Verification of angular dependence for track sensitivity on several types of CR-39

Radiation Measurements 43 (2008) S269 – S273 www.elsevier.com/locate/radmeas

Verification of angular dependence for track sensitivity on several types of CR-39 N. Yasuda a,∗ , D.H. Zhang b , S. Kodaira c , Y. Koguchi d , S. Takebayashi d , W. Shinozaki d , S. Fujisaki d , N. Juto d , I. Kobayashi e , M. Kurano a , D. Shu a , H. Kawashima a a National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan b Institute of Modern Physics, Shanxi Normal University, Linfen, Shanxi Province 041004, China c Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan d Chiyoda Technol Corporation, 3681 Narita-cho, Oarai-machi, Higashi Ibaraki Gun, Ibaraki 311-1313, Japan e Nagase Landauer, Ltd., 11-6 Hisamatsu-cho, Nihonbashi, Chuou-ku, Tokyo 103-8487, Japan

Abstract We verified the angular dependence of track registration sensitivity for four types of CR-39 plastic nuclear track detectors using heavy ion beams. These detectors have been launched with several types of luminescence detectors on the Russian segment of the International Space Station (ISS) to intercompare passive radiation dosimeter components from several institutions. CR-39 detectors were irradiated by several kinds of heavy ions with several hundred MeV/n. All the etched tracks were analyzed using a high-speed microscope (HSP-1000) with ellipse fitting software (PitFit). Reduction of sensitivity was observed near the region of the critical angle. Corrections were made using an empirical method based on the known structure of the galactic cosmic ray spectrum. © 2008 Elsevier Ltd. All rights reserved. PACS: 29.40.Wk; 29.40.Gx; 87.66.Pm; 87.53.Qc Keywords: CR-39; Solid state detector; Space radiation dosimetry

1. Introduction The CR-39 (allyl diglycol carbonate) nuclear track detector has been used for various fields. This detector is a particularly useful tool to measure LET information for space radiation dosimetry (Benton, 1984; Benton et al., 2002a, b; Doke et al., 2002; Tawara et al., 2002) and to measure the elemental and isotopic compositions of ultra-heavy cosmic rays for astrophysics (Hasebe et al., 2006). Since the track formation sensitivity (S ≡ Vt /Vb − 1; where Vt and Vb denote the track etch rate and the bulk etch rate, respectively) is essentially constant for relativistic nuclei with identical charge, we can get physical values using this technique. In these fields, however, it is well known that the track formation sensitivity (detector response) depends on the dip angle (Hayashi and Doke, 1980; ∗ Corresponding author. Tel.: +81 043 206 3479; fax: +81 043 206 3514.

E-mail address: [email protected] (N. Yasuda). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.03.027

Doke et al., 1997), and the correction of angular dependence of response is a key point in obtaining physical values. As verified by Hayashi and Doke using cosmic ray data and accelerated ion exposures, different types of CR-39 detector show specific characteristics for angular dependence. They developed an empirical method for the correction of angular dependence, which they used to explain the difference in LET distribution as measured by the silicon telescope type real-time radiation monitoring device (RRMD) and by CR-39 detectors in spacecraft. They also stated that the angular dependence will be dependent on the hardness of detectors. The hardness against etching may not be the same in the vicinity of the surface and inside of the detector. We have studied the track evolution at the early stage of etching (the bulk etch is about 20–1000 nm) for high energy C and Si ions in CR-39 detectors using an atomic force microscope (Yamamoto et al., 1999). According to our results, the track diameters increased linearly with the amount of bulk etch, but retardation of track length growth was observed at

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Fig. 1. Track formation sensitivities as a function of the restricted energy loss (calibration curve) at dip angle of 90◦ for different types of CR-39: HARZLAS TD-1, BARYOTRAK-P(CR), TT-A and TT-S.

the early stage of the etching. The probable cause of this track length growth retardation was considered to be due to the insufficient erosion and exchange of etchant into the cone end of the track at the early stage of the etching process (Yasuda et al., 2001). The reduced sensitivity at the reduction of dip angle might be explained by this qualitative explanation. Several variations of CR-39 track detectors were developed and commercialized in Japan with different track formation sensitivities and varying track registration thresholds for use in specific applications. For example, the HARZLAS TD-1 was developed by Fukuvi Chemical Industry in 1995. Details of the curing cycle for polymerization of HARZLAS TD-1 and its response to low energy protons have been described (Ogura et al., 1997, 2001). Hayashi and Doke (1980) verified the angular dependence of the detector made from the TS-16N type monomer (allyl diglycoal carbonate) which was provided by Tokuyama Soda Co., Ltd. The “Early BARYOTRAK” (manufacturing period: 1987–1992) was an improved product of the TS-16N detector by Fukuvi Chemical Industry, and appeared as “Lantrak” elsewhere (Ipe et al., 1992). From 1993 to 1996, the “BARYOTRAK type-P(MR)” was developed with a purified (99.4%) MR-3 type monomer (allyl diglycoal carbonate; Mistsui Touatsu Chemical Co.). In 1997, the BARYOTRAK typeP(CR) was fabricated from purified (99.4%) CR-39姠 monomer (PPG Industries). This series of detector does not include any antioxidant and is mainly used for routine neutron dosimetry. We have recently developed a new CR-39 detector with two variations. TechnoTrak type-A (TT-A) consists of purified

