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Journal of Instrumentation      
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This content was downloaded from IP address on 22/01/2018 at 12:53
Published by IOP Publishing for Sissa Medialab
Received: September 30, 2017 Revised: October 11, 2017
Accepted: November 24, 2017 Published: December 22, 2017
19th International Workshop on Radiation Imaging Detectors 2–6 July 2017 AGH University of Science and Technology, Kraków, Poland
Development of a hemispherical rotational modulation collimator system for imaging spatial distribution of radiation sources
M. Na,a,1 S. Lee,a,1 G. Kime,1 H.S. Kim,b,c J. Rho,d and J.G. Oka,2
aDepartment of Mechanical and Automotive Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Seoul, 01811 Korea
bProgram in Biomedical Radiation Sciences, Department of Transdisciplinary Studies, Graduate School of Convergence Science and Technology, Seoul National University, 103 Daehak-ro 8-ga-gil, Seoul, 08826 Korea
cBiomedical Research Institute, Seoul National University Hospital, 101 Daehak-ro 8-ga-gil, Seoul, 03080 Korea
dDepartment of Mechanical Engineering and Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Pohang, 37673 Korea
eDepartment of Nuclear Engineering, Sejong University, 209 Neungdong-ro, Seoul, 05006 Korea
E-mail: jgok@seoultech.ac.kr
c© 2017 IOP Publishing Ltd and Sissa Medialab https://doi.org/10.1088/1748-0221/12/12/C12050
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Abstract: Detecting and mapping the spatial distribution of radioactive materials is of great importance for environmental and security issues. We design and present a novel hemispherical rotational modulation collimator (H-RMC) system which can visualize the location of the radiation source by collecting signals from incident rays that go through collimator masks. The H-RMC system comprises a servomotor-controlled rotatingmodule and a hollow heavy-metallic hemisphere with slits/slats equally spaced with the same angle subtended from the main axis. In addition, we also designed an auxiliary instrument to test the imaging performance of the H-RMC system, comprising a high-precision x- and y-axis staging station on which one can mount radiation sources of various shapes. We fabricated the H-RMC system which can be operated in a fully-automated fashion through the computer-based controller, and verify the accuracy and reproducibility of the system by measuring the rotational and linear positions with respect to the programmed values. Our H-RMC system may provide a pivotal tool for spatial radiation imaging with high reliability and accuracy.
Keywords: Detector design and construction technologies and materials; Gamma telescopes; Overall mechanics design (support structures and materials, vibration analysis etc); Search for radioactive and fissile materials
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4 Conclusions 6
1 Introduction
Radiation imaging, namely, detecting and mapping the spatial distribution of various radioactive materials is of great significance for environmental and security issues. Most radiation imaging techniques are based on collimation and selection of radiation-induced signals from radiation detectors. Two main categories for collimation are mechanical and electronic collimation methods. Mechanical collimation refers to the technique which, in advance, blocks the noise signals that will not contribute to formation of the radiation image; therefore the radiation detector will not generate signals from the blocked radiation particles in principle. Coded-aperture imaging (CAI) is one of the popular radiation imaging methods based on mechanical collimation [1, 2]. Meanwhile, electronic collimation-based methods allow all the rays incident on the direction of the radiation detector to create radiation-induced signals first, and then, those which can be utilized for the formation of the radiation image will be algorithmically selected with signal processing scheme and used for radiation image reconstruction. The most representative example of a radiation imaging method using electronic collimation is the Compton Camera [3, 4].
The typical examples for radiation imaging methods given above, however, require position- sensitive radiation detectors as an essential part of the apparatus. Position-sensitive detectors can exist either as an array of many detectors or as an internally segmented structure to form a pixelated or a double-strip type device [5–9]. Position-sensitive detectors not only require com- plex processes in fabrication and system composition stages, but also demand implementation of a position-sensing scheme specific to the detector type which often involves algorithmic techniques in the signal processing stage. The rotational modulation collimator (RMC)-based radiation imaging approach introduced in this work is a relatively simple and quick imaging method that does not accompany position-sensitive radiation detectors. This method is one of mechanical collimation- based approaches; however, one exploits temporal modulation of radiation-induced signal intensity depending on the rotational angle of the spinning mechanical collimators instead of the accumu- lated spatial modulation profile to reconstruct the radiation image. The RMC approach has been extensively investigated in a few recent studies for the homeland security application [10–12]. One drawback of the conventional RMC-based radiation imaging is the limited field-of-view (FOV) im- posed by the cylindrical geometry of two collimator masks and flight tube. Most of the conventional
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RMC systems involving cylindrical geometry have the FOV of less than 20◦ from the central axis. An alternative design for the RMC, which can essentially extend the FOV nearly up to 90◦ (∼ 2π in solid angle) by utilizing hemispherical configuration, has recently been proposed, yet still is waiting for reliable instrumentation [13].
