Ⅲ-4. Radiation Chemistry & Biology

RIKEN Accel. Prog. Rep. 49 (2016)

Microbeam divergence
divergence fromfrom glass capillaries
capillaries compared
Microbeamcomparedwith
withsimulation
simulationforforbiologicaluse
biological use
T. Ikeda,＊1 R. J. Bereczky,＊1 T. Kobayashi,＊1 Y. Sakai,＊2 and T. Abe＊1
Heavy-ion beam irradiation at relatively low doses
induces mutations at a high rate without severely inhibiting
growth. As DNA, which is relevant to mutation, is localized
in a cell, micrometer-sized beams should be used toirradiate
theDNAeffectively.
Whenamegaelectronvolt-energy ion beam is transmitted through atapered glass capillary with a micrometer-sized outlet, theextracted beam can be utilized as an ion microbeam for cellirradiation 1). The capillary is closed by a thin end-windowat the outlet to maintain the inner vacuum so that the outlet
can be close to the target cells in water solution. In the caseof thinner capillaries (order of 1 m), the divergence of thetransmitted beam has been investigated. However, larger
outlets (10 m or more) with end-windows need further
study because the probability of mishitting the neighboring
cells due to larger lateral momentum components of ionsshould be determined before experiments.
In our experiment, He2+ ions with an energy of 4.5 MeV
from the Pelletron accelerator at RIKEN were used andtransmitted through capillaries with different outlet
diameters of the order of 10 m, where the inlet diameterand the length were 800 m and ~5 cm, respectively. The
beam profile was obtained with pieces of a plastic tracking
detector, CR-39, in air 6 mm downstream of the capillaryand then compared with simulated data using SRIM(TRIM) code. The distance between the capillary outlet andthe target was determined so that the ion hit points are
sufficiently separated from each other to be observed by amicroscope with good statistics. The beam divergenceinside the capillary is parameterized in the simulation,
where ions start at the upstream side surface of theend-window with a homogeneous distribution for initial
positions and with a distribution of lateral momentum
components expressed by a single parameter,  , thestandard deviation of a Gaussian distribution. Most of theexperimental hit point distributions on the CR-39 pieces arewell reproduced even when single was selected properly.
In this case, a difference in of 0.1o reflects a large change
in chi-square for the comparison 2). However, some of thecapillaries showed more out-going events. This fact implies
the divergence inside the capillary may havemulti-parameter ’s. This idea has been reported by
Hasegawa et al. 3), who introduced a ‘core’ component aswell as a ‘halo’ component. Figure 1 shows the hit point
distributions of the experimental and simulated results as afunction of the distance between a hit point and the beamaxis (or the weighted center for experiments), r inmicrometers. As a trial,  of 1.35o for Fig.1(a) and  of＊1
＊2

RIKEN Nishina Center
Department of Physics, Toho University- 267 -3.00o for (b) were selected after a rough scan in . Bothplots do not show an agreement between the experimentaland simulated data. Figure 1(c) shows that simulated data oflinear combination (a) ×1 + (b) ×0.32 reproduce theexperimental spectrum, where the coefficient of the
combination was obtained by chi-square fitting. It showsthat more than one component of the beam spread during
the transmission is needed. In order to reduce the halocomponent, how and what amount of such events isgenerated and what shape of the capillary is best should beinvestigated. We have started a realistic GEANT4
simulation that allows us to install the complicated capillary
shape 4). Figure 1(d) shows a typical event display with acapillary outlet size of 20 m and end-window of 3 mT.
After the installation of the measured capillary, a simulation
with not only parallel incoming beams but also
spread/focused beams adjusted by a Q-magnet upstream of
the capillary is scheduled.

Fig. 1. Comparison between experimental (green) andsimulated (purple) spectra. The experimental spectra are thesame through (a)-(c). The heights of the spectra for thesimulated results in (a) and (c) are normalized so that thepeak heights are similar to those of the experimental results.
The normalization factor for (b) is the same as that for (a).
(d) A typical event display of GEANT4 for the trajectories
of He2+ with 4.5 MeV stopping (at yellow points) in watersolution.
References

1) R. J. Bereczky et al.: Abstract of 12th Int. Workshop on
Microbeam Probes of Cellular Radiation Response, Fukui,Japan, (2015), p. 74.
2) R. J. Bereczky et al., RIKEN Accel. Prog. Rep. 48, 316 (2015).
3) J. Hasegawa et al., J. Appl. Phys. 110, 044913 (2011).
4) T. Ikeda et al., Surf. Coat. Technol. 206, 859 (2011).