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Track sensitivity and the surface roughness measurements of CR-39 with atomic force microscope
Radiation Measurements
PERGAMON
RadiationMeasurements31 (1999) 203-208
TRACK SENSITIVITY AND THE SURFACE ROUGHNESS MEASUREMENTS OF CR-39 WITH ATOMIC FORCE MICROSCOPE N. YASUDA *, M. YAMAMOTO *, K. AMEMIYA **, H. TAKAHASHI ** A. KYAN *** AND K. OGURA **** * National Institute of Radiological Science, Chiba 263-8555, Japan ** Quantum Engineering and Systems Science, University of Tokyo, Tokyo 113-0033, Japan *** Graduate School of Science and Engineering, Ibaraki University, Ibaraki 310-0056, Japan **** College of Industrial Technology, Nihon University, Chiba 275-0006, Japan ABSTRACT Atomic Force Microscope (AFM) has been applied to evaluate the surface roughness and the track sensitivity of CR-39 track detector. We experimentally confirmed the inverse correlation between the track sensitivity and the roughness of the detector surface after etching. The surface of CR-39 (CR-39 doped with antioxidant (HARZLAS (TD-1)) and copolymer of CR-39/NIPAAm (TNF-I)) with high sensitivity becomes rough by the etching, while the pure CR-39 (BARYOTRAK) with low sensitivity keeps its original surface clarity even for the long etching. KEYWORDS CR-39; Atomic Force Microscope (AFM); roughness; track sensitivity.
INTRODUCTION CR-39 plastic track detector is utilized in many fields, such as space dosimetry and fast neutron dosimetry, and radiography, etc. In case of the optical microscopic observation, the thickness of material removed by etching is usually ranging from 10 to 50 p,m for CR-39. Consequently, the ragged edge of the opening mouth of the etched track and the rough and uneven surface of the detector are observed. Therefore, it is difficult to distinguish the shallow tracks from the background of the rough surface. This thing will be hard to determine the threshold REL (Restricted Energy Loss) value for the track regislxation of the detector. Moreover the ragged mouth of track and locally non-uniform etching raise to the fluctuation in track sensitivity and limit the charge and mass resolution of CR-39 as a consequence. The research for lowering CR-39 track registration threshold is important in order to extend the field of the application. The CR-39 (HARZLAS (TD-1)) manufactured by Fukuvi Chemical Industry Co. Ltd., Japan has the high sensitivity to low LET (Linear Energy Transfer) particle, recording normally incident protons up to the energy of 20 MeV. Recently, new CR-39 (TNF-I) developed by Ogura et al. (1997) can record normally incident protons up to the energy of 27 MeV. It is empirically known that there is the close relation between the surface roughness after the chemical etching and the track sensitivity. By clarifying the relationship between sensitivity and roughness of the surface, it is possible to improve efficiently the sensitivity and quality of the surface of CR-39. By using the AFM (Atomic Force Microscope) technique, in contrast to the optical microscope (OPT), it 1350-4487/99/$ - see frontmatter© 1999 ElsevierScienceLtd.All rights reserved.
