Measurement of 59Ni and 63Ni by accelerator mass spectrometry at CIAE

Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

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Measurement of 59Ni and 63Ni by accelerator mass spectrometry at CIAE Xiaoming Wang a, Ming He a,⇑, Xiangdong Ruan b, Yongning Xu a, Hongtao Shen c, Liang Du a, Caijin Xiao a, Kejun Dong a, Shan Jiang a, Xuran Yang a, Xiaoxi Lan b, Shaoyong Wu a, Qingzhang Zhao a, Li Cai b, Fangfang Pang c a b c

China Institute of Atomic Energy, P.O. Box 275(50), Beijing 102413, China College of Physics and Technology, Guangxi University, Nanning 530004, China College of Physics and Technology, Guangxi Normal University, Guilin 541004, China

a r t i c l e

i n f o

Article history: Received 14 November 2014 Received in revised form 19 March 2015 Accepted 19 March 2015 Available online xxxx Keywords: 59 Ni 63 Ni AMS Q3D magnetic spectrometer Measurement

a b s t r a c t The long lived isotopes 59Ni and 63Ni can be used in many areas such as radioactive waste management, neutron dosimetry, cosmic radiation study, and so on. Based on the large accelerator and a big Q3D magnetic spectrometer, the measurement method for 59Ni and 63Ni is under development at the AMS facility at China Institute of Atomic Energy (CIAE). By using the DE-Q3D technique with the Q3D magnetic spectrometer, the isobaric interferences were greatly reduced in the measurements of 59Ni and 63Ni. A four anode gas ionization chamber was then used to further identify isobars. With these techniques, the abundance sensitivities of 59Ni and 63Ni measurements are determined as 59Ni/Ni = 1  1013 and 63 Ni/Ni = 2  1012, respectively. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction

(GFM) technique was developed for eliminating the isobar interference [9–13]. Based on this technique, sensitivities for 59Ni/Ni and 63 Ni/Ni of 1014 were reported [10,11]. In order to measure 59 Ni and 63Ni at CIAE AMS lab, the method, named DE-Q3D, was developed for effective suppression of isobaric interferences. This method was successfully used for the measurements of 32Si and 53 Mn [14,15]. In this paper, the details for measuring 59Ni and 63 Ni by using the DE-Q3D technique are presented.

The long lived isotopes 59Ni and 63Ni can be used in a number of fields, including low-level radioactive waste management [1], cosmic radiation study [2], neutron dosimetry [3–5] and astrophysics [6,7]. Although AMS has the potential of providing necessary sensitivity for these applications, the sensitivity for medium mass isotopes, such as 59Ni and 63Ni, is usually limited by high background of their stable isobars. When passing through a medium the relative difference in the differential energy loss of isobars decreases with the increase of mass, which makes isobaric suppression more difficult by just energy loss measurement. Different techniques for reducing the isobaric interference were applied at AMS laboratories for the measurements of medium mass isotopes. The projectile X-ray method was used for isobar identification [3,8]. However, due to the limited identification power and low X-ray detection efficiency, their sensitivity for 59Ni and 63Ni was limited to about 1011 [8]. A full-stripping technique was used in AMS measurement of 59Ni for reducing the 59Co interference at the Argonne National Laboratory, utilizing particle energy of 641 MeV [7]. This technique requires very high ion energies, which are unreachable in most existing AMS systems. A gas-filled magnet

2. The AMS system at CIAE The schematic diagram of the AMS system is shown in Fig. 1. A dedicated ion injection system is used for AMS measurement. This injection system consists of a 40-sample NEC MC-SNICS ion source, a 90° spherical electrostatic analyzer with radius of 75 cm and a double-focusing 112° injection magnet with radius of 80 cm. The HI-13 tandem accelerator can work at the terminal voltage up to 12.5 MV. Carbon foils of thickness of 3 lg/cm2 are used as stripper at the terminal. The high-energy analysis system consists of a 90° double-focusing magnet with a radius of 127 cm, followed by a switching magnet that direct the beam to different beam lines. Two beam lines are now used for AMS experiments. The first AMS line has a 15° electrostatic deflector, time of flight system and a gas ionization chamber. The second line with a Q3D spectrometer is a multi-task setup. The Q3D spectrometer was

⇑ Corresponding author. E-mail address: [email protected] (M. He). http://dx.doi.org/10.1016/j.nimb.2015.03.057 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

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Fig. 1. The schematic diagram of AMS system at CIAE.

