides), etching solution temperature and dose absorbed by the PC foil have been studied. The results obtained show the current drain variation for a voltage of 200–1600 V, a frequency of 2–10 kHz, effective area with a diameter of 2–12 cm, PC thickness of 125–250–375–500 m and a temperature of etching solution of 25–35 ◦ C. Lexan PC foils were exposed to doses of 0.1, 1, 2.5, 3, 4, 5, 10 rad (0.001 . . . 0.1 Gy) of neutrons. The unexposed foils were considered as the background (BG) foils. Most of the experiments were performed at a voltage of 800 V, a frequency of 2 and 8 kHz, foil thickness of 250 m, diameter of effective etched area of foils of 2, 6 and 12 cm, temperatures of 25 and 35 ◦ C and the etching process from 0 up to overload stage. Overload stage occurs when the foil becomes so thin due to growth of the tracks that it leads to sparking between phase and null that makes a hole in the foil. Current drain curves versus the function of the etching time are absolutely different for various doses from zero (BG) to 10 rad (BG up to 0.1 Gy). This is true especially for the time interval from 3 h of etching up to overload stage. In this way, it is possible to obtain a calibration of PC detector net current drain based on its absorbed dose. In this experiment, the number and diameter of tracks and their relation with drain current and PC foil residual thickness at overload stage have been studied. The same experiment has been performed for various concentrations of radon gas (Bq/m3) as well. © 2005 Elsevier Ltd. All rights reserved. Keywords: Electrochemical etching (ECE); Current; Polycarbonate; Radon; Neutron
1. Introduction The chamber used for electrochemical etching (ECE) of charge particle tracks in polymers has been advanced in
∗ Corresponding author. Tel.: +98 21 634023; fax: +98 21 8009502. E-mail address: [email protected] (G. Zainali).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.03.010
design and operational characteristics from the time of its invention (Tommasino, 1970; Sohrabi, 1974). In the original design, the chamber holds a detector tight between two semichambers, filled with an etchant , to insulate them electrically from each other. ECE chambers have been designed for processing of only one detector (Tommasino, 1970; Somogyi, 1977; Sohrabi and Zainali, 1999), and of larger detectors (Turek, 1992; Sohrabi and Zainali, 1995, 1999). The rubber “O” rings on
338
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342 5 2 kHz 4 kHz 6 kHz 8 kHz 10 kHz
current (mA)
4 3 2 1 0 0
200 400 600 800 1000 1200 1400 1600 1800
Fig. 1. Schematic design of the ECE system.
flat washers serve only to hold the detector tight for insulation and the semichambers hold the etchant plus the detector. In this paper, two semichambers and two flat rubber washers with different diameter have been used for ECE (Fig. 1) (Sohrabi and Zainali, 1999).
current (mA)
(a) 22 20 18 16 14 12 10 8 6 4 2 0
2 kHz 3 kHz 4 kHz 5 6 7 8 9
0
kHz kHz kHz kHz kHz
200 400 600 800 1000 1200 1400 1600 1800 voltage (V)
(b)
current (mA)
2. Experimental procedure and results In this paper, an attempt has been made to estimate the neutron dose by the current drain variation during ECE on the PC detector. Since the parameters such as voltage, frequency, thickness and etched area have an effect on the value of the above current at the beginning of the ECE process, their effects have been considered (Figs. 2 and 3). During this process the other parameters, such as the solution temperature, neutron dose, and whether a one-sideor two-side-etched area is used, have a strong effect on the current variation through the detector. In this experiment a PTWFG5 high-voltage generator is used for detector current measurement. This current depends on the etched area, recoil tracks, and whether it is etched in PEW solution (15 g KOH + 40 g C2 H5 OH + 45 g H2 O) on both sides of the detector, or PEW is applied on one side of the detector and an HCL (3%) solution on the other. The current has been measured by using a 250 m PC detector which has a diameter (D) of 2 cm. The applied signal has an amplitude of 200–1600 V at a frequency of 2–10 kHz (Fig. 2a). Similar experiments have been performed using detectors of diameters 6 and 12 cm which are shown in Figs. 2b and c. An exposed or unexposed detector has been used for a 2-minute experiment at this stage. Fig. 3 shows the current as a function of detector thickness with a diameter of 6 cm
voltage (V)
60 55 50 45 40 35 30 25 20 15 10 5 0
2 kHz 3 kHz 4 kHz 5 kHz 6 kHz 7 kHz 8 kHz 9 kHz 10 kHz
0
(c)
200 400 600
800 1000 1200 1400 1600 1800 voltage (V)
Fig. 2. Current through the detector as a function of voltage for various frequencies with respect to detector diameter. (a) D =2 cm, (b) D = 6 cm, (c) D = 12 cm.
