Measuring the Th-229 isomer energy with a microcalorimeter

钍-229同位素的核能级方案预计将以一种长寿的异构态为特征，非常接近核基态。目前最被接受的异构体能量值为7.8 eV，对应于160 nm的波长。这种Th-229异构体可能是所有同位素中激发态最低的，可以被激光操纵，在原子和核物理之间建立了令人兴奋的联系。然而，目前还没有明确的证据证明这种状态的存在，确切的异构体能量仍然难以确定。伽马探测器技术的进步引发了测定异构体能量的进展。所有现有的实验数据都依赖于在10-1000keV范围内测量在铀233的阿尔法衰变中激发的Th-229核结构的更高水平。然后，通过将对应于分别进入基态和异构态的衰变路径的伽马能量相减，间接确定EV水平上的异构体能量。测量高能以得出一个小的差异显然会导致很大的误差。此外，衰变路径(带间跃迁)的一些细节还无法解决，因此特定分支比的理论吸收进入了能量确定，因此这是一个严重争议的问题。在这里，我们建议使用最先进的磁微量热计来解析Th229的29.19keV二重态，它只有一条直接的衰变路径进入基态或同分异构态。解决这一双重问题将提供最终的证据，并测量异构体的能量，不涉及进一步的假设和准确，这将使直接激光光谱研究。该项目将作为维也纳科技大学(项目负责人)和海德堡大学之间的国际合作执行。维也纳小组将在原子和亚原子物理研究所生产U-233样品并对其进行表征，协助在海德堡进行测量，并进行数据分析。海德堡团队将提供低温微量热计，针对此处描述的项目对其进行优化，并进行测量。正如项目合作伙伴在联合可行性研究ARXIV：1306.3069中所证明的那样，利用现有的探测器技术可以成功地进行测量，然而，目前正在开发一种更专用的探测器。结合维也纳和海德堡现有的专业知识和设备，只需18个月即可完成测量，几乎不需要额外资源。

The nuclear level scheme of the Thorium-229 isotope is expected to feature a long-lived isomer state, extremely close to the nuclear ground state. The corrently most accepted Isomer energy value is 7.8 eV, corresponding to a wavelength of 160 nm. Probably the lowest excited nuclear state of all isotopes, this Th-229 isomer could be accessible to laser manipulation, creating an exciting link between atomic and nuclear physics. However, there is yet no unambiguous proof of the existence of this state, and the exact isomer energy remains elusive.Progress in determining the isomer energy is triggered by advances in gamma detector technology. All available experimental data relies on measuring higher levels of the Th-229 nuclear structure in the 10-1000 keV regime, excited in the alpha decay of Uranium-233. The isomer energy on the eV level is then determined indirectly by substracting gamma energies corresponding to decay paths into the ground and isomer state respectively. Measuring high energies to derive a small difference obviously leads to large errors. Furthermore, some of the details of the decay paths (interband transitions) could not yet be resolved, so theoretical asumptions for specific branching ratios enter the energy determination, which is hence heavily disputed.Here we propose to use a state-of-the-art magnetic microcalorimeter to resolve the 29.19 keV doublet of Th229, that only has a direct decay path into either the ground, or the isomer state. Resolving this doublet will provide ultimate proof for the existence and measure the isomer energy without involving further assumptions and with an accuracy, that will enable direct laser spectroscopy investigations.The project will be carried out as an international collaboration between the Vienna University of Technology (project leader) and the University of Heidelberg. The Vienna team will produce and characterize the U-233 samples at the Institute for Atomic and Subatomic Physics, assist in the measuremens in Heidelberg, and perform the data analysis. The Heidelberg team will provide the cryogenic microcalorimeter, optimize it for the project described here, and perform the measurement. As the project partners have demonstrated in a joint feasability study arXiv:1306.3069, the measurment can be successful with the already available detector technology, however an even more dedicated detector is currently under development. Combining the expertise and equipment available in Vienna and Heidelberg, the measurement can be performed in only 18 months and very little additional resources.