An international team at the ISOLDE facility showed that some atomic nuclei can assume asymmetric, "pear" shapes. The observations contradict some existing nuclear theories and will require others to be amended. Published in the journal Nature, a technique pioneered at ISOLDE was used successfully to study the shape of the short-lived isotopes Radon 220 and Radium 224.

Most nuclei have the shape of a rugby ball. While state-of-the-art theories are able to predict this behaviour, the same theories have predicted that for some particular combinations of protons and neutrons, nuclei can also assume asymmetric shapes, like a pear.  In this case there is more mass at one end of the nucleus than the other.

In contradiction with some nuclear theories, Radium 224 is pear-shaped, Radon 220 does not assume the fixed shape of a pear but rather vibrates about this shape.  

Such exotic atoms with pear-shaped nuclei could help with the searches for electric dipole moments (EDMs), that is the separation of positive and negative charges within the atom. 

The ISOLTRAP collaboration publishes in the journal Nature the mass of exotic calcium nuclei using a new instrument installed at the ISOLDE facility. The results cast light on how nuclei can be described in terms of the fundamental strong force.

The ISOLTRAP team used the ISOLDE facility to make exotic isotopes of calcium, which has the magic number of 20 protons in a closed shell. Their goal was to find out how the shell structure evolves with increasing numbers of neutrons. Standard calcium with 20 neutrons is doubly magic, and a rare long-lived isotope has 28 neutrons – another magic number.

Now, the ISOLTRAP team has determined the masses of calcium isotopes all the way to calcium-54, which has 34 neutrons in addition to the 20 protons. The measurements not only reveal a new magic number, 32, but also pin down nuclear interactions in exotic neutron-rich nuclei.

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The new CERN-MEDICIS (Medical Isotopes Collected from ISOLDE) facility produced radioisotopes for medical research for the first time. These radioisotopes are destined primarily for hospitals and research centres in Switzerland and across Europe. 

“Radioisotopes are used in precision medicine to diagnose cancers, as well as other diseases such as heart irregularities, and to deliver very small radiation doses exactly where they are needed to avoid destroying the surrounding healthy tissue,” said Thierry Stora, MEDICIS project coordinator. “With the start of MEDICIS, we can now produce unconventional isotopes and help to expand the range of applications.”

The first batch produced was Terbium 155, which is considered a promising radioisotope for diagnosing prostate cancer.

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ISOLDE reports about a phenomenon unique to mercury isotopes where the shape of the atomic nuclei dramatically moves between a football and rugby ball. Isotopes with extreme neutron to proton ratios are typically very short-lived, making them difficult to produce and study in the laboratory. The experiment reproduced one of ISOLDE’s flagship results of 40 years ago. The result showed that although most of the isotopes with neutron numbers between 96 and 136 have spherical nuclei, those with 101, 103 and 105 neutrons have strongly elongated nuclei, the shape of rugby balls. Several theories had tried to describe what was happening, but none was able to provide a full explanation.

Using one of the world’s most powerful supercomputers, theorists in Japan performed the most ambitious nuclear shell model calculations to date. These calculations identified the microscopic components that drive the shape shifting; specifically, that four protons are excited beyond a level predicted by expectations of how other stable isotopes in the nuclear landscape behave. These four protons combine with eight neutrons and this drives the shift to the elongated nuclear shape. In fact, both nuclear shapes are possible for each mercury isotope, depending on whether it is in the ground or excited state, but most have a football shaped nucleus in their ground state. The surprise is that Nature chooses the elongated rugby ball shape as the ground state for three of the isotopes.

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CERN's nuclear physics facility, ISOLDE, began producing ion beams at higher energies. The first cryomodule of the new HIE-ISOLDE (High-Intensity and Energy ISOLDE) accelerator is up and running, increasing the beam energy from 3 to 4.3 MeV per nucleon. 

These first beams are the result of eight years of development and manufacturing. The assembly of this first cryomodule presented CERN’s teams with numerous technical challenges. It contains five accelerating cavities and a solenoid magnet that focuses the beam, all of which are superconducting. The cavities were particularly complex to build, and the cryomodule is made up of no fewer than 10 000 components! It was transported to the ISOLDE hall on 2 May and coupled to the existing accelerator. The commissioning began in the summer, culminating in the acceleration of the first radioactive beam on 22 October.

Radioactive ion beams from many chemical elements are produced at ISOLDE and more than 1000 Radioactive Ion Beams (RIBs) are available for the users.  With a Carbon nanotube target the element boron could be produced as a RIB for the first time and the isotope ⁸B (T_1/2=770 ms) could be observed. With this addition to the palette of ISOLDE beams the Facility can now provide beams from 74 chemical elements to the user community.

In the middle of 2013 the success of combined technical and physical efforts was demonstrated in three papers published in Nature within the space of one month.

1. The acceleration of the hitherto heaviest post-accelerated beams, ²²⁰Rn and ²²⁴Rn and detection of gamma rays from Coulomb excitation in the MINIBALL Germanium detector array showed octupole deformed (pear-like) shapes of the nuclei (Nature 497 (2013) 199).
2. With the very successful mass spectrometer ISOLTRAP the mass of the exotic nuclide ⁵⁴Ca was determined. The mass systematics confirmed the existence of a new magic number N=32 and provides a validation of three-body forces using chiral perturbation theory (Nature 498 (2013) 346).
3. In a high-precision study via Rydberg states with the laser ion-source (RILIS), the ionisation potential for the element Astatine, the least abundant chemical element on earth, was determined (Nature Communications 4 (2013) 1835).

(Image: Groundbreaking for the CERN-MEDICIS building. From left - R. Meuli, Chef du Département de Radiologie Médicale, CHUV, D. Hanahan, Director, Swiss Institute for Experimental Cancer Research R. Heuer, Directeur général, CERN Y. Grandjean Secrétaire général, HUG P. Piet Van Duppen, Nuclear Spectroscopy Group, Katholieke Universiteit Leuven. Credit: Maximillien Brice/ CERN)

CERN-MEDICIS will use the primary proton beam at ISOLDE to produce radioisotopes for medical research. A second target will be placed after the one at ISOLDE as the high energy proton beam only deposits 10% of its intensity and energy when traversing the standard type of ISOLDE target and thus the protons passing through the target can still be used. An automated conveyor will then carry this second target to the CERN-MEDICIS infrastructure, where the radioisotopes will be extracted.

(Image: The ISOLDE beamline, equipped with the first HIE-ISOLDE cryomodule in its light grey cryostat)

The HIE-ISOLDE project is a major staged upgrade of the existing ISOLDE facility. It includes an energy increase of the REX post-accelerator up to 5.5 MeV/u with a future option of going to 10 MeV/u, as well as an upgrade of the REX low energy stage capacity. The beam quality will be improved e.g. with the installation of a RFQ cooler and a new Resonant Laser Ionization System. The driver intensity will be increased from the new Linac-4 and upgrades in the intensity and energy of the PS Booster. 

(Image: ISCOOL, an ion cooler and buncher installed at ISOLDE)

An ion cooler and buncher, ISCOOL, is installed in the HRS section of ISOLDE. Beams with strongly reduced emittances and energy spreads are now available for all experiments downstream the beam line.

Using its bunching capabilities, ISCOOL permits an increase in the sensitivity for experiments such as those devotde to collinear laser spectroscopy. The first physics run with this device took place in July 2008 where the isotope ⁷⁷Ga (T_1/2=13 s) was studied with the COLLAPS laser spectroscopy setup.