Development and Comparison of Fiber-Optic Beta Radiation Sensors with Different Diameters of Their Sensing Probes

A fiber-optic radiation sensor (FORS) was developed for remote and real-time measurements of beta radiation from radioactivecontaminated soil. The sensing probe consisted of a bundle of organic scintillators and a mixture of epoxy resin to improve the detection efficiency. The measurement system consisted of a sensing probe with an aluminium foil reflector, a transmitting plastic optical fiber, and a light-measuring system comprising a photomultiplier tube, a preamplifier, a multichannel analyzer, and a laptop computer. Several sensing probes, whose dead-end diameters were 26mm (bundle type I), 36mm (bundle type II), and 46mm (bundle type III), were prepared and characterized to identify the best sensing probe in terms of its radiation detection efficiency. The reproducibility of the FORS for the measurement of beta radiation was confirmed using a χ-test. The measurements showed that the FORS sensing probe with a diameter 46mm has the best detection performance.


Introduction
Radioactive-contaminated sites can be released from regulatory control only if the remaining radioactivity is less than the regulatory limit [1].Prior to its decontamination, the radiation from the site needs be identified, which provides accurate information on the radionuclides to be decontaminated.The radionuclides frequently found at the contaminated sites of foreign nuclear facilities include Am-241, Cs-137, Co-60, H-3, I-129, I-131, Pu, Ra, Rn, Sr-90, Tc-99, Th, and U [2].The radionuclides found at the decommissioned site of an old Korean research reactor, TRIGA MARK III, were Co-60, Cs-134, Cs-137, Sr-90, Cr-51, and Fe-59 [3].
Most radionuclides found at the contaminated site are gamma-emitters, but some are beta-emitters that need to be considered in the subsequent decontamination stage.Sr-90, which emits a 0.546 MeV beta particle, has chemical similarity to calcium.The radionuclide accumulates in bones and irradiates the bone marrow causing its high radiotoxicity [4].Accordingly, this study selected Sr-90 as the target radionuclide to be detected by a fiber-optic radiation sensor (FORS).
Several fiber-optic radiation sensors have been developed for the remote and real-time measurements.Park and Joo developed an underwater beta radiation monitoring sensor using a CaF 2 : Eu, YAG : Ce, YAP : Ce scintillator [5].Seo et al. developed a thin film type of plastic scintillation detector [6].Jang et al. fabricated a fiber-optic radiation sensor for the detection of tritium [7].Kim et al. developed a ZnS(Ag)/BC-408 phoswich detector for monitoring radioactive contamination inside pipes [8].Hong et al. developed a fiber-optic dual detector to measure beta-and gamma-rays simultaneously [9].
In this study, a FORS was developed for the remote and real-time measurement of beta radiation from radioactivecontaminated soil.The sensing probe consisted of a bundle of organic scintillators and a mixture of epoxy resin to improve its detection efficiency of lights.Several sensing probes, whose dead-end diameters were different, were prepared and characterized to select the best sensing probe in terms of its radiation detection efficiency.

Experimental Setup
The organic scintillator used was a plastic scintillating fiber (BCF12, Saint-Gobain).Despite being more expensive than a liquid scintillator, the plastic scintillating fiber is chemically  stable as a solid material and is easy to fabricate [10].Table 1 lists its physical properties [11].The light generated from scintillation was transmitted to the light guide, which was fabricated with transparent epoxy resin that can transmit the light efficiently to the optical fiber owing to its small optical loss resulting from the absorption and leakage of the light.Its characteristics depend on the epoxy resin to hardener mixing ratio.To meet the characteristics required for the light guide, the epoxy resin (YD-128, Kukdo., Korea) and hardener (D-230, Kukdo., Korea) were mixed at a ratio of 5 : 1 [12].The mixture of epoxy resin was prepared in a truncated conical shape.
Table 2 shows cross-sectional views of two parts of the FORS sensing probes.The overall diameters of the sensing probes were 26 mm (Bundle type I), 36 mm (Bundle type II), and 46 mm (Bundle type III).The bundles of the scintillating fibers were attached to the broader end of the epoxy resin mixture.The sides of the sensing probes were wrapped with aluminium foil (3 m, Samwha) as a reflector to reduce light leakage.
A plastic optical fiber (#53-833, Edmund optics) was used as a transmitting optical fiber.The plastic optical fiber has small transmission loss with a peak value of 435 nm.Table 3 lists the properties of the plastic optical fiber.The FORSs were applied to measure beta radiation from a Sr-90 disk source (0.1 Ci, 2014.09,Spectrum Technique).The FORSs were installed in a dark room to reduce the influences of external light.The sensing probes of the FORS were located 30 mm above the beta disk source to reduce the effects of backscattered radiation [13].For comparison, beta radiation from a Sr-90 disk source was also detected using a commercial GM counter (GM214007, NEO).
Because it cannot help being affected by the background radiation, the radiation measurement system should distinguish between background radiation and the radiation to be measured.The critical level (  ) is defined as the net count rate which must be exceeded before the sample can be said, with a certain level of confidence, to contain measurable radioactivity above background.For the critical level, this paper used (1) to check if the FORS can measure the radiation from the beta source distinguishable from the background radiation, as suggested by [14].
where   is the total background counts.

