INTRODUCTION
The discharge of nuclear fuel material into the environment from a nuclear power plant requires a rapid and sensitive analysis method for detecting fission products in samples taken from the environment. The nuclide 135Cs with a half-life of 2.3×106 yr is interesting because it plays an important role in many research fields, especially in nuclear environmental studies, due to its high fission yield and long half-life. In addition to formation by direct fission, it is produced as a daughter nuclide in the decays of 135Xe and 135I, so the concentration of 135Cs increases during the operation of a reactor. Therefore, 135Cs may be the appropriate tracer for the leakage of nuclear material from a reactor. However, 135Cs is a pure β-decay nucleus and the lack of γ emission makes its detection by radiometric methods very difficult. Traditional mass spectrometry methods such as thermal ionization mass spectrometry (TIMS) (Lee et al. Reference Lee, Ku, Lu and Chen1993) and inductively coupled plasma mass spectrometry (ICP-MS) (Zheng et al. Reference Zheng, Bu, Tagami, Shikamori, Nakano, Uchida and Ishii2014; Liezers et al. Reference Liezers, Farmer and Thomas2009) have been used for 135Cs measurement. However, the interferences of stable molecular and atomic isobar ions strongly limit the detection sensitivity. Accelerator mass spectrometry (AMS) may be a promising method to detect 135Cs (Eliades et al. Reference Eliades, Zhao, Litherland and Kieser2013; Lachner et al. Reference Lachner, Kasberger, Martschini, Priller, Steier and Golser2015).
In this paper, we report the experimental results for different samples of Cs, which were measured with the HI-13 tandem AMS facility at the China Institute of Atomic Energy (CIAE). In order to improve the sensitivity, a new conducting medium, Fe powder, was used for the first time in this experiment.
EXPERIMENT AND RESULTS
Ion Extraction
To improve detection sensitivity, interference should be removed or decreased. In AMS measurement, isobaric interferences can be removed or decreased by using suitable negative ions. 135Ba is the main interference in the measurement of 135Cs with AMS, however, it is very difficult to differentiate between 135Cs and 135Ba by using the ΔE-E method. For the purpose of finding the most favorable ions for 135Cs measurements, which should maximize Cs ion currents and minimize currents of Ba, the beam current of stable isotope 133Cs, such as Cs−, CsF−,
$$\rm C{\rm sF}_{2}^{{\minus}} $$
, and
$$\rm C{\rm sF}_{{\rm 3}}^{{\minus}} $$
were tested for 135Cs AMS measurement. It should be noted that
$$\rm C{\rm sF}_{2}^{{\minus}} $$
was used for 135Cs AMS measurement in the previous study by using PbF2 as conducting material (Eliades et al. Reference Eliades, Zhao, Litherland and Kieser2013; MacDonald et al. Reference MacDonald, Charles, Zhao, Kieser, Cornett and Litherland2015a, Reference MacDonald, Charles, Cornett, Zhao, Kieser and Litherland2015b; Zhao et al. Reference Zhao, Charles, Cornett, Kieser, MacDonald, Kazi and St-Jean2016). Since the usual Cs sputtering would obscure the 135Cs/133Cs ratio of a sample, Rb sputtering was successfully applied and tested also for various other typical AMS elements. Cs− currents of several 10 nA were extracted over hours from milligram amounts of Cs2SO4 material by Lachner et al. (Reference Lachner, Kasberger, Martschini, Priller, Steier and Golser2015). However, Cs− has been selected as the suitable choice for the present 135Cs AMS measurements based on our previous experimental results. The details of experiment can be found in Yin et al. (Reference Yin, He, Dong, Dou, Lan, Pang, Wu and Jiang2015). The beam current was measured before and after the measurement. The ion beam current was relatively stable and long-lasting during the measurement time, as shown in Table 1.
Table 1 Ion beam current of Cs− from different samples.
Background Measurements
To measure the 135Ba background level, CsNO3 with different conducting media were measured with HI-13 tandem AMS system by extracting Cs− ions. The 135Ba background from the different conducting media, Fe powder (99.9%), and Ag powder (99.9%) were also measured by AMS. The system comprises an injection system with a 90º spherical electrostatic deflector in front of a double-focusing 112º injection magnet with mass resolution, M/ΔM = 400 to 800 (He et al. Reference He, Ruan, SL, Wang, Li, Shen, Wang, Lu, Wu, He and Jiang2010). A high terminal voltage, up to 13 MV, is produced in the middle of the acceleration tube. The high-energy beam analysis system consists of a 90º double-focusing magnet followed by a switching magnet which transports the beam to the AMS beam line with a 15º electrostatic deflector and the surface barrier detector (SBD).
