1. Introduction
Gamma-rays emitted from radioactive sources have specific energies. Due to the fact that gamma-rays are uncharged and indirectly ionizing radiation, gamma detection depends on causing the gamma-ray photons to undergo interactions in the absorbing material. When gamma-rays interact with a scintillator, they produce charged electrons mainly by three different interactions, a photoelectric effect, Compton scattering, and pair production. These electrons give rise to scintillating light, making it is possible to analyze and compare radioactive isotopes by measuring the total energy absorbed per interaction (
i.e., pulse height) according to the specific energies of gamma-rays [
1].
For radiation detection or gamma-ray energy spectroscopy, a scintillation detector using a thallium-activated sodium iodide (NaI:Tl) crystal is traditionally used in nuclear medicine and environmental measurements. Although an NaI:Tl crystal-based energy spectrometer has high efficiency and adequate energy resolution, it requires a large sensing volume for gamma-ray energy spectroscopy because the density and the effective atomic number (
i.e., Z-number) of the NaI:Tl crystal are relatively lower than those of other inorganic scintillator crystals, such as bismuth germinate (BGO), cerium-doped gadolinium oxyorthosilicate (GSO:Ce), cerium-doped yttrium orthosilicate (YSO:Ce), and cerium-doped lutetium yttrium orthosilicate (LYSO:Ce) [
2]. Its large sensing volume and lack of flexibility prohibit the detection of gamma-rays under challenging geometrical conditions. Furthermore, the NaI:Tl crystal must be sealed within an air-tight container, as it has a hygroscopic characteristic [
1,
2,
3,
4]. To overcome these problems and also allow real-time and remote measurements in harsh environments, a newly designed gamma-ray spectrometer with small size, flexibility, and non-hygroscopicity is required.
Radiation sensors using an optical fiber have been developed in conjunction with many kinds of organic or inorganic scintillators; most of them, however, can only measure scintillating light intensity [
5,
6,
7]. Although existing fiber-optic radiation sensors (FORSs) have many advantages, such as small sensing volume, high spatial resolution, good flexibility, real-time sensing, remote operation, and immunity to high electromagnetic interference (EMI) [
8,
9,
10], they have not been used for accurate discrimination of radioactive isotopes using a spectroscopic technique [
11,
12,
13,
14]. In previous reports, for radiation energy measurements, an optical fiber was only used to transmit a digital signal from an NaI:Tl scintillation detector module as a fiber-optic data link [
15]. A scintillation counter that uses a Perspex light guide with a diameter of 5.08 cm, a plastic scintillator with 5.08 cm diameter and 2.54 cm thickness, and a pulse height analyzer was also introduced [
16]. However, very thick and rigid Perspex with reflective wrapping was used as a light guide instead of an optical fiber and, accordingly, the total internal reflection ratio and the transmission rate of the scintillating light were low. Consequently, it could obtain the pulse height according to the length of the light guide but could not obtain the accurate gamma-ray energy spectrum.
Recently, a feasibility experiment on a novel FORS for gamma-ray spectroscopy was carried out by our research team [
17]. In this experiment, the scintillating light generated from three types of inorganic scintillators, BGO, YSO:Ce, and LYSO:Ce, was measured and we selected LYSO:Ce crystal as an adequate scintillator because it provided the highest scintillating light output. Also, the gamma-ray energy spectra for sodium-22 (Na-22), cesium-137 (Cs-137), and cobalt-60 (Co-60) were measured successfully by using the FORS. However, FORS for gamma-ray spectroscopy have still not been deployed on a commercial scale due to both their small sensing volume, which cannot completely absorb the charged particle energy, and the light attenuation rate in the receiving optical fiber.
The main purpose of the present study is to demonstrate that the proposed FORS can discriminate different gamma-ray emitters by measuring their inherent energy spectra, even when radioactive isotopes are mixed. For remote sensing and gamma-ray energy spectroscopy, we fabricated a small, flexible, and insertable FORS using a sensing probe, a plastic optical fiber (POF), a photomultiplier tube (PMT)-amplifier system, and a multichannel analyzer (MCA). By using the POF to transmit scintillating light with the total internal reflection, the FORS can be applied to measure various radioactive isotopes and radioactive contamination in inaccessible locations, such as narrow areas, pipes, and holes. In the proposed sensor, a POF is used to guide scintillating light generated from an inorganic scintillator crystal in the sensing probe to a PMT. A PMT-amplifier system is used to convert light signals to electrical signals and the amplified voltage signals are measured by an MCA. In this study, we determined the length of the inorganic scintillator crystal to obtain the gamma-ray energy spectrum of radioactive isotopes considering the absorption efficiency and evaluated the performance of the fabricated FORS according to the length and diameter of the POF. First, we measured the energy spectra of radioactive isotopes using the scintillating light transmitted via the POF and compared the results with the gamma-ray energy spectra obtained using a conventional high-purity germanium (HPGe) detector. The response of the proposed FORS according to the radioactivity was then obtained. In addition, we demonstrated that the species of radioactive isotopes can be discriminated using the measured gamma-ray energy spectra.