Addressing the aforementioned issues, we newly design and fabricate the hemispherical rota- tional modulation collimator (H-RMC) system and demonstrate its reliable mechanical operation. More specifically, we first perform the geometrical H-RMC design and prototype fabrication of the H-RMC system centrally including a controlled rotating module where a mechanically precision- machined H-RMC is mounted. Then we additionally create an auxiliary tool to test the spatial imaging performance of the fabricated H-RMC system, comprising a high-precision x- and y-axis staging station on which one canmount radiation sources of various shapes. We integrate the system with a computer-based controller, aiming a fully-automated, thus more reliable and reproducible operation, and finally evaluate the performance of a completed instrument by measuring both the rotational and linear positions with respect to the computer-programmed values.
2 Design and fabrication of the H-RMC system
Figure 1 illustrates the overall H-RMC system operation principle. Particularly, the geometrical design of an H-RMC body appears to be a hollow heavy-metallic (i.e., lead-coated stainless steel) hemisphere having two concentric layers of slits/slats that are equally spaced with the same angle subtended from the main axis. While the current geometry of an H-RMC body is specified in figure 1, its size and shape can be modulated depending on the imaging condition [13], which can be experimentally furthered in the future work. A radiation detector is placed at the center inside an H-RMC body. When radiation is incident from outside while an H-RMC body is rotating, the detector collects signals that go through collimator slits, thereby spatially imaging the radiation according to the incident position and distance [13].
Figure 1. Schematic diagram of the H-RMC system operation.
The H-RMC system designed and fabricated in this work consists of two main instruments: a controlled rotating module equipped with a mechanically machined H-RMC body and a radiation detector (figure 2), and a high-precision x- and y-staging module where a radiation source is to be mounted for verifying the H-RMC module performance (figure 3). The three-dimensional
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(3D) computer-aided design (CAD) drawings (Pro/Engineer, PTC) and the fabricated prototypes for those two instruments are shown in figures 2 and 3. We first describe in this section how the system can be designed and manufactured in a more robust and precise fashion, along with a computer-based controller for automated operation. Then in the next section, we validate the operation performance of the fabricated system by comparing the computer-input values with the real positions, systematically in two aspects: the rotational position of an H-RMC, and the linear position of a radiation source operator.
The driving of the H-RMC system is implemented by rotating a servo motor (HF-KP053, Mitsubishi) using a control software (MR Configurator), which is then rotating an H-RMC body according to the gear ratio through the timing belt (figure 2). The gear ratio between the motor and the H-RMC body is designed to be 1:6; when the motor rotates one time, the H-RMC rotates by 60◦, which enables more delicate H-RMC control, for instance, at 1◦ per second. The bearing unit (UCP-312, MISUIMI; inner diameter of 60mm) and the rotating shaft (hollow for securing a detector’s view from the backside: inner and outer diameters of 48 and 60mm, respectively) are carefully designed and fabricated to support the heavy, laterally-suspended H-RMC body while keeping its rotating axis horizontal, which is crucial for accurate radiation detection and imaging later. To facilitate smooth and precise control of an H-RMC body with minimal vibration during its rotation, the load of an H-RMC body is distributed to multiple supports, and the timing belt and the motor axis are tightly coupled with each other.
Figure 2. (a) 3D CAD modeling of the H-RMC system and (b) assembled prototype.
