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From Isotopes to the Stars

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From Isotopes to the Stars COMMENT ENVIRONMENT The age of FILM Werner Herzog on POLICY Overhauling the COMMUNICATION Public mistrust cheap, abundant cave painting and the NIH will not fix the broken of Japanese officials is not water is ending p.27 brutality of nature p.30 US health pipeline p.31 irrational but sensible p.31 THE GUARDIAN D. TUFFS/ Hans Geiger (left) and Ernest Rutherford’s experimental work revealed the nucleus at the centre of atoms. From isotopes to the stars Creating more exotic isotopes will reveal the stellar formation of atoms — a fitting tribute to Ernest Rutherford, say Michael Thoennessen and Bradley Sherrill. hundred years have passed since standard model to describe these particles nuclides — atoms with a specific number of Ernest Rutherford published his and their interactions. This quest to find protons and neutrons in their nuclei. In 2010, discovery of the atomic nucleus nature’s ultimate building blocks is being led for the first time, more than 100 new unstable Ain May 1911 and started a journey to the by experiments at CERN, Europe’s particle- isotopes — nuclides with different numbers centre of the atomic world. In Rutherford’s physics laboratory near Geneva, Switzerland. of neutrons — were discovered in a single MANCHESTER UNIV. famous experiment, a stream of α-particles Despite this progress, some basic questions year (see ‘The nuclide trail’). We expect more was aimed at a very thin sheet of gold foil. remain unanswered. It is not known how than 1,000 new isotopes, including some of Some of the particles were deflected at angles Rutherford’s nucleus can result from quarks the most scientifically interesting to date, to that suggested they had collided with a small, and the strong force. It is not even known in be discovered over the next decade or so. dense atomic core. As Rutherford remarked: detail how the strong force binds quarks to Initially, the search for new isotopes was “It was almost as if you fired a 15-inch shell make neutrons and protons, or how it results driven by the quest for the unknown, to at a piece of tissue paper and it came back in the forces that hold together protons and make something nobody else had made and and hit you.” The experiment supported neutrons in the nucleus. Even simpler ques- the urge to understand the underlying forces. a planetary model of the atom — the idea tions, such as how many elements might But they have enormous practical applica- that most of the mass is concentrated in a be possible, or how many neutrons a given tions too: in nuclear energy, medical imaging nucleus, with even smaller electrons orbiting number of protons can bind, are currently and treatment, carbon dating and tracer it like planets around the Sun. unanswerable. elements. The international effort to create Over the century since, scientists have So, away from the high-profile, high- new isotopes will push our understanding of probed to sizes 1,000 times smaller than energy frontier, a small army of machines atom formation and nuclei to new levels, but Rutherford managed — to the level at which is quietly advancing understanding of the may also lead to applications. quarks are important — and developed the atomic nucleus by generating new and rare Even before Rutherford’s experiment, 5 MAY 2011 | VOL 473 | NATURE | 25 © 2011 Macmillan Publishers Limited. All rights reserved COMMENT THE NUCLIDE TRAIL Isotope-discovery technique Isotope discovery over the past 100 years (below) has jumped with each introduction of new technology. Some Light particle reactions Neutron reactions 2,700 radioactive isotopes have been discovered so far (below right), but about 3,000 more are predicted to exist. Fusion Fragmentation/spallation 120 120 2010: rst year in which Stable and naturally more than 100 isotopes 100 100 existing radioactive were discovered isotopes 80 80 Second World War 60 60 40 40 5-year Number of protons running Predicted, but as yet 20 average 20 unconrmed, isotopes Number of isotopes discovered 0 0 1900 1920 1940 1960 1980 2000 0 20 40 60 80 100 120 140 160 180 Year Number of neutrons radioactive-decay studies showed that a other fundamental astrophysics questions not be allowed to halt the machine builders. given element can exist in different forms. on where in the cosmos these isotopes are The facility in Germany still needs to secure The discovery of the neutron in 1932 revealed created, why stars explode, the nature of sufficient funding to start operations by the that the nucleus of an atom was composed of neutron stars and what the first stars in the end of the decade. Isotope discovery over protons and neutrons. Soon after, Irène Curie Universe were like. the past 100 years has been a worldwide and Frédéric Joliot used α-particles from