Characterisation of sealed radioactive source for nuclear forensic purposes

A study characterized sealed radioactive sources (60Co, 137Cs, and 241Am) for nuclear forensic analysis, focusing on their physical, macroscopic, and radiological properties. Results revealed specific features, such as stamped or engraved information, that served as valuable signatures for nuclear forensic investigation. Corrosion observations provide valuable insights into the condition of the source and historical context. Gamma-ray spectra analysis revealed clear distinctions, including a scattering peak in cylindrical 60Co and 137Cs disk sources (200-500 keV), which was absent in the cup-style 241Am source. Unusual peaks in the gamma spectra of 60Co, 137Cs, and 241Am sources suggested potential impurities or background radiation. This was indicated by non-Gaussian shapes and significant counts per second at specific energy levels. X-ray radiography effectively highlights nuclear material characteristics, emphasizing density, uniformity, and internal structure. This analysis contributed to the development of a National Nuclear Forensic Library (NNFL) and enhanced Malaysia’s capability in nuclear forensic investigations related to radioactive sources found outside regulatory control.


Introduction
Nuclear forensic analysis aims to determine the origin and history of nuclear and radioactive materials.This analysis involves characterizing samples of nuclear or radioactive material to determine their inherent signature.Validated signatures measured on known material of the same type as the material under investigation assist in interpreting the analytical data obtained in nuclear forensic investigations [1].This scientific finding can be utilized to answer forensic questions about the materials, such as their composition, origin, production methods, and intended use.In general, the characterization of nuclear and radioactive materials involves determining their specific physical properties, chemical and elemental composition, and isotope ratios, which can be achieved through appropriate analyses.[2].
For sealed radioactive sources, the signatures can provide valuable information for nuclear forensic analysis.Radiological forensic signatures for sealed radioactive sources focus on the physical construction and chemical composition of the radiation capsule and container, as well as physical markings indicating an owner or manufacturer, and isotopically determined material age [3].Therefore, all characteristics of sealed radioactive sources serve as signatures that can help answer nuclear forensic questions and provide valuable clues to law enforcement agencies about the origin of these sealed radioactive sources that are out of regulatory control.1308 (2024) 012016 IOP Publishing doi:10.1088/1757-899X/1308/1/012016 2 Sealed radioactive sources are used in various sectors, including industry, medicine, agriculture, and research.Standardized sealed radioactive sources do not exist, and many of these sources are made up of small stainless welded metal components containing radioactive material [3].Furthermore, significant variations exist across all observable and measurable parameters that characterize these sources.In industrial settings, sealed radioactive sources serve various purposes, including industrial measurements, smoke detectors, irradiation, non-destructive testing, and gamma radiography.The radioisotopes used differ not only in their levels of radioactivity but also in size and mass.Moreover, the construction, materials, and encapsulation methods vary among different types of radiation sources.The physical characteristics of sealed radioactive sources include size, shape, number of capsules, manufacturer, dimensions, weld details, spacer placement, engravings, and construction materials [4].Consequently, the variations in the physical characteristics of sealed radioactive sources can serve as signatures for specific source models and manufacturers, and can be utilized in nuclear forensic investigations for the source that found outside regulatory control.
Various techniques can be used to identify the signatures of sealed radioactive sources.The selection of a method for characterizing a sealed radioactive source depends on the specific parameters to be determined, whether they are physical, chemical, or radiological.In a nuclear forensics study conducted by Gregor et al., a high-purity germanium detector was used to analyze the gamma-ray spectra of nine different 252 Cf sources [5].In another study, Cummings et al. analyzed a 137 Cs source using ICP-MS to determine its age [6].Notably, nuclear forensic studies are still pending in Malaysia.The present study aims to characterize sealed radioactive sources, specifically 60 Co, 137 Cs, and 241 Am, to identify their inherent signatures.The sealed radioactive sources were characterized according to their physical, macroscopic, and radiological properties.It is significant that no previous studies have been conducted in Malaysia to characterize sealed radioactive sources.Signatures obtained from nuclear forensic analysis are compiled in the Nuclear Forensic Library (NFL).This specialized library is used to identify nuclear and other radioactive materials found outside regulatory control [7].Therefore, the findings of this study provide valuable insights for nuclear forensic analysis in Malaysia, especially in the establishment of a National Nuclear Forensic Library (NNFL).As a result, this information could help identify or at least narrow down possibilities when dealing with an unregulated radioactive source.

