- Email: [email protected]
Measurement of 182Hf with HI-13 AMS system
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 259 (2007) 246–249 www.elsevier.com/locate/nimb
Measurement of Jiuzi Qiu
a,b
182
Hf with HI-13 AMS system
, Shan Jiang a,*, Ming He a, Xinyi Yin a, Kejun Dong a, Yongjing Guan c, Yiwen Bao a, Shaoyong Wu a, Jian Yuan a, Bingfan Yang a a
c
Department of Nuclear Physics, China Institute of Atomic Energy, Beijing, 102413, China b Chinese People’s Armed Police Force Academy, Langfang, Hebei, 065000, China The College of Physics Science and Engineering Technology, Guangxi University, Nanning 530004, China Available online 8 February 2007
Abstract 182
Hf with half-life of about (8.90 ± 0.09) Ma is an extinct radionuclide and can only be produced by a supernova explosion in nature. Hf is one of a few radionuclides in the million-year half-life range for tracing a possible supernova event in the vicinity of the Earth within the last 100 million years. This may be accomplished by finding measurable traces of live 182Hf in suitable terrestrial archives. With accelerator mass spectrometry (AMS), an ultra-sensitive nuclear analytical technique, it is possible to detect minute amounts of 182Hf. The detection method of 182Hf with HI-13 AMS system at China Institute of Atomic Energy (CIAE) and the chemical procedures to reduce 182W interference are presented. 2007 Elsevier B.V. All rights reserved. 182
PACS: 07.75; 26.30 Keywords:
182
Hf; Accelerator mass spectrometry AMS; Supernova; Isotope ratio
1. Introduction 182
Hf is a long-lived radionuclide of particular interest in the study of supernova explosion events. 182Hf is believed to be produced by r-process nucleosynthesis, but it can also be produced by a fast s-process in massive stars [1]. During a supernova explosion, a certain amount of 182Hf could be injected into the surrounding interstellar medium (ISM). If such an event took place in the vicinity of the Earth within a few half-lives of 182Hf, a signal should be detectable in appropriate archives. The fact that primordial 182Hf had already decayed, together with supernova as the only known production source in nature, makes 182Hf an ideal candidate as an indicator of a possible supernova explosion in the vicinity of the Earth within the last 100 million years. Recently, an indication for a nearby supernova explosion
*
Corresponding author. Tel.: +86 10 69358335; fax: +86 10 69357787. E-mail address: [email protected] (S. Jiang).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.228
has been found through the detection of 60Fe (t1/2 = 1.6 Ma) in terror-manganese crusts [2]. But more measurements are needed. One advantage of 182Hf compared to 60 Fe is the possibility to detect signals from older supernova events because of its longer half-life. In any production scenario, live 182Hf is expected to be present in the ISM as a result of recent nucleosynthesis. Gamma-ray detection of 182Hf is not feasible due to its overall low activity. However, the deposition of ISM grains by accretion onto Earth could make direct detection of live 182 Hf possible in slow-accumulating reservoirs such as deep-sea sediments. With accelerator mass spectrometry it is possible to detect minute amounts of 182Hf. 182Hf detection by AMS was first presented by Christof Vockenhuber [3] at the Vienna Environmental Research Accelerator (VERA), a dedicated AMS facility based on a 3-MV tandem accelerator. Vockenhuber tried to direct 182Hf in deep-sea sediment samples, but failed to obtain satisfactory results due mainly to the insufficient sensitivity and the interference from the isobaric nuclide 182W.
J. Qiu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 246–249
In this paper, a method for the detection of 182Hf with a 13-MV tandem accelerator (HI-13) mass spectrometer and the chemical procedures to reduce W content are described.
