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,＊3

Y. Yanagisawa,＊3 T. Kambara,＊3 and A. Yoshida＊3

The polyimide Kapton retains its excellent physical, electrical, and mechanical properties over a wide temperature range between 4 and 673 K, and hence, it is considered 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 with heavy 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 of the polyimide films as etched-track detectors for research on ultra-heavy cosmic rays has also been suggested; in this case, relatively long etchings are performed prior to the surface 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) with a 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. After the exposure, the films were etched in a sodium hypochlorite solution kept at 55˚C.

Fig. 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. ＊1 Graduate School of Maritime Sciences, Kobe University ＊2 National Institute of Radiological Sciences ＊3 RIKEN Nishina Center

Figure 1 shows typical growth curves of etch pit radius ragainst the thickness of layer removed G for U ions and other indicating heavy ions. During the etching, the films were reduced in thickness by 2G. With increasing energy of U ions, the fitted slope for each data set decreases. The observed linear relation allowed us to use the conical assumption in evaluating the etch rate ratio V, which is the ratio of the track etch rate Vt to the bulk etch rate Vb. 7) The etch rate ratio was assessed by the following relation:

V={1+( dr/dG)2} /{1-( dr/dG)2} (1) where (dr/dG) is the slope of the fitted line. The sensitivity of etch pit formation is defined as V-1. Figure 2 indicates the sensitivity of U ions, as well as other heavy ions, as a function of the stopping power. The threshold of U ions for etch pit formation is 3,439 keV/µm, which is higher than that of other heavy ions. The threshold is also observed in the 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.

Fig. 2. Sensitivity of Kapton for U ions and other heavy ions against the stopping power.

References 1) P. Vater: Nucl. Tracks Radiat. Meas. 15, 743 (1988). 2) C. Trautmann et al.: Nucl. Instr. Meth. B 111, 70 (1996). 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).

0

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2

3

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7

0 2 4 6 8 10 12 14

Thickness of layer removed, G (m)

Etch

pit

radi

us, r

(m

)

U(14.7 MeV/n)

(2.3 MeV/n)

(2.5 MeV/n) U(17.1 MeV/n)

(3.5 MeV/n)(3.7 MeV/n)

U(337.6 MeV/n)

1.88 µm

10-2

10-1

100

101

103 104

Sens

itivi

ty

Stopping power (keV/m)

U

3x104

3439 keV/µm

High-density n-type doping of diamond by nitrogen beamimplantation

H. Ueno,∗1 R. Kato,∗2 H.M. Yamamoto,∗2,∗3 A. Yoshimi,∗1,∗4 K. Morimoto,∗1 Y. Ichikawa,∗1 Y. Ishibashi,∗1,∗5

Y. Ohtomo,∗1,∗6 and T. Suzuki∗1,∗6

After the discovery of the superconductivity ofboron-doped (p-type) diamond in 20041), studies havebeen extensively conducted to raise the Tc of diamondsuperconductivity. These studies have indicated that ahigh doping concentration of boron is favorable for in-creasing the Tc of superconductivity. High doping con-centration can be introduced by the chemical-vapor-deposition (CVD) method2,3). Based on this tech-nique, Tc has so far reached 11.4 K4) at doping con-centration ρ =8.4×1021 cm−3. Theoretically, it hasbeen predicted that a very high critical temperatureTc ≥ 100 K is possible for doped diamond5), becausethe high phonon frequency of diamond is favorable forincreasing the Tc of superconductivity.Unfortunately, the doping concentration achievable

by CVD is limited to less than ρ = 1022 cm−3 forboron and, more seriously, ρ = 1020 cm−3 for phospho-rus (n-type). With higher concentration, the dopantsare known to form dimers, causing localization of thedoped carriers, and thus cannot be used for electricconduction. In view of electronic applications, both p-type and n-type dopings are necessary. From an anal-ogy to other superconductors such as high-Tc cuprates,it is natural to expect n-type superconductivity in dia-mond. To date, however, high-density doping of n-typecarrier has not been achieved.