2. Materials and Experimental Setup
As a sensing element of the FORS probe, an LYSO:Ce crystal (LYSO:Ce, Advanced Microwave Technologies Solution Co., Ltd., Seoul, Korea) was used for gamma-ray energy spectroscopy.
Table 1 shows the physical properties of the LYSO:Ce inorganic scintillator crystal used in this study. The colorless and transparent LYSO:Ce crystal has many advantages including low cost, high density, high effective Z-number, high light output, fast decay time, and good energy resolution. In addition, it has a non-hygroscopic characteristic and high sensitivity against gamma-rays. The light yield of the LYSO:Ce crystal is 85% relative to NaI:Tl. However, LYSO:Ce has about two and three times more light output compared to GSO and BGO, respectively. The peak emission wavelength and the decay time of an LYSO:Ce crystal are about 402 nm and 40 ns, respectively.
In order to transmit scintillating light with the total internal reflection, a step-index multimode fiber (CK-120, Mitsubishi Rayon Co., Ltd., Tokyo, Japan) was chosen as the POF in this study. Diameters are 2.95 ± 0.18 mm for the core only and 3.00 ± 0.18 mm including the cladding. The materials of the core and the cladding are polymethylmethacrylate (PMMA) and fluorinated polymer, respectively. Accordingly, the refractive indices of the core and the cladding are 1.49 and 1.402, respectively, and the numerical aperture (NA) is approximately 0.5. The maximum transmission loss of the POF is 200 dB/km when used with 650 nm collimated light. In a previous report, no significant degradation in the attenuation of POF was observed for irradiation up to 3.5 kGy [
18].
Table 1.
Physical properties of LYSO:Ce crystal.
Table 1.
Physical properties of LYSO:Ce crystal.
Density (g/cm3) | Melting Point (°C) | Refractive Index | Decay Time (ns) | Peak Emission (nm) | Light Yield (%) (Relative to Nal:Tl) |
---|
7.40 | 2050 | 1.82 | 40 | 402 | 85 |
Figure 1 shows a schematic diagram of the sensing probe of the FORS proposed in this study. The square-shaped LYSO:Ce crystal with dimensions of 3 × 3 × 15 mm
3 was glued with optical grease (BC-630, Saint-Gobain Ceramic & Plastics, Inc., Hiram, OH, USA) to the distal end of a POF. The surface of the POF was polished with optical graded pads before coupling. The outer surface of the LYSO:Ce crystal was surrounded by polytetrafluoroethylene (PTFE, Teflon
®, Wilmington, DE, USA) reflector tape (BC-642, Saint-Gobain Ceramic & Plastics, Inc.) to increase the scintillating light-collection efficiency. Furthermore, an aluminum foil, black shielding tape, and a black PMMA holder were used to intercept ambient light noise, as illustrated in
Figure 1.
Figure 1.
Schematic diagram of a sensing probe of a FORS.
Figure 1.
Schematic diagram of a sensing probe of a FORS.
As gamma-ray emitters, two solid-disc type radioactive isotopes with the same dimension (Spectrum Techniques, LLC., Oak Ridge, TN, USA), Cs-137 and Co-60, were used. The diameter and the thickness of this solid disc-type gamma source are 25.6 mm and 2.68 mm, respectively. The physical properties of commercially available gamma-ray emitters used in this study are listed in
Table 2. The photopeak (or full-energy peak) of gamma-rays generated from Cs-137 is 661.6 keV and Co-60 produces two distinct gammas, 1173 and 1332 keV [
3].