(99.7%) CR-39姠 monomer (PPG Industries) with a phenolic antioxidant (0.05 wt%), and type-S (TT-S) consists of nonpurified CR-39姠 monomer with 0.01 wt% antioxidant. Track registration threshold and anti-aging effects of the phenolic antioxidant were previously examined (Koguchi et al., 2005). Surface roughness of the detectors after long etching was examined and reported in separated papers (Yasuda et al., 1999; Koguchi et al., 2005). These newly developed detectors have been launched with several types of luminescence detectors on the Russian segment of ISS to intercompare passive radiation dosimeter components from several institutions as a series of experiments to establish a “reference standard” for space radiation monitoring as part of the ICCHIBAN project (Uchihori and Benton, 2004; Yasuda et al., 2006). As part of this work, an investigation was carried out to calibrate for four types of commercialized CR-39 detectors using heavy ion beams. The angular dependence of track registration sensitivity was also verified as a first step to check their characteristics for practical use for space dosimetry. 2. Experiments We used four types of CR-39 track detectors in this study: HARZLAS TD-1 and BARYOTRAK type-P(CR) from Fukuvi Chemical Industry and TT-S and TT-A from Chiyoda Technol Corporation. To calibrate these detectors, we used heavy ions from HIMAC and protons from the cyclotron facility in NIRS. The detectors were exposed to heavy ions from helium to iron

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Fig. 2. Dip angle dependences for different types of CR-39: HARZLAS TD-1, BARYOTRAK-P(CR), TT-A and TT-S.

with energy below 500 MeV/n, and to 10–20 MeV protons. The ion exposures were performed with various incident (dip) angles from 30◦ –90◦ to the detector surface. HARZLAS TD-1 and BARYOTRAK-P(CR) detectors were etched in 7 N sodium hydroxide at 70 ◦ C. TT-A and TT-S type detectors were etched in 5 N sodium hydroxide at 70 ◦ C. Each of four detector types was etched for 24 h. The major (DA ) and minor (DB ) axes of etch pits for each detector were measured using a high-speed optical microscope (HSP-1000) and ellipse fitting software (PitFit) (Yasuda et al., 2005). Then the track formation sensitivity (S ≡ Vt /Vb − 1) was calculated using a geometrical relation (Fleisher et al., 1975);  2 D2 16DA B S= + 1 − 1, (1) (4B 2 − DB2 )2 where B is the amount of bulk etch. 3. Results and discussion After 24 h of etching, the amount of bulk etch for each detector was measured to be 42, 45, 21 and 24 m for HARZLAS TD-1, BARYOTRAK-P(CR), TT-A and TT-S, respectively. The track formation sensitivities of the different types of CR-39 detectors at dip angle of 90◦ are plotted as a function of the restricted energy loss (REL; Fowler et al., 1979) in Fig. 1. The new detectors (TT-A and TT-S) have almost the same sensitivities as those of the HARZLAS TD-1 detector. As described

in separate papers (Yasuda et al., 1999; Koguchi et al., 2005), TT-A and TT-S have a flat surface even after long etching in contrast to the HARZLAS TD-1 detector. This may be an advantage when using automated measurement image analysis. The BARYOTRAK-P(CR) has a steep response to REL, and its detection threshold is expected to be ∼ 200 MeV cm2 /g. This detector will be a candidate for ultra-heavy cosmic rays detection and/or higher LET component (> 100 keV/m) in space radiation dosimetry, since it has a low and steep response with excellent flatness. Dip angle dependences of the track formation sensitivity for the four types of detector are shown in Fig. 2. Despite a common tendency for decreasing sensitivity with the reduction of dip angle, each detector has an individually different character. Each detector therefore has its own critical angle. Increased sensitivity is also observed from 500 MeV/n Fe exposures in both new detectors. This may not be a reason for the relationship between angular dependence track length growth retardation in the early stage of etching as described above. Further systematic study is required for etch pit growth related to the etching process. 4. Correction method Using an empirical method (Doke et al., 1997), we tried to obtain the correction factor  for LET distribution as follows. The critical angle, the minimum detectable value of the incident dip angle of particle to the detector surface, is expressed

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Fig. 3. An empirical method for correction of dip angle dependence. The solid line shows the limit of track recording (critical angle). A track formation sensitivity at dip angle of 90◦ gives a critical angle i when the sensitivity has no dip angle dependence (ideal case). Calibration results (white dots) show reduction of sensitivity due to the dip angle dependence, the extrapolation of measured sensitivities gives a different critical angle r (real case).

