The AMS measurement of ²³⁶U at CIRCE

The AMS measurement of 236 U at CIRCE 1 corresp first-author Corresponding author. E-mail address: yjguan@gxu.edu.cn Corresponding author. E-mail address: yjguan@gxu.edu.cn yjguan@gxu.edu.cn 1 2 3 Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science & Technology, Guangxi University, Nanning 530004, China Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science & Technology, Guangxi University, Nanning 530004, China Department of Diagnostic Methodologies and Measurement Techniques Italian Aerospace Research Centre (CIRA), Capua (CE), 81043, Italy and INFN, Napoli 80126 Department of Diagnostic Methodologies and Measurement Techniques Italian Aerospace Research Centre (CIRA), Capua (CE), 81043, Italy and INFN, Napoli 80126 CIRCE, Department of Mathematic and Physics, II University of Naples, Caserta 81100, Italy CIRCE, Department of Mathematic and Physics, II University of Naples, Caserta 81100, Italy Accelerator mass spectrometry (AMS) is an ultrasensitive technique for measuring long-lived actinides, e.g. 236 U, 237 Np and Pu isotopes. In order to improve the detection limit for actinides abundance, and to increase the detection efficiency in actinides AMS measurement, a 16-strip silicon detector was used to identify actinides at the Center for Isotopic Research on Cultural and Environmental heritage (CIRCE) in Caserta, Italy. The sensitivity of 236 U/ 238 U was 1×10 −11 by spacial resolution and 5.0×10 −12 by time resolution. The pulse height defect of 236 U in an ion-implanted silicon detector in the low energy range with 17.26 MeV is presented. Uranium Accelerator mass spectrometry Sensitivity Pulse height defect 1 Introduction 1 The long-lived anthropogenic radionuclides of 236 U ( T 1/2 =2.348×10 7 a), 237 Np ( T 1/2 = 2.1×10 6 a), 239 Pu ( T 1/2 = 2.4×10 4 a) and 240 Pu ( T 1/2 = 6.6×10 3 a) have been released into the environment by nuclear weapons testing, nuclear accidents, fuel reprocessing and decommissioning of nuclear power plants. These radionuclides play important roles in environmental monitoring, nuclear safeguards and nuclear forensic studies. The main sources of 236 U are from the thermal neutron reaction of 235 U and the alpha decay of 240 Pu. The nature abundances of 238 U, 235 U and 234 U are approximately 99.272%, 0.72% and 0.054%, respectively. Due to the difference of 236 U concentration between natural uranium and the spent uranium by 7 to 11 orders of magnitude, 236 U/ 238 U isotopic ratio is a powerful indicator of uranium resource. Usually, the activity of 236 U in environmental samples is much lower than the exemption limit, and analyzing 236 U does not require special radiochemical laboratories. However, determination of 236 U is not an easy task, because low quantities of 236 U are usually mixed with the high abundance of 238 U and 235 U, considered as potential sources of strong interferences. Since the energy difference between the alpha particles emitted by 236 U and 235 U is less than 0.10 MeV, detection of 236 U alpha particles emission is interfered with 235 U alpha particles and other isotope emissions, caused by energy tailing. The 236 U/ 238 U isotopic ratio is limited in 2.6×10 −5 in alpha spectrometry. With the improvements in sample preparation methods and detection techniques, thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS) are becoming the most popular and powerful tools for the determination of uranium isotopic measurements. ICP-MS and TIMS have been employed for measuring 236 U, in sensitivities of 10 −8 and 10 −10 , respectively, though [ 1 1 ] . The important factors affecting the detection sensitivity of 236 U in ICP-MS and TIMS are molecular ions (e.g. 235 UH + , 234 H 2 + ) and high abundance isotope peak tail of 238 U and 235 U on mass 236. To improve the 236 U/ 238 U detection sensitivity, multiple collector ICP-MS (MC-ICP-MS) have