Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 00 (0)Gamma-ray inspection of rotating object
T. Kambara,∗1 H. Takeichi,∗1 and A. Yoshida∗1Radioisotopes (RIs) have long been used as tracers
for wear diagnosis of mechanical parts. We proposeda surface activation method that utilizes RI beam implantation1) instead of the conventional ion-beam irradiation.
The degree of wear is determined by the decreaseof the radioactivity of the object part or the increaseof the radioactivity of the lubricant, through external gamma-ray measurements. Therefore, a lubricant
circulation system is required for removal of activatedsurface debris from the machine. If the spatial distribution of the radioactivity in a running machine can beobtained, wear diagnosis can be performed for a closed
system without a circulation system.
In many cases, the mechanical parts being subject
to wear diagnosis work in continuous and periodical motions such as rotation. We are developing a
new method to determine the spatial distribution of
positron-emitting RIs on periodically-moving objects
in a closed system, which is based on the same principle as medical PET systems but is simpler and lessexpensive.

reconstructed from the sinogram. Therefore, with only
two detectors, the RI distribution on a rotating object
contained in a vessel can be inspected without stopping the rotation, if y and ϕ at the time of coincidence
detection are determined.

Fig. 2. Prototype setup.

In order to prove the feasibility of the method, wehave constructed a prototype (Fig. 2). A pair of NaI
scintillator detectors are placed on the opposite sidesof a rotating turntable (diameter of 14 cm) that holds
RI sources and moves back and forth. Gamma raysfrom the sources are collimated by a pair of Pb blocks
placed in front of each detector. A pin fixed to theturntable generates a pulse signal from a photoelectric
sensor at each turn. The orientation of the turntable isdetermined by a clock-pulse counter that is started by
the photoelectric sensor. At each coincidence detection
the orientation and the position of the turntable andthe pulse heights from the detectors are recorded.

Fig. 1. Geometrical Concept.

Figure 1 shows the geometrical concept. A positronemitting point source is located at (r, θ) in the polar
coordinate fixed to the object. The orientation of theobject is denoted by ϕ. A pair of gamma-ray detectors
are located at both sides of the object to detect the511-keV photons from positron annihilations. Since
the photons are emitted in the opposite directions, thecoincident detection is allowed only when the source ison the straight line between the detectors (line of response: LOR). This condition is followed by an equation y = r cos(θ + ϕ), where y is the distance between
LOR and the rotation center. If the pair of detectors
is moved in parallel so that LOR scans the object andthe coincidence rate is measured as a function of ϕand y, the coincidence events from a point source fallon a sinusoidal curve in the ϕ-y plane. If the sourceis spatially distributed on the object, the coincidence
rate on the ϕ-y plane yields a diagram called sinogram,which is a superposition of the sinusoidal curves. Conversely, the spatial distribution of the source can be∗1Fig. 3. Sinogram (left) and reproduced image (right) wherethe circles show the turntable and the RI sources.

Figure 3 shows a sinogram and a reconstructed radioactivity distribution for two 22 Na sources, (A) 65
kBq and (B) 1.55 kBq, fixed on the turntable thatrotates at 150 rpm and moves back and forth by 2mm step/minute over a 140-mm range. The aperture
width of the collimator is 6 mm. The sinusoidal curves
marked A and B in the sinogram correspond to eachsource. The positions of the sources are reconstructedwithin 3.5 mm. Details of the reproduction algorithm
are described elsewhere.
References1) A. Yoshida, et al,: Nucl. Instr. Meth. B 317, 785 (2013).

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