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2017 JINST 12 C12050

Published by IOP Publishing for Sissa Medialab

Received: September 30, 2017Revised: October 11, 2017

Accepted: November 24, 2017Published: December 22, 2017

19th International Workshop on Radiation Imaging Detectors2–6 July 2017AGH University of Science and Technology, Kraków, Poland

Development of a hemispherical rotational modulationcollimator system for imaging spatial distribution ofradiation 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

1These authors equally contributed to this work.2Corresponding author.

c© 2017 IOP Publishing Ltd and Sissa Medialab https://doi.org/10.1088/1748-0221/12/12/C12050

2017 JINST 12 C12050

Abstract: Detecting and mapping the spatial distribution of radioactive materials is of greatimportance for environmental and security issues. We design and present a novel hemisphericalrotational modulation collimator (H-RMC) system which can visualize the location of the radiationsource by collecting signals from incident rays that go through collimator masks. The H-RMCsystem comprises a servomotor-controlled rotatingmodule and a hollow heavy-metallic hemispherewith slits/slats equally spaced with the same angle subtended from the main axis. In addition, wealso 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 sourcesof various shapes. We fabricated the H-RMC system which can be operated in a fully-automatedfashion through the computer-based controller, and verify the accuracy and reproducibility of thesystem 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 reliabilityand accuracy.

Keywords: Detector design and construction technologies and materials; Gamma telescopes;Overall mechanics design (support structures and materials, vibration analysis etc); Search forradioactive and fissile materials

2017 JINST 12 C12050

Contents

1 Introduction 1

2 Design and fabrication of the H-RMC system 2

3 Performance evaluation of the fabricated H-RMC system 4

4 Conclusions 6

1 Introduction

Radiation imaging, namely, detecting and mapping the spatial distribution of various radioactivematerials is of great significance for environmental and security issues. Most radiation imagingtechniques are based on collimation and selection of radiation-induced signals from radiationdetectors. 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 willnot contribute to formation of the radiation image; therefore the radiation detector will not generatesignals from the blocked radiation particles in principle. Coded-aperture imaging (CAI) is one of thepopular radiation imaging methods based on mechanical collimation [1, 2]. Meanwhile, electroniccollimation-based methods allow all the rays incident on the direction of the radiation detector tocreate radiation-induced signals first, and then, those which can be utilized for the formation ofthe radiation image will be algorithmically selected with signal processing scheme and used forradiation image reconstruction. The most representative example of a radiation imaging methodusing 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 detectorscan exist either as an array of many detectors or as an internally segmented structure to form apixelated 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 aposition-sensing scheme specific to the detector type which often involves algorithmic techniques inthe signal processing stage. The rotational modulation collimator (RMC)-based radiation imagingapproach introduced in this work is a relatively simple and quick imaging method that does notaccompany position-sensitive radiation detectors. This method is one of mechanical collimation-based approaches; however, one exploits temporal modulation of radiation-induced signal intensitydepending 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 beenextensively investigated in a few recent studies for the homeland security application [10–12]. Onedrawback 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 insolid angle) by utilizing hemispherical configuration, has recently been proposed, yet still is waitingfor 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 theH-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 spatialimaging performance of the fabricated H-RMC system, comprising a high-precision x- and y-axisstaging station on which one canmount radiation sources of various shapes. We integrate the systemwith a computer-based controller, aiming a fully-automated, thus more reliable and reproducibleoperation, and finally evaluate the performance of a completed instrument by measuring both therotational 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 geometricaldesign 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 anglesubtended from the main axis. While the current geometry of an H-RMC body is specified infigure 1, its size and shape can be modulated depending on the imaging condition [13], which canbe experimentally furthered in the future work. A radiation detector is placed at the center insidean H-RMC body. When radiation is incident from outside while an H-RMC body is rotating, thedetector collects signals that go through collimator slits, thereby spatially imaging the radiationaccording 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: acontrolled rotating module equipped with a mechanically machined H-RMC body and a radiationdetector (figure 2), and a high-precision x- and y-staging module where a radiation source is tobe 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 prototypesfor those two instruments are shown in figures 2 and 3. We first describe in this section howthe system can be designed and manufactured in a more robust and precise fashion, along witha computer-based controller for automated operation. Then in the next section, we validate theoperation performance of the fabricated system by comparing the computer-input values with thereal positions, systematically in two aspects: the rotational position of an H-RMC, and the linearposition 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 bodyaccording to the gear ratio through the timing belt (figure 2). The gear ratio between the motorand the H-RMC body is designed to be 1:6; when the motor rotates one time, the H-RMC rotatesby 60◦, which enables more delicate H-RMC control, for instance, at 1◦ per second. The bearingunit (UCP-312, MISUIMI; inner diameter of 60mm) and the rotating shaft (hollow for securing adetector’s view from the backside: inner and outer diameters of 48 and 60mm, respectively) arecarefully designed and fabricated to support the heavy, laterally-suspended H-RMC body whilekeeping its rotating axis horizontal, which is crucial for accurate radiation detection and imaginglater. To facilitate smooth and precise control of an H-RMC body with minimal vibration duringits rotation, the load of an H-RMC body is distributed to multiple supports, and the timing belt andthe 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 twomotorized linear actuators for its x- and y-axis transport; three sets of these are assembled togetherin the current version for versatile operations handling multiple sources (figure 3a–3b). The linearactuators (LX2005, MISUMI; linear stroke up to 300mm) are chosen so as to secure sufficientstrokes of radiation sources for real spatial imaging. The radiation source mounting plate (sourcemounter, 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 commercialsource sizes, the geometry of this rectangular source mounter is designed to 100 × 60mm. Forthe snug fit of sources, the source grooves are precisely machined with the fine tolerance of +0.1to +0.3mm.

