Offline investigation of thermodynamic and chemical processes in targets for radioactive beam production

用高能质子束辐照同位素分离靶以制备稀有同位素束（RIB）是ISAC/TRIUMF建立的一种行之有效的方法。亚原子物理学研究中一些最具挑战性的项目需要具有极端数量的质子和中子的外来元素，以深入了解物质的稳定性和元素的形成。为了提供非常短的RIB的前沿研究，开发更好的靶标以适应它们的有效释放是必不可少的。即使有了最近的重大改进，如复合陶瓷碳化铀靶材的发展，它变得越来越难以适应高规格的RIB实验的要求，例如非常富中子的Ca和Be束。虽然对靶内产率所能做的相对较少，但通过将热力学和化学过程转向所需方向，可以显著减少同位素向离子源的渗出时间，从而增加产率，特别是短寿命同位素的产率。然而，在工作温度高达2300°C的ISOL目标中，对这些过程的理解是有限的。由质子束引起的核反应导致元素周期表上的每一种元素都比目标材料本身轻。每种元素的渗出过程都是独特的，取决于其挥发性、温度、化学反应和化学计量比。一些初步的在线调查，例如，用CF4将氟化物的形成引导到所需的方向，是有希望的，但由于在线测试时间有限，不是决定性的。我提出了一个研究项目，在实际操作条件下（温度，压力等）离线调查热力学和化学过程，在那里它们可以更准确地控制，并且许多样品的处理不受稀缺和昂贵的光束时间的限制。实验将使用基于商用四极杆质谱仪（QMS）的台式仪器进行，该仪器具有电离器单元和高温真空反应柱，该反应柱作为在线目标的简化模型。固体样品，挥发物和/或反应气体将通过快速进样引入色谱柱，以研究挥发和保留时间。这种设备的开发将把我的专业知识与原子束源和质谱结合起来，以及在TRIUMF的高温炉和目标制造方面的专业知识，例如钽的电子束焊接。这种高温炉质谱计（HTF-MS）为物理或/和化学专业的研究生提供了研究各种有趣问题的机会，旨在从ISOL目标中挥发折射物质。特别是富中子的Ca和Be同位素的挥发具有很高的优先权。它们最好以足够的速率从锕系氧化物或碳化物靶中生产，缺点是Be或Ca作为相对不移动的氧化物或碳化物保留在靶基体中。挑战在于研究Ca和Be作为氟化物快速挥发的最佳条件。此外，许多其他分子也将被研究。

The irradiation of ISOL (isotope separation on-line) targets with high-energy proton beams for the production of rare isotope beams (RIB) is a well-established method at ISAC/TRIUMF. Some of the most challenging projects in subatomic physics research are in need of exotic elements with extreme numbers of protons and neutrons to gain insight in the stability of matter and the formation of the elements. In order to provide cutting-edge research with very short-lived RIB, the development of better targets to accommodate their efficient release is essential.Even with recent major improvements, such as the development of composite ceramic uranium carbide targets, it becomes increasingly difficult to accommodate requests of high-profile experiments for RIB, e.g. very neutron-rich Ca and Be beams. While relatively little can be done about in-target production rates, the effusion times of isotopes to the ion source could be decreased significantly by steering thermodynamic and chemical processes into the desired direction, thus, increasing the yields especially of short-lived isotopes. However, the understanding of these processes in ISOL targets with operating temperatures as high as 2300° C, is limited.The nuclear reactions induced by the proton beam lead to the production of every element on the periodic table lighter than the target material itself. The effusion process is unique for every element and depends on its volatility, temperature, chemical reactions and stoichiometric ratios. Some initial on-line investigations, for example, steering the formation of fluorides into the desired direction with CF4, were promising, but not conclusive due to the limited availability of online testing time.I propose a research project to investigate thermodynamic and chemical processes offline under realistic operating conditions (temperature, pressure, etc.) where they can be controlled much more accurately and where the processing of many samples is not limited by scarce and expensive beam time. The experiments will be performed with a table-top apparatus based on a commercial quadrupole mass spectrometer (QMS) with an ionizer unit and a high-temperature vacuum reaction column which serves as a simplified model for an online target. Solid samples, volatiles and/or reaction gases will be introduced into the column via a fast injection for the investigation of volatilization and retention times. The development of such an apparatus will combine my expertise with atomic beam sources and mass spectrometry and the know-how at TRIUMF on high-temperature furnaces and target fabrication, e. g. e-beam welding of tantalum.Such a high-temperature furnace mass spectrometer (HTF-MS) offers the opportunity for graduate students with a physics or/and chemistry major to work on a variety of interesting questions, aiming at the volatilization of refractive species from an ISOL target. In particular the volatilization of the neutron-rich Ca and Be isotopes has a high priority. They are best produced with sufficient rates from actinide oxide or carbide targets with the disadvantage that Be or Ca are retained within the target matrix as relatively immobile oxides or carbides. The challenge is to investigate the optimal conditions for the fast volatilization of Ca and Be as fluorides. Additionally, a multitude of other molecules will be investigated as well.