Measuring the Th-229 isomer energy with a microcalorimeter
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Final Report Abstract
The nuclear level scheme of the isotope Thorium-229 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 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 the development of dedicated tunable high power lasers to drive and probe this transition appear like a hard and inefficient task. In order to make a significant step forward, we developed a first 2d-array of cutting-edge magnetic microcalorimeters to resolve the 29.19 keV doublet of 229Th that only has a direct decay path into either the ground, or the isomer state. Resolving this doublet would provide a 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 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 at the Institute for Atomic and Subatomic Physics, assisted in the measurements in Heidelberg, and performed data analysis. The Heidelberg team developed and micro-fabricated the cryogenic micro-calorimeter and performed the measurements. The working plan got significantly confused within the first 12 months, when it turned out that our previous supplier of dc-SQUID magnetometers, a key component of the amplifier chain of magnetic calorimeters, had serious fabrication problems and that he would not be able to support this project. To overcome this problem we established a process for dc-SQUID fabrication in our cleanroom and were able to produce sufficient devices for the 32 amplifier chains of the detector. The cost for the project was 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 also to the fact that we have not been able to reduce the uncertainty of isomer energy with the spectroscopic data taken during the project period. Within this project 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 detector, maXs-30, consists of 8×8 dense packed x-ray absorbers made of 15 micron thick gold providing a total active area of 4mm×4mm 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. All together this is a worldwide unique combination of properties making this dense-packed detector array well suited for high resolution spectroscopy on 229Th as well as for a multitude of precision experiments in atomic and nuclear physics. Pushed forward by the project partners at TU Wien , we established also procedures to produce and handle 233U in electro-plated as well as liquid form allowing for fast and efficient chemical cleaning of the source material before future experiments. Careful chemical removal of all daughter products immediately before an experiment will be a 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 spring 2017 to significantly reduce the present uncertainty of the isomer energy - somewhat delayed, but with the originally proposed high resolution.
Publications
- Direct-current superconducting quantum interference devices for the readout of metallic magnetic calorimeters, Supercond. Sci. Technol. 28, 045008 (2015)
S. Kempf, A. Ferring, A. Fleischmann, C. Enss
(See online at https://doi.org/10.1088/0953-2048/28/4/045008) - Sie messen, was sie heiß macht, Physik Journal 5, 27 (2016)
A. Fleischmann, L. Gastaldo, S. Kempf, C. Enss
