Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Spot size estimation for laser aiming system of ion microbeam
irradiation using a tapered glass capillary optics
K. Sato,∗1,∗2 T. Ikeda,∗1,∗2 K. Hirose,∗1,∗2 M. Matsubara,∗2 T. Masuyama,∗2 T. Minowa,∗2 and W.-G. Jin∗2
In order to investigate the response of living cells toradiation, micrometer-sized beams are used to shoot asmall structure inside the cell. A microbeam irradiation
system has been developed at RIKEN, employing MeV
H/He ions generated by the Pelletron accelerator and tapered glass capillary optics, whose beam inlet and outlet
diameters are ∼1 mm and several micrometer, respectively.1) HeLa cells2) and E-coli. cells3) were irradiatedusing this system. Since the high accuracy needed toshoot the targets should be achieved easily, the installation of an aiming system utilizing laser micro spot has
been scheduled. The capillary can transmit both ionsand laser at the same time. The aiming system provides
the laser microbeam needed to spotlight the target prior
to ion irradiation. When the excitation light of a speciﬁc
ﬂuorescent protein or ﬂuorescent dye is selected as thespotlight, only the labeled target in a microscopic view
will be irradiated by the ion microbeam.
The laser transmission experiments have been carriedout with tapered glass capillary optics in Toho University. The power of the transmitted beam was wellreproduced by a simulation with a precisely measured
capillary shape.4) The beam power was measured by using a power meter based on a photodiode whose sensitive area was 10 mm × 10 mm. In order to determine
the laser spot size, a microscopic imaging technique isneeded. Figure 1 shows our method, which records thespot shape on a ﬂuorescent-bead screen located L mmdownstream of the capillary outlet; it was set up at theQuantum Electronics Lab. in Toho Univ., using a laser
beam from an Ar+ laser source (wavelength λ = 488 nm,
CW power = 15 mW). The screen consists of ﬂuorescentbeads, 2 µm in diameter, which can shift the λ from
488 nm (input laser) to around 508 nm (ﬂuorescence).
A band pass ﬁlter attached at the eye piece suppresses
the input laser intensity by 10−6 , except for λ = 510
with a width of 20 nm. The spot images were taken
by a digital camera and analyzed for L > 1 mm in aprevious work.5)
This year, we introduced another microscope to determine L precisely and succeeded in achieving measurements of L down to 17 µm, which is short enough tospotlight the cell targets in the range of 4 MeV He2+ ionsin water. The spot shape for L > 1 mm was similar tothat of a Fraunhofer diﬀraction pattern, which is known
as the ring images for a parallel laser beam entering asmall aperture. Although a ﬁnite beam divergence during transmission does not follow the Fraunhofer formula,the obtained similarity for L > 1 mm inspires Fresnel
pattern for L < 100 µm, where higher-order outer rings∗1∗2RIKEN Nishina Center
Department of Physics, Toho University- 21 -Fig. 1. Fluorescent beads, 2 µm in diameter, shift the wavelength from 488 nm (input laser) to around 508 nm (fluorescence) in order to observe the spot shape at a specificdistance.

Fig. 2. The spot size as a function of the capillary outlet
diameter at L = 17 µm and λ = 488 nm.

are strongly suppressed. This is highly advantageous for
spotlighting a small target. We succeeded in performing
spot size estimation, for the ﬁrst time, using the precise
L-determination system. Figure 2 shows the results ofspot size as a function of outlet diameter at L = 17 µm.
The full spot width at half (or 20%) maxima for eachspot is represented by a square (or circular) symbol. The
dashed line is a guide to show the case when the spot sizeis equal to the outlet size. We conﬁrmed that smaller
spots are obtained for smaller outlet capillaries without
any higher-order rings. The estimation included the calibration of non-linearity of light intensity at the camera
and the suppression of the halation eﬀect due to crosstalk between the ﬂuorescent beads. The installation ofthe system to a beam line of the Pelletron accelerator isin progress.
References
1) V. Mäckel et al., Rev. Sci. Instrum. 85, 014302 (2014).
2) Y. Iwai et al., Appl. Phys. Lett. 92, 023509 (2008).
3) M. Kato et al., Appl. Phys. Lett. 100, 193702 (2012).
4) W.-G. Jin et al., J. Phys. Soc. Jpn 84, 114301 (2015).
5) M. Koushima et al., J. Phys. Conf. Ser. 875, 112004(2017).