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Bulk etching rate of LR115 detectors
Applied Radiation and Isotopes 57 (2002) 275–278
Technical note
Bulk etching rate of LR115 detectors D. Nikezic! a,*, A. Janic! ijevic! b a
Faculty of Science, Department of Physics, R. Domanovic 12, 34000 Kragujevac, Yugoslavia b Faculty of Technology and Metallurgy, 11000 Belgrade, Yugoslavia
Received 9 October 2001; received in revised form 26 November 2001; accepted 6 February 2002
Abstract The thickness of the layer of LR115 detector removed during etching was measured with a very precise instrument. Dependence of the bulk etch rate on temperature of the etching solution was investigated. It has been found that the etch rate is (3.2770.08) mm/h at 601C in 10%NaOH of water solution. It was also found that the track density in the detector irradiated to radon and its progeny increases linearly with the removed layer. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: LR115 detector; Bulk etch rate; Form Talysurf; Radon measurements
1. Introduction
2. The measurements of the etching rate
It was theoretically established that the thickness of the layer removed during the etching of a solid state nuclear detector is one of the main factors influencing the track parameters, shape and characteristics (Somogyi and Szalay, 1973; Somogyi, 1980). Other factors which determine the track parameters and shape are incident particle characteristics, energy, charge, mass and impact angle. The removed layer of LR115 detector is also a decisive factor influencing the detector performances relevant for alpha detection. The thickness of the layer of LR115 detector removed by etching is very important when this detector is applied for radon measurements. Determination of bulk removing rate (or bulk etching rate in mm/h) is the objective of this work. We have used a non-strippable LR115 detector, type II, with declared thickness of 12 mm, manufactured by KodakPathe, France. This detector is based on cellulose nitrate.
There are different results in the literature on the etching rate of LR115 detector. Various methods were applied for the determination of etching rate: some methods use the mass difference before and after the etching; other methods are based on the measurements of the diameter of the track opening after the irradiation by heavy ions (Durrani and Bull, 1987). All these methods are indirect, involving some uncertainty (possible large) in the final results. The newest approach is to use an atomic force microscope for removing rate measurements (Yasuda et al., 1998; Vazquez-Lopez et al., 2001). We have intended to made direct measurements of the etching rate. For this purpose the instruments ‘‘Form Talysurf’’1 have been applied. The measuring system is based on a two-axis laser interferometer instrument. The system is controlled and the data are processed by a personal computer. The instrument belongs to the class of the profile graph measuring devices. The accuracy of the instrument is 0.004 mm which is actually better than is needed. During the measurements a sensitive needle of the instrument passes slowly across some surface, and a
*Corresponding author. Tel.: +381-33-50-49; fax: +381-3350-50. E-mail address: [email protected] (D. Nikezi!c).
1
Trade mark of ‘‘Rank Taylor Hobson’’, Leicester, England.
0969-8043/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 2 ) 0 0 1 0 9 - 4
276
D. Nikezi!c, A. Jani!cijevi!c / Applied Radiation and Isotopes 57 (2002) 275–278
graph as an output is obtained, showing the profile of the scanned surface. The instrument is primarily used in precise metal industry for quality control. In order to determine the thickness of the LR115 detector, one edge of the detector sensitive layer (redcolored layer) was cut and removed so that the supporting plastic of the detector could be seen. Then the detector was placed on the measuring table; the sensitive needle of the measuring device moved slowly across the detector until the end of the sensitive layer. Then, the needle slipped down from the sensitive layer onto the supporting plastic. This fall is clearly seen in the output graph and it can be used for the determination of
Fig. 1. The output of Tylor Hobson instrument. The figure represents the measurements of the initial detector thickness (before etching). The starting detector thickness is 50.015– 38.010=12.005 mm. The roughness of the detector surface is seen (upper level). The distance which the sensitive needle passed is given on abscissa axis. So 3.988 mm is the distance which is travelled by the needle. The vertical displacement of the needle is given on the ordinate.
