Radiative Decay Study of the low-lying thorium-229 isomer

Project Information

RaDTho

Grant agreement ID: 101026762

DOI

10.3030/101026762

Project closed

EC signature date 9 March 2021

Start date 1 August 2021

End date 31 July 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 166 320,00

EU contribution

€ 166 320,00

Coordinated by

KATHOLIEKE UNIVERSITEIT LEUVEN
Belgium

Periodic Reporting for period 1 - RaDTho (Radiative Decay Study of the low-lying thorium-229 isomer)

Reporting period: 2021-08-01 to 2023-07-31

Summary of the context and overall objectives of the project

The radionuclide thorium-229 features an extremely low exciation siomer with an energy of around 8 eV. This unique low-lying isomer features an expected radiative lifetime as high as 10000 s. This results in a a very narrow linewidth of this transition. The possibility of a direct exciation of this nuclear transition with a vacuum-ultraviolet (VUV) laser and the robustness of nuclear transitions to the environmental conditions makes this isomer of thorium-229 a possible building block of a new frequency standard - a nuclear clock. Such nuclear clocks could match, or even exceed the precision of the current stat-of-the-art optical clocks and will provide an excillent tool for the test of fundamental physics.

This project aims to characterize this special nuclear isomer and has two main objectives:

1. Radioactive ion beams of mass A=229, consisting of francium and radium, are implanted into a large band-gap solid state crystals such as calcium fluoride and magnesium fluoride at the ISOLDE flacility in CERN. These implanted crystals are placed in a VUV-spectrometer and the potons stemming from the decay of the isomer are wavelength selected giving a precise energy measurement of the isomer. Such a spectrometer enables the measurement of the excitation energy of the isomer by directly detecting the photons stemming from the isomer's radiative decay. Following the VUV-signal over time the radiative livetime of the isomer will be measured.

2. To measure the hyperfine structure of the singly charged thoirum-229 isomer and ground state to extract the magnetic dipole moment and the electric quadrupole moment using the in-gas-jet spectroscopy setup at KU Leuven.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A new vacuum-ultraviolet spectroscopy setup (Resonance Ltd, VM180) was characterized and connected to the beamline at ISOLDE, CERN. Unfortunately, pure actinium-229 beam was not available at ISOLDE, but a beam of mass A=229, mostly comprising of francium-229 and radium-229, both of which decay into actinium-229 was available. During the experimental campaign IS658, beam of mass A=229 was implanted onto calfium fluoride, (5 mm thick crystal from Thorlabs) as well as 50 nm thick foil on silicon substrate produced at IMEC and magnecium fluoride crystals (5mm crystal from Thorlabs). The VUV photons from the de-excitaion of the isomer was observed for the first time enabling the measurement of the isomer's excitation energy to a precision of 0,4 nm, dominated by systematic uncertainties. The measured excitaion energy value is a seven fold improvement in the precision compared to the previous results. The lifetime of this VUV-signal of the thorium-229 isomer implanted on a magnesium fluoride was also measured and the measured values are in good agreement with the theoretical predictions.

Improvements in the VUV-spectrometer as well as the data acquisition system was performed after the IS658 measurements at ISOLDE. To improve the precision of the isomer's excitation energy, new optimized calibration method and experimental procedures were developed to minimize the systematic uncertainty. Another experiment campaign IS715 was performed in July, 2023, end of the fellowship period. Several different crystals were tested during this experiment campaign and the VUV-efficiency and the isomer' lifetime have been measured in different crystals.

The second part of this project was the measurement of the hyperfine structure of the isomer and the ground state of a singly charged thorium-229 ion. An important first step for this part was a develpment of an efficient ionization scheme. Due to the lack of such efficient ionization scheme, this part of the project was delayed in order to search for an efficient ionization shcme and to measure the second ionization potential of thorium-229. A more efficient ionization scheme has been developed and the measurements for the second ionization potential is still ongoing.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Several important developments which go beyonf the state-of-the-art were made during this project. A new VUV-spectrometer has been characrized and setup in KU Leuven. The excitation energy of the thorium-229 isomer was measured for thr first time by observing the photons from the radiative decay of the isomer. The precision in the isomer's excitaion energy will help norrow the scan range for a direct excitation of the isomer with a VUV laser. The radiative decay lifetime of the isomer was measured for the first time and is consistent with the theoretical calculation. On one hand the exictation energy will help narrow down the scan range for a direct excitation of the isomer while on the other, the VUV-efficiency of different crystals will help search for the best possible candidate for a solid state nuclear clock.

VUV-Spectrometer