1 Introduction

Industrial computed tomography (ICT) is a new nondestructive testing (NDT) technology that combines radiation science with computer technology to produce sharper and more informative images than those provided by conventional projection radiology by eliminating structural superposition. Moreover, the resulting images correspond more closely to the way humans visualize two-dimensional structures, as shown in Fig. 1, making them easier to interpret than conventional radiological images [1, 2]. Due to these specific benefits, ICT has been applied as an advanced tool for verifying product quality in various industries such as spaceflight, navigation, petroleum, materials, and armaments [3,4]. The use of ICT in China has rapidly expanded in recent decades with the dramatic expansion of nuclear technology and increasing economic development.

Fig. 1Full-screen View

Typical CT images with a resolution 2.0 lp/mm (courtesy of CD-650BX Systems, ICT Research Center, Chongqing University): (a) camera (2048×2048); (b) Automobile engine (2048×2048).

Compared to X-ray ICT, γ-ray ICT has the advantages of small physical size, light weight, low power requirements, simplicity, low price, and high stability of output [5-7]. In addition, due to the use of monochromatic sources, γ-ray ICT can easily obtain high-quality images employing simple correction methods in the image construction process [8,9]. In contrast, X-ray ICT adopts polychromatic sources, and this leads to numerous insurmountable problems including cupping artifacts and metal strip artifacts that result from beam hardening, all of which require complex correction methods [10,11]. Based on these outstanding advantages, γ-ray ICT has been widely used in various fields as an ideal tool for NDT [6-9,11]. Nevertheless, γ-ray ICT also has some distinctive drawbacks, like low efficiency, and, in particular, the safety associated with radioactive isotopes has caused increasing concerns [12,13].

This paper describes the γ-ray ICT system in brief in Section 2. Sections 3 and 4 then summarize our design experience, and present pertinent examples for significant radiation protection measures, which offer practical guidance for satisfying established safety requirements regarding the design and shielding of γ-ray ICT facilities. Measurement methods and radiation dose results are discussed in Section 5, and, finally, conclusions are presented in Section 6.

2 Gamma-ray ICT System

The typical γ-ray ICT system shown in Fig. 2 is mainly comprised of a sealed source, rear collimator, detectors, specimen stage, readout electronics, and computer. This system is quite different from other industrial radiography systems as well as medical CT systems in structure and in the type of scanning model employed. Hence, for a γ-ray ICT system, radiation protection planning cannot be sacrificed for the sake of CT image quality [14].

Fig. 2Full-screen View

Schematic illustration of a standard γ-ray ICT system

An appropriate scanning model is selected for γ-ray ICT according to nature of the test object, which determines the required structural radiation shielding for the workspace, the sealed source, and the source container. Therefore, it is necessary to consider the effects of the scanning model prior to designing a γ-ray ICT system. For γ-ray ICT systems, two types of CT scanning models are generally employed [15]. The first type of model is similar to the scanning model employed in medical CT, where the radiation source and detector system are both translated and rotated about a stationary test object, which is herein denoted as the SDTR model. In this case, the trajectory of the source is circular, which must be carefully considered when conducting shielding design for the workspace. The other type of model is where the radiation source and the detectors remain stationary, and the object is moved in a direction perpendicular to the γ-ray beam, which is herein denoted as the OTR model. Because the direction of γ-ray irradiation is fixed, the OTR model provides for easier and lower cost radiation shielding compared to the SDTR model.

3 Structural Shielding Design for a γ-ray ICT System

3.1 Sealed radioactive sources

The activity of the radioactive source is a fundamental factor that must be taken into account in the design of radiation protection for γ-ray ICT systems. In general, the required activity and dimensions of the radioactive source are determined by the physical dimensions and characteristics of the test object such as its size, weight, density, and material type. The size of a test object can range from several micrometers in cases such as an integrated circuit conductor to several meters in cases such as a railroad casting, and materials can include not only high density metals, but also low density materials such as wood and plastics. Therefore, the careful selection of a radioactive source with an appropriate type (γ-ray energy) and activity is of fundamental importance. Typically recommended sources are ¹⁹²Ir (photon average energy of 0.32 MeV), ¹³⁷Cs (photon average energy of 0.66 MeV), and ⁶⁰Co (photon average energy of 1.25 MeV). The range of radioactive source activity must span from $3.7\times {10}^8$ Bq to $7.4\times {10}^{12}$ Bq.