the detector thickness. The difference between the two height levels, that is seen on the ordinate axis, is the detector thickness. As an example the output of the instrument is shown in Fig. 1. The higher (leftmost) part of the graph represents the detector surface. Some roughness of the detector surface is seen. Horizontal lines at 38.010 and 50.015 mm were added by the authors for the reason of better presentation. The starting thickness (fresh, not etched detector) of sensitive layer found by this method is (1270.01) mm. At last 10 samples were measured and the results given here are arithmetical means. The error 0.01 is the standard deviation. This value (1270.01) mm is in excellent agreement with the thickness declared by the manufacturer (12 mm). This conformity can be regarded, also, as a test of validity of our method for the thickness measurements. In the literature can be found that the thickness of the LR115 detector is between ‘‘12 and 13 mm’’. After this measurement we can claim that the initial thickness of LR115 is 12 mm. The measurements of the etching rate have been performed by using the following schedule. Totally 15 detectors were separated in three groups, each one containing five detectors. The initial thicknesses of all detectors were measured before the etching. Each group of the detectors were etched in 10% solution of NaOH for some time in a bath at a different temperature: 501C, 601C or 701C. The temperature was kept constant with an accuracy of 711C. Then the etching was stopped and the detector were taken out from the bath and carefully washed in distillated water. The residual thickness of the detector were determined as was described above, and the etching process was continued again for some time.
Fig. 2. The thickness of removed layer as a function on etching time for three different temperatures of the etchant solution.
D. Nikezi!c, A. Jani!cijevi!c / Applied Radiation and Isotopes 57 (2002) 275–278
This multistage etching-residual thickness measurement procedure was continued until the sensitive layer of the detector was removed completely. The removed layer thickness was determined by subtracting the residual thickness from starting thickness (12 mm). The obtained results are shown in Fig. 2. One can se in Fig. 2, the sensitive layer will disappear after (9073) min if the temperature of the etching solution is 701C. The detector etched on 601C would dissolve completely after (22075) min, while the detector etched on 501C will disappear after (41078) min. The bulk etching rate can be determined by using these data and the results are given in Table 1. Etching rate is very dependent on the temperature. The most frequently used condition for LR115 etching is just 10%NaOH at 601C; so the etching rate for this condition is (3.2770.08) mm/h.
3. Repercussion for radon measurements LR115 detector was used for radon measurements by many investigators in the field for the survey, see Nikolaev and Ilic (1999). Frequently, the data about
Table 1 Etching rate of LR115 detector at various temperatures of 10%NaOH etchant Temperature of the etchant 10%NaOH (1C)
Etching rate (mm/h)
50 60 70
1.7570.03 3.2770.08 870.3
277
removed layer of the detector during the etching process are not given (possibly because the removed layer was not controlled). However, the track density and calculated radon concentration strongly depend on the thickness of removed layer. In order to study this, the following experiment was performed. The set of 15 LR115 detectors (split in three groups with five detectors each) were exposed to the radon and its short-lived progeny, in a box with high radon concentration. Then, the detectors were etched in 10% solution of NaOH; the first group was etched on 501C, the second on 601C and the last one at 701C. The etching process was interrupted, detectors were washed and dried, and the residual thickness and the track density were measured. Then, the etching was continued for some time and so on. This process, multi-step etching-track counting-residual thickness measurements, was continued until the detectors were etched completely. Track reading was performed by using an optical microscope with magnification 400. Only tracks that completely perforated the sensitive layer have been counted. They are easily recognized because the light from the microscope lamp is seen (in yellow or green color) on the lower end of the tracks. The results of these experiments are given in Fig. 3. The track density increases linearly with the removed layer. It was expected that the track density would increase with removed layer thickness, but the character of the increment is not known before. The slope of the lines in Fig. 3 is large, so a small variation of removed layer can influence very much on the results of radon measurements. The second finding is that track density depends on the temperature of the etchant; this is a little unexpected. This means that Vt =Vb ¼ V depends on the temperature (Vt and Vb are track and bulk etch rate, respectively). If
Fig. 3. Track density as a function of etching time.
