Nuclear Shapes of Heavy Atoms and Proton-Emitting nuclei

Project Information

NSHAPE

Grant agreement ID: 101088504

DOI

10.3030/101088504

EC signature date 27 February 2023

Start date 1 April 2023

End date 31 March 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 500 000,00

EU contribution

€ 2 500 000,00

Coordinated by

KATHOLIEKE UNIVERSITEIT LEUVEN
Belgium

Periodic Reporting for period 1 - NSHAPE (Nuclear Shapes of Heavy Atoms and Proton-Emitting nuclei)

Reporting period: 2023-04-01 to 2025-09-30

Summary of the context and overall objectives of the project

By combining atomic, nuclear and particle physics techniques, I shall perform the experimental study of the shape of key atomic nuclei, to challenge our understanding of the nucleus. This combined effort will reach unprecedented sensitivity, precision and accuracy to determine the shape observables (charge radii, electric quadrupole moments) and compare them to state-of-the-art nuclear models (spherical shell model, density functional theory, ab initio models based on chiral effective field theory). This programme will combine different techniques at various accelerator facilities where I employ and develop unique approaches and instrumentation:
- At CERN ISOLDE (Geneva, CH), high-resolution laser resonance ionization spectroscopy will be performed with the Resonant Ionization Laser Ion Source (RILIS) combined with the Perpendicularly-Illuminated Laser Ion Source and Trap (PI-LIST) to study the onset of octupole deformation and proceed towards the proton drip line with selected nuclei.
- At GANIL SPIRAL2 (Caen, FR), high-sensitivity laser resonance ionization spectroscopy will be performed in the supersonic gas jet of the gas cell at the focal plane of the Super Separator Spectrometer (S3) to study the most exotic isotopes not available at ISOLDE, reaching proton-unbound nuclei at the drip line.
- At PSI (Villigen, CH), muonic x-ray spectroscopy will be performed on key isotopes to measure absolute charge radii that are crucial to complete the analysis of the NSHAPE isotopes. This work will combine a strong experimental development in target production for muX, in the detector array, and in the analysis tools. Combining the high-resolution laser spectroscopy and the high-accuracy from μx-ray spectroscopy is a unique programme that only NSHAPE can fully realise, providing radii and moments with unprecedented accuracy.
From those results, I shall obtain a deeper understanding of the strong interaction at work in the nuclear medium.

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

In this first period of NSHAPE, the focus has been on preparing the experimental campaign, technical developments, and initial data collection.
At KU Leuven, a new laser laboratory is under construction for the development of ionisation schemes for laser spectroscopy. This laboratory should enter full operation in the summer of 2025.
At CERN ISOLDE, tests have been performed to demonstrate the feasibility of the study of neutron-deficient lutetium isotopes with the PI-LIST device and a proposal was subsequently accepted to perform that study.
Meanwhile, the analysis of the data collected on the neutron-rich polonium and actinium isotopes with the PI-LIST is near completion. Those results have been presented at the International Nuclear Physics Conference 2025 and the preparation of publications of those results are in preparation.
At GANIL, the commissioning of the facility is facing some delays. To mitigate the impact on the NSHAPE programme, some initial measurements have been proposed at GSI in Germany, using the SHIP velocity filter, to investigate the most neutron-deficient lutetium isotopes, beyond what can be reached at ISOLDE. The beam time is planned for 2026.
At PSI, test data sets have been collected for muonic x-ray spectroscopy on the naturally available La-139, Th-232, and U-238 to demonstrate the feasibility of study more exotic isotopes of those elements. Furthermore, a full data set has been collected with natural and enriched lutetium, to complete the study of Lu-175,176. Combined with the laser spectroscopy data that should be collected at CERN ISOLDE and GSI, this would offer a comprehensive view of this isotopic chain.
Meanwhile, extensive technical developments are ongoing to support the muonic x-ray spectroscopy programme: a systematic study of target production techniques has demonstrated that implanted beams, as obtained from mass separation, are directly applicable in muonic x-ray spectroscopy experiments; this would enable the direct use of target prepared for NSHAPE with that technique. Moreover, development work has been performed to achieve the efficient mass separation of lanthanum isotopes, especially to produce a high-purity La-138 samples for the upcoming campaign. Monte Carlo simulations have been performed to demonstrate the improvement that can be achieved with high-purity germanium clover detectors instead of the current standard detectors used at PSI (BEGe, SEGe, REGe); this study has motivated to bring the ISOLDE Decay Station array to PSI for the 2026 campaign, to be complemented by 2 new detectors acquired from NSHAPE.
Finally, the full analysis pipeline for the determination of absolute charge radii from muonic x-ray spectroscopy has been established, using the somewhat simpler case of Cl-35,3y and K-39,40,41. This paves the way for the full analysis of the isotopes of interest to NSHAPE, while also helping identify the challenges that are faced by the theory groups.

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)

The demonstration of the direct applicability of implanted targets for muonic x-ray spectroscopy measurements opens the door to many cases beyond those proposed within NSHAPE. This ensures that the samples do not require post-chemical treatment and hereby reduces the losses and impact on the preparatory work. This is sure to enable new scientific cases for the future.
Concerning the extraction of radii from muonic x-ray spectroscopy data, the full pipeline that has been developed has also revealed shortcomings from the data extraction in the previous century. This has brought to light that many results currently in data table - which are considered reference points with which nuclear models are defined - are inaccurate and that many uncertainties are underestimated. This generates a global call to reevaluate those absolute radii that is resonating with the entire community.