at several voltages and frequencies. In Figs. 2 and 3, the net current drain (the current when the chambers are full of the etchant minus the current without the etchant) is plotted which shows the starting point of the current for different parameters. Some 250 m PC detectors, which have been exposed to fast neutrons from an Am–Be source at different doses (0.001–0.1 Gy), were etched by applying 800 V with a frequency of 8 kHz at 25 ◦ C (Fig. 4). Fig. 4 shows that for
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342
339
Fig. 3. Current through the detector as a function of various thickness for different voltages and frequencies.
higher doses, the current drain will also have a higher amplitude and the time of overload will be shorter.Figs. 5 and 6 show the current drain variation for D = 12 cm at 25 ◦ C and 35 ◦ C. Figs. 7 and 8 show them for D = 6 cm and D = 2 cm. A glove box containing Ra-226 powder with a radon con-
centration of 32 kBq/m3 was considered. Some 250 m PC detectors were exposed to radon, which was in equilibrium, at 6, 24 and 128 h, respectively, inside the glove box (Fig. 9). The response of track detectors is proportional to the integral Rn concentration.
340
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342
40
BG 0.001 Gy 0.01 Gy 0.03 Gy 0.04 Gy 0.05 Gy 0.1 Gy
BG 0.025 Gy 0.1 Gy
35 current (mA)
current (mA)
25
20
1 side etched
30 25 20
15
15
0
50
100
150
200
250
300
etching time (min)
0
current (mA)
Fig. 4. Current through the detector as a function of etching time for various neutron doses on 250 m PC detectors at 800 V, 8 kHz, 25 ◦ C, two-side-etched areas with diameter D = 6 cm.
30
100
BG 0.025 Gy 0.1 Gy
150 200 250 etching time (min)
300
350
BG 0.025 Gy 0.1 Gy
28 current (mA)
38 36 34 32 30 28 26 24 22 20 18 16 14
50
(a)
26
2 sides etched
24 22 20 18
1 side etched
16 14 0
20
40
(b)
0
100
(a)
200 300 etching time (min)
400
500
60 80 100 120 etching time (min)
140
160
Fig. 6. Current through the detector as a function of etching time for 250 m PC detector with various neutron doses at 35 ◦ C, 800 V, 2 kHz one- and two-side-etched areas, D = 12 cm.
32
15.0
30
BG 0.025 Gy 0.1 Gy
26
BG 0.01 Gy 0.03 Gy 0.1 Gy
13.5
2 sides etched
current (mA)
current (mA)
28
24 22 20 18
12.0 10.5 9.0 7.5
16
6.0
14 0 (b)
50
100
150 200 etching time (min)
250
300
Fig. 5. Current through the detector as a function of etching time for various neutron doses on 250 m PC detectors at 800 V, 2 kHz, 25 ◦ C, one- and two-side-etched areas with diameter D = 12 cm.
0
50
100 150 200 etching time (min)
250
300
Fig. 7. Current through the detector as a function of etching time for various neutron doses at 25 ◦ C, 800 V, 2 kHz, two-side-etched areas, D = 6 cm PC thickness = 250 m.