Figure 2 shows the experimental setup for measuring gamma-ray energy spectra using the FORS and gamma-ray emitters. A PMT-amplifier system (Hamamatsu Photonics K.K. Co., Shizuoka, Japan) was used as a light-measuring device to measure scintillating light because of its high internal gain and reasonable quantum efficiency. This photosensor system is composed of a compact side-on PMT module (H9305-03), a low-noise amplifier (C7319), and a high-voltage power supply (C7169) that can provide the PMT with driving and control voltages. The PMT equipped with an optical fiber adapter, a subminiature type A (SMA) 905 connector, has a spectral response ranging from 185 to 900 nm and a peak sensitive wavelength of 450 nm. The generated scintillating light signals due to the interactions between the gamma-rays emitted from the radioactive isotope and the LYSO:Ce crystal in the sensing probe are guided to the PMT via the POF and converted to electric current signals [
1]. The current signals from the PMT are then converted to voltage signals and amplified by the amplifier. Finally, the amplified voltage signals are measured by a compact stand-alone MCA (ORTEC
® EASY-MCA-8k, Advanced Measurement Technology, Inc., Oak Ridge, TN, USA) with a fast conversion time of less than 2 μs and the energy spectrum is displayed and stored through emulation software (ORTEC
® MAESTRO
®-32, Advanced Measurement Technology, Inc.).
Figure 2.
Experimental setup for measuring gamma-ray energy spectra using a FORS and gamma-ray emitters.
Figure 2.
Experimental setup for measuring gamma-ray energy spectra using a FORS and gamma-ray emitters.
Table 2.
Physical properties of two gamma-ray emitters.
Table 2.
Physical properties of two gamma-ray emitters.
Radioactive Isotope | Half-Life (years) | Gamma Energies (keV) | Radioactivity (μCi) |
---|
Cs-137 | 30.1 | 661.6 | 0.23 |
0.46 |
0.93 |
4.66 |
Co-60 | 5.27 | 1173 | 0.93 |
1332 | 50.77 |
4. Discussion
Many kinds of radiation sources produce gamma-rays and often contaminate environments. Therefore, it is important to have an accurate method to determine the species of radionuclide that emits the gamma-rays. The conventionally-applied method for detecting and discriminating gamma-rays in nuclear and medical facilities is a conventional gamma-ray spectroscopy technique using fixed-type radiation detection systems including NaI:Tl and HPGe detectors. This method should be performed on site or at a laboratory, as it is necessary to take and transport the radioactive sample. Therefore, it is impossible to perform gamma-ray energy spectroscopy for the radioactive isotopes located in the inaccessible places due to the large sensing volume of the NaI:Tl and HPGe detectors. Furthermore, there is a critical disadvantage in that human radiation exposure is inevitable when the radioactivity of the gamma emitter is quite high. In order to overcome these problems, many different types of FORSs have been investigated and developed by many research teams. However, most FORSs can only measure the light intensity of scintillation signals or the total counts of the energy spectrum; therefore, they have not been used for accurate discrimination of the radioactive isotopes.
The FORS system proposed by our research team is composed of a small LYSO:Ce crystal with dimensions of 3 × 3 × 15 mm3, a 1 m-long POF with a diameter of 3 mm, a PMT-amplifier system with a control voltage of 915 mV, and an MCA. In this FORS system, the compositions and each parameter were optimized to measure the energy spectra of the gamma-rays emitted from Cs-137 and Co-60 with minimized dead time. Therefore, if the energy of a radioactive isotope is changed, the species or the dimensions of both the scintillator and the POF and the control voltage of the PMT-amplifier system also should be changed because a small sized scintillator in the sensing probe of the FORS cannot completely absorb high-energy radiation and the transmission or attenuation rate of the scintillating light with low intensity is dependent on the physical and optical properties of the scintillator and the POF.
To compare the gamma-ray energy spectrum measured by the FORS, unfortunately, we used a Ge semiconductor diode-based HPGe detector (
i.e., standard electrode coaxial Ge detector) instead of a scintillation detector based on an NaI:Tl crystal. Accordingly, it was only possible to compare each photopeak measured by using the FORS and the HPGe detector. From the experimental results, the photopeaks of the gamma-ray energy spectra measured by using the FORS were well matched with those of the HPGe detector; however, the FWHM related to the energy resolution was much larger in the FORS compared to the HPGe detector. From the viewpoint of energy resolution, a scintillator detector offers relatively low resolution as a radionuclide identifier (RID) compared to a semiconductor detector [
22]. However, the diameter and the length of the coaxial HPGe detector used in this study are 76 mm and 110 mm, respectively, and hence its sensing volume is much larger than that of the FORS. Furthermore, the HPGe detector must be cooled to avoid excessive thermally-generated leakage current while the proposed FORS can measure energy spectra at room temperature without cooling.