Source description
Four different types of sealed radioactive sources were selected for this study (Fig. 1).Disused Sealed Radioactive Sources (DSRS) were collected from the Waste Technology Development Centre (WasTeC) of the Malaysian Nuclear Agency.The sources include two units of 60 Co sources (Source 1 and Source 2), a unit of 137 Cs source (Source 3), and a unit of 241 Am source (Source 4).

Physical characterization
Physical characterization methods for the source include visual inspection, photography, weight determination, and dimensioning [2].Visual inspection was conducted to identify the key features visible to the naked eye and to assess the condition of the source.This information includes labeling, engraving, manufacturer, source geometry, and container type.All information gathered during the visual inspection was documented.During the visual inspection, photographs of each source were also taken using a digital camera (Nikon D600).The weight of each source was determined using an analytical balance (Mettler Toledo model AB204-S).The dimensions of the sources were determined using a digital micrometer (Mitutoyo model DC-1MXT) and a digital caliper (Mitutoyo model CD-15APX.

Gamma spectrometric analysis
All the sources were measured using a high-resolution coaxial high-purity germanium (HPGe) detector coupled with a multi-channel analyser, featuring a relative efficiency of 25% and a resolution of 1.9 keV at 1332.[8].Before starting source counting, three known standard calibration sources with specified activities ( 241 Am, 137 Cs, and 60 Co) provided by the manufacturer were included for validation and quality control.The peak energy of each calibration source determined the identity of the source.Specifically, 241 Am appeared at 60 keV, 137Cs at 662 keV, and 60 Co at two energy peaks simultaneously at 1173.2 and 1332.5 keV.Measurements for each source were conducted over an approximate duration of 24 hours, ensuring continuous and reliable evaluation.The acquired spectra were processed and analyzed using specialized spectroscopy software, namely Gamma Vision.The software enables precise measurement of the full width at half maximum (FWHM) of the observed gamma-ray peaks.A smaller FWHM indicates higher resolution, thus improving the discrimination of energy levels.This comprehensive approach enabled the assessment of resolution for each gamma source, providing valuable insight into the precision of energy level discrimination in nuclear forensic investigations.

Radiographic method
Radiographic experiments were performed on sources using an X-ray machine (Dandong Aolong model XXG-2205, China), a directional X-ray tube with a maximum of 200 kV and a fixed current of 5 mA.The film radiography method was selected for this experiment, and digitization and image enhancement were conducted for the purpose of record-keeping.Proper settings were necessary to obtain high-quality images.Thus, the voltage applied ranged from 80 kV to 120 kV, and the exposure time varied from 0.2 up to 0.8 minutes, depending on the source types.Figure 2 shows the setup of the system and the arrangement of objects for radiographic imaging.

Physical characteristic of sources
The physical characterization results are presented in Table 1.Upon visual inspection, notable differences were observed in the construction of each radioactive source.For instance, variations were found in the type of shielding used for the sources.Some sources were safeguarded within sealed stainless-steel capsules, while others consisted of metal foils or discs.Source 1, containing 60 Co with an activity of 74 kBq, was in the form of a rod-shaped radioactive source enclosed within a sealed stainlesssteel capsule, further held in a metal holder.The rod-shaped source had a cylindrical form (Fig. 3a), with the radioactive end positioned at the lower part within the cylinder shaft.Source 2, also a 60 Co source with an activity of 185 kBq, was enclosed in a metal foil and secured at the bottom of a metal cup using a circlip.A wire mesh provided protection for the foil located at the front of the cup, and the source was equipped with a 4 mm diameter handle for easy handling (Fig. 3b).Additionally, the sources commonly featured stamped or engraved information about the radionuclide and its initial activity (Fig. 4).This physical characteristic can serve as a valuable signature in nuclear forensic investigations.Figure 4a clearly shows the presence of corrosion on the surface of the 241 Am cup-style source.
Additionally, the surface of the cup-style 60 Co source exhibited green carbonate rust, a clear indicator of ongoing corrosion.Corrosion could provide crucial information about the source condition and historical context, making it a relevant aspect of forensic investigations.Consequently, other methods, such as XRF techniques, might be needed to characterize the corrosion product.