designed by H.A. Enge and manufactured by Scanditronix. The main components are three dipolar magnets which have the same radium of 100 cm, but deflection angles of 65°, 65° and 25°, respectively. The feature of the spectrometer was described in detail in reference [16]. The Q3D magnetic spectrometer has very high dispersion, which along its focal plane is DX/(DP/P) = 11.37 cm%1. DX is the separation distance alone the focal plane, P is the momentum of ions. The DE-Q3D technique used for 59Ni and 63Ni measurement was developed based on the high dispersion of the Q3D spectrometer. The DE-Q3D system consists of the Q3D magnetic spectrometer with an absorber placed at its entrance, which is used to absorb part of the ions’ energy, and a four-anode gas ionization chamber with an entrance window of 70 mm  40 mm at the center of its focal plane. A movable surface barrier detector (SBD) with a diameter of 12 mm is also mounted on the focal plane. The SBD is used for measuring the ion spatial distribution along the focal plane. In order to improve the position resolution, the SBD is covered with a slit of 7 mm  12 mm.

position of the focal plane and detected by the multi-anode ionization chamber. The interfering isobars are then focused onto different positions of the focal plane. Tails of the isobar(s), and other interfering ions due to straggling and scattering effects, are further separated in energy spectrum by the gas ionization chamber. In order to separate isobars with the DE-Q3D technique, the difference of energy losses (dDE) between isobars should be larger than the energy straggling after passing through the absorber. The uniformity and thickness of the absorber are the two key factors. Carbon foils were tested at first. It was found that the energy straggling was much larger than expected. This is mainly a result of the inhomogeneous thickness of the carbon foil. Silicon nitride (Si3N4) membranes (from Silson Ltd, England) were then tested and found to work very well due to their very homogeneous thickness. The suitable thickness of the absorber is another factor. The dDE increases linearly with foil thickness, while energy loss straggling is proportional to the square root of foil thickness. This means that the thicker the absorber the larger the dDE and the better the isobar separation. However, as the absorber thickness increases it will broaden the beam spot and decrease the ion energy, thus decreasing the detection efficiency and isobar identification ability of the gas ionization chamber. So, one has to find a compromise among the position separation, isobar identification and detection efficiency. Based on a series of experiments it is found that the suitable absorber thickness is the one that causes about 1/3 of the ion energy to be lost in the absorber and the rest 2/3 deposited in the detector. In the case of 59Ni measurement, 5 lm Si3N4 foil was used as the absorber. Respective energies of 50.1 MeV and 48 MeV were lost after 59Ni and 59Co ions with the same initial energy of 149.5 MeV passed through the absorber. Then the 59Ni and 59Co ions of the selected charge state were separated on the focal plane of the Q3D due to their different remaining energies. The movable SBD was then used to measure the position distributions of 59Ni and 59 Co on the focal plane. The results are shown in Fig. 2. Due to the strong background of 59Co and low count rate of 59Ni, 59Ni peak was not recognized by the SBD along the focal plane. 60Ni ions with the same magnetic rigidity as that for 59Ni ions were used to simulate the position of 59Ni. Fig. 2 shows that the peaks of 59Ni and 59Co on the focal plane were separated by 132 mm, which is a little more than the theoretical estimation (115 mm), and each peak has a width (FWHM) of about 60 mm. The results show that the DEQ3D method is effective for separating isobars in this mass range.

3. Isobar separation with DE-Q3D technique The gas-filled magnet (GFM) technique has been developed successfully for isobar separation for medium mass nuclides measurement in some AMS labs [10,12]. We have tried the GFM technique with the Q3D magnetic spectrometer, but found it unsuitable according to our experimental result. The reason is that the magnetic field of the Q3D is arranged such that more than half the ion path in the gas-filled region is not under the influence of the magnetic field. The beam is therefore broadened due to multiple scattering, and no isobar separation takes place. The characteristic high dispersion of the Q3D has only a minor influence on the resolution of isobars. In order to take full advantage of the high dispersion, the DE-Q3D technique was developed. The basic principle of the DE-Q3D technique is described as follows. A membrane with a suitable thickness is used as absorber placed at the entrance of the Q3D magnetic spectrometer. When isobars with the same energy pass through the absorber, their energy losses are different. The Q3D magnetic spectrometer is used to separate isobars due to their different residual energies. Because of the charge state redistribution after ions passing through the absorber, only one charge state with the highest stripping probability for the interested nuclide is selected and focused onto a designated Please cite this article in press as: X. Wang et al., Measurement of http://dx.doi.org/10.1016/j.nimb.2015.03.057