A set of the radiation source operator comprises a source mounting plate connected to two motorized linear actuators for its x- and y-axis transport; three sets of these are assembled together in the current version for versatile operations handling multiple sources (figure 3a–3b). The linear actuators (LX2005, MISUMI; linear stroke up to 300mm) are chosen so as to secure sufficient strokes of radiation sources for real spatial imaging. The radiation source mounting plate (source mounter, hereafter) is designed to have three grooves that can accommodate various types (i.e.,
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chalk and coin shapes) of experimental radiation sources (figure 3c). Considering the commercial source sizes, the geometry of this rectangular source mounter is designed to 100 × 60mm. For the snug fit of sources, the source grooves are precisely machined with the fine tolerance of +0.1 to +0.3mm.
We select the motor used to drive the above module as a stepping motor controlled by pulse waves with a unit inherent split angle (SST43D3000, D&J); six motors are deployed for three source mounters: one motor per one linear actuator. This motor operates the designated actuator with a discrete open-loop basis, namely, moving only at multiples of the unit angle (i.e., 1mm for the actuator), which has an advantage over DC or servo motors in terms of accuracy as well as durability while a high-speed operation is not critical for the radiation source transport. By using the Arduino board having multiple communication channels (Mega 2560 R3, Arduino), six motors can be operated by the centralized control through Arduino IDE software (Arduino) [14]. Since each axis is individually controlled by the designated motor, precise spatial manipulation of up to three radiation sources is possible.
Figure 3. (a) 3D CAD modeling of the radiation source operator and (b) assembled prototype.
3 Performance evaluation of the fabricated H-RMC system
In order to evalutate the performance of the completed H-RMC system, we perform two mea- surements for two aspects: rotating angle of an H-RMC body and linear position of each source mounter, while paying attention to the positon error and operation reproducibility with respect to the computer-input values. First, the error and reproducibility compared to the programmed rotation angle of an H-RMC body are measured using a digital protractor (Pro3600, M-D Building Products, Inc.). As schematically shown in figure 4, a digital protractor detects and monitors the readings of the angular position of a rotating H-RMC body at an 1◦ interval for a 360◦ rotation. Figure 5 plots the evaluation result; notably, measurement errors are displayed ten times larger than the real values because they are otherwise too small to indicate. This confirms that the rotational operation can be conducted with an error suppressed to less than 0.1◦ with an aid of precise PID control of the servo motor [15].
We now evaluate the error and reproducibility of the source mounter operation, by comparing the computer-programmed input values with the measured positions of the source mounter in both
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Figure 4. Schematic drawing of a digital protractor measurement for the rotational H-RMC operation evaluation.
Figure 5. Measurement values compared to a programmed 360◦ rotation of an H-RMC body (errors are ten times multiplied for readability).
x- and y-directions, as schematically depicted in figure 6. A laser distance sensor (CD22-100- 485, FASTUS) is used to record the center positions of the source mounter. Figure 7 shows the measurement data for three source mounters (as shown in figure 3) where the positions are measured at every 5mm increment for a total moving stroke of 100mm along x- and y-directions; here again, error values are ten times multiplied for readability. This result clearly demonstrates that all source
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mounters can operate with a very small error of 0.2mm or less with respect to the input values. This presents that the fabricated H-RMC system, along with the auxiliary source mounter tool, can realize highly accurate and reproducible radiation imaging in conjunction with the radiation detector and sources.
Figure 6. Schematic drawing of a laser distance sensor measurement for the linear source mounter operation evaluation.
Figure 7. Measurement values compared to programmed linear strokes of three source mounters (errors are ten times multiplied for readability).
4 Conclusions
In summary, we designed, fabricated, and evaluated the H-RMC system for developing a spatial radiation imager. The system centrally includes a servo motor-driven rotational module to control an H-RMC body at high precision, along with the motorized source mounting instrument that can control the spatial positions of multiple radiation sources through linear actuators. By measuring the rotating angles of an H-RMC body and linear positions of source mounters with respect to the computer-programmed values, we confirmed that the completed system can be operated with high accuracy and reproduciblity. The fabricated H-RMC system may provide a core instrument to
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many fields involving environmental, space, nuclear, sensor, and security applications, for reliable detection and spatial imaging of various radioactive targets.
This work is supported by the National Research Foundation of Korea (NRF) Grants funded by the Korean Government (MSIP) (No. 2015M2A2A4A01045225, No. 2016R1C1B2016182, and No. 2015R1A5A1037668).
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