Measuring the Th-229 isomer energy with a microcalorimeter

The nuclear level scheme of the Thorium-229 isotope is expected to feature a long-lived isomer state, 229mTh, extremely close to the nuclear ground state. The currently 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 229 Th 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 229 Th 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 subtracting 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 assumptions 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 229 Th, 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 233U samples at the Institute for Atomic and Subatomic Physics, assist in the measurements 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 feasibility study arXiv:1306.3069, the measurement 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.

The nuclear level scheme of the isotope Thorium-229 features a long-lived isomer state, 229mTh, extremely close to the nuclear ground state. The predicted isomer energy value is 7.8 eV. Probably the lowest nuclear state of all isotopes, this 229Th isomer could be accessible to laser manipulation, creating an exciting link between atomic and nuclear physics. However, the present level of uncertainty of this isomer energy of 0.5eV makes is too large to envision direct laser spectroscopy. To make a significant step forward, we developed a first 2d-array of cutting-edge magnetic micro-calorimeters to resolve the 29.19 keV doublet of 229Th that has a direct decay path into either the ground, or the isomer state. The project was carried out as an international collaboration between the Vienna University of Technology and Heidelberg University. The Vienna team produced and characterized the 233U samples, assisted in the measurements in Heidelberg, and performed data analysis. The Heidelberg (Hd) team developed and micro-fabricated the cryogenic micro-calorimeter and performed the measurements.The working plan got significantly delayed when it turned out that the previous supplier of dc-SQUID magnetometers (PTB) had serious fabrication problems. We hence established a process for dc-SQUID fabrication in Hd and were able to produce sufficient devices for the 32 amplifier chains of the detector. This caused a significant delay and the noise performance of this first generation of devices being about a factor of 10 worse than originally planned. Both problems together lead to the fact that we were not able to reduce the uncertainty of isomer energy during the project period. However, we successfully designed, micro-fabricated and operated the first 2-dimensional array of metallic magnetic micro-calorimeters for the high resolution detection of single x-ray and gamma-ray photons. The maXs-30 detector consists of 88 dense packed x-ray absorbers made of 15 micron thick gold providing a total active area of 4mm4mm and high stopping power for photons with energies up to 30 keV. The instrumental lineshape is well described buy a Gaussian with a FWHM of 7.8eV. This is a worldwide unique combination of properties, ideal for high resolution spectroscopy on 229Th as well as for a multitude of precision experiments in atomic and nuclear physics. At TU Wien, we established procedures to produce and handle 233U in electro-plated as well as liquid form for fast and efficient chemical cleaning of the source material. Careful chemical removal of all daughter products immediately before an experiment will be key to all spectroscopic experiments on 233U aiming for low background and high resolution. As the recent fabrication runs also yielded low noise SQUIDs, we now have all ingredients at hand to perform high resolution spectroscopy on 229Th in summer 2017 to significantly reduce the present uncertainty of the isomer energy --- somewhat delayed, but with the originally proposed high resolution.