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

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Improvement of Spatial Resolution of PIXE-CT
at TIARA
T. Satoh, M. Kohka, A. Yokoyama, T. Ohkubo, A. Yamazaki, Y. Ishii and T. Kamiya
Department of Advanced Radiation Technology, TARRI, JAEA
Introduction
We develop a technique for three-dimensional (3D)
measurement of trace elements in a minute sample (e.g. a
biological cell) using micro-PIXE and STIM. In our
previous report 1, 2) , we applied not only CT (computed
tomography) technique but also STIM (scanning
transmission ion microscopy) to micro-PIXE analysis for 3D
measurement taking account of such parameters as the 3D
densities of major elements, the energy of an incident
particle, the X-ray production cross section and the X-ray
attenuation coefficient, and we used FBP (filtered back
projection) algorithm, with which 3D reconstruction is
relatively simple and fast. As a result, the spatial resolution
of about 10 μm was achieved but it is not enough for a
biological cell. To make it better, we improved rotation
mechanism of a sample, and used ML-EM (maximum
likelihood expectation maximization) iterative algorithm
instead of FBP because ML-EM is well suited to rare event
analysis such as micro-PIXE.
Experiment
We used micro-PIXE system connected to the 3 MV
single-ended accelerator at TIARA, and a leg of an insect
was used as a test sample. It is useful for confirming
spatial resolution of this 3D analysis because it has hollow
tubular structure and includes various elements. It was
bombarded by 3 MeV proton microbeam in micro-PIXE and
STIM.
In this development, the sample must be rotated
somehow in micro-PIXE and STIM. In our previous work,
it was glued on a metal needle and was rotated directly by
hand. The vibration of the rotation caused a bad effect on
spatial resolution. Therefore, we used an ultrasonic motor
because it has good rotational accuracy and very low
mechanical vibration.
For STIM experiment, the surface barrier detector was
placed just behind the target, while a Faraday cup was
placed at the same position in the case of micro-PIXE
analysis.
Result
Figure 1 shows a microscope image of the test sample.
Figure 2 is the 3D image which was reconstructed from
twelve micro-PIXE images using ML-EM method. The
number of iteration was 10.
Cross-sectional images along the white line of Fig. 2
were shown in Fig. 3. The left and right images in Fig. 3
were reconstructed using ML-EM and FBP, respectively.
As shown in Fig. 3, many radial artifacts were produced
JAEA-Review 2010-065
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using FBP. On the other hand, we were able to get a better
spatial resolution and to recognize the boundary of the
sample easily because there was no artifact in case of
ML-EM.
Fig. 1 Optical microscope image of the sample. It
was glued on the top of a metal needle. The white
rectangle shows the measured area (200 μm ×
200 μm).
Fig. 2 The result of the 3D reconstruction from
twelve micro-PIXE images with ML-EM.
Fig. 3 Crossl-sectional image at the white line in
Fig. 2. The left and right images in Fig. 3 were
reconstructed using ML-EM and FBP, respectively.
References
1) T. Satoh et al., Nucl. Instrum. Meth. B 267 (2009) 2125.
2) T. Satoh et al., JAEA Takasaki Ann. Rep. 2008 (2009)
110.