JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Fast Single-Ion Hit System for Heavy-Ion Microbeam
at TIARA Cyclotron (III)
W. Yokota, T. Satoh, S. Okumura, S. Kurashima, N. Miyawaki, H. Kashiwagi,
K. Yoshida, M. Kohka, K. Takano and T. Kamiya
Department of Advanced Radiation Technology, TARRI, JAEA,
The focusing microbeam system and the fast single-ion
hit system are being developed for this decade at the TIARA
cyclotron. Microbeams and single-ion hit of 260 MeV-
20 7+ 40 14+
Ne and/or 520 MeV- Ar were utilized to irradiate
living biological cells for study of radiation biology,
semiconductor devices for study of radiation effects such as
Single Event Upset and micro-fabrication of polymer films
in fiscal 2009. The experimental results of these studies
are described elsewhere. The microbeam size used in those
experiments was 1 - 2 m in diameter in vacuum. It was
extracted out to the atmosphere through a 0.2 m thick
silicon nitride window for biological experiment. The
beam size grew several times as large mainly by scattering
in air depending on the distance between the window and
the target. Although these beams were generally supplied
smoothly, there are still technical matters to improve for
better beam quality and wider application, each of which is
described below.
Instability of microbeam sometimes appeared as frequent
decrease of intensity. The instability was synchronizing
with change of trajectory or profile of beam on the way of
beam transport from the ECR ion source to the cyclotron.
Although any change related to the ion source such as
intensity of light and electric potential of plasma was not
observed, change in the plasma condition is the most
suspicious as an origin of the instability since ECR plasma is
easy to be influenced by temperature of ambient air and
1)
coolant . Strict control of these temperatures will be
introduced in the near future.
Miss hit, no ion hit at a targeted point, appeared on
occasion at more than ten percents of the total hit number
depending on tuning of beam focusing or transport as
2)
reported in the last annual report . This can affect
irradiation experiment seriously and it is desired to be
reduced down to less than 1%. Miss hit took place
obviously when an ion hit other than a targeted point, and a
candidate of the cause described in the report was scattering
of the ion at the micro slit (see Fig.1 for the composition of
the microbeam system). Every ion focused on a target in
an area of a microbeam spot size must pass both the micro
slit and the divergence defining slit even if it is scattered by
the micro slit. Only the possibility, therefore, that every
ion reaches other than the targeted point is change of ion
charge state at the micro slit, which brings different focusing
force of the micro lens from that for the original charge state.
The distance of ion shift was examined by applying the
electrostatic scanner voltage using 260 MeV- 20 Ne 7+ beam
focused on CR-39 target by a quadrupole lens upstream.
JAEA-Review 2010-065
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The ions concentrated in a limited area without the voltage,
while the area broke up into four groups when the voltage
was turned on. The distance of the shift of the groups from
the original position was proportional to successive integers
corresponding to the charge state. The ion concentration of
each group depended on opening of the micro slit. These
results clearly indicate that ion charge state was changed
into the four values by scattering at the micro slit.
Expansion of ion beam species and energies is desired to
proceed continuously. Development of usual beam,
non-microbeam, is made for two or three ion species or
energies every year. That for microbeam is more
time-consuming because it includes an experiment of
microbeam formation. Therefore, a new microbeam will
be added every year. In fiscal 2009, 220 MeV- 12 C 5+ ,
required from radiation biology study, was developed and
2 m diameter was attained. If a cocktail beam
acceleration technology is applied, a number of microbeams
can be developed more easily 3) . For example, a mass to
charge ratio of 260 MeV- 20 Ne 7+ is about 2.86 (≈ 20/7), and
ions having similar ratio, such as 190 MeV- 14 N 5+ with
M/Q ≈ 2.8, can be accelerated and focused using the similar
operational parameters with less tuning.
References
1) K. Yoshida et al., Rev. Sci. Instrum. 81, 02A312 (2010).
2) Y. Yokota et al., JAEA Takasaki Ann. Rep. 2008 (2009)
165.
3) S. Kurashima et al., Nucl. Instrum. Meth. B 267 (2009)
2024.
Fig. 1 Schematic diagram of the microbeam
system at the TIARA cyclotron.