PII:S1350-4487(99)00089-X
204
N. Yasuda et aL / Radiation Measurements 31 (1999) 203-208
is possible to observe the three-dimensional profile of the track and the etched surface in the early stage of the etching process (M. Yamamoto et al., 1997; N. Yasuda et aL, 1998). In this study, AFM has been applied to evaluate the surface roughness and track sensitivity for CR-39 detectors. EXPERIMENTAL
The AFM (Nanoscope III; Digital Instruments) equipped with a 125 p.m cantilever having a typical tip length of 10 ~m was operated in the tapping mode which has been developed as a method to achieve high resolution without inducing destructive frictional forces. The cantilever was oscillated near its resonant frequency (-250 kHz) which allowed the whole surface of the sample to be scanned. The driving of the cantilever was done by piezoceramic actuators, which change their dimensions according to the applied voltage. The applied voltage to the piezo scanner was automatically converted into the height (depth) information of the surface of the sample. The calibration of a piezoceramic scanner was carded out by scanning the standard samples such as diffraction gratings (10.0_+0.3 p.m pitch, 180.0+5.4 nm high and 1.0L~0.1 ~tm pitch, 1.00_+0.05 p.m high). The surface measurements have been done for three types of CR-39, pure CR-39 (BARYO TRACK), CR-39 doped with antioxidant (HARZLAS (TD-1)) and copolymer of CR-39/NIPAAm (TNF-I), which were manufactured by Fukuvi Chemical Industry Co. Ltd., Japan. The samples of CR-39 were prepared by curing a sheet (0.9 mm thick) into square-shaped pieces of 1 x 1 cm2. These pieces were etched in 7N NaOH solution at 70°C using a water bath incubator. The etching time was varied from 1 to 24 hours. The surface roughness was measured by AFM. . In order to compare the track sensitivity, two types of CR-39 (BARYOTRAK and TD-I) were exposed to 530, 200 MeV/n Ar ions, 470 MeV/n Si ions and 200 MeV/n Ne ions from NIRS(National Institute of Radiological Science)-HIMAC (Heavy Ion Medical Accelerator in Chiba). These stacks were etched in 7N NaOH solution at 70°C for 24 hours. The track etch pits were measured by AFM and OPT. RESULTS AND DISCUSSION
Figure 1 shows 3-D images of the detector surface taken with a scanning area of 10 x 10 p.m2 of CR-39 which was non-etched and etched for 2, 10 and 24 hours, respectively. Height scale perpendicular to the detector surface was normalized to 1.4 p.m. The images displayed as a viewer 30°above the surface would see it. It is clearly seen that the surface condition of TD-I was getting rough as the course of the etching time in contrast to the surface of pure CR-39 (BARYOTRAK). The RMS roughness (Rq), the amount of bulk etch and track registration threshold for three types of CR-39 are summarized in Table 1. The Rq is the standard deviation of the Z (height) valuewithin the scanned area and is calculated as
Rq = R(F(~" Z i - Zave)2,N)),
(l)
where Zave is the average of the height value within the scanned area, Zi is the observed height value, and N is the number of pixels. It is proven that the surface of BARYOTRAK is almost flat even ff the etching lime is prolonged, while the roughness of surface of TD-1 and TNF-I become significant as the course of the etching time. It was experimentally and quantitatively confirmed that the CR-39 having high sensitivity shows rough surface after etching and the CR-39 with low sensitivity keeps good quality of the surface even for the long time etching. This tendency of the material is empirically known among the CR-39 user so far.
N. Yasuda et aL / Radiation Measurements 31 (1999) 203-208
Fig. 1. 3-D images of the detector surface (BARYOTRACK (left side) azld TD-1 (right side)) taken with a scanning area of 10 x 10 grn 2 of CR-39 which was non-etched and etched for 2, 10 and 24 hours, respectively. Height scale perpendicular to the detector surface was normalized to 1.4 gm.
