First measurement of the degree of polarization of radioactive ions with the MORA experiment

The MORA experiment, coordinated by GANIL, has just reached a decisive milestone with the first measurement of the degree of polarization of radioactive ²³Mg⁺ ions in its trap, carried out at the IGISOL facility of the University of Jyväskylä (Finland). This result, published in European Physical Journal A, constitutes the first experimental demonstration of laser polarization of radioactive ions trapped in a transparent Paul trap.

A Novel and Highly Efficient Laser Polarization Technique

The MORA experiment aims to search for a violation of the fundamental CP symmetry in the β⁺ decay of ²³Mg⁺ ions, as a possible clue to explaining the matter–antimatter asymmetry observed in the Universe. It relies on measuring the angular correlation between the recoil nucleus and the emitted positron in the decay of strongly polarized ions. The setup uses a transparent Paul trap, enabling the polarization of radioactive ions through their interaction with pulsed laser beams. Thanks to ion trapping, the polarization method achieves an efficiency several orders of magnitude higher than that of classical techniques using neutral atom traps (magneto-optical traps). As in those systems, the MORA apparatus allows prolonged exposure of the ions to the laser beams, leading to strong polarization.

The recent measurement indicates a polarization degree between 55% and 100%, with a 90% confidence level, and a trapping efficiency of about 30%. For comparison, magneto-optical traps also achieve very high polarization degrees (often above 90%), but with trapping efficiencies below a fraction of a percent. This first measurement of the polarization degree thus provides convincing proof of the exceptional potential of the laser polarization technique implemented in the trap of MORA.

A Milestone Toward the Search for CP Violation

Installed at IGISOL, University of Jyväskylä, since 2022, the MORA experiment has achieved several important steps. After the system was commissioned, significant efforts were made to improve the ion beam purity, a limiting factor for trapping. In 2024, the team observed for the first time coincidences between positrons and recoil ions, indicating an initial orientation of the spin of the trapped ions. This observation was confirmed in March 2025 by the measurement of the polarization degree.

The ions’ polarization degree is inferred from an asymmetry (A) between the numbers of positrons detected by two segmented silicon detectors placed on opposite sides of the trap (Fig. 1a, Si1 and Si2):

where N₁ and N₂ are the numbers of positrons detected in opposite directions (Fig. 1b).

For a fully polarized cloud of radioactive ions, one expects A = 0.51 ± 0.01. The observed agreement confirms that the spin of the ²³Mg⁺ ions is strongly aligned along the trap axis, demonstrating the effectiveness of the laser system.

Toward a Precise Measurement of the D Correlation in Nuclear β Decay

This first polarization measurement marks a key step toward searching for a nonzero D-correlation in the β decay of ²³Mg⁺ ions, which would be a signature of CP symmetry violation.

In July and October 2025, further improvements led to an additional reduction in beam contamination, paving the way for the start of symmetry-violation measurements—the central objective of the MORA scientific program.

An International Collaboration with Promising Prospects

MORA is coordinated by GANIL (Grand Accélérateur National d’Ions Lourds) in collaboration with LPC Caen, University of Jyväskylä (Finland), KU Leuven (Belgium), and IJCLab. Currently hosted at Jyväskylä, the experiment will move to GANIL after the DESIR beamline is commissioned (expected in 2028). It will then benefit from the intense SPIRAL1 beams, purified by the High Resolution Separator (HRS) of DESIR, allowing unprecedented precision measurements.

The project has been supported by the Normandy Region (2018–2020) and by the French National Research Agency (ANR, 2020–2025) through dedicated projects. It has recently received renewed support from the ANR through the ACCLAIM-MORA project (AdvanCed CaLcium radioActive Isotope Manipulation for MORA).

The Normandy Region also funds the PhD thesis of Luis Miguel Motilla Martinez, a joint project between GANIL and the University of Jyväskylä, which led to the beam purification and the laser polarization proof of principle. The University of Caen supports the PhD of Marah Jbayli, focused on data analysis for the D-correlation measurement.

Learn more about the measurement:

Goyal et al., “Performance of the MORA apparatus for testing time-reversal invariance in nuclear beta decay,” Eur. Phys. J. A 61 (10), 221 (2025). DOI: 10.1140/epja/s10050-025-01694-3

Contact: Pierre Delahaye pierre.delahaye@ganil.fr