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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Profile measurements of dual-microbeams generated by glass capillaries
M. Mori,∗2,∗1 T. Ikeda,∗1,∗2 and W. -G. Jin∗2
Tapered glass capillary optics is known to be asimple and reliable microbeam generator for ion/laser
beams.1,2) The DNA of living cells is seriously damaged by not only ion-beam irradiation but also UV light.
However, to avoid mis-hitting of a target cell nucleus, alaser sight of a visible micrometer-sized spot is needed
prior to the UV microbeam shooting, where the visible
light never damages the DNA. We have proposed a dualmicrobeam system of visible light + ion beam (producedat Pelletron in RIKEN Nishina Center) and visible + UV
light because any quantum beams can be transmitted
through the capillary optics. The transmission characteristics and the microbeam proﬁles of visible laser wereinvestigated until 2017,3) followed by the similar study
for UV microbeams.4) To realize the system of visible+ UV laser, the two microbeam spots should be on thesame position. In this report, the displacement between
the two spots as a function of possible mis-alignment of
the input beam is described, although a laser beam iswell-guided by the capillary optics.
Figure 1 shows the setup in Toho University. A UV
laser with a wavelength λ = 375 nm from a source(THORLABS L375P70MLD) entered a glass capillarywith an outlet diameter of 60 µm, where the capillary
was aligned with an accuracy better than the smallest
scale of 0.34 mrad in the mirror tuning. The microbeam
proﬁle approximately 3-mm downstream of the capillary
was obtained as shown in Fig. 2 by a knife-edge method
employing a motorized stage (SURUGASEIKI XY620G-N) and a photodiode (OPHIR PD300) connected toa power meter display. The central sharp peak like anairy disk, which is known in Fraunhofer diﬀraction fora circular aperture, was used to extract the full widthat half maximum (FWHM) as the spot size after Gaussian ﬁtting. A visible light laser (λ = 488 nm) from asource (Photochemical Research Associates Inc. LA15R)
also entered the capillary with a half mirror, which wastilted horizontally to emulate the mis-alignment with aFig. 2. Horizontal profile of a UV microbeam. The spot size
is defined as the FWHM of the central sharp peak.

Fig. 3. Displacement of the visible spot from the UV spot as
a function of θ, along with the spot sizes.

step of 0.68 mrad. The peak position and the spot size ofeach laser were measured according to the tilting angle
θ, where θ = 0 when the UV and visible laser enteredcoaxially.
The displacements of the visible laser spot from theUV spot as a function of θ are plotted in Fig. 3. Thegreen and purple symbols show the peak positions ofvisible and UV lasers, respectively, where the UV peak
positions were set to zero. The vertical bars correspondto the spot sizes, which depend on the initial beam divergences. As expected, the UV spot sizes were almost
constant. We conﬁrmed that the visible laser peak positions were measured to be constant in the larger θ region(≥1.36). The guiding eﬀect of the laser beam throughthe capillary optics is clearly proven.5) However, for UV
spot sizes of ∼30 µm, the mis-alignment θ should be suppressed to less than 1.36 mrad to obtain the same spotpositions so that the mis-hitting in the UV microbeam
irradiation is avoided.
References

Fig. 1. Experimental setup with a knife-edge method to obtain the beam profiles. The distance L ∼ 3 mm.
∗1∗2

RIKEN Nishina Center
Department of Physics, Toho University

1) T. Ikeda et al., Appl. Phys. Lett. 89, 163502 (2006).
2) W. -G. Jin et al., J. Phys. Soc. Jpn. 84, 114301 (2015).
3) M. Koushima et al., J. Phys. Conf. Ser. 875, 112004 (2017).
4) S. Kawamura et al., J. Phys. Soc. Jpn. 89, 055002 (2020).
5) M. Mori et al., RIKEN Accel. Prog. Rep. 53, 143 (2020).

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