Gamma spectrum evaluation
Gamma-ray spectra analysis revealed clear peaks for each gamma source, indicating good resolution (Fig. 5 to 8).The peaks for the cylindrical 60 Co and 137 Cs disk sources (200-500 keV) were well-defined, allowing for easy identification and differentiation of energy levels.In contrast, the cup-style 241 Am source exhibited lower resolution with broader and less defined peaks.This information is crucial for accurate identification and analysis in nuclear forensic investigations.Notably, within the energy range of 200-500 keV, a scattering peak was observed in the cylindrical 60 Co source (Fig. 5a and 6a) and the 137 Cs disk source (Fig. 7a), but not in the cup-style 241 Am source (Fig. 8a).These observations indicate that the source design can influence the observed spectrum.Moreover, the presence of the Compton edge was detected in both the 137 Cs plastic disc source (1321.3keV) (Fig. 7a) and the 241 Am cup-style source (125.3 keV) (Fig. 8a), although the energies at which they occurred showed slight variations between the two sources.
The gamma spectra of the 60 Co, 137 Cs, and 241 Am sources unveiled additional peaks not originating from these sources.These unexpected peaks can be attributed to various factors in nuclear spectroscopy.The unusual non-Gaussian shapes of these peaks often point to the influence of impurities or background radiation.In Source 1 with 60 Co, significant counts per second (cps) at 1115.1 keV (0.090) and 310.5 keV (0.146) suggest the presence of additional gamma-ray emissions.Similarly, Source 3 with 137 Cs exhibits notable cps at 471 keV (0.248), 473.5 keV (1.091), and 477 keV (0.660), indicating unexpected peaks.Additionally, Source 4 with 241 Am shows significant cps at 101.1 keV (0.110).The potential presence of impurities within the sources is a common phenomenon in radioactive materials produced through irradiation in a nuclear reactor [3].These impurities can originate from various sources, including raw materials, accidental contamination during the manufacturing process, or intentional additives aimed at modifying material properties or providing support.The trace elements introduced through these means, along with their activated products and decay progeny, contribute to the observed gamma-ray peaks, creating a distinct signature that reflects a combination of the target material and the reactor [3].Further analysis should be conducted to confirm the presence of impurities and radionuclides in these sources, as demonstrated by Apostol et al. who observed the existence of 241 Am and 154 Eu in californium neutron sources using high-resolution gamma spectrometry [9].

X-Ray imaging
X-ray radiography provides the information about the physical characteristics of nuclear materials, including their density, uniformity, and internal structure.Fig. 9 (a) provides valuable information about the internal structure of the disc-type radioactive sources.The X-ray image accurately portrays the location and active area of the 241 Am and 60 Co radioactive sources within the discs.However, the image does not successfully display these attributes for the 137 Cs source.The adjustments made in terms of energy source and exposure time were likely influenced by factors such as the absorption properties of the different radioactive materials and the sensitivity of the X-ray imaging system employed.These

Conclusion
In summary, the physical and radiological attributes of sealed radioactive sources, specifically 60 Co, Cs, and 241 Am, provide essential characteristics for their identification, with signatures such as construction, shielding types, and engraved details playing a key role in differentiation.Gamma-ray spectral analysis reveals unique characteristics such as scattering peaks and non-originating peaks, indicating the influence of source design and potential impurities, as reflected in elevated counts per second (cps) values.X-ray radiography enhances the investigation by providing valuable insights into the uniformity and internal structure of sealed radioactive sources.Together, this comprehensive analysis contributes significantly to the establishment of a National Nuclear Forensic Library (NNFL) and enhances Malaysia's capability to identify and analyze radioactive sources that are out of regulatory control.Future research is required to assess the viability of alternative methods for characterizing sealed radioactive sources for nuclear forensic purposes.

4 Figure 1 .
Figure 1.Types of sealed radioactive sources used in this study (a) and (b) are 60 Co, (c) 137 Cs and (d) 241 Am

FFD =Focal to film distance Figure 2 .
The setup of the system and the arrangement of objects for radiographic imaging.

Figure 3 .Figure 4 .
Figure 3. (a) A cylindrical 60 Co source of the 74 kBq type pulled out of its metal holder and (b) a 185 kBq 60 Co source enclosed in a metal foil disc.

Figure 9 .
the optimal settings required to capture clear images of the radioactive sources.Fig.9 (b) presents the image of the 60 Co cylindrical source in the aluminium shield, captured at 80 kV with an exposure time of 0.5 min.The internal structure of the source was clearly identifiable, and it was positioned within a rectangular space.(a) X-ray imaging setup of 241 Am, 137 Cs plastic, and 60 Co disk sources and (b) the 60 Co cylindrical source within an aluminum shield, along with (c) and (d) representing the X-ray images of the radioactive sources.

Table 1 .
Physical Characteristics of sealed radioactive sources.