247
was prepared using non-irradiated enriched HfO2 with the same chemical procedures as for the standard sample. 2.2. Column separation procedure
2. Experimental 182
The two prerequisites for AMS measurement of Hf are high mass resolution to reduce the interference from the stable neighboring isotopes, mainly 180Hf and isobar separation to reduce the interference from the stable isobar 182 W. The AMS facility of CIAE could satisfy mass resolution for 182Hf measurement by narrowing the image slits of injection magnet and analyzing magnet. If the width of the slit is reduced to 2 mm, the mass resolution of our AMS facility can be increased to 220, whereas the transmission is still above 80%. However the energy of less than 100 MeV available at the AMS facility of CIAE can not separate the stable isobar 182W from 182Hf in the final detector system. According to Vockenhuber [3], a 182W suppression of about 6000 can be achieved by using sample material of HfF4 and extracting negative ions of HfF5 from the ion source. Although the main interference to detection can be significantly reduced by using HfF4 sample material and extracting HfF5 beam from ion source, we found that chemical separation is still necessary. 2.1. Preparation of samples In this experiment, 182Hf was produced by irradiating 50-mg HfO2, enriched in 180Hf to 98.3%, with the high neutron flux of the heavy water research reactor at CIAE for eighteen days in December 2002. The reactor neutron flux is about 4.54 · 1013 n cm 2 s 1 at the sample irradiation site. In the reactor, 180Hf may capture a neutron to produce 181 Hf, and the produced 181Hf may capture a second neutron to produce 182Hf. After a cooling time of 920 days, the sample was purified with chemical procedures to reduce W, the ratio of 182Hf/180Hf was (1.628 ± 0.011) · 10 6 determined with a thermal ionization mass spectrometry (TIMS). Standard samples with 182Hf/180Hf ratios of (3.03 ± 0.03) · 10 8 and (3.00 ± 0.03) · 10 10 were prepared using a series dilution of the irradiated sample with non-irradiated enriched HfO2 powder. Meanwhile the 182 W/183W ratio in samples measured with TIMS was 1.78. Approximately 10 mg of the HfO2 standard sample material was dissolved in a 5-ml 40% HF and 5-ml 63% HNO3 mixed solution. The solution was heated on a hot plate, and evaporated to about 2ml, another 5-ml 40% HF and 5-ml 63% HNO3 was added and evaporated to approximately 1ml, then 2-ml 40% HF and 2-ml 63% HNO3 was added and evaporated to near dryness. After that, 2-ml 40% HF was added to dissolve the residue and was then evaporated to dryness. Finally the sample was roasted in oven for 2 h at 120 C to obtain desiccated HfF4 powder. The blank sample material of HfF4 powder
The HfF4 samples prepared above were respectively redissolved in 10-ml, 1-M HF solution for column separation. A 1-ml sample solution was loaded onto an anion exchange column. The column was rinsed with 10 ml of 1-M HF. Hf was then eluted by 30 ml of 0.01-M HF 9-M HCl, while W and Ta retained on the column. Tracer experiments showed that the average chemical yield of Hf was greater than 95%, and the decontamination factors for W and Ta were larger than 1000. The Hf sample purified with this procedure was transformed to HfF4 powder again and it was satisfactory for eliminating isobaric interferences for AMS determination of 182Hf. 2.3. Measurement of sputter and ionization yield Sample material of HfF4 was mixed with 1:1 w/w silver powder and pressed firmly into Al-target holders of the 40 position MC-SNICS source. The silver powder was served as both an electrical and thermal conductor. The Hf isotopes of interest were sputtered by Cs+ as negatively charged HfF5 and extracted with about 15 kV from the ion source. On the low-energy side, the beam was analyzed by means of a 90 magnetic deflector. The sputter and ionization yield for HfF5 ions was measured to be about 3.4 · 10 3 with a target of known sample mass. The typical HfF5 beam current was about 150 nA. The current for the whole lifetime of the target was collected and the amount of extracted 180 HfF5 ions was calculated. 2.4. Simulation transport of
182
Hf beam