278
D. Nikezi!c, A. Jani!cijevi!c / Applied Radiation and Isotopes 57 (2002) 275–278
the ratio Vt =Vb is not dependent on the temperature the three lines in Fig. 3 would be parallel. With increasing the temperature, the track etch rate Vt increases more than Vb ; Vt and Vb depend on the temperature in different ways.
4. Conclusion The thickness of removed layer is an important factor in all applications of solid state detectors. It has been demonstrated in the case of radon measurements where the track density increases linearly with the removed layer. The thickness of removed layer was directly determined by the means of a high-class instrument. For the most frequently used etching condition for LR115 detector, 10%NaOH at 601C, we have found that the etching rate is (3.2770.08 ) mm/h. This value can be taken as a reference in this field.
References Durrani, S.A., Bull, R.K., 1987. Solid State Nuclear Track Detection: Principles, Methods and Applications. Pergamon Press, Oxford. Nikolaev, V.A., Ilic, R., 1999. Etched track radiometers in radon measurements; a review. Radiat. Meas. 30 (1), 1–13. Somogyi, G., 1980. Development of etched nuclear tracks. Nucl. Instrum. Methods 173, 21–42. Somogyi, G., Szalay, A.S., 1973. Track diameter kinetics in dielectric track detector. Nucl. Instrum. Methods 109, 211–232. Vazquez-Lopez, C., Fragaso, R., Golzarri, J.I., Castillo-Mejia, F., Fujii, M., Espinoza, G., 2001. The atomic force microscope as a fine tool for nuclear track studies. Radiat. Meas. 34, 189–191. Yasuda, B., Yananoto, M., Miyahara, N., Ishigure, N., Kanai, R., Ogura, K., 1998. Measurement of bulk etch rate of CR39 with atomic force microscopy. Nucl. Instrum. Methods B 142, 111–116.
Technical note
Bulk etching rate of LR115 detectors D. Nikezic! a,*, A. Janic! ijevic! b a
Faculty of Science, Department of Physics, R. Domanovic 12, 34000 Kragujevac, Yugoslavia b Faculty of Technology and Metallurgy, 11000 Belgrade, Yugoslavia
Received 9 October 2001; received in revised form 26 November 2001; accepted 6 February 2002
Abstract The thickness of the layer of LR115 detector removed during etching was measured with a very precise instrument. Dependence of the bulk etch rate on temperature of the etching solution was investigated. It has been found that the etch rate is (3.2770.08) mm/h at 601C in 10%NaOH of water solution. It was also found that the track density in the detector irradiated to radon and its progeny increases linearly with the removed layer. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: LR115 detector; Bulk etch rate; Form Talysurf; Radon measurements
1. Introduction
2. The measurements of the etching rate
It was theoretically established that the thickness of the layer removed during the etching of a solid state nuclear detector is one of the main factors influencing the track parameters, shape and characteristics (Somogyi and Szalay, 1973; Somogyi, 1980). Other factors which determine the track parameters and shape are incident particle characteristics, energy, charge, mass and impact angle. The removed layer of LR115 detector is also a decisive factor influencing the detector performances relevant for alpha detection. The thickness of the layer of LR115 detector removed by etching is very important when this detector is applied for radon measurements. Determination of bulk removing rate (or bulk etching rate in mm/h) is the objective of this work. We have used a non-strippable LR115 detector, type II, with declared thickness of 12 mm, manufactured by KodakPathe, France. This detector is based on cellulose nitrate.
