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

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JAEA-Review 2010-065
Incident Energy Dependence of Nuclear Reaction
Imaging of Boron Doped in Iron
H. Shibata a) , Y. Kohno b) , T. Satoh c) , T. Ohkubo c) , A. Yamazaki c) , Y. Ishii c) ,
A. Yokoyama c) and M. Kohka c)
a) Graduate School of Engineering, Kyoto University,
b) Department of Materials Science and Engineering, Muroran Institute of Technology,
c) Department of Advanced Radiation Technology, TARRI, JAEA
An addition of a trace amount of boron to iron improves
mechanical properties. Behavior of boron additive, however,
is not sufficiently understood because of the difficulty of
microscopic analysis, although boron treatment substantially
may prevent hydrogen from segregating at grain boundaries.
Recently imaging of boron distribution in a cancer cell has
been also required to elucidate the buildup mechanism for
boron neutron capture therapy (BNCT). These requirements
to analyze the behavior of several tens ppm boron with a good
spatial resolution stimulates us for developing an imaging
technique of a trace amount of boron distribution by using
PIGE or α-particle detection by nuclear reaction.
A proton micro-beam from 3 MV single-ended electrostatic
accelerator of TIARA facility was used for microanalysis of a
trace amount of boron. The imaging techniques of a trace
amount of boron (several tens ppm) distribution by detecting
428 keV γ-ray emitted from 10 B (p, α’ γ) 7 Be or α-particle from
11 8
B (p, α) Be nuclear reaction have been developed. In the
case of γ-ray measurement, X-rays from the same sample was
also measured simultaneously for heavier elemental analysis.
A typical current of several pA at the beam diameter of about
2 ~ 3 μm was used for mapping area of 250 μm × 250 μm in
this experiment. The overall spatial resolution of the proton
beam can be kept nearly 3 μm.
A hp-Ge γ-ray detector (Ortec 1601-1231- S-2), which has
100 cc crystal volume, is remodeled by Raytech corporation.
The endcap of the detector is converted to L-shape to set the
detector crystal just behind the sample. The resolution of this
detector is 1.7 keV at 1.33 MeV with a cooled FET
pre-amplifier. A Si(Li) detector is also installed for
micro-PIXE analysis of heavier elements. The simultaneous
measurements of X-ray and γ-ray can be performed in this
study.
In 2009, in order to obtain information of depth profile of
boron distribution, γ-ray images dependent on incident beam
energy were measured. 1.5 ~ 2.3 MeV proton micro-beams
were used to determine depth position of boron by measuring
γ-rays from the inside of the specimen. An iron specimen
(10 × 10 × 1 mm) used in this study contained 100 ppm boron
and trace amounts of C, Si, Mn, P, S, N, Cr, W, Co and V.
Figure 1 shows total cross section of 10 B(p, α'γ) 7 Be reaction.
One broad resonance peaked at 1.5 MeV superposing smooth
slope beginning from around 700 keV can be seen in this
energy region. Irradiation was performed in the energy range
from 1.5 to 2.3 MeV in every 200 keV interval.
Penetration depths of protons for nuclear reaction were
~ 8 μm for 1.5 MeV incident energy, ~10 μm for 1.7 MeV,
~ 13 μm for 1.9 MeV, ~15 μm for 2.1 MeV and ~19 μm for
2.3 MeV that were calculated by using SRIM considering
nuclear reaction threshold of about 750 keV. The relation
between these images is clearly understood because of
2 ~ 4 μm depth intervals.
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Fig. 1 Total cross-section of 10 B(p, α'γ) 7 Be reaction 1) .
Typical γ-ray images taken by (a) 1.5, (b) 1.7, (c) 1.9 ,
(d) 2.1 and (e) 2.3 MeV proton irradiation are shown in
Fig. 2. Several tens micron of segregated boron blocks
were observed at each incident energy, and some
correlations between boron images at different incident
energies could be clearly seen. This segregation may
appear along with iron grain boundaries. Sizes of these
blocks are from several μm to ~10 μm. In this study,
any iron grains cannot be imaged, therefore, these
blocks cannot be determined their locations. As
intensities of γ-ray signals does not calibrate to the
absolute value of boron density, concentration of boron
in a block does not estimated.
(a) (b)
(c) (d) (e)
250μm
Fig. 2 Typical γ-ray images of 100 ppm boron contained
steel specimen bombarded by (a) 1.5, (b) 1.7, (c) 1.9 ,
(d) 2. 1 and (e) 2.3 MeV proton micro-beam.
Reference
1) R. Mateus et al., Nucl. Instrum. Meth. Phys. Res. B
219-220 (2004) 519-523.