The measurement of 182Hf was performed with a 13-MV tandem accelerator (HI-13) mass spectrometer at CIAE [4]. On the high-energy side, the beam was analyzed by means of a 90 analyzing magnet with a mass-energy product of 200-MeV amu and a 17 electrostatic deflector. The value of terminal voltage was dictated by the maximum mass-energy product of the high-energy beam-transport system. For 182Hf9+ ions, the maximum usable terminal voltage was 8.5 MV, which corresponds to a final energy of 82.1 MeV. In the terminal of the HI-13 tandem accelerator, a carbon foil of 3 lg cm 2 thickness was used as a stripper. At the high-energy side, 182Hf9+ ions were analyzed. At the beginning of the research, the 182Hf beam transport was simulated with sample material of 180HfF4 and extracting ions of 180 HfF5 . Due to the significant scattering induced by the carbon foil and Coulomb explosion,the beam current in high-energy side was too small to tune for beam transport. In order to make the adjustment of beam transport easier and maximize 180Hf9+ current for
248
J. Qiu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 246–249
analysis, the simulation transportation was divided into three steps. First, the sample material of 180HfO2, instead of 180HfF4, was used and 180 HfO2 ions were extracted from ion source to simulate 180Hf9+ beam transport of sample material of 180HfF4 and 180 HfF5 ions, because the 180 HfO2 current is much larger than 180 HfF5 at low-energy side. In the simulation transport, 180Hf9+ ions had the same energy with the 182Hf 9+ ions, and the parameters of ions optics system were tuned for the optimum state of beam transport. Secondly, sample material of 180HfF4 was used and 180 HfF5 ions extracted to transport 180Hf9+ beam with the same energy as 182Hf9+ beam. The parameters of the ion optics system were finely tuned for the optimum state of 180Hf beam transport. Thirdly, sample material of 180 HfF4 was used and 180 HfF5 ions extracted to simulate 182 Hf beam transport of sample material of 182HfF4 and 182 HfF5 ions. In the third step, 180Hf9+ ions had the same momentum as 182Hf 9+ ions, and the parameters of electronic and magnetic elements after analyzing magnet were further tuned for optimal statue of 180Hf9+ beam transport. The experiment showed that the three-step method for the optimization of beam simulation transport makes the adjustment much easier. The transmission efficiency from low-energy side Faraday-cup to detector was measured to be about 5.0 · 10 3 for 180HfF4 sample and 180Hf9+ ions.
Fig. 1. Two-dimension spectrum of
182
Hf.
3. Results The energy and time of flight two-dimension spectra of Hf (shown in Fig. 1) and 183W were obtained, respectively. The interference from 182W was corrected by using the measured 183W/180Hf ratio and the 182W/183W ratio of 1.78 for the samples. The detection limit for the 182 Hf/180Hf ratio was 2.2 · 10 10. 182
4. Summary 2.5. Detector system 182
The Hf ions were detected by a gold-silicon surface barrier detector (SBD) for energy determination, and a time of flight (TOF) detector for isotope identification. Flight length of the TOF detector was 2 m. The energy detector with energy resolution of about 4% was used to separate the nuclides having large mass difference and different charge states with 182Hf9+. The TOF detector was used to distinguish the adjacent nuclides with the same charge state, such as Hf isotopes, which have a small difference in energy but a large difference in time of flight with 182 Hf. The start detector of TOF was a micro-channel plate (MCP) detector with a carbon foil of 10 lg cm 2 thickness, and the stop detector was a SBD (meanwhile used as energy detector). The time resolution of TOF detector was better than 1.0 ns, and the difference of time of flight between 180Hf and 182Hf was about 2.4 ns. Therefore, the TOF detector was sufficient to distinguish 182Hf from 180 Hf. The detector efficiency was measured to be about 60%. The overall efficiency for 182Hf measurement with our setup was about 8.5 · 10 6. 2.6. Correction for
182
W interference
The stable isobar 182W is the main interference for 182Hf detection. In the experiment, the stable Tungsten isotope of 183 W was measured for the accurate subtraction of the 182 W contribution to the mass 182 events with the known 182 W/183W isotope ratio of 1.78 measured by TIMS.
