Improvement The AMS Method For ¹⁸²Hf

¹⁸²Hf with half-life of(8.90±0.09)Ma is an important chronometers,it has great potential applications in astrophysics and physical geography.By measuring ¹⁸²Hf in sediment that produced in possible supernova event may help to better understand the stellar production mechanism and the development of the galaxy within the last 100 million years.In addition,¹⁸²Hf is a long-lived radionuclide of particular interest for nuclear engineering as hafnium is always used as reactor control rods and neutron flux monitor.It is very important for reactor operation and design by measurement isotope changes of hafnium in control rods.By accuracy measure the long-lived ¹⁸²Hf in surroundings,we can also get some useful information(e.g.neutron flux)in early fusion reaction.For surrounding medium,detection of ¹⁸²Hf through theirβ^- decay orγ-ray is unfeasible due to its overall low activity.As the existence of the stable and common isobar ¹⁸²W and interference of some molecular ions,it is difficult to detect ¹⁸²Hf in surroundings with conventional mass spectrometry(MS)technique.Accelerator Mass Spectrometry(AMS)is an ultra-sensitive nuclear analytical technique started about 30 years ago.It evolved into the most versatile method to measure minute isotope ratios(10^-15)in small samples.AMS is one of the most promising methods to detect minute amounts of ¹⁸²Hf.The most important goal for develop the AMS method of ¹⁸²Hf is put it into application (e.g.measure supernova signal).Furthermore,it not only improving the AMS method for heavy nuclide,but also can provide a dependable analytical method for nuclear diagnosis and supernova explosions study.The previous obtained sensitivity(¹⁸²Hf/¹⁸⁰Hf=2.2×10^-10)can not meet requirement of applications.The work in this thesis followed the previous studies that have carried out at China Institute of Atomic Energy(CIAE).An enhanced detection limit and overall efficiency is expected during development of AMS method for ¹⁸²Hf after the new injector was put into use.Mass resolution and transport efficiency of the new injector were measured in this work.As shown by experiment,mass resolution of the injector can reach 430 when the image point slit of the injector magnet is±1mm while optical and image point of electrostatic analyzer(ESA)is almost completely opened.If image point slit of ESA is±2.5 mm,we can get a mass resolution of 630.Compared with the condition of slits are completely opened up, only losing 5%of beam current.The high mass resolution can accomplish heavy nuclide measurement on AMS.In this work,a "solid-phase" reaction equipment dedicated for AMS has been set up for the first time based on optimized reaction conditions:using NH₄HF₂ and HfO₂ powder instead of commonly used liquid-phase reaction to preparation HfF₄,the reaction lasts for 4 hours at 200℃,then separate excessive NH₄HF₂ for 2 hours at 250℃.F^-/O^- of solid-phase reaction samples are two to three times of that prepared by liquid-phase reaction.Also, negative ion current of ¹⁸⁰HfF₅^- can reach up to 150nA for naturally abundance HfF₄ sample, this is two times larger than preprared by liquid-phase reaction.HfF₄ samples were mixed with equally amounts of silver powder and pressed in a target holder made of aluminum by NEC Co.Target needles made of high purity nickel wire and supporting plates without metal gibbous were used when pressing powder into sample holder. To lower the background level of tungsten,a glove bag filled with argon gas was used during the process of samples pressing.For depress the interference of stable hafnium isotopes,time of flight(TOF)detector that used in previous work was replaced by using injector and analytical magnet with improved mass resolution as well as the ESA with improved energy resolution.In order to enhance the transmission efficiency from low-energy side Faraday cup to detector,the maximum usable terminal voltage 8.5MV were used,¹⁸⁰HfO₂^- were extracted to directly simulate transmission of ¹⁸²Hf⁹⁺by using HfO₂ samples instead of that extracted ¹⁸⁰HfF₅^-from HfF₄.By this method,simulate of ¹⁸²Hf⁹⁺becomes more easily and more convenient.In this paper,HfF₄ samples were prepared by "solid-phase" reaction from HfO₂,which are provided by China Academy of Engineering Physics(CAEP)and have already been decontaminated.At present,results for ¹⁸²Hf/Hf ratio of 4.5×10^-11were obtained,with total efficiency of 2.1×10^-6(under very narrow slits).This result is based on interference from ¹⁸²W was corrected by measuring the other stable tungsten isotopes.Hence,the improved AMS method could apply to detection ¹⁸²Hf in surroundings samples,a measurement of rock samples may soon be possible.