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

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Analysis of diﬀraction patterns of laser spots in dual-microbeams
generated by glass capillary optics for future biological use
K. Inayoshi,∗2,∗1 T. Ikeda,∗1,∗2 K. Ono,∗2,∗1 Y. Kozu,∗2 K. Sangu,∗2,∗1 Y. Hikima,∗2,∗1 and W. -G. Jin∗2
We have developed an ion microbeam irradiation system employing tapered glass capillary optics for biological use in the RIKEN Pelletron accelerator facility in theNishina R&D Building. Glass capillary optics is a simple
and reliable tool to generate ion/laser microbeams,1–3)
where both the ion and laser can be extracted from thecapillary outlet simultaneously. When an end window is
installed at the outlet, the microbeams can be applied totargets in the liquid/gas phase. At the microbeam irradiation port, cultivated living cells in a liquid medium orparts of small living insects in air are targets.4) The corresponding irradiation lengths are expected to be a few
micrometers or submillimeters for liquid and air, respectively. The ion species are H and He ions with energies
of a few MeV so that the range in water (approximately
100 and 30 μm for H and He ions, respectively) approximately corresponds to the sizes of the biological targets.
The maximum linear energy transfer (LET) is selected to
be 80 or 240 keV/μm for H or He ions, respectively, which
is suﬃciently high to cause serious damage to DNA. In
this ion irradiation, any mishits should be avoided because such a single ion hit may cause an undesirable effect on the living target. To realize precise ion targeting,a laser microbeam extracted from the same capillary isused as a laser sight of a μm-order scale. The laser microspot on the target consists of a center spot and higher
order rings of diﬀraction, which are diﬀerent from thoseof Fraunhofer diﬀraction. The higher order rings deteriorate the sharpness of the laser spot, which then reduces
the accuracy of the laser sight. In this study, to understand the mechanism of ring generation, the brightnessof each ring was measured according to the distance fromthe capillary outlet so that the results could be discussed
based on ray tracing simulations. For this purpose, thedistance range was selected to be in the order of 100 mm
because larger ring images can provide detailed information of light trajectories. The end window was not
introduced to obtain clear ring images.
Figure 1 shows the setup at Toho University. A visible
light laser with a wavelength λ = 488 nm from a source(Photochemical Research Associates Inc. LA15R) entered a glass capillary with an outlet diameter of 62 μm,
as shown in Figs. 2(a)–(c). The width of the input beam
was approximately 570 μm at the full width at half maximum (FWHM).2) Laser spots on a transparent screen
were recorded by a digital camera for a distance L ranging from 120 to 300 mm, where L was deﬁned as thedistance between the capillary outlet and the screen. An
ND ﬁlter was used to reduce the intensity of the input
beam so that rings of a lower brightness were clearly∗1∗2

RIKEN Nishina Center
Department of Physics, Toho UniversityFig. 1. Experimental setup to obtain diﬀraction patterns oflaser spots. The distance L ranged from 120 to 300 mm.

Fig. 2. (a) Capillary inlet, (b) capillary outlet with a diameter of 62 μm, (c) the downstream view of the outlet, (d)a typical laser spot on the screen, and (e) relative densityof the brightness as a function of L.

recorded with an 8-bit dynamic range of the image ﬁles.
A typical laser spot image is shown in Fig. 2(d) alongwith the deﬁnition of the center spot (0th region) andhigher order rings (1st, 2nd, and 3rd rings). The total
brightness of each component (0–3) was obtained by integrating over the area, and the brightness density wascalculated by dividing by the area. To compare the Ldependence of each density, the density was normalized
by that at L = 120 mm, as shown in Fig. 2(e) with adashed line corresponding to L−2 . The density for the
3rd ring (#3) was found to follow the L−2 curve; thatis, #3 was emitted from a point-like source, which may
be owing to reﬂection at the inner surface around theoutlet. Although curve #0 was expected to be constantbecause it was the component without any reﬂection inside the capillary, the curve was found to depend on L.
The obtained behaviors of the curves will be comparedwith ray tracing simulation results.
References
1) T. Ikeda, Quantum Beam Sci. 4(2), 22 (2020).
2) W.-G. Jin et al., J. Phys. Soc. Jpn. 84, 114301 (2015).
3) S. Kawamura et al., J. Phys. Soc. Jpn. 89, 055002 (2020).
4) T. Ikeda et al., Nucl. Instrum. Methods Phys. Res. B 470,
42 (2020).

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