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342 7
30
0.025 Gy 0.1 Gy
6
6 hr Rn 24 hr Rn 168 hr Rn
28 26
current (mA)
T= 25° current (mA)
341
5 4
24 22 20 18
3
16
2
14
0
50
(a)
100 150 200 etching time (min)
250
300
current (mA)
100
200
300
400
etching time (min)
Fig. 9. Current through the detector as a function of etching time for 250 m PC detectors with various radon concentrations at 25 ◦ C, one-side-etched area, 800 V, 2 kHz, D = 12 cm.
7 BG 0.025 Gy 0.1 Gy
6
0
T= 35°
5
Different ranges for voltage, frequency and thickness have been studied at the beginning of the ECE process, but in practice, the optimum parameters for 250 m PC foils are 800 V and 2 kHz for achieving a high detection efficiency and suitable track diameters.
4 3 2 0
50
100
150
3. Conclusion
(b)
etching time (min) Fig. 8. Current through the detector as a function of etching time for a 250 m PC detector with various neutron doses at 25 and 35 ◦ C, two-side-etched areas, 800 V, 2 kHz, D = 2 cm.
For determination of neutron doses from an Am–Be source or radon doses, some parameters such as, voltage, frequency, solution temperature, etc. should be considered. According to Fig. 5a, a calibration curve can result for current variation versus the dose at a point of 5 or 6 h (this point should be determined so that ECE overlap does not occur at any dose). The linearity of the response is reliable up to 0.025 Gy and more than one, the exponential behavior is considered as occurring because of track saturation on the detector. For Figs. 4 and 6b, the suitable time should be 3 and 2 h. The best etching conditions are similar to Figs. 5a or 9. As shown in Fig. 8, the current sensitivity for lower diameters is lower; it could be compensated if the voltage and frequency are increased (in comparison with Figs. 4 and 7). Increasing current depends directly on the track density and their diameters. For a certain etching time (e.g. 6 h), it can be assumed that the mean track diameters for different doses are the same, so that the current drain is dependent on the track density.
(1) The one-side-etched PC foils with respect to the twoside-etched PC foils have a longer overload time with a higher current drain. (2) The overload time decreases with increasing solution temperature. (3) The increase of the current drain is proportional to the increase of the track density of PC foils at any etching time. (4) The residual thickness of PC foils at the overload stage is 35 m ± 5%.
References Sohrabi, M., 1974. Electrochemical etching amplification of recoil particle tracks in polymers and is applications in fast neutron personnel dosimetry. Health Phys. 27, 598–600. Sohrabi, M., Zainali, Gh., 1995. A new multi-detector ECE processing chamber system for large scale radon and neutron dosimetry applications. Nucl. Tracks Radiat. Meas. 25, 463–464. Sohrabi, M., Zainali, Gh., 1999. Versatile electrochemical etching chamber (VECEC) system for multi-size and/or multi-shape detector processing. Nucl. Tracks Radiat. Meas. 31, 153–156. Somogyi, G., 1977. Processing of plastic track detectors. Nucl. Track Detection 1, 3–18.
342
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342
Tommasino, L., 1970. Electrochemical etching of damage track detectors by H.V. pulse and sinusoidal wave forms. Proceedings of the Seventh International Colloqiuium on Corpuscular and Photochemical Solid Detectors, Barcelona.
Turek, K., 1992. Universal multi-detector etching stand for electrochemically etched plastic track detectors. Nucl. Tracks Radiat. Meas. 20, 601–604.