been used, with better precision. The outstanding feature of AMS method is the significant suppression of scattering tails due to its high ion energies and the complete elimination of molecular interference. AMS is an effective and robust technique for studying long-lived actinides, e.g. 236 U, 237 Np and 239 Pu. Of the all available detection techniques, AMS is presently the most sensitive technique for 236 U measurement, 236 U/ 238 U ratios down to the level of 10 −13 was reported by Wilcken et al [ 2 2 ] . More and more AMS laboratories performed the actinides research [ 3 3 - 10 10 3-10 ] . In this paper we present the performance of 236 U AMS measurement, developed at the Center for Isotopic Research on Cultural and Environmental heritage (CIRCE) in Caserta, Italy. A 16-strip ion-implanted silicon detector is used to identify actinides by spacial and time resolution, respectively. The pulse-height defect of 236 U ions in the detector with the energy of 17.26 MeV is analyzed. 2 AMS facility and detection system 1 CIRCE is a dedicated AMS facility based on a 3MV-tandem accelerator, built by National Electrostatics Corporation. The schematic layout of the present setup is shown in Fig. 1 Fig. 1 . The AMS facility is an ideal system for actinides measurement, which consists of high resolution injection magnet, analyzing magnet and electrostatic analyzer. The cesium sputter ion source is a 40-sample MC-SNICS normally biased at −50 kV for 236 U measurement. In our case, the sample preparation provides uranium in the form of U x O y +Fe 2 O 3 , and an output from a Φ 1 mm aluminum cathodes for 238 U 16 O − ions in Faraday Cup 2 (FC-2) is in the range of 50–300 nA with a total injection energy of 50 keV. The injection magnet object and image slits in vertical and horizontal directions are all close to ±1 mm, mass resolution injection system is M /Δ M =500. High terminal voltage is helpful to obtain lower background and higher ion transmission, but requires a higher field in the analyzing and switching magnet. According to parameters of the analyzer components, 236 U 5+ is transmitted at a terminal voltage of 2.900 MV. The double focusing 90° high energy bending magnet is of ρ =1.27 m, ME / Z 2 =176 and M /Δ M =725, so that, e.g. 236 U 5+ at terminal voltage of 3 MV can be analyzed with a beam spot size of 3.5 mm. The two 45° electrostatic spherical analyzers ( ρ =2.54 m and gap=3 cm) are operated at ±40 kV, and energy resolution is E /Δ E =700. The actinides beam line, mounted after the switching magnet, consists of a microchannel plate detector (MCP) and a position-sensitive 16-strip silicon detector. The detector, with an active area of 58 mm×58 mm, is a partially-depleted passivated implanted planar silicon (PIPS) detector (Canberra PF-16CT-58*58-300EB). The 16-strip detector is used to measure the ion energy and the time signal. The 236 U, 235 U and 238 U ions hit different positions of the strip detector due to the different E / q ratios, so the position information can be served as uranium isotopes identification. The time of flight (TOF) detection system with a flight path of 1.5 m is installed to discriminate 236 U ions from 235 U, 238 U and other interferences. Details about the TOF detection system can be found in Ref.[ 11 11 ]. 3 236 U AMS measurement 1 The principle of AMS and general tuning procedure were described clearly in earlier work [ 12 12 , 13 13 ] . The sensitivity of AMS is mainly limited by the interferences from isobars, isotopes and other backgrounds. The source of interferences was discussed in details and a program was designed for experimental spectra in AMS measurement [ 14 14 ] . In order to optimize the beam transmission through the various apertures, we tuned the ion-optical components with 238 U 16 O − at terminal voltage of 2.875 MV. The focusing and steering elements were optimized by maximizing the current in