4. New Method for High Resolution Autoradiography Using CR-39 Solid State Track Detectors and Atomic Force Microscopy

Kuniaki Amemiya^1,2, Nakahiro Yasuda¹, Mikio Yamamoto¹, Hiroyuki Takahashi², Masaharu Nakazawa² and Koichi Ogura³
(¹NIRS; ²Univ. of Tokyo; ³Nihon Univ.)

Keywords: CR-39, atomic force microscope, radiography; autoradiography

Recently, the atomic force microscope (AFM) has been applied to measurements of very small etch pits on CR-39 solid state track detectors. In this technique, the CR-39 is etched for a short time and then the etched surface is scanned with the AFM. The technique also allows high resolution imaging of charged particle tracks without a complicated setup and it is expected to be applicable to high resolution charged particle autoradiography.

Autoradiographic applications often require accurate positioning between the sample and the particle tracks in order to determine the radiation dose distribution inside the sample. As a fiducial marker for positioning, aluminum patterns were deposited on the CR-39 surface using a photolithography technique. The marked CR-39 was etched in 7N NaOH solution at 70°C for 5 minutes. The aluminum pattern was dissolved during the etching process leaving pattern-shaped steps on the surface of the CR-39 detector as shown in Fig. 4. The level difference of the step was about 70 nm. Thus, it is a suitable marker that can be measured using AFM and positioning between the sample and the etch pits in autoradiography should be possible with good accuracy.

The resolution of the imaging mentioned above is determined by the size of the etch pits which can be observed with the AFM. A small CR-39 (BARYOTRAK-P) cut into 1 x 1 cm² was irradiated with 1 MeV helium ions using the Tandetron accelerator at RCNST. Irradiated CR-39 was etched in 7N NaOH solution at 70°C for 2 minutes and observed with the AFM. Fig. 5 shows a typical cross-sectional view of an etch pit observed with the AFM. The diameter of the mouth of the etch pit was about 80 nm. In this condition, the position resolution of imaging for the charged particle track was 30 nm (FWHM). Because the intrinsic resolution of the AFM is extremely high, varying etching conditions (i.e. the concentration and the temperature of etchant and the etching time) should make the etch pit smaller than in the present result. The surface roughness of BARYOTRAK-P was about 2 nm even after etching process. Therefore the ultimate resolution of this method was thought to be of the order of several nanometers.

This method provides a new technique for subcellular scale radiation imaging that is required in microdosimetry.

Fig.4. The AFM image of the dark band of the pattern with etch pits for 1 MeV carbon ions.

Fig.5. The cross-sectional AFM image of an etch pit for 1MeV helium ions.

Publications:
1) Takahashi H., Amemiya K., Nakazawa M., Yasuda N., Yamamoto M., et al.: Nucl. Instr. and Meth., A422, 751-755, 1999.
2) Amemiya K., Takahashi H., Nakazawa M., Yasuda N., Yamamoto M., Ogura K., et al.: Nucl. Instr. and Meth., B, in press.