The detection limit of 182Hf depends on the W content in the sample material and the background of neighboring isotope, mainly 180Hf. Chemical separation is a direct method to effectively reduce W isotopes from sample material. In addition, if we know the 182W/183W ratio in the samples, we can correct for the interference from the remaining 182W by measuring the 183W. In the AMS measurement of 182Hf, the main difficulty is the interference from the intense beam of 180Hf. Although most of the 180 Hf is suppressed by the injection and analyzing magnets, a small fraction of this intense beam can still interfere with the 182Hf measurement. The main reason for the leakage of interfering ions is the charge changing processes induced by the residual gas within the relevant optical elements. Angular scattering on the residual gas, electrodes, slits or vacuum chamber walls can also allow the background to pass a filter. However, the scattering cross-section is in the order of 10 20b whereas the cross section of charge changing is 10 16–10 15b [5]. However, in the TOF detector, the angular scattering on the carbon is serious. The leakage can be reduced by simply adding more filter elements and choosing the thinner carbon foils, such as 4 lg cm 2. Each additional filter element can provide leakage suppression for several orders of magnitude. A new ion injector with mass resolution of larger than 380 is being constructed, and the thinner carbon foils from ACF-Metals Arizona Carbon Foil Co., Inc. are expected to be mounted in our AMS at CIAE for suppressing the interference from neighboring isotopes .
J. Qiu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 246–249
Acknowledgements We acknowledge Weizhi Tian from CIAE for his helpful discussion, Chunhua Zhang and Jinsong Zhang from NPIC for their help in sample preparation. References [1] B.S. Meyer, D.D. Clayton, Space Sci. Rev. 92 (2000) 133.
249
[2] K. Knie, G. Korschinek, T. Faestermann, et al., Phys. Rev. Lett. 83 (1999) 18. [3] Christof Vockenhuber, Max Bichler, Robin Golser, et al., Nucl. Instr. and Meth. B 223–224 (2004) 823. [4] Jiang Shan, He Ming, Jiang Songsheng, et al., Nucl. Instr. and Meth. I B 172 (2000) 87. [5] H.-D. Betz, Rev. Mod. Phys. 44 (1972) 465.
Nuclear Instruments and Methods in Physics Research B 259 (2007) 246–249 www.elsevier.com/locate/nimb
Measurement of Jiuzi Qiu
a,b
182
Hf with HI-13 AMS system
, Shan Jiang a,*, Ming He a, Xinyi Yin a, Kejun Dong a, Yongjing Guan c, Yiwen Bao a, Shaoyong Wu a, Jian Yuan a, Bingfan Yang a a
c
Department of Nuclear Physics, China Institute of Atomic Energy, Beijing, 102413, China b Chinese People’s Armed Police Force Academy, Langfang, Hebei, 065000, China The College of Physics Science and Engineering Technology, Guangxi University, Nanning 530004, China Available online 8 February 2007
Abstract 182
Hf with half-life of about (8.90 ± 0.09) Ma is an extinct radionuclide and can only be produced by a supernova explosion in nature. Hf is one of a few radionuclides in the million-year half-life range for tracing a possible supernova event in the vicinity of the Earth within the last 100 million years. This may be accomplished by finding measurable traces of live 182Hf in suitable terrestrial archives. With accelerator mass spectrometry (AMS), an ultra-sensitive nuclear analytical technique, it is possible to detect minute amounts of 182Hf. The detection method of 182Hf with HI-13 AMS system at China Institute of Atomic Energy (CIAE) and the chemical procedures to reduce 182W interference are presented. 2007 Elsevier B.V. All rights reserved. 182
PACS: 07.75; 26.30 Keywords:
182
Hf; Accelerator mass spectrometry AMS; Supernova; Isotope ratio
1. Introduction 182
Hf is a long-lived radionuclide of particular interest in the study of supernova explosion events. 182Hf is believed to be produced by r-process nucleosynthesis, but it can also be produced by a fast s-process in massive stars [1]. During a supernova explosion, a certain amount of 182Hf could be injected into the surrounding interstellar medium (ISM). If such an event took place in the vicinity of the Earth within a few half-lives of 182Hf, a signal should be detectable in appropriate archives. The fact that primordial 182Hf had already decayed, together with supernova as the only known production source in nature, makes 182Hf an ideal candidate as an indicator of a possible supernova explosion in the vicinity of the Earth within the last 100 million years. Recently, an indication for a nearby supernova explosion