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5. Measurement of

4. Sample preparation The 59Ni and 63Ni samples were produced by neutron-induced Ni(n,c)59Ni and 62Ni(n,c)63Ni reactions. A high-purity natural Ni foil together with Zr and Fe foils was irradiated with well-thermalized neutron flux from a reactor at CIAE. The thermal and epithermal neutron fluxes were calculated by measuring the c-rays emitted by 95Zr, 97Zr, and 59Fe with a HPGe detector and using the well-known thermal neutron capture cross-sections and epithermal neutron resonance integrals of 94Zr(n,c)95Zr, 96 Zr(n,c)97Zr and 58Fe(n,c)59Fe reactions [17,18]. The 59Ni/Ni and 63 Ni/Ni ratios of the irradiated sample were calculated from the obtained neutron fluencies and the thermal neutron capture cross-sections of 58Ni(n,c)59Ni and 62Ni(n,c)63Ni [11,19]. The 59 Ni/Ni and 63Ni/Ni of the irradiated sample were determined to be (2.06 ± 0.09)  107 and (3.72 ± 0.17)  108, respectively. The sample was then used to prepare a series of laboratory reference samples by the following procedures. First, the irradiated sample was dissolved with 9 N HCl. Subsequently, a series of reference solutions with different ratios of 59Ni/Ni and 63Ni/Ni were prepared by stepwise dilution with the NiCl2 solution with an accurately known concentration. Each reference solution was then transferred onto an AG1 anion exchanger column for removing Co and Cu. After passing the column, the eluate was collected and ammonia was added. Nickel was precipitated with the addition of dimethylglyoxime. This complex was centrifuged and ashed at 600 °C to produce NiO powder, which served as target material for the AMS measurement. A series of standard samples were prepared with 59Ni/Ni and 63Ni/Ni ratios between 108 and 1012. An unirradiated high-purity natural Ni foil that has gone through the same chemical procedures as for the reference sample preparation was used as the blank sample. The NiO powder was mixed with the same weight of high purity Ag powder and pressed into target holders used for AMS measurement. In order to reduce the 63Cu and 59Co interferences in the measurements of 59Ni and 63Ni, it is necessary to reduce the 63Cu and 59Co background that originate from the copper and cobalt materials used in the ion source. Copper- and cobalt-free AMS target holders must be used. In this work, high purity electrolytic Aluminum (99.99%) was used as the target holders. Despite of its poor mechanical strength as target holder, the high purity Aluminum target holder proved working satisfactorily by careful operation. 58

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Ni ions were extracted from the sample in the Cs sputter source and pre-accelerated to 95 KeV. After passing through the electrostatic analyzer and the injection magnet, 59Ni ions were selected and injected into the HI-13 Beijing tandem accelerator, meanwhile 58Ni and 60Ni were inward and outward deflected relative to the 59Ni trajectory, and two movable offset Faraday cups at image points of the magnet were used to record the beam currents of 58Ni and 60Ni. The terminal voltage of the accelerator was set at 11.5 MV. The ions were accelerated to 11.5 MeV at the terminal, where they passed through the carbon stripper foil to produce Ni ions with positive charge states and were further accelerated. 59Ni ions with charge state of 12+ were selected by the 90° double focusing analyzing magnet. After passing the switching magnet, 59Ni12+ ions with energy of 149.5 MeV were transported to the second AMS beam line and entered the Q3D magnetic spectrometer. A Si3N4 foil with a thickness of 5 lm (a stack of three, each 1 lm thick) was used as an absorber at the entrance of the Q3D magnetic spectrometer. 59Ni19+ ions with a stripping probability of 20% were analyzed by the Q3D magnetic spectrometer. According to the energy loss calculation of 59Ni and 59Co in the absorber by the SRIM-2000 program [20], 59Ni19+ and 59Co19+ ions with respective energies of 99.4 MeV and 101.5 MeV after passing through the absorber were separated on the focal plane of Q3D, the four-anodes gas ionization chamber mounted at the center of the focal plane was used for 59Ni determination. In order to optimize the magnetic field of the Q3D, the distributions of 59Ni and 59Co as functions of magnetic field of Q3D were determined with the gas ionization chamber by using a sample of 59Ni/Ni = (8.53 ± 0.51)  109. The results are shown in Fig. 3. The optimum magnetic field for 59Ni is 0.4835T which is 1.14% lower than that of 59Co, this means that the peak distance between 59Co and 59Ni on the focal plane is 130 mm. This value coincides well with the value of SBD measurement. When the magnetic field of the Q3D was set to 0.4835T, 75% of 59Ni19+ ions can be detected in the gas ionization chamber, while most of 59Co ions cannot enter the gas ionization chamber. A suppression factor of more than 1000 for 59Co has been obtained. By the DE-Q3D technique alone the separation between 59Co and 59Ni is not sufficient due to straggling and scattering effects. The 59Ni and remaining 59Co were further identified with the four-anodes gas ionization chamber. Five signals, i.e. four energy loss signals (E1, E2, E3, E4) from the anodes and a total energy signal (Et) from the cathode, were used. Fig. 4 shows the plot of energy

Fig. 2. After passing through 5 lm SiN absorber with initial energy of 149.5 MeV, the spatial distributions of 59Ni19+ and 59Co19+ on the focal plane of the Q3D magnetic spectrometer. 60Ni ions with the same magnetic rigidity as that for 59Ni ions were used for simulating 59Ni ions.

