CERN Accelerating science

HIE-ISOLDE: a unique window into the nucleus

by Karl Johnston (CERN)

With the recent completion of the second phase of HIE-ISOLDE, CERN’s ISOLDE facility has a machine capable of answering long-unanswered questions about the nature of the nucleus. The installation of 4 cryomodules in 2018 allowed for the acceleration of radioactive isotopes up to 7.4 MeV/u for A/q = 4.5. This new accelerator, in combination with ISOLDE’s expertise for producing radioisotopes, allows for the re-acceleration of the widest variety of radioactive isotopes worldwide.

The ISOLDE community is now beginning to take advantage of these new opportunities offered at HIE-ISOLDE to peer into the nucleus with a fresh eye. The first two papers from HIE-ISOLDE experiments have recently been published and highlight its potential in two different experiments, both employing the Miniball gamma detector array which can be seen in figure 1.

Figure 1: The Miniball array, used for all the measurements described in this article.

The first results arose from IS551, which reported on the Coulomb excitation of the very neutron-rich  132Sn, an isotope with 50 protons and 82 neutrons [1]. This nucleus has been long identified as a doubly-magic nucleus, but never directly proved to be so. Of the 3200 known nuclei, only 10 possess the properties of being doubly magic: displaying increased stability compared to their neighbouring isotopes due to the perfect filling of proton and neutron shells within the nucleus (similar to the electrons filling atomic shells, making the noble gasses the most stable and least reactive of all elements). The importance of studying 132Sn extends beyond nuclear structure: it is also at a critical path in the r-process, which governs the astrophysical process for the production of elements heavier than iron   in the universe. The production path around 132Sn is not fully understood; the precise study of this nucleus is therefore also crucial for nuclear astrophysics.  The recent results have shown beyond any doubt that it is indeed doubly-magic.
 
The experiment employed  a novel production technique for the radioactive isotope, using a molecular form of Sn – in the form of 132Sn34S. This allowed the experiment to obtain a purer beam of 132Sn, because the unwanted isobaric 132Cs does not form such molecules, and therefore is not present after the mass separation of a mass 166 beam. The molecule was subsequently broken up in the low energy part of the HIE-ISOLDE accelerator before the 132Sn beam was re-accelerated to an energy of 5.45MeV/u and directed onto a 206Pb target in the vacuum chamber inside the Miniball arry. This reaction excited the nucleons in the 132Sn nuclei to higher-energy states. These collective excitations, which have low chances of occurring, decayed with emission of gamma-ray photons, which MINIBALL detected. By analysing the number of gamma-ray photons detected, the authors measured the strengths of these excitations with an order of magnitude higher precision that before, thanks to the reaction being more probably at these higher beam energies. From these transition strengths, they found more pronounced excitations in 132Sn compared to those of its nuclear neighbours. This was as predicted by theory and is a crucial feature of doubly magic nuclei. It thus confirms the doubly magic nature of 132Sn. An example spectrum is shown in figure two, where the key gamma rays are indicated.
 
Figure 2: Two Gamma rays from the coulomb excitation of 132Sn (a) indicating the 132Sn 21+ -> 0g.s+ and 3-1 -> 0g.s+ . transitions and (b) same as for (a) but with the restriction of scattering angles ensuring pure electromagnetic excitation [1].

 

The second paper reports on the Coulomb excitation of even Rn isotopes and is from experiment IS552. Nuclei are known to  take on many forms and their shape is dependent on the underlying nuclear structure. E.g. doubly-magic nuclei like 132Sn are spherical, but others cn be prolate (rugby-ball-like) shaped or oblate (pancake-like) deformed.  An additional category are nuclei which lack the reflection symmetry of the rugby-ball-like or pancake-like nuclei; these nuclei can take an octupole (pear-like) shape. Nuclei with such an asymmetric static pear-like shape have attracted considerable interest for their role in the search for permanent electric dipole moments (EDMs), as such a moment is expected to be amplified in octupole deformed nuclei. Ra and Rn nuclei have been identified as suitable candidates in the search for EDMs and, when the isotope possesses a static octupole shape, the nuclear Schiff moment due to a non-zero EDM, could be enhanced by a factor of 100-1000. The identification of pear-shaped nuclei is already a specialism of ISOLDE: the observation by Gaffney et al [2] of static octupole deformation in the isotopes of 224Ra and 226Ra was a milestone measurement a few years ago. Now, a new paper builds on this work, and reports on the first spectroscopy of the excited states of even Rn isotopes: 224Rn and 226Rn [3].

Figure 3: (a) Gamma spectrum from the even Rn isotopes measured at HIE-ISOLDE. (b) Cartoon of phonon coupling in octupole deformed nuclei and the difference in aligned spin for negative- and positive-parity states in 218-224Rn. The dashed line at Δix=0 is the expected value for isotopes possessing static-octupole deformation, indicating that the investigated Rn nuclei do not possess this property.  

Rn beams were produced by irradiating a ThC target coupled to a cold plasma ion source. The ions were reaccelerated to 5.08MeV/u and directed towards a target of 120Sn. The re-acceleration – and indeed production – of such heavy ions is unique to ISOLDE worldwide and is among the many aspects where ISOLDE continues to lead the field for radioactive ion beam facilities. A typical spectrum is shown in figure 3 showing the rich gamma spectrum under consideration. Unlike other isotopes – such as 224Ra, which display static octupole deformation [2] –  the Rn isotopes do not display this property. Thus, these isotopes are less sensitive for searches for an EDM, contrary to earlier claims.

HIE-ISOLDE has already shown with these first two results the impact which this new accelerator will have, not only in nuclear physics, but beyond. A rich bounty of results is expected from other experiments which have had the opportunity to run before LS2 including the ISOLDE solenoidal spectrometer and the versatile scattering chamber. The first experiments undertaking transfer studies at the high energies offered by HIE-ISOLDE are still being analysed and the user community is eagerly awaiting the restart of the machine after LS2 when it can be exploited to its full potential.

References:

[1] D. Rosiak et al. Phys. Rev. Lett. 121, 252501

[2] Gaffney, L. P. et al. Studies of pear-shaped nuclei using accelerated radioactive beams. Nature 497, 199–204 (2013).

[3] P. Butler et al  The observation of vibrating pear-shapes in radon nuclei Nature Communications 10, Article number: 2473 (2019)