Exciting new opportunities for experiments with beta-decaying atomic nuclei have arisen at the ISOLDE facility with the recently developed new spectroscopy station called DeVITO [1]. The novelty of the setup lies in its integration with the VITO beamline for laser-polarisation of radioactive beams [2]. This unique combination enables spectroscopy measurements with spin-oriented nuclei that, due to the parity non-conserving nature of the weak interaction, emit radiation anisotropically. The ability to exploit the directional distribution of radiation represents a significant advance over conventional beta-decay experiments, which greatly benefit from the high angular-momentum selectivity of the process but often struggle to unambiguously infer nuclear spins and parities from observed beta-decay feeding intensities [3]. These crucial quantum numbers, essential for discussing complex phenomena observed in nuclei, can be deduced from experimental beta-decay asymmetries measured in coincidence with delayed radiation [4, 5]. This novel approach to beta-decay measurements with polarised nuclei was pioneered by a group from the University of Osaka, which successfully applied it to studies of allowed beta transitions [5-7]. It has been shown that the key ingredient towards unambiguous spin-parity assignments is the high polarisation of the beam. The VITO beamline, where spin orientation in atoms or ions is induced via optical pumping with laser light, has proven to be an excellent place for implementing this technique for the first time in Europe [1] and further developing its extension.

The DeVITO station receives a laser-polarised beam from the optical-pumping section of the VITO beamline [2]. One vital function of the station is to provide a magnetic field that adiabatically decouples the electron and nuclear spins and maintains the nuclear-spin polarisation after ions or atoms are implanted into a host material. Another essential aspect is the efficient detection of radiation emitted in the beta decay of neutron-rich nuclei, including electrons, gamma rays, and neutrons. The magnetic field and laser light present in the experimental chamber impose strict requirements for unobstructed measurements of beta-particle emission asymmetry in coincidence with gamma rays and neutrons for which energy information is registered. The new station built at VITO required solutions that are not standard in conventional beta-decay experiments, so its key components were custom-made and manufactured locally at CERN.

Figure 1: Top: The newly developed end station integrated with the VITO beamline at ISOLDE. Bottom-left: Main components of the DeVITO station, including the detector arrays surrounding the implantation chamber, which is positioned inside the magnet. Bottom-right: Schematic drawing highlighting the key components of the detection setup: three HPGe Clovers for gamma-ray energy measurements (shown in blue) and the neutron time-of-flight VANDLE detectors (presented in violet).

Figure 1 shows the DeVITO station ready for the first physics experiment in April 2024, preceded by a successful commissioning run with a radioactive beam in July 2023. At the heart of the station is an implantation chamber mounted inside a compact water-cooled magnet that provides a field of 800-1000 gauss. This chamber contains a beam collimator, implantation-host system, and two plastic scintillators coupled with silicon photomultiplier boards (see Figure 2). These scintillators, placed at 0° and 180° angles relative to the polarisation-axis direction, are used to measure beta particles emitted following atom or ion implantation into a solid sample at the centre of the chamber. Gamma rays and neutrons emitted in the beta decay of the spin-oriented nuclei are detected by the spectrometers surrounding the magnet. Three high-purity germanium (HPGe) detectors equipped with four crystals allow precise measurement of the gamma-ray energy [8]. Two neutron time-of-flight detector arrays, VANDLE [9], are used to measure the energies of neutrons. Each array consists of thirty plastic scintillator bars, each equipped with a photomultiplier at both ends. The detector modules are mounted in arch-shaped frames providing a radius of 100 cm for neutron time-of-flight measurements with sub-nanosecond time resolution [10]. Over 170 detector output and logic signals are recorded in trigger-less mode using the data acquisition system 250-MHz XIA PIXIE16.

The part of the analysis that goes beyond traditional beta-decay spectroscopy includes the determination of experimental beta-particle asymmetries [4, 5]. The coincidence condition on the energy of delayed radiation allows the selection of a particular beta transition, feeding a nuclear level of interest that immediately de-excites by emitting gamma rays or neutrons. From experimental asymmetries measured for individual beta transitions, the degree of nuclear spin polarisation and asymmetry parameters can be determined. While the former quantity describes the spin-oriented precursor (polarisation degree is the same for all beta transitions), the latter parameter depends on spins and parities of nuclear levels involved in the beta transition. This dependence, particularly simple for allowed transitions [4, 5], paves the way for unambiguous spin-parity assignments for excited states in nuclei.

Figure 2: Left: In-chamber assembly for radioactive beam implantation and beta-decay asymmetry measurements. Right: Implantation host (cubic crystal, KCl) installed between plastic scintillators for beta-particle asymmetry measurements.

