RIKEN Accel. Prog. Rep. 50 (2017)

Ⅰ. HIGHLIGHTS OF THE YEAR

Gamma-ray inspection of a rotating object
T. Kambara∗1 and A. Yoshida∗1
around the highest peak.

We are developing a method called gamma-ray inspection of a rotating object (GIRO) to image the spatial distribution of γ-ray sources. The setup employs
two sets of collimated γ-ray detectors on the opposite
sides of an object with RI sources that continuouslyrotates. The principle of the method and some resultsof test measurements have been reported.1,2)
We replaced the original detectors with four NaI(Tl)scintillation detectors (V51B102/2M-X from Scionix,
51 mm × 51 mm × 102 mm). Two detectors werestacked on either side of a turntable. The energy resolution of the new detectors was about 7.2 %-7.8 %full-width half maximum (fwhm) for 511-keV γ rays,
which is better than the 17 %-18 % fwhm of the previous detectors.
The γ-ray collimator in the original setup was a 4mm wide vertical aperture between two 3-cm thicklead blocks. The γ-ray photons that penetrated theedge of the block resulted in a dull collimation andcaused a background near a strong source in the reconstructed image. To improve the collimation, weattached a parallel pair of wolfram plates (W-plates)to the lead blocks. The W-plates were 2-mm thick,
110-mm high, and 100-mm deep and separated by 4mm, as shown in Fig. 1.

Fig. 2. Reconstructed image of three 22 Na sources with collimators (a) without and (b) with the W-plates. Leftis the two-dimensional image, right is its projection toy-axis and its ten-fold magniﬁcation.

The measurements described above were performedin the PET mode with coincidence measurements of
two 511-keV photons from the positron annihilation.
GIRO also allows SPECT-mode measurements, wheresingle γ-ray photons are detected by each detector.
The SPECT mode has a lower spatial resolution for theRI distribution and higher background than the PET
mode, but it is applicable to various γ-ray emitting nuclides and its measurement time is much shorter. Figure 3 shows SPECT-mode images of the same setup as
shown in Fig. 2 taken by 511-keV photons. The measurement took about 27 min. The W-plates improve
the resolution of the SPECT-mode image, which shows
not only the 309 kBq source but also the 29 kBq source.

Fig. 1. GIRO detector setup with wolfram plates.

Figure 2 compares reconstructed images taken (a)without and (b) with the W-plates for three point-like22
Na sources (309 kBq, 29 kBq, and 2.6 kBq) on theturntable. The turntable rotated at 150 rpm and thedetectors moved by 2 mm/step and 1 step/10 s overa ±74-mm range. The measurement time was about
10.3 h without the W-plates and 20.7 h with them.
The two-dimensional image shows three spots corresponding to the sources. In the image without theW-plates, a halo-like background surrounds the spotof 309 kBq, but it disappears in the image with theW-plates. The y-axis projections of the image on theright show that the W-plates reduced the background∗1Fig. 3. SPECT-mode image of the same source shown inFig. 2 (a) without and (b) with the W-plates.

References
1) T. Kambara, A. Yoshida, and H. Takeichi: Nucl. Instr.
Meth. A 797, 1 (2015).
2) T. Kambara: Nuclear Physics News 26, No. 4, 26(2016).

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