The design and manufacture of the sealed radioactive source for γ-ray ICT should comply with all relevant regulations and national standards [16,17,18]. The effective spot size is that region of the source from which gamma rays effectively emanate. The spot size is an important determinant of the aperture function, and, in general, the spatial resolution distribution improves with a decreasing spot size [6]. The effective spot size is generally determined by the material, shape, and thickness of the radioactive source shell. For instance, based on our experience, a radioactive source shell designed according to Fig.3 for a ⁶⁰Co source should meet the requirements established by the national standards for sealed source safety, and can obtain a smaller effective spot size as well as a sharp edge and symmetric shape for the γ-ray intensity distribution curve. The shaded region in Fig. 3 represents the radioactive source active region that is 1.0 mm in diameter and 4.0 mm in length. After passing through the 1.0 mm thick titanium shell, the gamma photon loss is less than 2.5%.

Fig. 3Full-screen View

Sectional drawing of a sealed radioactive source for γ-ray ICT (⁶⁰Co, activity 3.0×10¹¹ Bq). The shell is made of titanium with a thickness of 0.4–1.0 mm, and is sealed by welding to keep the source completely stable inside (dimensions in mm).

The sealed source is installed in a source rod made of a tungsten alloy, as shown in Fig. 4. The translational motion of the source rod allows for switching γ-ray emission on and off, as shown in Fig. 5.

Fig. 4Full-screen View

Sectional schematic of the source rod of a standard γ-ray ICT (⁶⁰Co, activity 3.0×10¹¹ Bq; dimensions in mm).

Fig. 5Full-screen View

Structure of a source container for a standard γ-ray ICT (⁶⁰Co, activity 3.0×10¹¹ Bq; dimensions in mm).

3.2 Source container

The source container of a γ-ray ICT plays an important role for radiation safety during operation, storage, and equipment transport. The main shielding material of a source container usually consists of a high density and high atomic number metal that has a pronounced capacity to screen gamma radiation, such as lead, tungsten alloys, and uranium [14,20].

An example of a source container structure is illustrated in Fig. 5. The radioactive source is ⁶⁰Co whose activity is 3.0×10¹¹Bq. To maintain safety under standard conditions, and even under conditions of somewhat heavy vibration and high temperature, the source container should be comprised of a steel casing filled with lead or a tungsten alloy. The reasonable shielding thickness of the source container should be established by referring to pertinent publications [13, 17, 20]. As shown in Fig. 5, moving the source rod to the left places the sealed source in alignment with the collimator channel, such that the radiation will directly pass outward through the channel. This represents the radiation ON status. To avoid the unintended absorption of radiation by the channel wall, the error in source rod motion should be less than ±0.03 mm. Moving the source rod to the right places the sealed source in the storage position, where the source is fully embedded within the γ-ray blocking material and the collimator channel is blocked by the tungsten alloy source rod. This represents the radiation OFF status. The ON or OFF status of the unit must be clearly indicated on the exterior of the radiation source container. The OFF position can be secured by a cylinder lock or padlock. The ON position can be secured by a cylinder lock, a padlock, or a locking bolt, as shown in Fig. 6.

Fig. 6Full-screen View

Image of a source container for a γ-ray ICT (structure and parameters shown in Fig. 5)

3.3 Control of source status switching

The source container control system, which is mainly comprised of a control box, stepping motor, door interlock, sound and light alarms, and a computer, implements the control of γ-ray status switching and safety interlocks. A functional block diagram is shown in Fig. 7. If the source switching conditions, such as door interlock, CT enable signal, and emergency stop button, are all ready, the control box sends a control command to switch the source to the ON status. In particular, the control system must immediately switch the unit to the OFF status in the event of an emergency.