There are different results in the literature on the etching rate of LR115 detector. Various methods were applied for the determination of etching rate: some methods use the mass difference before and after the etching; other methods are based on the measurements of the diameter of the track opening after the irradiation by heavy ions (Durrani and Bull, 1987). All these methods are indirect, involving some uncertainty (possible large) in the final results. The newest approach is to use an atomic force microscope for removing rate measurements (Yasuda et al., 1998; Vazquez-Lopez et al., 2001). We have intended to made direct measurements of the etching rate. For this purpose the instruments ‘‘Form Talysurf’’1 have been applied. The measuring system is based on a two-axis laser interferometer instrument. The system is controlled and the data are processed by a personal computer. The instrument belongs to the class of the profile graph measuring devices. The accuracy of the instrument is 0.004 mm which is actually better than is needed. During the measurements a sensitive needle of the instrument passes slowly across some surface, and a
*Corresponding author. Tel.: +381-33-50-49; fax: +381-3350-50. E-mail address: [email protected] (D. Nikezi!c).
1
Trade mark of ‘‘Rank Taylor Hobson’’, Leicester, England.
0969-8043/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 2 ) 0 0 1 0 9 - 4
276
D. Nikezi!c, A. Jani!cijevi!c / Applied Radiation and Isotopes 57 (2002) 275–278
graph as an output is obtained, showing the profile of the scanned surface. The instrument is primarily used in precise metal industry for quality control. In order to determine the thickness of the LR115 detector, one edge of the detector sensitive layer (redcolored layer) was cut and removed so that the supporting plastic of the detector could be seen. Then the detector was placed on the measuring table; the sensitive needle of the measuring device moved slowly across the detector until the end of the sensitive layer. Then, the needle slipped down from the sensitive layer onto the supporting plastic. This fall is clearly seen in the output graph and it can be used for the determination of
Fig. 1. The output of Tylor Hobson instrument. The figure represents the measurements of the initial detector thickness (before etching). The starting detector thickness is 50.015– 38.010=12.005 mm. The roughness of the detector surface is seen (upper level). The distance which the sensitive needle passed is given on abscissa axis. So 3.988 mm is the distance which is travelled by the needle. The vertical displacement of the needle is given on the ordinate.
the detector thickness. The difference between the two height levels, that is seen on the ordinate axis, is the detector thickness. As an example the output of the instrument is shown in Fig. 1. The higher (leftmost) part of the graph represents the detector surface. Some roughness of the detector surface is seen. Horizontal lines at 38.010 and 50.015 mm were added by the authors for the reason of better presentation. The starting thickness (fresh, not etched detector) of sensitive layer found by this method is (1270.01) mm. At last 10 samples were measured and the results given here are arithmetical means. The error 0.01 is the standard deviation. This value (1270.01) mm is in excellent agreement with the thickness declared by the manufacturer (12 mm). This conformity can be regarded, also, as a test of validity of our method for the thickness measurements. In the literature can be found that the thickness of the LR115 detector is between ‘‘12 and 13 mm’’. After this measurement we can claim that the initial thickness of LR115 is 12 mm. The measurements of the etching rate have been performed by using the following schedule. Totally 15 detectors were separated in three groups, each one containing five detectors. The initial thicknesses of all detectors were measured before the etching. Each group of the detectors were etched in 10% solution of NaOH for some time in a bath at a different temperature: 501C, 601C or 701C. The temperature was kept constant with an accuracy of 711C. Then the etching was stopped and the detector were taken out from the bath and carefully washed in distillated water. The residual thickness of the detector were determined as was described above, and the etching process was continued again for some time.
Fig. 2. The thickness of removed layer as a function on etching time for three different temperatures of the etchant solution.
D. Nikezi!c, A. Jani!cijevi!c / Applied Radiation and Isotopes 57 (2002) 275–278
This multistage etching-residual thickness measurement procedure was continued until the sensitive layer of the detector was removed completely. The removed layer thickness was determined by subtracting the residual thickness from starting thickness (12 mm). The obtained results are shown in Fig. 2. One can se in Fig. 2, the sensitive layer will disappear after (9073) min if the temperature of the etching solution is 701C. The detector etched on 601C would dissolve completely after (22075) min, while the detector etched on 501C will disappear after (41078) min. The bulk etching rate can be determined by using these data and the results are given in Table 1. Etching rate is very dependent on the temperature. The most frequently used condition for LR115 etching is just 10%NaOH at 601C; so the etching rate for this condition is (3.2770.08) mm/h.