Study of current drain during electrochemical etching of polycarbonate detectors Gh. Zainali∗ , A. Afkar National Radiation Protection Department, Atomic Energy Organization of Iran, P.O. Box 14155-4494, Tehran, Iran Received 27 August 2004; received in revised form 27 February 2005; accepted 23 March 2005
Abstract Polycarbonate (PC) detector is one of the common detectors for neutron and radon gas detection. Using this detector it is possible to measure the dose in mSv, by counting tracks/cm2 on an etched surface. In this paper, a special procedure has been suggested to determine the dose based on current drain during the etching process. In these experiments the effects of voltage, frequency, effective etched area, PC detector’s thickness, etched area (one side or two sides), etching solution temperature and dose absorbed by the PC foil have been studied. The results obtained show the current drain variation for a voltage of 200–1600 V, a frequency of 2–10 kHz, effective area with a diameter of 2–12 cm, PC thickness of 125–250–375–500 m and a temperature of etching solution of 25–35 ◦ C. Lexan PC foils were exposed to doses of 0.1, 1, 2.5, 3, 4, 5, 10 rad (0.001 . . . 0.1 Gy) of neutrons. The unexposed foils were considered as the background (BG) foils. Most of the experiments were performed at a voltage of 800 V, a frequency of 2 and 8 kHz, foil thickness of 250 m, diameter of effective etched area of foils of 2, 6 and 12 cm, temperatures of 25 and 35 ◦ C and the etching process from 0 up to overload stage. Overload stage occurs when the foil becomes so thin due to growth of the tracks that it leads to sparking between phase and null that makes a hole in the foil. Current drain curves versus the function of the etching time are absolutely different for various doses from zero (BG) to 10 rad (BG up to 0.1 Gy). This is true especially for the time interval from 3 h of etching up to overload stage. In this way, it is possible to obtain a calibration of PC detector net current drain based on its absorbed dose. In this experiment, the number and diameter of tracks and their relation with drain current and PC foil residual thickness at overload stage have been studied. The same experiment has been performed for various concentrations of radon gas (Bq/m3) as well. © 2005 Elsevier Ltd. All rights reserved. Keywords: Electrochemical etching (ECE); Current; Polycarbonate; Radon; Neutron
1. Introduction The chamber used for electrochemical etching (ECE) of charge particle tracks in polymers has been advanced in
∗ Corresponding author. Tel.: +98 21 634023; fax: +98 21 8009502. E-mail address: [email protected] (G. Zainali).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.03.010
design and operational characteristics from the time of its invention (Tommasino, 1970; Sohrabi, 1974). In the original design, the chamber holds a detector tight between two semichambers, filled with an etchant , to insulate them electrically from each other. ECE chambers have been designed for processing of only one detector (Tommasino, 1970; Somogyi, 1977; Sohrabi and Zainali, 1999), and of larger detectors (Turek, 1992; Sohrabi and Zainali, 1995, 1999). The rubber “O” rings on
338
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342 5 2 kHz 4 kHz 6 kHz 8 kHz 10 kHz
current (mA)
4 3 2 1 0 0
200 400 600 800 1000 1200 1400 1600 1800
Fig. 1. Schematic design of the ECE system.
flat washers serve only to hold the detector tight for insulation and the semichambers hold the etchant plus the detector. In this paper, two semichambers and two flat rubber washers with different diameter have been used for ECE (Fig. 1) (Sohrabi and Zainali, 1999).
