2. Track Sensitivity and Surface Roughness Measurement of CR-39 Surface with Atomic Force Microscope

Nakahiro Yasuda , Mikio Yamamoto, Mieko Kurano, Kuniaki Amemiya ¹, Hiroyuki Takahashi ¹ and Koichi Ogura ²
(¹ Univ. of Tokyo; ² Nihon Univ.)

Keywords: CR-39, atomic force microscope, roughness measurement, track sensitivity

Research on lowering CR-39 track registration threshold is important in order to extend its application fields. The CR-39 (HARZLAS (TD-1)) manufactured by Fukuvi Chemical Industry Co. Ltd., Japan has the high sensitivity to low LET (Linear Energy Transfer) particles, recording normally incident protons up to the energy of 20 MeV. Recently, new CR-39 (TNF-1) developed by Ogura et al. (1997) can record normally incident protons up to the energy of 27 MeV. Since these tracks are observed as shallow tracks, it is difficult to distinguish them from background of the rough surface caused by chemical etching. Moreover, the ragged mouth of the track and locally non-uniform etching cause fluctuation in track sensitivity and limit the charge and mass resolution of CR-39 as a consequence. 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 is possible to observe a three-dimensional profile of the track and the etched surface in the early stage of the etching process. In this study, AFM has been applied to evaluate the surface roughness and track sensitivity for CR-39 detectors.

The surface measurements have been done for three types of CR-39, pure CR-39 (BARYOTRACK), CR-39 doped with antioxidant (HARZLAS (TD-1)) and copolymer of CR-39/NIPAAm (TNF-1); all were manufactured by Fukuvi Chemical Industry Co. Ltd., Japan. The samples of CR-39 were prepared by cutting a sheet (0.9 mm thick) into square-shaped pieces of 1 x 1 cm ². 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-1) were exposed to several kinds of heavy ions from shown in Table.1. These samples were etched in 7N NaOH solution at 70°C for 24 hours. The track etch pits were measured by AFM and OPT.

The surface roughness was evaluated using the RMS roughness (Rq). The surface of BARYOTRAK is almost flat even if the etching time is prolonged, while the surface roughness of TD-1 and TNF-1 becomes significant with passage of etching time. It was experimentally and quantitatively confirmed that the CR-39 having high sensitivity shows a rough surface after etching and the CR-39 with low sensitivity keeps its good surface quality even for a long etching time. Fig.2 shows AFM images of a track etch pit with the surface profiles on BARYOTRAK and TD-1. The samples were irradiated with 200 MeV/n Ne ions and etched for 24 hours. Due to the rough surface, the shape of the track edge on TD-1 is not so clear in contrast to BARYOTRAK. A ragged feature appears on the fringe of the opening mouth of the track on TD-1 in the cross-sectional image of a track. The curing conditions of polymerization for TD-1 and BARYOTRAK were the same, but differed 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 in the manufacturing process of the materials.

The variations of sensitivities are summarized with ion energy in Table.1. The track sensitivities of BARYOTRAK are lower than those of TD-1 and BARYOTARCK shows a steep response for the LET value in contrast to TD-1. Its track registration threshold is about 40 keV/m. An inverse correlation was experimentally confirmed between the track sensitivity and the roughness of the detector surface after etching, i.e. the surface of CR-39 with high sensitivity is roughened by etching, while the BARYOTRAK with low sensitivity keeps its original surface clarity, even for a long etching. The characteristics of BARYOTRAK, such as the surface clarity and the steep response curve, are expected to make it ideal for the detection of cosmic-ray Fe isotopes.

Fig.2. AFM images of a track etch pit with the surface profiles on BARYOTRAK(a) and TD-1(b).
The samples were irradiated with 200MeV/n Ne ions and etched for 24 hours.
Height scale perpendicular to the fetector surface was normalized to 1.4 m.

Publications:
1) Takahashi H., Amemiya K., Yasuda N., Yamamoto M., et al.: Nucl. Inst. and Meth., A422, 751-755, 1998.
2) Yamamoto M., Yasuda N., et al.: Nucl. Inst. and Meth., B152, 349-356, 1999.