3. Repercussion for radon measurements LR115 detector was used for radon measurements by many investigators in the field for the survey, see Nikolaev and Ilic (1999). Frequently, the data about
Table 1 Etching rate of LR115 detector at various temperatures of 10%NaOH etchant Temperature of the etchant 10%NaOH (1C)
Etching rate (mm/h)
50 60 70
1.7570.03 3.2770.08 870.3
277
removed layer of the detector during the etching process are not given (possibly because the removed layer was not controlled). However, the track density and calculated radon concentration strongly depend on the thickness of removed layer. In order to study this, the following experiment was performed. The set of 15 LR115 detectors (split in three groups with five detectors each) were exposed to the radon and its short-lived progeny, in a box with high radon concentration. Then, the detectors were etched in 10% solution of NaOH; the first group was etched on 501C, the second on 601C and the last one at 701C. The etching process was interrupted, detectors were washed and dried, and the residual thickness and the track density were measured. Then, the etching was continued for some time and so on. This process, multi-step etching-track counting-residual thickness measurements, was continued until the detectors were etched completely. Track reading was performed by using an optical microscope with magnification 400. Only tracks that completely perforated the sensitive layer have been counted. They are easily recognized because the light from the microscope lamp is seen (in yellow or green color) on the lower end of the tracks. The results of these experiments are given in Fig. 3. The track density increases linearly with the removed layer. It was expected that the track density would increase with removed layer thickness, but the character of the increment is not known before. The slope of the lines in Fig. 3 is large, so a small variation of removed layer can influence very much on the results of radon measurements. The second finding is that track density depends on the temperature of the etchant; this is a little unexpected. This means that Vt =Vb ¼ V depends on the temperature (Vt and Vb are track and bulk etch rate, respectively). If
Fig. 3. Track density as a function of etching time.
278
D. Nikezi!c, A. Jani!cijevi!c / Applied Radiation and Isotopes 57 (2002) 275–278
the ratio Vt =Vb is not dependent on the temperature the three lines in Fig. 3 would be parallel. With increasing the temperature, the track etch rate Vt increases more than Vb ; Vt and Vb depend on the temperature in different ways.
4. Conclusion The thickness of removed layer is an important factor in all applications of solid state detectors. It has been demonstrated in the case of radon measurements where the track density increases linearly with the removed layer. The thickness of removed layer was directly determined by the means of a high-class instrument. For the most frequently used etching condition for LR115 detector, 10%NaOH at 601C, we have found that the etching rate is (3.2770.08 ) mm/h. This value can be taken as a reference in this field.
References Durrani, S.A., Bull, R.K., 1987. Solid State Nuclear Track Detection: Principles, Methods and Applications. Pergamon Press, Oxford. Nikolaev, V.A., Ilic, R., 1999. Etched track radiometers in radon measurements; a review. Radiat. Meas. 30 (1), 1–13. Somogyi, G., 1980. Development of etched nuclear tracks. Nucl. Instrum. Methods 173, 21–42. Somogyi, G., Szalay, A.S., 1973. Track diameter kinetics in dielectric track detector. Nucl. Instrum. Methods 109, 211–232. Vazquez-Lopez, C., Fragaso, R., Golzarri, J.I., Castillo-Mejia, F., Fujii, M., Espinoza, G., 2001. The atomic force microscope as a fine tool for nuclear track studies. Radiat. Meas. 34, 189–191. Yasuda, B., Yananoto, M., Miyahara, N., Ishigure, N., Kanai, R., Ogura, K., 1998. Measurement of bulk etch rate of CR39 with atomic force microscopy. Nucl. Instrum. Methods B 142, 111–116.