current (mA)
(a) 22 20 18 16 14 12 10 8 6 4 2 0
2 kHz 3 kHz 4 kHz 5 6 7 8 9
0
kHz kHz kHz kHz kHz
200 400 600 800 1000 1200 1400 1600 1800 voltage (V)
(b)
current (mA)
2. Experimental procedure and results In this paper, an attempt has been made to estimate the neutron dose by the current drain variation during ECE on the PC detector. Since the parameters such as voltage, frequency, thickness and etched area have an effect on the value of the above current at the beginning of the ECE process, their effects have been considered (Figs. 2 and 3). During this process the other parameters, such as the solution temperature, neutron dose, and whether a one-sideor two-side-etched area is used, have a strong effect on the current variation through the detector. In this experiment a PTWFG5 high-voltage generator is used for detector current measurement. This current depends on the etched area, recoil tracks, and whether it is etched in PEW solution (15 g KOH + 40 g C2 H5 OH + 45 g H2 O) on both sides of the detector, or PEW is applied on one side of the detector and an HCL (3%) solution on the other. The current has been measured by using a 250 m PC detector which has a diameter (D) of 2 cm. The applied signal has an amplitude of 200–1600 V at a frequency of 2–10 kHz (Fig. 2a). Similar experiments have been performed using detectors of diameters 6 and 12 cm which are shown in Figs. 2b and c. An exposed or unexposed detector has been used for a 2-minute experiment at this stage. Fig. 3 shows the current as a function of detector thickness with a diameter of 6 cm
voltage (V)
60 55 50 45 40 35 30 25 20 15 10 5 0
2 kHz 3 kHz 4 kHz 5 kHz 6 kHz 7 kHz 8 kHz 9 kHz 10 kHz
0
(c)
200 400 600
800 1000 1200 1400 1600 1800 voltage (V)
Fig. 2. Current through the detector as a function of voltage for various frequencies with respect to detector diameter. (a) D =2 cm, (b) D = 6 cm, (c) D = 12 cm.
at several voltages and frequencies. In Figs. 2 and 3, the net current drain (the current when the chambers are full of the etchant minus the current without the etchant) is plotted which shows the starting point of the current for different parameters. Some 250 m PC detectors, which have been exposed to fast neutrons from an Am–Be source at different doses (0.001–0.1 Gy), were etched by applying 800 V with a frequency of 8 kHz at 25 ◦ C (Fig. 4). Fig. 4 shows that for
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342
339
Fig. 3. Current through the detector as a function of various thickness for different voltages and frequencies.
higher doses, the current drain will also have a higher amplitude and the time of overload will be shorter.Figs. 5 and 6 show the current drain variation for D = 12 cm at 25 ◦ C and 35 ◦ C. Figs. 7 and 8 show them for D = 6 cm and D = 2 cm. A glove box containing Ra-226 powder with a radon con-
centration of 32 kBq/m3 was considered. Some 250 m PC detectors were exposed to radon, which was in equilibrium, at 6, 24 and 128 h, respectively, inside the glove box (Fig. 9). The response of track detectors is proportional to the integral Rn concentration.
340
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342
40
BG 0.001 Gy 0.01 Gy 0.03 Gy 0.04 Gy 0.05 Gy 0.1 Gy
BG 0.025 Gy 0.1 Gy
35 current (mA)
current (mA)
25
20
1 side etched
30 25 20
15
15
0
50
100
150
200
250
300
etching time (min)
0
current (mA)
Fig. 4. Current through the detector as a function of etching time for various neutron doses on 250 m PC detectors at 800 V, 8 kHz, 25 ◦ C, two-side-etched areas with diameter D = 6 cm.
30
100
BG 0.025 Gy 0.1 Gy
150 200 250 etching time (min)
300
350
BG 0.025 Gy 0.1 Gy
28 current (mA)
38 36 34 32 30 28 26 24 22 20 18 16 14
50
(a)
26
2 sides etched
24 22 20 18
1 side etched
16 14 0
20
40
(b)
0
100
(a)
200 300 etching time (min)
400
500
60 80 100 120 etching time (min)
140
160
Fig. 6. Current through the detector as a function of etching time for 250 m PC detector with various neutron doses at 35 ◦ C, 800 V, 2 kHz one- and two-side-etched areas, D = 12 cm.
32
15.0
30
BG 0.025 Gy 0.1 Gy
26
BG 0.01 Gy 0.03 Gy 0.1 Gy
13.5
2 sides etched
current (mA)
current (mA)
28
24 22 20 18
12.0 10.5 9.0 7.5
16
6.0
14 0 (b)
50
100
150 200 etching time (min)
250
300
Fig. 5. Current through the detector as a function of etching time for various neutron doses on 250 m PC detectors at 800 V, 2 kHz, 25 ◦ C, one- and two-side-etched areas with diameter D = 12 cm.