205
206
N. Yasuda et al. / Radiation Measurements 31 (1999) 203-208
Table 1. The summaryof measured data. ThresholdREL values of TNF-1 and TD-1 w e r e measured by Ogura et al. (1997). Etching T'une
CR-39
(hours) TNF-1 0
10
24
Bulk
Rq
Threshold REl-~oo,v
(Ixm)
(nm)
(MeV cm2/g)
0
4.8
TD-1
0
2.3
BARYO
0
2.0
TNF-I
1.6
5.8
TD-I
1.4
4.6
BARYO
1.2
2.1
TNF-1
3.2
12.2
TD-1
3.1
10.6
BARYO
2.2
2.3
TNF-I
14.3
43.9
TD-I
13.7
33.2
BARYO
11.7
2.1
TNF-1
48.8
64.7
~10"
TD-1
44.8
16.9
-15"
BARYO
37.2
2.4
,.,2 x 102
~mm
Figure 2 shows the AFM images of track etch pit with the surface profiles on BARYOTRAK and TD1. The samples were irradiated to 230 MeV/n Ne ions and etched for 24 hours. In this etching condition, it was impossible to observe the tip of etch pit, because the etch pits on the CR-39 surface grew too large and was beyond the reaching limit of the cantilever tip. Due to a rough surface, the shape of the track edge in TD-1 is not so clear in contrast to BARYOTRAK. It is clearly seen in the crosssectional image of a track that the ragged feature appeared on the fringe of the opening mouth of the track in TD-1. The curing conditions of polymerization for TD-1 and BARYOTRAK are same, but different from TNF-1. The monomer used for the polymerization of TD-1 and BARYOTRAK was purified in order to exclude oligomers. The surface roughness and the track sensitivity would be affected by the difference of the manufacturing process of the materials. The track sensitivity S is denoted by the etch rate ratio of the track etch rate to the bulk etch rate. S for these ions were calculated by the following formula; 2
S =
1
(2)
where D and B denotes the track diameter and amount of bulk etch, respectively. The variations of sensitivities are summarized with ion energy and REL value in Table 2. The track sensitivities of
207
N. Yasuda et al. / Radiation Measurements 31 (1999) 203-208
BARYOTRAK are lower than those of TD-I and shows the steep response for the REL value m contrast to TD-1.
Fig. 2. AFM images of track etch pit with the surface profiles on BARYOTRAK(a) and TD-I(b). The samples were irradiated to 200 MeV/n Ne ions and etched for 24 hours. Height scale perpendicular to the detector surface was normalized to 1.4 gm.
Table 2. The results of sensitivity measurement. Exposed ion
Energy
REL:ooev
Sensitivity
Sensitivity
(MeWn)
(MeV cm2/g)
BARYOTRAK
TD- 1
Ne
200
233
0.05_+0.01
0.83_+0.03
Si
470
296
0.11_+0.01
0.86_-t-0.07
Ar
530
466
0.74_+0.05
1.49_+0.07
Ar
200
783
2.59_+0.18
4.05!-0.39
208
N. Yasudaet al. /Radiation Measurements 31 (1999) 203-208
CONCULUSIONS
Atomic Force Microscope (AFM) observation has been applied to evaluate the surface roughness and the track sensitivity for CR-39 detectors. The experiments have been done for three types of detector (pure CR-39, CR-39 doped with antioxidant and copolymer of CR-39/NIPAAm) varying with the etching time up to 24 hours in 7N-NaOH at 70°C. We experimentally confirmed the inverse correlation between the track sensitivity and the roughness of the detector surface after etching, i.e. the surface of CR-39 with high sensitivity becomes rough by the etching, while the pure. CR-39 (BARYOTRAK) with low sensitivity keeps its original surface clarity even for the long etching. The ragged edge of the opening mouth of the etch pit and locally non-uniform etching raise to the fluctuations in track sensitivity. The characteristics of BARYOTRAK, such as the surface clarity and the steep response curve, will be expected to be an ideal detector for the detection of cosmic-ray Fe isotopes. Acknowledgements - We would like to express our thanks to the staff of NIRS-HIMAC for their support during the experiment.W e alsoappreciatethe supportand encouragement provided by T. Mumkami andT. Nishio. The experiments were performed as one linkin the chainof Research Projectwith Heavy Ions atNIRS-HIMAC.
REFERENCES
Ogura K, Hattori T, Asano M, Yoshida M, Omichi H, Nagaoka N, Kubota H, Katakai R and Hasegawa H (I997) Proton response of high sensitivityCR-39 copolymer. Radiat. Meas. 28, 197200.
Yamamoto M, Yasuda N, Kaizuka Y, Yamagishi M, Kanai T, Ishigure N, Fumkawa A, Kurano M, Miyahara N, Nakazawa M, Doke T and Ogura K (1997) Sensitivity analysis on heavy ion beam with atomic force microscope. Radiat. Meas. 28, 227-230. Yasuda N, Yamamoto M, Miyahara N, Ishigure N, Kanai T and Ogura K (1998) Measurement of bulk etch rate of CR-39 with atomic force microscopy. Nucl. Instrum. and Meth. B142(1998) 111-116.