using track parameters or a track registration sensitivity by the following formulae:     1 −1 Vb −1 = sin . (2) c = sin Vt S+1 In Fig. 3, the solid curve denotes the critical angle as a function of dip angle of incident ion, and white dots show measured sensitivities with different dip angles corresponding to the same LET value. For the ideal case (no angular dependence), the sensitivity at 90◦ gives a critical angle i , and the extrapolation of measured sensitivities gives a new critical angle r in real case with angular dependence. From these angles i and r , we can obtain corresponding critical zenith angles i and r for each case. On the other hand, when we assume that the majority of incident particles have relativistic velocity with isotropic distribution, the geometrical factor ε (detection efficiency after correction of the critical angle c ) is given by the following equation: ε=

A (1 − cos 2c ), 2

(3)

where A gives the detector area. Thus, the correction factor  for LET distribution is given by the ratio i and r , so that =

1 − cos 2i . 1 − cos 2r

(4)

Thus, we obtained the correction factor  for LET value. As an example, Fig. 4 shows the variation of the correction factor  for the HARZLAS TD-1 detector as a function of LET value. For a

Fig. 4. Variation of correction factor  for HARZLAS TD-1 detector is plotted as a function of LET value. The solid line shows a fitting result of the obtained data from this study, and the dashed line shows the result obtained by a previous experiment (Doke et al., 1997).

particle with relativistic velocity, the LET value of the particle is proportional to the REL value; we used LET = 0.19 × REL, in this study. The solid line shows a best fit for the data from this study, and the dashed line shows the result obtained by a previous experiment (Doke et al., 1997). Fitting was done by assuming the formula: =1+

 , LET

(5)

where  is a fitting parameter. Although a possible qualitative explanation was given above based on AFM results in the early etching process for angular dependence, it is clear that the angular dependence is not unique as it depends on the characteristics of the detector, e.g. the type of monomer used and the purity of the CR-39. The dependence might also be influenced by the curing cycle of polymerization, and by the conditions of etching as shown in Fig. 2. Further systematic study is required to confirm this phenomenon. For practical use in space radiation dosimetry, we require precise calibration using a high energy accelerator as described here. 5. Conclusions We verified angular dependence of track registration sensitivity for four types of CR-39 plastic nuclear track detectors using heavy ion and proton beams. Newly developed detectors (TT-S and TT-A) have almost the same sensitivity as HARZLAS TD1 detector at dip angle of 90◦ exposure. The BARYOTRAKP(CR) (pure CR-39 detector without antioxidant) has a steep

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response to the REL, and its detection threshold is relatively low in contrast to the detectors that include an antioxidant. Each detector has its own characteristics for the angular dependence of incident particles. Critical angle from geometrical relation should be corrected for practical use in space radiation dosimetry. Acknowledgments We would like to express our thanks to the staff of NIRSHIMAC. This research (16P167) was performed as a part of the Research Project of Heavy Ions at NIRS-HIMAC. References Benton, E.V., 1984. Summary of current radiation dosimetry results on manned spacecraft. Adv. Space Res. 4 (10), 153–160. Benton, E.R., Benton, E.V., Frank, A.L., 2002a. Passive dosimetry aboard the Mir Orbital Station: internal measurements. Radiat. Meas. 35, 439–455. Benton, E.R., Benton, E.V., Frank, A.L., 2002b. Passive dosimetry aboard the Mir Orbital Station: external measurements. Radiat. Meas. 35, 457–471. Doke, T., Hayashi, T., Kobayashi, M., Watanabe, A., 1997. Dip angle dependence of track formation sensitivity in antioxidant doped CR-39 plates. Radiat. Meas. 28, 445–450. Doke, T., Hayashi, T., Kikuchi, J., Nagaoka, S., Nakano, T., Takahashi, S., Tawara, H., Terasawa, K., 2002. Dose equivalents inside the MIR Space Station measured by the combination of CR-39 plates and TLDs and their comparison with those on Space Shuttle STS-79, -84 and -91 missions. Radiat. Meas. 35, 505–510. Fleisher, R.L., Price, P.B., Walker, R.M., 1975. Nuclear Track in Solids. Principles and Applications. University of California Press, Berkley. Fowler, P.H., Clampham, V.M., Henshaw, D.L., Amin, S., 1979. Proc. 16th Int. Cosmic Ray Conf., Kyoto, Vol. 11, p. 97. Hasebe, N., Hareyama, M., Kodaira, S., Sakurai, K., Yamashita, N., Miyachi, T., Okudaira, O., Takano, M., Torii, S., Doke, T., Ogura, K., Yasuda, N., Uchihori, Y., Tawara, H., Nakamura, S., Shibata, T., Yanagimachi, T., Wanajo, S., 2006. Observation program of isotope composition in the ultra heavy cosmic rays. In: Proceedings of the 9th Conference on Astroparticles, Particle and Space Physics, Detectors, and Medical Physics Applications, pp. 223–228.

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