Different sample mixtures (as shown in Tables 1 and 2) were pressed into the standard NEC aluminum target holder. Cs− ion was used for analysis extracted from the commercial CsNO3 (99.99%) +Ag, CsNO3 (99.99%) +Fe, Ag, and Fe samples. The 133Cs− ions were selected by the injection magnet, a terminal voltage of 7.6 MV was used for acceleration and the 8+ charge state was selected for Cs ions. Transmission parameters of all equipment were set for 133Cs. Then the beam current of 133Cs8+ was measured by the AMS Faraday cup. It should be noted that the previous measurement for CsNO3+Ag sample (Yin et al. Reference Yin, He, Dong, Dou, Lan, Pang, Wu and Jiang2015) had been made using the 10+ charge state at 7.6 MV. Then the 135Cs− ions were selected by changing the injection magnet. Keeping other transmission parameters unchanged, only adjust the terminal voltage to 7.7 MV and change the electrostatic deflector voltage to V133×135/133. The counts of 135Ba were determined from the SBD detector, while the number of 133Cs ions was calculated from the beam current (nA) of 133Csq+ in the AMS Faraday cup. Measuring time is 300 sec for each sample. The 135Ba/133Cs ratio can be calculated by the formula below:
$$R={{{N \over {\rm t}}} \over {{\,I \over q}{\times}n}}$$
where N is the 135Ba total counts, t is the measuring time, I is the beam current intensity (nA) in the AMS Faraday cup, q is the charge state of the ions, and n=6.25×109, the particle counts per second for 1 nA beam current. The background of 135Ba from different samples is shown in Table 2.
Table 2 Background level of different samples.
DISCUSSION
According to Middleton et al. (Reference Middleton1989), the electron affinity of Ba is negative. This means that there should be no background of 135Ba in the case of extracting Cs− from the ion source. However, our results show that the 135Ba background still can be detected in the case of extracting Cs− although it is low. The representative energy spectrum of CsNO3+Ag and CsNO3+Fe samples detected with the SBD detector is shown in Figure 1. The two peaks of 118Sn and 135Ba are clearly visible. This implies that the Ba can be extracted as the form of Ba− and the electron affinity of Ba should be larger than 0. The new data of electron affinity of Ba is about 0.14 eV (Petrunin et al. Reference Petrunin, Voldstad, Balling, Kristensen, Andersen and Haugen1995), and our results support this data. Possible interfering ions were analyzed for 135Cs AMS measurement using our own computer programs, and the results are shown in Table 3. The analysis results show that the rigidity of 118Sn7+ is very similar to that of 135Cs8+. So 118Sn can be detected, but separated by the different energy (Figure 1).
Figure 1 Representative energy spectra of CsNO3+Ag and CsNO3+Fe samples.
Table 3 Possible interfering ions for 135Cs AMS measurement.
The importance of the conducting medium in AMS measurement cannot be neglected. In our previous experiments (Yin et al. Reference Yin, He, Dong, Dou, Lan, Pang, Wu and Jiang2015), Ag powder was used as the conducting medium. In order to affirm whether the Ag powder is the appropriate conducting medium or not, a different conducting medium, Fe powder, was used in the present experiment. As shown in Table 1, for the sample of CsNO3 with Ag, the beam current of Cs− is little higher than that from the CsNO3 with Fe. For the sample of Ag, the beam current of Cs− is almost equal to that of Fe. It can be seen from Table 2 that the background of 135Ba from CsNO3 mixed with Ag is ~10 times lower than that from CsNO3 mixed with Fe. Furthermore, the background of 135Ba from sample Ag is also ~10 times lower than that from sample Fe. Therefore, the results indicate that the conducting medium Ag powder is better than Fe powder in our AMS measurement, as shown in Tables 1 and 2.
In addition, the sensitivity of 1.8×10–10 (135Ba/Cs) can be obtained from the present experiment, which is slightly worse than that of previous result (Yin et al. Reference Yin, He, Dong, Dou, Lan, Pang, Wu and Jiang2015). The likely reason for this is that, in case of the Cs− ion with charge state 10+, an interference peak from the scattering of some ions with similar energy cannot be completely separated from the peak of 135Ba.
SUMMARY
The AMS measurement method of 135Cs has been studied at CIAE. The new conducting medium Fe powder was used for the first time in this experiment. For the sample of CsNO3 with Ag, the beam current of Cs− is slightly higher than that of CsNO3 with Fe. The background of 135Ba from the sample mixed with Ag is ~10 times lower than that from the sample mixed with Fe. The results indicate that the conducting medium Ag powder is better than Fe powder in AMS measurement for 135Cs. The 135Ba background level is about 1.8×10–10 (135Ba/Cs). However, more investigations in the future, such as the sample of Cs2SO4, may be useful to improve the sensitivity of AMS measurement. Additional measurements are needed in order to deal with the problem of cross contamination. We note that some recent works (MacDonald et al. Reference MacDonald, Charles, Zhao, Kieser, Cornett and Litherland2015a, Reference MacDonald, Charles, Cornett, Zhao, Kieser and Litherland2015b; Zhao et al. Reference Zhao, Charles, Cornett, Kieser, MacDonald, Kazi and St-Jean2016) have carried out detailed experiments on this issue.
Acknowledgments
The authors would like to thank the crew of the HI-13 tandem accelerator at the China Institute of Atomic Energy for steady operation of the accelerator. This work was supported by the National Science Foundation of China (NSFC) under Grant No. 11205250.