PERGAMON
RadiationMeasurements31 (1999) 203-208
TRACK SENSITIVITY AND THE SURFACE ROUGHNESS MEASUREMENTS OF CR-39 WITH ATOMIC FORCE MICROSCOPE N. YASUDA *, M. YAMAMOTO *, K. AMEMIYA **, H. TAKAHASHI ** A. KYAN *** AND K. OGURA **** * National Institute of Radiological Science, Chiba 263-8555, Japan ** Quantum Engineering and Systems Science, University of Tokyo, Tokyo 113-0033, Japan *** Graduate School of Science and Engineering, Ibaraki University, Ibaraki 310-0056, Japan **** College of Industrial Technology, Nihon University, Chiba 275-0006, Japan ABSTRACT Atomic Force Microscope (AFM) has been applied to evaluate the surface roughness and the track sensitivity of CR-39 track detector. We experimentally confirmed the inverse correlation between the track sensitivity and the roughness of the detector surface after etching. The surface of CR-39 (CR-39 doped with antioxidant (HARZLAS (TD-1)) and copolymer of CR-39/NIPAAm (TNF-I)) with high sensitivity becomes rough by the etching, while the pure CR-39 (BARYOTRAK) with low sensitivity keeps its original surface clarity even for the long etching. KEYWORDS CR-39; Atomic Force Microscope (AFM); roughness; track sensitivity.
INTRODUCTION CR-39 plastic track detector is utilized in many fields, such as space dosimetry and fast neutron dosimetry, and radiography, etc. In case of the optical microscopic observation, the thickness of material removed by etching is usually ranging from 10 to 50 p,m for CR-39. Consequently, the ragged edge of the opening mouth of the etched track and the rough and uneven surface of the detector are observed. Therefore, it is difficult to distinguish the shallow tracks from the background of the rough surface. This thing will be hard to determine the threshold REL (Restricted Energy Loss) value for the track regislxation of the detector. Moreover the ragged mouth of track and locally non-uniform etching raise to the fluctuation in track sensitivity and limit the charge and mass resolution of CR-39 as a consequence. The research for lowering CR-39 track registration threshold is important in order to extend the field of the application. The CR-39 (HARZLAS (TD-1)) manufactured by Fukuvi Chemical Industry Co. Ltd., Japan has the high sensitivity to low LET (Linear Energy Transfer) particle, recording normally incident protons up to the energy of 20 MeV. Recently, new CR-39 (TNF-I) developed by Ogura et al. (1997) can record normally incident protons up to the energy of 27 MeV. It is empirically known that there is the close relation between the surface roughness after the chemical etching and the track sensitivity. By clarifying the relationship between sensitivity and roughness of the surface, it is possible to improve efficiently the sensitivity and quality of the surface of CR-39. By using the AFM (Atomic Force Microscope) technique, in contrast to the optical microscope (OPT), it 1350-4487/99/$ - see frontmatter© 1999 ElsevierScienceLtd.All rights reserved.
