Nuclear physics

Weighing up the superheavies

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To discover superheavy elements and study their properties, we need to know the masses of the isotopes of elements heavier than uranium. Weighing these isotopes in an electromagnetic trap has now become possible.

Nuclear reactions allow us to create elements beyond uranium (element 92), the heaviest element in nature. However, we are far from knowing what the heaviest possible element is. In the sparsely explored territory of superheavy elements, an 'island of stability' is expected. This island would consist of isotopes of elements that are more strongly bound and longer-lived than the isotopes surrounding it. Challenging expeditions towards the island have so far led to the discovery of elements up to element 118. But knowing the masses of the isotopes of elements heavier than uranium (trans-uranium elements) is of great importance for the success of the journey. In this issue (page 785), Block and colleagues1 describe the first-ever direct measurement of the masses of isotopes of a trans-uranium element. They have used an ion trap as a high-precision scale for weighing isotopes of nobelium, an element that has ten more protons than uranium.

Chemical elements are sorted into a periodic table according to their properties. These properties reflect an atom's electronic structure, which is determined by the number of protons in the atomic nucleus. In a similar way, the approximately 3,000 known isotopes of the different elements are depicted in a proton number–neutron number diagram, the chart of nuclides. Most of these isotopes are radioactive and can be produced only artificially by using nuclear reactions. In exploring the limits to the existence of nuclides, physicists' expeditions have reached beyond uranium (92 protons) towards the northeast end of the chart of nuclides.

The existence of superheavy elements possessing many more protons than uranium was predicted four decades ago. Their increased stability against nuclear fission would originate from their nuclear shells being filled by protons and neutrons, like the electron orbits in an atom. For certain combinations of numbers of protons and neutrons — 'magic numbers' — a more strongly bound system would be formed that would also have a longer half-life. Accordingly, the superheavy elements are predicted to populate an island of stability located around proton number 120 and neutron number 184 (see Fig. 3 on page 787). Several exploratory groups have set sail2,3,4,5 for this destination, and in the course of their journey have discovered various superheavy elements, the latest being ununoctium, with 118 protons. However, we cannot be certain that this is the heaviest element. And progress towards setting foot on the island of stability is slow because of the painfully low rates, sometimes only one atom per week, at which these exotic atoms can be produced.

The more difficult the synthesis of heavier elements and the production of their isotopes, the more important is the availability of better information on their properties, either through direct measurements or by better theoretical prediction. The accurate knowledge of masses is particularly critical. Einstein's mass–energy equivalence relates the mass of an isotope directly to how strongly its protons and neutrons are bound; that in turn determines whether it can exist and its lifetime before it decays. This is where Block and colleagues1 have achieved a breakthrough — by performing the first direct mass measurement on the isotopes of a trans-uranium element and by providing the information needed to build a bridge to the island of stability. Furthermore, they have demonstrated that the technique chosen is indeed the most promising one for meeting the challenge of determining the masses of isotopes of superheavy elements.

Block et al. determined the masses of the isotopes 252No, 253No and 254No of nobelium. The result achieved for 253No led to a tenfold improvement in the accuracy of its mass, and as a consequence, to an improved knowledge of the masses of all isotopes in the α-decay chain of which it is a member. An α-decay chain is a sequence of radioactive decays in which elements transform into one another by emitting an α-particle. The measurements were performed with SHIPTRAP, a facility specifically developed for high-precision experiments using trapped ions of very heavy and superheavy elements.

For the mass determination, powerful Penning-trap mass spectrometry6 was used. In a Penning trap, charged particles can be confined and stored in a strong magnetic field under vacuum for long periods of time. The frequency of the circular, 'cyclotron' motion performed by a trapped ion is connected to its charge, its mass and the magnetic-field strength. By determining this frequency, it is possible to obtain the ion's mass. Such mass measurements can reach extremely high precision — relative uncertainties of less than one part in a billion have been achieved for stable ions7. The real challenge in applying this approach to superheavy elements is reaching a high enough efficiency in transferring these rare isotopes as ions into the trap. Here1, SHIPTRAP has set a new record for the lowest production rate for which a Penning trap has been successfully used to measure the mass of an unstable isotope.

Until now, superheavy elements were identified by their indirect connection by α-decay chains to known elements around uranium. Penning-trap mass measurements can tie down loose decay chains that are not yet connected to known elements. Block and colleagues' first direct mass measurements provide firm anchor points that are much closer to the superheavy elements than before, in addition to improving the accuracy of the mass values for all isotopes in these chains. And such Penning-trap mass measurements1 may become even more important in the long term. As the island of stability is approached, the lifetime of the nuclides is expected to become longer with the addition of more neutrons. Theory predicts half-lives as long as minutes to hours, a trend that is supported by experiments. For half-lives this long, the identification of superheavy elements on the basis of the radioactive decay of their isotopes will no longer be feasible. Identifying new superheavies by weighing them in a Penning trap may then be the only practical approach.

References

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Bollen, G. Weighing up the superheavies. Nature 463, 740–741 (2010) doi:10.1038/463740a

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