Research On Detection Method Of ～(182)Hf With Accelerator Mass Spectrometry

(182)^Hf with a half-life of about (8.90±0.09) Ma is an extinct radionuclide and can onlybe produced by a supernova explosion in nature. Therefore (182)^Hf is an ideal candidatenuclide for the study on possible supernova explosions in the vicinity of the Earth withinthe last 100 million years. In addition, (182)^Hf is a long-lived radionuclide of particularinterest for nuclear engineering. Accurate measurement of ultratrace (182)^Hf is veryimportant for reactor design, and studies on nuclear cosmo-physics and other fields.Detection of (182)^Hf byβorγray measurement is unfeasible due to its overall lowactivity. It should be possible to detect trace amounts of (182)^Hf with accelerator massspectrometry (AMS).In this work, the detection method of (182)^Hf with a 13 MV tandem accelerator (HI-13)mass spectrometry at China Institute of Atomic Energy (CIAE) is presented.The mass resolution of 80 for the current AMS facility at CIAE could not satisfy therequirement for (182)^Hf measurement. Therefore an injector dedicated for AMS with a 90°spherical electrostatic deflection and a 112°double-focusing analyzing magnet is nowbeing built with a designed maximum mass resolution of 430. In this work, themeasurement of (182)^Hf has been carried out on the old injector with some modifications.The widthes of the slits for the injector magnet and analyzing magnet were reduced to 2.0mm and 4.0 mm, respectively. As a result, a mass resolution of about 200 was obtained forthis AMS facility.(182)^Hf was produced through irradiating HfO₂, enriched in (180)^Hf to 98.28%, with aneutron flux of 4.85×10¹³s^-1.cm-2 of the heavy water research reactor at CIAE for eighteendays. After the irradiation, the ratio of (182)^Hf/(180)^Hf measured with a thermal ionization massspectrometry was (1.628±0.011)×10^-6.Standard samples with (182)^Hf/(180)^Hf ratios of (3.221±0.034)×10^-8 and (3.452±0.042)×10^-10 were prepared by series dilution of the irradiated sample with the enriched (180)^Hr.The main interference for (182)^Hf detection is stable isobar(182)^W. (182)^W can not beseparated from (182)^Hf in the final detector system. A (182)^W suppression of about 6000 can beachieved by using sample material of HIF₄ and extracting negative ions of HfFs from ionsource. In order to further suppress (182)^W, chemical separation was conducted before measurement.The samples were dissolved in 40% HF and 68% HNO₃ mixed solution and heated ona hot plate to obtain dry HfF₄ powder respectively. The HfF₄ samples were re-dissolved in1M HF solution for column separation, respectively. One mL sample solution was loadedonto an anion exchange column. The column was rinsed with 10 mL of 1M HF. Hf wasthen 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%,the decontamination factor for W was about 80 and the contents of both W and Ta in theresulting sample were at 10 ppb level. In order to further reduce the amount of ¹⁸²W, largeamount of ¹⁸⁶W (enriched to 96.7%) was added in the sample, the above separationprocedure was then repeated. In this way, the content of ¹⁸²W could be reduced to ppblevel.The sample material of HfF₄ was mixed with silver powder in the proportion of 1:1 byweight, and pressed firmly into Al-target holders of the 40 position MC-SNICS source.The sputter and ionization yield for ¹⁸⁰HfF₅^- ions was about 3.40×10^-3 in the source. Thetypical ¹⁸⁰HfF₅^- beam current was about 150 nA.An accelerator terminal voltage of 8.0 MV was applied, and a carbon foil of 3μg.cm²thick was used as a stripper in the terminal of the HI-13 tandem accelerator. At thehigh-energy side, ¹⁸²Hf⁹⁺ ions were analyzed.The ¹⁸²Hf beam transport was simulated with beams of ¹⁸⁰HfO₂^- and ¹⁸⁰HfF₅^- insuccession. The experiment showed that this sequential simulation method for optimizationof ¹⁸²HfF₅^- beam transportation makes the adjustment much easier than using ¹⁸²HfF₅^-beam only. The transmission efficiency from ion source to detector was about 2.62×10^-3with ¹⁸⁰Hf⁹⁺ beam.On the high-energy side of the AMS facility, the beam was analyzed by a 90°analyzing magnet and a 15°electrostatic deflector. Then the ¹⁸²Hf⁹⁺ ions were detectedby a time of flight (TOF) detection system, and a gold-silicon surface barrier detector(SBD) used as an energy detector (meanwhile as stop detector of TOF detection system).The flight length of the TOF detection system was 2.0m. The energy resolution of SBDwas about 3.8% for 77.4 MeV ¹⁸²Hf. The time resolution of the TOF detector was less than0.6 ns. The detector efficiency measured was about 75%. So the overall efficiency for¹⁸²Hf measurement with the AMS system was about 6.68×10^-6.The energy and TOF two-dimension spectra of ¹⁸²Hf and ¹⁸³W for blank and a series ofstandard samples have been obtained, respectively. The ¹⁸²W contribution to the counts in¹⁸²Hf peak was corrected for by ¹⁸³W normalization. The detection sensitivity of this AMS facility for ¹⁸²Hf/¹⁸³Hf ratio measurement isabout 4.15×10^-11 at present and a sensitivity of 1.0×10^-12 will be achievable when the newdedicated AMS injector is available.