PII:S1350-4487(99)00089-X
204
N. Yasuda et aL / Radiation Measurements 31 (1999) 203-208
is possible to observe the three-dimensional profile of the track and the etched surface in the early stage of the etching process (M. Yamamoto et al., 1997; N. Yasuda et aL, 1998). In this study, AFM has been applied to evaluate the surface roughness and track sensitivity for CR-39 detectors. EXPERIMENTAL
The AFM (Nanoscope III; Digital Instruments) equipped with a 125 p.m cantilever having a typical tip length of 10 ~m was operated in the tapping mode which has been developed as a method to achieve high resolution without inducing destructive frictional forces. The cantilever was oscillated near its resonant frequency (-250 kHz) which allowed the whole surface of the sample to be scanned. The driving of the cantilever was done by piezoceramic actuators, which change their dimensions according to the applied voltage. The applied voltage to the piezo scanner was automatically converted into the height (depth) information of the surface of the sample. The calibration of a piezoceramic scanner was carded out by scanning the standard samples such as diffraction gratings (10.0_+0.3 p.m pitch, 180.0+5.4 nm high and 1.0L~0.1 ~tm pitch, 1.00_+0.05 p.m high). The surface measurements have been done for three types of CR-39, pure CR-39 (BARYO TRACK), CR-39 doped with antioxidant (HARZLAS (TD-1)) and copolymer of CR-39/NIPAAm (TNF-I), which were manufactured by Fukuvi Chemical Industry Co. Ltd., Japan. The samples of CR-39 were prepared by curing a sheet (0.9 mm thick) into square-shaped pieces of 1 x 1 cm2. These pieces were etched in 7N NaOH solution at 70°C using a water bath incubator. The etching time was varied from 1 to 24 hours. The surface roughness was measured by AFM. . In order to compare the track sensitivity, two types of CR-39 (BARYOTRAK and TD-I) were exposed to 530, 200 MeV/n Ar ions, 470 MeV/n Si ions and 200 MeV/n Ne ions from NIRS(National Institute of Radiological Science)-HIMAC (Heavy Ion Medical Accelerator in Chiba). These stacks were etched in 7N NaOH solution at 70°C for 24 hours. The track etch pits were measured by AFM and OPT. RESULTS AND DISCUSSION
Figure 1 shows 3-D images of the detector surface taken with a scanning area of 10 x 10 p.m2 of CR-39 which was non-etched and etched for 2, 10 and 24 hours, respectively. Height scale perpendicular to the detector surface was normalized to 1.4 p.m. The images displayed as a viewer 30°above the surface would see it. It is clearly seen that the surface condition of TD-I was getting rough as the course of the etching time in contrast to the surface of pure CR-39 (BARYOTRAK). The RMS roughness (Rq), the amount of bulk etch and track registration threshold for three types of CR-39 are summarized in Table 1. The Rq is the standard deviation of the Z (height) valuewithin the scanned area and is calculated as
Rq = R(F(~" Z i - Zave)2,N)),
(l)
where Zave is the average of the height value within the scanned area, Zi is the observed height value, and N is the number of pixels. It is proven that the surface of BARYOTRAK is almost flat even ff the etching lime is prolonged, while the roughness of surface of TD-1 and TNF-I become significant as the course of the etching time. It was experimentally and quantitatively confirmed that the CR-39 having high sensitivity shows rough surface after etching and the CR-39 with low sensitivity keeps good quality of the surface even for the long time etching. This tendency of the material is empirically known among the CR-39 user so far.
N. Yasuda et aL / Radiation Measurements 31 (1999) 203-208
Fig. 1. 3-D images of the detector surface (BARYOTRACK (left side) azld TD-1 (right side)) taken with a scanning area of 10 x 10 grn 2 of CR-39 which was non-etched and etched for 2, 10 and 24 hours, respectively. Height scale perpendicular to the detector surface was normalized to 1.4 gm.