*
Corresponding author. Tel.: +86 10 69358335; fax: +86 10 69357787. E-mail address: [email protected] (S. Jiang).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.228
has been found through the detection of 60Fe (t1/2 = 1.6 Ma) in terror-manganese crusts [2]. But more measurements are needed. One advantage of 182Hf compared to 60 Fe is the possibility to detect signals from older supernova events because of its longer half-life. In any production scenario, live 182Hf is expected to be present in the ISM as a result of recent nucleosynthesis. Gamma-ray detection of 182Hf is not feasible due to its overall low activity. However, the deposition of ISM grains by accretion onto Earth could make direct detection of live 182 Hf possible in slow-accumulating reservoirs such as deep-sea sediments. With accelerator mass spectrometry it is possible to detect minute amounts of 182Hf. 182Hf detection by AMS was first presented by Christof Vockenhuber [3] at the Vienna Environmental Research Accelerator (VERA), a dedicated AMS facility based on a 3-MV tandem accelerator. Vockenhuber tried to direct 182Hf in deep-sea sediment samples, but failed to obtain satisfactory results due mainly to the insufficient sensitivity and the interference from the isobaric nuclide 182W.
J. Qiu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 246–249
In this paper, a method for the detection of 182Hf with a 13-MV tandem accelerator (HI-13) mass spectrometer and the chemical procedures to reduce W content are described.
247
was prepared using non-irradiated enriched HfO2 with the same chemical procedures as for the standard sample. 2.2. Column separation procedure
2. Experimental 182
The two prerequisites for AMS measurement of Hf are high mass resolution to reduce the interference from the stable neighboring isotopes, mainly 180Hf and isobar separation to reduce the interference from the stable isobar 182 W. The AMS facility of CIAE could satisfy mass resolution for 182Hf measurement by narrowing the image slits of injection magnet and analyzing magnet. If the width of the slit is reduced to 2 mm, the mass resolution of our AMS facility can be increased to 220, whereas the transmission is still above 80%. However the energy of less than 100 MeV available at the AMS facility of CIAE can not separate the stable isobar 182W from 182Hf in the final detector system. According to Vockenhuber [3], a 182W suppression of about 6000 can be achieved by using sample material of HfF4 and extracting negative ions of HfF5 from the ion source. Although the main interference to detection can be significantly reduced by using HfF4 sample material and extracting HfF5 beam from ion source, we found that chemical separation is still necessary. 2.1. Preparation of samples In this experiment, 182Hf was produced by irradiating 50-mg HfO2, enriched in 180Hf to 98.3%, with the high neutron flux of the heavy water research reactor at CIAE for eighteen days in December 2002. The reactor neutron flux is about 4.54 · 1013 n cm 2 s 1 at the sample irradiation site. In the reactor, 180Hf may capture a neutron to produce 181 Hf, and the produced 181Hf may capture a second neutron to produce 182Hf. After a cooling time of 920 days, the sample was purified with chemical procedures to reduce W, the ratio of 182Hf/180Hf was (1.628 ± 0.011) · 10 6 determined with a thermal ionization mass spectrometry (TIMS). Standard samples with 182Hf/180Hf ratios of (3.03 ± 0.03) · 10 8 and (3.00 ± 0.03) · 10 10 were prepared using a series dilution of the irradiated sample with non-irradiated enriched HfO2 powder. Meanwhile the 182 W/183W ratio in samples measured with TIMS was 1.78. Approximately 10 mg of the HfO2 standard sample material was dissolved in a 5-ml 40% HF and 5-ml 63% HNO3 mixed solution. The solution was heated on a hot plate, and evaporated to about 2ml, another 5-ml 40% HF and 5-ml 63% HNO3 was added and evaporated to approximately 1ml, then 2-ml 40% HF and 2-ml 63% HNO3 was added and evaporated to near dryness. After that, 2-ml 40% HF was added to dissolve the residue and was then evaporated to dryness. Finally the sample was roasted in oven for 2 h at 120 C to obtain desiccated HfF4 powder. The blank sample material of HfF4 powder
