RIKEN Accel. Prog. Rep. 48 (2015)

Ⅲ-1. Atomic & Solid State Physics (Ion)

Response of polyimide films to U ion beams as etched-track detectors
T. Yamauchi,＊1 T. Kusumoto,＊1 K. Matsukawa,＊1 Y. Mori,＊1 M. Kanasaki,＊1 K. Oda,＊1 S. Kodaira,＊2 K. Yoshida,＊3Y. Yanagisawa,＊3 T. Kambara,＊3 and A. Yoshida＊37Etch pit radius, r (m)

6(2.3 MeV/n)5(2.5 MeV/n)U(14.7 MeV/n)43(3.5 MeV/n)

1.88 µm

210468U3439 keV/µm103(3.7 MeV/n)

10104

Stopping power (keV/m)

3x104

Fig. 2. Sensitivity of Kapton for U ions and other heavy
ions against the stopping power.
12Thickness of layer removed, G (m)

Graduate School of Maritime Sciences, Kobe University
National Institute of Radiological Sciences＊3
RIKEN Nishina Center＊2U

10-2References

14Fig. 1. Etch pit growth curves for U ions (14.7, 17.1 and
337.6 MeV/n), Xe ions (2.3 MeV/n), Kr ions (2.5 MeV/n),
Si ions (3.5 MeV/n), and Al ions (3.7 MeV/n). Each energy
for other indicating ions is close to that of the Bragg peaks.
＊1100(17.1 MeV/n)

U2101

10-1

(337.6 MeV/n)
0Figure 1 shows typical growth curves of etch pit radius r
against the thickness of layer removed G for U ions andother indicating heavy ions. During the etching, the films
were reduced in thickness by 2G. With increasing energy ofU ions, the fitted slope for each data set decreases. The
observed linear relation allowed us to use the conicalassumption in evaluating the etch rate ratio V, which is theratio of the track etch rate Vt to the bulk etch rate Vb. 7) The
etch rate ratio was assessed by the following relation:(1)
V={1+( dr/dG)2} /{1-( dr/dG)2}
where (dr/dG) is the slope of the fitted line. The sensitivityof etch pit formation is defined as V-1. Figure 2 indicatesthe sensitivity of U ions, as well as other heavy ions, as afunction of the stopping power. The threshold of U ions foretch pit formation is 3,439 keV/µm, which is higher thanthat of other heavy ions. The threshold is also observed inthe growth curve (Fig. 1), as the intersect of the fitted line
for 337.6 MeV/n U ions with a depth of 1.88 µm.

Sensitivity

The polyimide Kapton retains its excellent physical,
electrical, and mechanical properties over a wide
temperature range between 4 and 673 K, and hence, it isconsidered an attractive candidate for a nuclear track
membrane. Size-controllable nuclear pores on the
sub-micron scale have been fabricated in the polyimide
films by chemical etching, subsequent to irradiation withheavy ions.1-3) Such nuclear membranes have been used in
nanopore membranes, templates for metallic nanowires,aerosol filters, and gas separation films.4-6) Applicability ofthe polyimide films as etched-track detectors for researchon ultra-heavy cosmic rays has also been suggested; in thiscase, relatively long etchings are performed prior to thesurface observations on the micron-scale under optical
microscopes.7) Few studies have been carried out, however,on the response of the polyimide for U ions, even as
fundamental studies,8,9) different from that on polyethylene
terephthalate.10) In this report, we describe the first result on
the detection threshold and sensitivity of Kapton for U ions.
Commercially available Kapton films (from Nilaco) witha thickness of 125 µm were stacked and exposed to 345
MeV/n U-238 beams in air at the port of BigRIPS(F12),
covering the stopping powers up to 20,000 keV/µm. Afterthe exposure, the films were etched in a sodium
hypochlorite solution kept at 55˚C.

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3) R. Sudowe et al.: Nucl. Instr. Meth. B 175-177, 564 (2001).
4) W. Ensinger et al.: Radiat. Meas. 40, 642 (2005).
5) W. Ensinger et al.: Surf. Coatings Technol. 201, 8442 (2007).
6) W. Ensinger et al.: Radiat. Phys. Chem. 79, 204 (2010).
7) T. Yamauchi et al.: Appl. Phys. Exp. 6, 046401 (2013).
8) C. Trautmann et al.: Nucl. Instr. Meth. B 116, 429 (1996).
9) S. A. Saleh and Y. Eyal: Nucl. Instr. Meth. B 208, 137 (2003).
10) D. Drach et al.: Nucl. Instr. Meth. B 28, 46 (1987).

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