205
206
N. Yasuda et al. / Radiation Measurements 31 (1999) 203-208
Table 1. The summaryof measured data. ThresholdREL values of TNF-1 and TD-1 w e r e measured by Ogura et al. (1997). Etching T'une
CR-39
(hours) TNF-1 0
10
24
Bulk
Rq
Threshold REl-~oo,v
(Ixm)
(nm)
(MeV cm2/g)
0
4.8
TD-1
0
2.3
BARYO
0
2.0
TNF-I
1.6
5.8
TD-I
1.4
4.6
BARYO
1.2
2.1
TNF-1
3.2
12.2
TD-1
3.1
10.6
BARYO
2.2
2.3
TNF-I
14.3
43.9
TD-I
13.7
33.2
BARYO
11.7
2.1
TNF-1
48.8
64.7
~10"
TD-1
44.8
16.9
-15"
BARYO
37.2
2.4
,.,2 x 102
~mm
Figure 2 shows the AFM images of track etch pit with the surface profiles on BARYOTRAK and TD1. The samples were irradiated to 230 MeV/n Ne ions and etched for 24 hours. In this etching condition, it was impossible to observe the tip of etch pit, because the etch pits on the CR-39 surface grew too large and was beyond the reaching limit of the cantilever tip. Due to a rough surface, the shape of the track edge in TD-1 is not so clear in contrast to BARYOTRAK. It is clearly seen in the crosssectional image of a track that the ragged feature appeared on the fringe of the opening mouth of the track in TD-1. The curing conditions of polymerization for TD-1 and BARYOTRAK are same, but different from TNF-1. The monomer used for the polymerization of TD-1 and BARYOTRAK was purified in order to exclude oligomers. The surface roughness and the track sensitivity would be affected by the difference of the manufacturing process of the materials. The track sensitivity S is denoted by the etch rate ratio of the track etch rate to the bulk etch rate. S for these ions were calculated by the following formula; 2
S =
1
(2)
where D and B denotes the track diameter and amount of bulk etch, respectively. The variations of sensitivities are summarized with ion energy and REL value in Table 2. The track sensitivities of
207
N. Yasuda et al. / Radiation Measurements 31 (1999) 203-208
BARYOTRAK are lower than those of TD-I and shows the steep response for the REL value m contrast to TD-1.
Fig. 2. AFM images of track etch pit with the surface profiles on BARYOTRAK(a) and TD-I(b). The samples were irradiated to 200 MeV/n Ne ions and etched for 24 hours. Height scale perpendicular to the detector surface was normalized to 1.4 gm.
Table 2. The results of sensitivity measurement. Exposed ion
Energy
REL:ooev
Sensitivity
Sensitivity
(MeWn)
(MeV cm2/g)
BARYOTRAK
TD- 1
Ne
200
233
0.05_+0.01
0.83_+0.03
Si
470
296
0.11_+0.01
0.86_-t-0.07
Ar
530
466
0.74_+0.05
1.49_+0.07
Ar
200
783
2.59_+0.18
4.05!-0.39
208
N. Yasudaet al. /Radiation Measurements 31 (1999) 203-208
CONCULUSIONS
Atomic Force Microscope (AFM) observation has been applied to evaluate the surface roughness and the track sensitivity for CR-39 detectors. The experiments have been done for three types of detector (pure CR-39, CR-39 doped with antioxidant and copolymer of CR-39/NIPAAm) varying with the etching time up to 24 hours in 7N-NaOH at 70°C. We experimentally confirmed the inverse correlation between the track sensitivity and the roughness of the detector surface after etching, i.e. the surface of CR-39 with high sensitivity becomes rough by the etching, while the pure. CR-39 (BARYOTRAK) with low sensitivity keeps its original surface clarity even for the long etching. The ragged edge of the opening mouth of the etch pit and locally non-uniform etching raise to the fluctuations in track sensitivity. The characteristics of BARYOTRAK, such as the surface clarity and the steep response curve, will be expected to be an ideal detector for the detection of cosmic-ray Fe isotopes. Acknowledgements - We would like to express our thanks to the staff of NIRS-HIMAC for their support during the experiment.W e alsoappreciatethe supportand encouragement provided by T. Mumkami andT. Nishio. The experiments were performed as one linkin the chainof Research Projectwith Heavy Ions atNIRS-HIMAC.
REFERENCES
Ogura K, Hattori T, Asano M, Yoshida M, Omichi H, Nagaoka N, Kubota H, Katakai R and Hasegawa H (I997) Proton response of high sensitivityCR-39 copolymer. Radiat. Meas. 28, 197200.
Yamamoto M, Yasuda N, Kaizuka Y, Yamagishi M, Kanai T, Ishigure N, Fumkawa A, Kurano M, Miyahara N, Nakazawa M, Doke T and Ogura K (1997) Sensitivity analysis on heavy ion beam with atomic force microscope. Radiat. Meas. 28, 227-230. Yasuda N, Yamamoto M, Miyahara N, Ishigure N, Kanai T and Ogura K (1998) Measurement of bulk etch rate of CR-39 with atomic force microscopy. Nucl. Instrum. and Meth. B142(1998) 111-116.