The HfF4 samples prepared above were respectively redissolved in 10-ml, 1-M HF solution for column separation. A 1-ml sample solution was loaded onto an anion exchange column. The column was rinsed with 10 ml of 1-M HF. Hf was then eluted by 30 ml of 0.01-M HF 9-M HCl, while W and Ta retained on the column. Tracer experiments showed that the average chemical yield of Hf was greater than 95%, and the decontamination factors for W and Ta were larger than 1000. The Hf sample purified with this procedure was transformed to HfF4 powder again and it was satisfactory for eliminating isobaric interferences for AMS determination of 182Hf. 2.3. Measurement of sputter and ionization yield Sample material of HfF4 was mixed with 1:1 w/w silver powder and pressed firmly into Al-target holders of the 40 position MC-SNICS source. The silver powder was served as both an electrical and thermal conductor. The Hf isotopes of interest were sputtered by Cs+ as negatively charged HfF5 and extracted with about 15 kV from the ion source. On the low-energy side, the beam was analyzed by means of a 90 magnetic deflector. The sputter and ionization yield for HfF5 ions was measured to be about 3.4 · 10 3 with a target of known sample mass. The typical HfF5 beam current was about 150 nA. The current for the whole lifetime of the target was collected and the amount of extracted 180 HfF5 ions was calculated. 2.4. Simulation transport of
182
Hf beam
The measurement of 182Hf was performed with a 13-MV tandem accelerator (HI-13) mass spectrometer at CIAE [4]. On the high-energy side, the beam was analyzed by means of a 90 analyzing magnet with a mass-energy product of 200-MeV amu and a 17 electrostatic deflector. The value of terminal voltage was dictated by the maximum mass-energy product of the high-energy beam-transport system. For 182Hf9+ ions, the maximum usable terminal voltage was 8.5 MV, which corresponds to a final energy of 82.1 MeV. In the terminal of the HI-13 tandem accelerator, a carbon foil of 3 lg cm 2 thickness was used as a stripper. At the high-energy side, 182Hf9+ ions were analyzed. At the beginning of the research, the 182Hf beam transport was simulated with sample material of 180HfF4 and extracting ions of 180 HfF5 . Due to the significant scattering induced by the carbon foil and Coulomb explosion,the beam current in high-energy side was too small to tune for beam transport. In order to make the adjustment of beam transport easier and maximize 180Hf9+ current for
248
J. Qiu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 246–249
analysis, the simulation transportation was divided into three steps. First, the sample material of 180HfO2, instead of 180HfF4, was used and 180 HfO2 ions were extracted from ion source to simulate 180Hf9+ beam transport of sample material of 180HfF4 and 180 HfF5 ions, because the 180 HfO2 current is much larger than 180 HfF5 at low-energy side. In the simulation transport, 180Hf9+ ions had the same energy with the 182Hf 9+ ions, and the parameters of ions optics system were tuned for the optimum state of beam transport. Secondly, sample material of 180HfF4 was used and 180 HfF5 ions extracted to transport 180Hf9+ beam with the same energy as 182Hf9+ beam. The parameters of the ion optics system were finely tuned for the optimum state of 180Hf beam transport. Thirdly, sample material of 180 HfF4 was used and 180 HfF5 ions extracted to simulate 182 Hf beam transport of sample material of 182HfF4 and 182 HfF5 ions. In the third step, 180Hf9+ ions had the same momentum as 182Hf 9+ ions, and the parameters of electronic and magnetic elements after analyzing magnet were further tuned for optimal statue of 180Hf9+ beam transport. The experiment showed that the three-step method for the optimization of beam simulation transport makes the adjustment much easier. The transmission efficiency from low-energy side Faraday-cup to detector was measured to be about 5.0 · 10 3 for 180HfF4 sample and 180Hf9+ ions.