Results and Discussion
Figure 2 shows the measurements by the FORSs with the three different diameters.Each measurement by each FORS was made for over 600 seconds and repeatedly 20 times.The total counts in Table 4 are the average of the 20 measurements obtained by using the FORSs.The total counts by the FORSs with 26 mm, 36 mm, and 46 mm in diameter were 15341, 16543, and 17004, respectively, which are summarized in Table 4, along with other measurement values.The FORS detected more radiation from the source as the diameter of its sensing probe was increased.For the bundle type III of the sensing probe, the peak values were higher than the other two sensing probes.Compared to that by using a commercial GM counter for over 600 seconds, the relative total counts obtained by using the FORSs with a diameter of 26 mm, 36 mm, and 46 mm were found to be 26.9%,29.0%, and 29.8%, respectively.The reproducibility of the FORS was tested by applying a  2 -test to the measurements as a minimum requirement for a radiation sensor.Because radioactive decay is a random process, any measurement of radiation is subject to a degree of the statistical fluctuation.A  2 -test is constructed from a sum of squared errors or through the sample variance.If the probability for the estimated  2 is in the range of 0.1 <  < 0.9, it can be determined that the radiation detection system produces reproducible measurement reflecting the statistical fluctuations of radioactive decay appropriately [15][16][17][18].Table 5 lists the 20 measurements by each FORS and the relevant statistics for the  2 -test.The  2 s for the measurements by the FORSs were 19.72, 18.55, and 14.70 for a diameter of 26 mm, 36 mm, and 46 mm, respectively.The probabilities of  2 's with 19 degrees of freedom were 0.54, 0.51, and 0.76, respectively.Figure 3 shows the distributions of a statistic (  −  av )/, where   is th measurement by the FORSs and GM counter,  av is the average, and  is the standard deviation of the 20 measurements.Figure 3 helps to check intuitively if the FORSs generate the statistically significant measurements, showing the difference between the distributions of the statistic and the normal distribution.

Conclusion
This study developed and characterized a FORS for the remote and real-time measurements of beta radiation.The FORS consisted of the sensing probe with an aluminium foil reflector, transmitting plastic optical fiber, and lightmeasuring system that comprised a photomultiplier tube, preamplifier, multichannel analyzer, and laptop computer.The measurements showed that the FORS could be used as a radiation sensor and that the FORS sensing probe with a diameter 46 mm has the best detection performance.

Figure 1
Figure 1 shows the experimental setup for measuring beta radiation, which consisted of a photomultiplier (PMT, R1924, Hamamatsu), preamplifier (Canberra amplifier model 2006), multichannel analyzer (MCA, Canberra Multiport II), and a laptop computer that was powered by a high-voltage power supplier (Canberra model 3102D).The light transmitted through the transmitting optical fiber to the PMT tube was converted to an electrical signal.This electrical signal was amplified by the preamplifier.The signals were acquired by MCA and analyzed using the GENIE 2000 program installed at the laptop computer.The FORSs were applied to measure beta radiation from a Sr-90 disk source (0.1 Ci, 2014.09,Spectrum Technique).The FORSs were installed in a dark room to reduce the influences of external light.The sensing probes of the FORS were located 30 mm above the beta disk source to reduce the effects of backscattered radiation[13].For comparison, beta radiation from a Sr-90 disk source was also detected using a commercial GM counter (GM214007, NEO).Because it cannot help being affected by the background radiation, the radiation measurement system should distinguish between background radiation and the radiation to be measured.The critical level (  ) is defined as the net count rate which must be exceeded before the sample can be said, with a certain level of confidence, to contain measurable radioactivity above background.For the critical level, this paper used(1) to check if the FORS can measure the radiation from the beta source distinguishable from the background radiation, as suggested by[14].

Figure 2 :
Figure 2: Comparison of (a) pulse height spectra and (b) measurements (cps) by the FORSs with different diameters and a commercial GM counter.

Figure 3 :
Figure 3: Distributions of a statistic (  −  av )/ for 20 measurements by the FORSs and a commercial GM counter.

Table 2 :
Cross-sectional views of two parts of the FORS sensing probes (unit: mm).

Table 4 :
Comparison of the measurements by the commercial detector and FORSs.

Table 5 :
The 20 measurements by the FORSs with different diameters and the relevant statistics.