FC-4, after the analyzing magnet. According to the currents in FC-2 and FC-4, the stripping yield of +5 charge state is about 3.3%, at 0.93 Pa of the gas stripper pressure. The stripper yield as a function of the charge states is shown in Fig. 2 Fig. 2 . The +5 charge state is the minimum permitted by the bending power of high energy analyzing magnet. The electrostatic analyzer, the switching magnet and all of the focusing and steering elements were optimized by maximizing the current in FC-5, before the MCP ( Fig. 1 Fig. 1 ). The transmission efficiency between FC-4 and FC-5 at the actinides beam line is about 80%. Using a fast beam-switching method, 238 U 16 O − and 236 U 16 O − ions were alternately injected and passed on different components, and detected by the Faraday cup or detector. Typical injection time was 120 s for 236 U, and 10 s for 238 U. 238 U 5+ at 17.07 MeV (i.e. terminal voltage 2.875 MV) and 236 U 5+ at 17.26 MeV (i.e. terminal voltage 2.900MV) are of the same rigidity, hence no need of changing the analyzing magnet field. For 236 U AMS, two kinds of detection methods were used. 3.1. 236 U detected by 16-strip detector 2 The transmission efficiency from low-energy side FC-2 to detector was measured at 2.5% for 236 U 5+ ions. Most of the 235 U, 238 U and other interference ions were suppressed by analyzing components before they transmitted to the detector. The 236 U ions were detected by the 16-strip detector. Two kinds of samples (i.e. KkU and standard) were measured alternately and repeated for several times under the same conditions. The ‘standard’ with a nominal ratio 236 U/ 238 U~1×10 − 8 , obtained by adding a spike of 236 U to the KkU VERA-in-house-U standard, and the KkU itself of 236 U/ 238 U= (6.98±0.32)×10 −11 , were measured [ 15 15 ] . Due to a real blank sample was not easy to get, the peak of ‘blank’ sample was almost the same as the KkU, under the same conditions except that the voltage of high energy electrostatic analyser was 0.8% less than that of 236 U transmission. The ion distributions on the strip detector of different samples are shown in Fig.3 Fig.3 . The Y axis is the normalized fraction (counts per strip over total counts of the 16 strip-detector). The distance between two adjacent strips is 3.625mm.The separation between 236 U and other isotopes are easy to calculate based on parameters of the electrostatic analyzer. The separation between 236 U 5+ and interfering 238 U 5+ on the detector is 34 mm, and between 236 U 5+ and interfering 235 U 5+ is 17 mm, which agree well with the experimental results. As shown in Fig.3 Fig.3 , 236 U ions are distributed from strip_8 to strip_12 (31–45 mm), and more than 95% ions are in the region of interest (ROI), from strip_9 to strip_11. Due to the charge exchange process and continuous momentum spectrum of interfering isotopes, part of 235 U 5+ and 238 U 5+ ions transmitted to the detector, and the interference peaks are marked in Fig.3 Fig.3 . The absolute measurement values of isotopic abundance of ‘standard’ and KkU sample are 236 U/ 238 U=(9.51±0.36)×10 −8 and 236 U/ 238 U=(8.23±0.60)×10 −11 , respectively. This provides a background level isotopic ratio of 236 U/ 238 U is about 1×10 −11 . 3.2. 236 U detected by TOF- E detection system 2 A TOF detection system with the flight length of 1.5m was used to discriminate uranium isotopes. Fig.4 Fig.4 shows the TOF spectrum of the silicon strip_10 with the highest 236 U 5+ counts for the KkU standards. From theoretical calculation, the different interference peaks between 236 U 5+ ions are Δ T ( 236 U 5+ - 235 U 5+ )∼1.70 ns, and Δ T ( 238 U 5+ - 236 U 5+ )∼3.40 ns. In Fig.4 Fig.4 each channel means 160 ps, and the peak of 235 U, 236 U and 238 U are marked. Although the resolution is not good enough to separate 235 U and 236 U by TOF, combining with the energy signal, the detection limit of 5×10 −12 was obtained. 