We select the motor used to drive the above module as a stepping motor controlled by pulsewaves with a unit inherent split angle (SST43D3000, D&J); six motors are deployed for threesource mounters: one motor per one linear actuator. This motor operates the designated actuatorwith a discrete open-loop basis, namely, moving only at multiples of the unit angle (i.e., 1mm forthe actuator), which has an advantage over DC or servo motors in terms of accuracy as well asdurability while a high-speed operation is not critical for the radiation source transport. By usingthe Arduino board having multiple communication channels (Mega 2560 R3, Arduino), six motorscan be operated by the centralized control through Arduino IDE software (Arduino) [14]. Sinceeach axis is individually controlled by the designated motor, precise spatial manipulation of up tothree 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 sourcemounter, while paying attention to the positon error and operation reproducibility with respect to thecomputer-input values. First, the error and reproducibility compared to the programmed rotationangle 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 ofthe angular position of a rotating H-RMC body at an 1◦ interval for a 360◦ rotation. Figure 5 plotsthe evaluation result; notably, measurement errors are displayed ten times larger than the real valuesbecause they are otherwise too small to indicate. This confirms that the rotational operation can beconducted with an error suppressed to less than 0.1◦ with an aid of precise PID control of the servomotor [15].

We now evaluate the error and reproducibility of the source mounter operation, by comparingthe computer-programmed input values with the measured positions of the source mounter in both

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2017 JINST 12 C12050

Figure 4. Schematic drawing of a digital protractor measurement for the rotational H-RMC operationevaluation.

Figure 5. Measurement values compared to a programmed 360◦ rotation of an H-RMC body (errors are tentimes 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 themeasurement data for three source mounters (as shown in figure 3) where the positions are measuredat 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 radiationdetector and sources.

Figure 6. Schematic drawing of a laser distance sensor measurement for the linear source mounter operationevaluation.

Figure 7. Measurement values compared to programmed linear strokes of three source mounters (errors areten times multiplied for readability).

4 Conclusions

In summary, we designed, fabricated, and evaluated the H-RMC system for developing a spatialradiation imager. The system centrally includes a servo motor-driven rotational module to controlan H-RMC body at high precision, along with the motorized source mounting instrument that cancontrol the spatial positions of multiple radiation sources through linear actuators. By measuringthe rotating angles of an H-RMC body and linear positions of source mounters with respect tothe computer-programmed values, we confirmed that the completed system can be operated withhigh 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 reliabledetection and spatial imaging of various radioactive targets.

Acknowledgments

This work is supported by the National Research Foundation of Korea (NRF) Grants funded bythe Korean Government (MSIP) (No. 2015M2A2A4A01045225, No. 2016R1C1B2016182, andNo. 2015R1A5A1037668).

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