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Fig. 3. After passing through 5 lm SiN absorber with initial energy of 149.5 MeV, the spatial distributions of 59Ni19+ and 59Co19+ as functions of magnetic field of the Q3D. 63

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loss of E1 verses E4 for a standard sample with 59Ni/Ni ratio of (3.53 ± 0.20)  1010. It is shown that the 59Ni and 59Co are clearly identified. The spectrum for the blank sample is shown in Fig. 5. A multi-parameter data acquisition and analysis system was used to extract 59Ni counts with appropriate gates on these five signals, meanwhile the background was greatly removed. A suppression factor of 3  104 for 59Co can be achieved with the gas detector. In order to determine the background level for 59Ni measurement, the standard sample and blank sample were measured alternately. During the 59Ni measurement with the four-anodes gas ionization chamber, the beam current of 58Ni was measured simultaneously in the offset Faraday cups at image points of the injection magnet. After normalizing with the standard sample, the value of 59Ni/ Ni = 1.0  1013 for the blank sample was obtained.

6. Measurement of

63

Fig. 5. A plot of energy loss E1 versus E4 for a blank sample.

Ni

Similar to 59Ni measurement, Ni ions were extracted from ion source. After passing through the electrostatic analyzer and injection magnet, 63Ni ions were injected into the tandem accelerator which was set to a terminal voltage of 11.5 MV. Then ions with charge state of 12+ were selected with the high energy analyzing magnet. After passing the switching magnet, 63Ni12+ ions with energy of 149.5 MeV were transported to the DE-Q3D detection system. After passing through the Si3N4 foil with the thickness of 5 lm, ions with charge states of 18+ were analyzed by the Q3D magnetic spectrometer. According to the energy loss calculation of 63Ni and 63Cu in the absorber by the SRIM-2000 program, 63 Ni18+ and 63Cu18+ ions with respective energies of 98.8 MeV and 96.8 MeV were separated on the focal plane of Q3D. The distributions of 63Ni and 63Cu as functions of magnetic field of Q3D were measured with the gas ionization chamber by scanning the magnetic field of the Q3D using a sample of 63Ni/ Ni = (1.54 ± 0.09)  109. As shown in Fig. 6, the optimum magnetic field for 63Ni is about 1.1% higher than that of 63Cu, which means the distance between of 63Ni and 63Cu peaks on the focal plane is about 125 mm. The results also show that the 63Cu interference is very high. The count rate of 63Cu is about 50 times higher than that of 59Co. The main source for the copper ions appears to be the sample and target holder, but it requires further studies. After setting the optimum Q3D magnetic field for 63Ni measurement with the gas ionization chamber, about 103 of 63Cu ions can enter the gas ionization chamber. 63Ni and 63Cu were thus further identified with the four-anodes gas ionization chamber. The five signals, which were used for 59Ni measurement, were also used to identify 63Cu and 63Ni. Fig. 7 shows the plot of energy loss of

Fig. 6. After passing through 5 lm SiN absorber with initial energy of 149.5 MeV, the spatial distributions of 63Ni18+ and 63Cu18+ as functions of magnetic field of the Q3D.

Fig. 7. A plot of total energy Et versus energy loss E4 for a sample with 63Ni/Ni ratio of 1.5  109.

Et verses E4 for the laboratory reference sample with 63Ni/Ni ratio of (1.54 ± 0.09)  109. The spectrum for a blank sample is shown in Fig. 8. With proper coincidence of the five signals the background of 63Cu can be greatly reduced and the genuine 63Ni counts extracted. Due to the very strong 63Cu interference, the detection sensitivity is limited at present to be 63Ni/Ni = 2  1012.

Fig. 4. A plot of energy loss E1 versus E4 for a laboratory reference sample with 59Ni/ Ni ratio of 3.53  1010.

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Fig. 8. A plot of total energy Et versus energy loss E4 for a blank sample.

7. Conclusion A combination of large accelerator, DE-Q3D system and fouranodes gas ionization chamber was used to measure 59Ni and 63 Ni. The developed method based on the DE-Q3D system is effective for removing isobaric interferences in the measurements of 59 Ni and 63Ni. The abundance sensitivities of 59Ni and 63Ni measurements are 1  1013 and 2  1012, respectively. Methods for reducing Cu contents in samples and target holders are being studied to further improve the AMS measurement sensitivity of 63Ni. Applications of the 59Ni and 63Ni AMS measurements are being explored [21]. Acknowledgements This work was financially supported in part by National Natural Science Foundation of China under grant numbers 11175266, 11375272 and 11265005.

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