3.3. Pulse height defect 2 Since the energy peak of 236 U ions did not agree with the expect result, the pulse height defect (PHD) of the passivated implanted planar silicon strip detector was measured. The PHD is the difference in detected energy between heavy ions and light ions (e.g. alpha particles) of the same kinetic energy. The PHD of heavy ions in silicon detector consists of three contributions. (1) Window defect E w , that is the energy loss of the heavy ions in the dead layer on the front surface of the detector. (2)The nuclear stopping loss E N , for low energy nuclear stopping is dominant, the defect is due to non-ionizing collisions of the slow heavy ions with the atoms of the detector. (3) The recombination of electron-hole pairs E re . Along the path of and ionizing heavy particle, a plasma column of high density is formed. The recombination of electron-hole pairs lead to incomplete charge collection and a reduction of detected pulse height. Pulse height calibration were performed by recording of 5.486 MeV alpha particles from an 241 Am source, and 9.535 MeV 14 C 3+ and 13.527 MeV 14 C 4+ ions from the tandem accelerator. The peak position was determined by fitting a Gaussian to each peak separately. The detail energy losses in dead layer of the detector, the nuclear stopping loss and the electronic stopping loss are given in Table 1 Table 1 . E w , of the above ions passing through the dead layer with the maximum thickness (i.e. 50 nm) of detector was calculated with SRIM 2013. E N and E e means the energy loss caused by nuclear stopping and electronic stopping, respectively. The values of E N and E e were obtained by integrate the energy loss curve derived from the SRIM calculation results. For 236 U, with high mass number and low incidence energy, the energy loss contribution from nuclear stopping is about 6.336 MeV, only 10.232 MeV, the energy loss by electronic stopping can be collected. Fig 5 Fig 5 shows the calibration curve of energy loss ( E e ) versus channel number for alpha, 14 C and 236 U ions. E e is the collected net energy from electronic stopping loss. After considering the nuclear stopping loss, there is an excellent agreement between E e and peak in Fig. 5 Fig. 5 . In this case, the contribution of the recombination of electron-hole pairs ( E re ) effect to the PHD can be ignored. 1 1 Ions 1 1 Energy 1 1 Peak(Ch.) 1 1 E W 1 1 E N 1 1 E e 1 1 α 1 1 5.486 1 1 703.5 1 1 0.022 1 1 0.01 1 1 5.454 1 1 14 C 3+ 1 1 9.535 1 1 1247 1 1 0.165 1 1 0.073 1 1 9.297 1 1 14 C 4+ 1 1 13.527 1 1 1782 1 1 0.163 1 1 0.078 1 1 13.286 1 1 236 U 5+ 1 1 17.26 1 1 1348 1 1 0.692 1 1 6.336 1 1 10.232 4 Conclusion: 1 236 U AMS measurement was performed at CIRCE by the position sensitivity detection method with a background level of 236 U/ 238 U is 1×10 −11 , and the TOF detection method with a background level of 236 U/ 238 U is 5×10 −12 , respectively. The pulse height defect is obvious in 236 U energy measurement, and the energy difference is 6.336 MeV. The result show that the contribution of pulse height defect is mainly from the nuclear stopping loss. References P. Steier , F. Dellinger , O. 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Instrum Methods B . 266 , 2246 - 2250 ( 2008 ). doi: 10.1016/j.nimb.2008.03.002 http://doi.org/10.1016/j.nimb.2008.03.002 Natural and anthropogenic 236U in environmental samples 10.1007/s41365-017-0252-5 This work was supported by the National Science Foundation of China (Nos.41166002 and 11665006), and partly supported by the Guangxi Natural Science Foundation (No.2012GXNSFFA060005) and Provincial Key laboratory Project. 2016-09-09 2016-10-25 2016-11-02 2017-06-02 2017-07-01 2021-04-25 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2017 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-017-0252-5 http://dx.doi.org/10.1007/s41365-017-0252-5 . 2017 Nuclear Science and Techniques 07 28