0
50
100 150 200 etching time (min)
250
300
Fig. 7. Current through the detector as a function of etching time for various neutron doses at 25 ◦ C, 800 V, 2 kHz, two-side-etched areas, D = 6 cm PC thickness = 250 m.
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342 7
30
0.025 Gy 0.1 Gy
6
6 hr Rn 24 hr Rn 168 hr Rn
28 26
current (mA)
T= 25° current (mA)
341
5 4
24 22 20 18
3
16
2
14
0
50
(a)
100 150 200 etching time (min)
250
300
current (mA)
100
200
300
400
etching time (min)
Fig. 9. Current through the detector as a function of etching time for 250 m PC detectors with various radon concentrations at 25 ◦ C, one-side-etched area, 800 V, 2 kHz, D = 12 cm.
7 BG 0.025 Gy 0.1 Gy
6
0
T= 35°
5
Different ranges for voltage, frequency and thickness have been studied at the beginning of the ECE process, but in practice, the optimum parameters for 250 m PC foils are 800 V and 2 kHz for achieving a high detection efficiency and suitable track diameters.
4 3 2 0
50
100
150
3. Conclusion
(b)
etching time (min) Fig. 8. Current through the detector as a function of etching time for a 250 m PC detector with various neutron doses at 25 and 35 ◦ C, two-side-etched areas, 800 V, 2 kHz, D = 2 cm.
For determination of neutron doses from an Am–Be source or radon doses, some parameters such as, voltage, frequency, solution temperature, etc. should be considered. According to Fig. 5a, a calibration curve can result for current variation versus the dose at a point of 5 or 6 h (this point should be determined so that ECE overlap does not occur at any dose). The linearity of the response is reliable up to 0.025 Gy and more than one, the exponential behavior is considered as occurring because of track saturation on the detector. For Figs. 4 and 6b, the suitable time should be 3 and 2 h. The best etching conditions are similar to Figs. 5a or 9. As shown in Fig. 8, the current sensitivity for lower diameters is lower; it could be compensated if the voltage and frequency are increased (in comparison with Figs. 4 and 7). Increasing current depends directly on the track density and their diameters. For a certain etching time (e.g. 6 h), it can be assumed that the mean track diameters for different doses are the same, so that the current drain is dependent on the track density.
(1) The one-side-etched PC foils with respect to the twoside-etched PC foils have a longer overload time with a higher current drain. (2) The overload time decreases with increasing solution temperature. (3) The increase of the current drain is proportional to the increase of the track density of PC foils at any etching time. (4) The residual thickness of PC foils at the overload stage is 35 m ± 5%.
References Sohrabi, M., 1974. Electrochemical etching amplification of recoil particle tracks in polymers and is applications in fast neutron personnel dosimetry. Health Phys. 27, 598–600. Sohrabi, M., Zainali, Gh., 1995. A new multi-detector ECE processing chamber system for large scale radon and neutron dosimetry applications. Nucl. Tracks Radiat. Meas. 25, 463–464. Sohrabi, M., Zainali, Gh., 1999. Versatile electrochemical etching chamber (VECEC) system for multi-size and/or multi-shape detector processing. Nucl. Tracks Radiat. Meas. 31, 153–156. Somogyi, G., 1977. Processing of plastic track detectors. Nucl. Track Detection 1, 3–18.
342
G. Zainali, A. Afkar / Radiation Measurements 40 (2005) 337 – 342
Tommasino, L., 1970. Electrochemical etching of damage track detectors by H.V. pulse and sinusoidal wave forms. Proceedings of the Seventh International Colloqiuium on Corpuscular and Photochemical Solid Detectors, Barcelona.
Turek, K., 1992. Universal multi-detector etching stand for electrochemically etched plastic track detectors. Nucl. Tracks Radiat. Meas. 20, 601–604.