Fig. 1. Two-dimension spectrum of
182
Hf.
3. Results The energy and time of flight two-dimension spectra of Hf (shown in Fig. 1) and 183W were obtained, respectively. The interference from 182W was corrected by using the measured 183W/180Hf ratio and the 182W/183W ratio of 1.78 for the samples. The detection limit for the 182 Hf/180Hf ratio was 2.2 · 10 10. 182
4. Summary 2.5. Detector system 182
The Hf ions were detected by a gold-silicon surface barrier detector (SBD) for energy determination, and a time of flight (TOF) detector for isotope identification. Flight length of the TOF detector was 2 m. The energy detector with energy resolution of about 4% was used to separate the nuclides having large mass difference and different charge states with 182Hf9+. The TOF detector was used to distinguish the adjacent nuclides with the same charge state, such as Hf isotopes, which have a small difference in energy but a large difference in time of flight with 182 Hf. The start detector of TOF was a micro-channel plate (MCP) detector with a carbon foil of 10 lg cm 2 thickness, and the stop detector was a SBD (meanwhile used as energy detector). The time resolution of TOF detector was better than 1.0 ns, and the difference of time of flight between 180Hf and 182Hf was about 2.4 ns. Therefore, the TOF detector was sufficient to distinguish 182Hf from 180 Hf. The detector efficiency was measured to be about 60%. The overall efficiency for 182Hf measurement with our setup was about 8.5 · 10 6. 2.6. Correction for
182
W interference
The stable isobar 182W is the main interference for 182Hf detection. In the experiment, the stable Tungsten isotope of 183 W was measured for the accurate subtraction of the 182 W contribution to the mass 182 events with the known 182 W/183W isotope ratio of 1.78 measured by TIMS.
The detection limit of 182Hf depends on the W content in the sample material and the background of neighboring isotope, mainly 180Hf. Chemical separation is a direct method to effectively reduce W isotopes from sample material. In addition, if we know the 182W/183W ratio in the samples, we can correct for the interference from the remaining 182W by measuring the 183W. In the AMS measurement of 182Hf, the main difficulty is the interference from the intense beam of 180Hf. Although most of the 180 Hf is suppressed by the injection and analyzing magnets, a small fraction of this intense beam can still interfere with the 182Hf measurement. The main reason for the leakage of interfering ions is the charge changing processes induced by the residual gas within the relevant optical elements. Angular scattering on the residual gas, electrodes, slits or vacuum chamber walls can also allow the background to pass a filter. However, the scattering cross-section is in the order of 10 20b whereas the cross section of charge changing is 10 16–10 15b [5]. However, in the TOF detector, the angular scattering on the carbon is serious. The leakage can be reduced by simply adding more filter elements and choosing the thinner carbon foils, such as 4 lg cm 2. Each additional filter element can provide leakage suppression for several orders of magnitude. A new ion injector with mass resolution of larger than 380 is being constructed, and the thinner carbon foils from ACF-Metals Arizona Carbon Foil Co., Inc. are expected to be mounted in our AMS at CIAE for suppressing the interference from neighboring isotopes .
J. Qiu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 246–249
Acknowledgements We acknowledge Weizhi Tian from CIAE for his helpful discussion, Chunhua Zhang and Jinsong Zhang from NPIC for their help in sample preparation. References [1] B.S. Meyer, D.D. Clayton, Space Sci. Rev. 92 (2000) 133.
249
[2] K. Knie, G. Korschinek, T. Faestermann, et al., Phys. Rev. Lett. 83 (1999) 18. [3] Christof Vockenhuber, Max Bichler, Robin Golser, et al., Nucl. Instr. and Meth. B 223–224 (2004) 823. [4] Jiang Shan, He Ming, Jiang Songsheng, et al., Nucl. Instr. and Meth. I B 172 (2000) 87. [5] H.-D. Betz, Rev. Mod. Phys. 44 (1972) 465.