The commissioning and the first physics run [11] aimed to demonstrate the capability of the experimental approach pioneered by the Osaka group [5-7] and guide the development of its possible extensions. To facilitate proof-of-concept measurements, we used beams of neutron-rich potassium isotopes, which are abundantly produced at ISOLDE, and two of them have already been successfully laser-polarised in previous experiments at VITO [12]. The lightest of the studied isotopes, ⁴⁷K, emits gamma rays following its beta decay. This nucleus represents a great case to validate our experimental approach for measuring beta-decay asymmetry in coincidence with gamma rays and to demonstrate the feasibility of determining the multipolarity of gamma rays from the observed asymmetry in gamma-ray emission. Heavier potassium isotopes, ⁴⁹K and ⁵¹K, which emit neutrons from unbound states fed by the beta decay, are excellent cases to validate the analysis of beta-decay asymmetries in coincidence with neutrons to determine spins and parities of neutron-emitting states, which currently remain unknown.

The new setup for beta-decay spectroscopy at ISOLDE allows various end-station configurations and a diverse research programme addressing aspects of nuclear structure, astrophysics, as well as weak interaction studies. The current configuration of the DeVITO station and the initiated research programme [11] are targeted at studies of very neutron-rich nuclei to provide robust experimental data to gain insight into the mechanism of beta-delayed neutron emission, which is the primary decay mode of exotic nuclei involved in one of the astrophysical processes responsible for the formation of about half of the chemical elements heavier than iron. Recent neutron-spectroscopy results from the ISOLDE Decay Station have challenged existing beta-delayed neutron emission models, revealing two distinct behaviours among strong beta-delayed neutron emitters. These experimental results underscore the need for developing both advanced theoretical frameworks providing new insights into the decay mechanism as well as advanced experimental techniques measuring observables capable of benchmarking beta-decay calculations on a more fundamental level.

Left: Spokesperson of the DeVITO IS733 experiment (Monika Piersa-Siłkowska, CERN fellow awarded the Marie Skłodowska-Curie Individual Fellowship for the station construction), with the CERN PhD student (Ilaria Michelon, Univ. of Geneva), working on the project. Right: CERN fellow in charge of the setup integration with VITO (Nikolay Azaryan).

The DeVITO spectroscopy station was developed by the local VITO team from ISOLDE in collaboration with the University of Tennessee, the University of Warsaw, the University of York, and IFIN-HH Bucharest. The compact magnet was designed by Farhad Saeidi and Mojtaba Mohammadi Najafabadi from the Iranian Light Source Facility (ILSF). This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreement No. 101032999 (BeLaPEx) and the European Union’s Horizon Europe Framework research and innovation programme under grant agreement No. 101057511 (EURO-LABS).

Further Reading
 

1. BeLaPEx project, Marie Skłodowska-Curie Individual Fellowship (H2020-MSCA-IF-2020); Grant agreement 101032999. https://doi.org/10.3030/101032999
2. M. Kowalska et al., Phys. G: Nucl. Part. Phys. 44, 084005 (2017). https://doi.org/10.1088/1361-6471/aa77d7
3. S. Turkat et al., At. Data Nucl. Data Tables 152, 101584 (2023). https://doi.org/10.1016/j.adt.2023.101584
4. K. S. Krane, In: H. Postma, N. J. Stone, Low Temp. Nucl. Orient., North Holland, 1986.
5. H. Miyatake et al., Phys. Rev. C 67, 014306 (2003). https://doi.org/10.1103/PhysRevC.67.01430
6. Y. Hirayama et al., Phys. Lett. B 611, 239 (2005). https://doi.org/10.1016/j.physletb.2005.02.040
7. H. Nishibata et al., Phys. Rev. C 99, 024322 (2019). https://doi.org/10.1103/PhysRevC.99.024322
8. https://www.mirion.com/products/clover-detectors-four-coaxial-germanium (2003).
9. W. A. Peters et al., Nucl. Instrum. Methods Phys. Res., Sect. A 836, 122 (2016). https://doi.org/10.1016/j.nima.2016.08.054
10. S. Paulauskas et al., Nucl. Instrum. Methods Phys. Res., Sect. A 737, 22 (2014). https://doi.org/10.1016/j.nima.2013.11.028
11. M. Piersa-Siłkowska, M. Madurga, M. Kowalska, N. Azaryan et al., Tech. rep. CERN-INTC-2023-026, Geneva (2023). https://cds.cern.ch/record/2846000?ln=en
12. M. Jankowski, ISOLDE Workshop and Users meeting 202, 2 December 2022. https://indi.to/DJJG9
13. Z. Y. Xu et al., Phys. Rev. Lett. 133, 042501 (2024). https://doi.org/10.1103/PhysRevLett.133.042501
14. J. Heideman et al. (IDS Collaboration), Phys. Rev. C 108, 024311 (2023). https://doi.org/10.1103/PhysRevC.108.024311
15. Z. Y. Xu et al., Phys. Rev. Lett. 131, 022501 (2023). https://doi.org/10.1103/physrevlett.131.02250