A new approach expected to realize high-densitydoping is heavy-ion implantation, because i) a sharpfall-off at the distal edge in the stopping range (i.e.,Bragg peak), which becomes remarkable for heavy-ionbeams, is favorable for the purpose of high doping con-centration, and ii) dopant dimerization should not oc-cur very frequently because of the high randomnessof the implanted atoms. Taking these advantages, wehave been conducting doped-diamond studies in orderto realize an n-type diamond superconductivity withTc as high as possible. Furthermore, it is also impor-tant to reproduce the increase in Tc in boron-dopedp-type region, in particular, outside the CVD limit,under the scheme of heavy-ion implantation utilizingthe high beam current of various heavy ions availableat the RIBF facility. In parallel, the optimization ofconditions for heavy-ion implantation including an an-

∗1 RIKEN Nishina Center∗2 Condensed Molecular Materials Laboratory, RIKEN∗3 Research Center of Integrative Molecular Systems (CIMoS),

Institute for Molecular Science∗4 Research Core for Extreme Quantum World, Okayama Uni-

versity∗5 Institute of Physics, University of Tsukuba∗6 Department of Physics, Tokyo Institute of Technology

nealing process to make the diamond lattice as cleanas possible should be also investigated.

In the present work, we report on the first nitrogen-ion implantation experiment at RIBFa) to investigate apossible onset of n-type superconductivity of diamond.A beam of 14N3+ delivered at energy E/A = 450keV from the RILAC accelerator at RIBF was im-planted into (100)-faceted single crystals of type Ib di-amond6). This beam energy was set to be much lowerthan the Coulomb barrier to avoid the radioactivaionof samples. The crystals were prepared in the formof 4 mm × 4 mm × 0.3 mm(t), and mounted on aCu ladder placed in the center of the GARIS targetchamber. The implantation was carried out at roomtemperature at beam current I = 3 ∼ 10 µA.

The density ρ(14N) of the implanted 14N particleswas simply calculated by

ρ(14N) =I · T

q · S ·∆R, (1)

where I is the beam current, T is the irradiationtime, q = 3+ is the charge state of the beam, S ≃π×4 mm×6.5 mm is the beam spot size, and ∆R isthe stopping-range distribution of the beam in thecrystal. Taking a value of ∆R ≃ 0.2 µm at themean stopping range R ≃ 2.5 µm, calculated withthe simulation code SRIM7), we determined ρ(14N)of five nitrogen-implanted diamond crystals, rangingfrom ρ = 7.8 × 1020 to 1.9 × 1022 cm−3. These densi-ties are approximately the same as the reported CVD-doped boron densities at which superconductivity ofdiamond has been observed1,4,8). Offline resistivitymeasurements of the obtained nitrogen-doped crystalsas a function of temperature are in progress.

References1) E.A. Ekimov et al.: Nature 428, 542 (2004).2) B.V. Spitsyn et al.: J. Cryst. Growth 52, 219 (1981).3) M. Kamo et al.: J. Cryst. Growth 62, 642 (1983).4) H. Umezawa et al.: cond-mat 0503303 (2005).5) T. Shirakawa et al.: J. Phys. Soc. Jpn. 76, 014711

(2007).6) Sumicrystal: Sumitomo Electric Industries, Ltd., Os-

aka, Japan.7) J. Ziegler: SRIM―The stopping and range of ions in

matter, http://www.srim.org/ (2008).8) E. Bustarret et al.: Phys. Rev. Lett. 93 237005, (2004).

a) The experiment was performed under the Experimental Pro-gram ML1006-LINAC29.

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Ⅲ-1. Atomic & Solid State Physics (Ion) RIKEN Accel. Prog. Rep. 48 (2015)RIKEN Accel. Prog. Rep. 48 (2015) Ⅲ-1. Atomic & Solid State Physics (Ion)

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