Fig. 7Full-screen View

Functional block diagram for the source status switching control system

4 Structural Shielding Design of Workspaces

The location and architecture of a γ-ray ICT workspace are significant features that must be considered to ensure radiation protection and environmental protection. Site selection should take the local geological environment into account, and select regions that demonstrate stable geological conditions. Moreover, workspaces should be located as far as possibly from flammables and explosives, as well as sensitive areas such as schools, hospitals, and beadhouse. In general, γ-ray ICT workspaces are usually located on the periphery of factories in isolation from the workers [20].

In addition, factors including the size and weight of the γ-ray ICT system, radiation activity, γ-ray energy, system orientation, and ray source switch mode, should be fully considered when designing the layout of the workspace and shielding. For rooms on or above ground level, outside walls always require shielding, and additional structural support may be required for heavy equipment and for the additional weight of the shielding barriers [20]. Other aspects of γ-ray ICT workspaces, including interlocks, warning signs, warning lights, emergency buttons, electrical safety, and room lighting, should be carefully implemented.

Figure 8 presents an example of a typical γ-ray ICT workspace (radiation source ⁶⁰C, activity 3.0×10¹¹ Bq), consisting of a test room, control room, and other auxiliary rooms. The thickness and structure of the shielding walls are also shown in the figure, which are determined according to the calculational methods described in pertinent publications [13,17, 20]. The γ-ray CT host equipment is installed in the test room, and the control components are arranged in the control room. The primary radiation (useful beam) emitted directly from the source establishes the orientation of the primary barrier (wall A in Fig. 8), which must be directed away from the control room and any pathways of egress from the test room. According to the national standard for radiation protection provisions [21], the operators in these areas must be trained specifically in the use of these facilities.

Fig. 8Full-screen View

Typical layout of a γ-ray ICT workspace (⁶⁰Co, activity 3.0×10¹¹ Bq). The building material is concrete with a density not less than 2,300 kg·m^–3 (dimensions in mm).

5 Measurements

The dose rate distribution diagram of a source container for a typical γ-ray ICT (⁶⁰Co, activity 3.0×10¹¹ Bq (8Ci)) is shown in Fig. 9. The local dose rate is measured according to the methods and requirements established by the pertinent standards [14, 22, 23] at a distance of 1.0 m from the surface of the radiation source container designed according to the parameters illustrated in Fig. 5 with the radiation source in the OFF status. The measurement results indicate that the maximum leakage radiation detected is 4.9 μGy h^-1, and that the radiation dose level over a 360° span at 1 m is less than that permitted by Chinese national regulations (i.e., ≤ 20.0 μmGy h^-1 at 1 m) [14]. The measurement error, mainly owing to instrumental error, is less than 15%.

Fig. 9.Full-screen View

A dose rate diagram specifies the local dose rate at a distance of 1.0 m from the surface of the radiation source container with the radiation source in the OFF status. Here, the radiation source is ⁶⁰Co, with an activity of 3.0×10¹¹ Bq.

Test points A, B, C, D, and E in the layout of the workspace for the γ-ray ICT system shown in Fig. 8 are established in accordance with the requirements and methods given by the pertinent national standards for surrounding environmental radiation. The measurement data listed in Table 1 shows that the radiation dose rates do not exceed the value permitted by standards [22,23].

6 Conclusion

Radiation protection planning cannot be sacrificed for the sake of CT image quality during the design, manufacture, and layout of γ-ray ICT systems. This paper discussed the predominant measures associated with the radiation protection of γ-ray ICT systems in accordance with the pertinent Chinese standards. In addition, based on experience and pertinent examples, the design requirements for ensuring the radiation safety of a sealed radioactive source, source container, and workspace were defined in detail. The design examples and dose rate measurements conducted in conjunction with a γ-ray ICT system and workspace employing the proposed design standards have illustrated that the proposals provided in this paper are reasonable, feasible, and safe, and are therefore meaningful for the design, manufacture, and layout of γ-ray ICT systems.