Efficient formation of negative ions for plutonium AMS

IF 1.4 3区 物理与天体物理 Q3 INSTRUMENTS & INSTRUMENTATION
Michael Hotchkis , Keita Richardson , David Child , Dominik Koll , Anton Wallner , Klaus Wilcken
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Abstract

In an Accelerator Mass Spectrometry (AMS) instrument, the detection efficiency for rare radioisotopes is determined by a combination of ionisation efficiency, charge state yield and beam transmission. Of these, ionisation efficiency remains as the principal limitation, with few instances where efficiency greater than 1 % has been reported. Using the Vega AMS system at ANSTO, we have achieved reproducible ionisation efficiency, for formation of PuO anions, of 3–4 %. However, the achievement of high overall efficiency has come at the cost of operational efficiency, as it can take at least 10 h to consume each sample.
We have performed a series of tests to understand what determines ionisation efficiency for plutonium AMS. In the standard method used at ANSTO, plutonium is dispersed in iron oxide and mixed with niobium as ‘binder’. The overall efficiency for samples run to exhaustion is found to be linearly proportional to the total mass of loaded mixture. We have performed a series of tests investigating the effects of: (i) recess depth of material in the cathode; (ii) use of layered samples; (iii) binder / iron oxide mix; (iv) cathode materials; (v) different binder; (vi) sample surface area. We have also determined the molecular composition of the Pu anions: PuOx of extracted beams for x  = 0 to 3. The results are compared to a sputtering model that has been developed to account for the observed variation in count rates versus time as the sample is consumed. Ionisation efficiency up to 6.5 % has been observed for PuO anions.
钚AMS中负离子的有效形成
在加速器质谱(AMS)仪器中,稀有放射性同位素的检测效率是由电离效率、电荷态产率和光束透射率共同决定的。其中,电离效率仍然是主要的限制,很少有效率大于1%的例子被报道。使用ANSTO的Vega AMS系统,我们已经实现了3 - 4%的可重复电离效率,用于形成PuO阴离子。然而,实现高整体效率是以操作效率为代价的,因为消耗每个样品至少需要10小时。我们进行了一系列测试,以了解是什么决定了钚AMS的电离效率。在ANSTO使用的标准方法中,钚被分散在氧化铁中,并与作为“粘合剂”的铌混合。样品运行至耗尽的总效率与加载混合物的总质量成线性正比。我们进行了一系列试验,研究以下因素的影响:(i)阴极中材料的凹槽深度;(ii)分层样品的使用;(iii)粘结剂/氧化铁混合物;(四)正极材料;(v)不同的粘结剂;(vi)样品表面积。我们还测定了x = 0 ~ 3时所提取光束中Pu阴离子的分子组成:PuOx−。结果与已经开发的溅射模型进行了比较,该模型用于解释样品消耗时计数率随时间的观察变化。PuO阴离子的电离效率可达6.5%。
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来源期刊
CiteScore
2.80
自引率
7.70%
发文量
231
审稿时长
1.9 months
期刊介绍: Section B of Nuclear Instruments and Methods in Physics Research covers all aspects of the interaction of energetic beams with atoms, molecules and aggregate forms of matter. This includes ion beam analysis and ion beam modification of materials as well as basic data of importance for these studies. Topics of general interest include: atomic collisions in solids, particle channelling, all aspects of collision cascades, the modification of materials by energetic beams, ion implantation, irradiation - induced changes in materials, the physics and chemistry of beam interactions and the analysis of materials by all forms of energetic radiation. Modification by ion, laser and electron beams for the study of electronic materials, metals, ceramics, insulators, polymers and other important and new materials systems are included. Related studies, such as the application of ion beam analysis to biological, archaeological and geological samples as well as applications to solve problems in planetary science are also welcome. Energetic beams of interest include atomic and molecular ions, neutrons, positrons and muons, plasmas directed at surfaces, electron and photon beams, including laser treated surfaces and studies of solids by photon radiation from rotating anodes, synchrotrons, etc. In addition, the interaction between various forms of radiation and radiation-induced deposition processes are relevant.
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