effort, with more than 3,000 scientists in 125 polonium and targets of boron, magnesium MARCH OF MACHINES laboratories in 27 countries contributing. It and aluminium to create the first radioactive The first particle accelerators, developed will be a shame if the German facility — an isotopes in the laboratory. The new isotopes in the early 1930s, revealed many new international collaboration from its outset — of nitrogen, aluminium and phosphorus had isotopes. The Second World War delayed does not move forward expeditiously. one neutron fewer than the normal stable progress but, afterwards, neutron-capture Pushing science to the limits produces nuclides of these elements. and neutron-fission reactions in nuclear surprises. We have already learned that rare Since then, researchers have been reactors continued the exploration. The next nuclei with extreme proton-to-neutron ratios searching for the limits of nuclear existence, advance was the development of heavy-ion don’t always follow the textbook behaviour of to discover what element may have the most accelerators in the 1960s, which produced known stable isotopes. For example, the size protons and what are the largest (and small- heavy neutron-deficient isotopes in fusion of stable nuclei is proportional to their mass est) number of neutrons for a given element. evaporation reactions. — it scales as A1/3 (where A is the mass num- Even today, the limit to the number of With higher-energy accelerators in the ber of neutrons and protons). However, this neutrons that an element can bind is known 1990s, scientists could create more neutron- simple relationship ignores any differences only for the lightest elements, from hydro- rich nuclei during in-flight fission or projec- between neutrons and protons. Some rare gen to oxygen. That is one very small corner tile fragmentation of high-energy heavy ions. nuclei that exist only fleetingly have proved of the possible nuclear landscape (see ‘The This has been the most productive route to to be significantly larger. nuclide trail’). isotope discovery in recent times. But in the Other surprises may be in store. Hope- There are almost 300 stable nuclides on past decade, the rate of discovery dropped fully, the next-generation facilities will Earth and another 2,700 radioactive isotopes to levels not seen since the 1940s. It became create more than 1,000 new isotopes, and have been identified so far. This represents obvious that dedicated rare-isotope accelera- the limit of nuclear existence will be pushed perhaps only half of all predicted isotopes. tors were needed to make further progress. towards heavier elements, up to zirconium Around 3,000 have yet to be discovered (it The first of these facilities, the Rare Isotope (40 protons) but still some way from gold might be as many as 5,000 or as few as 2,000). Beam Factory, came online in 2007 in Wako, (79 protons). Fundamental phenomena Although the different masses of isotopes do Japan. In 2010, it reported the discovery of are waiting to be discovered, and increased not influence their chemistry much, the pro- 45 new neutron-rich isotopes. production of rare isotopes will bring new duction and study of rare isotopes is crucial To ensure that this is the beginning of a applications in medicine and other fields. We to understanding the process of nature that new era rather than just a discovery spike, are confident that in the next 10–15 years, makes atoms in their birthplace. it is crucial to continue efforts worldwide. most of the isotopes needed to answer the Most of the elements in nature are created Centres are under development, such as the question ‘What is the origin of elements in in stars and stellar explosions, and the iso- Facility for Antiproton and Ions Research the cosmos?’ will be created in the lab for the topes involved are often at the very limits of in Darmstadt, Germany, SPIRAL2 in Caen, first time. A fitting tribute to Rutherford. ■ stability. The next generation of rare-isotope France, and the Facility for Rare Isotope accelerators will create, for the first time on Beams at Michigan State University in East Michael Thoennessen and Bradley Earth, most of the isotopes that are formed Lansing. Scientists in the United States have Sherrill are at the National Superconducting in stellar environments. Where physicists been trying to build a rare-isotope accel- Cyclotron Laboratory and the Department currently have to rely on theoretical mod- erator for almost 20 years. Funding for an of Physics and Astronomy at Michigan State els based on extrapolations, they will soon earlier facility was halted during a previous University, East Lansing, Michigan 48824, USA. measure the properties of most of these period of austerity. e-mails: [email protected]; isotopes directly. It could help to answer Today’s difficult financial conditions must [email protected] 26 | NATURE | VOL 473 | 5 MAY 2011 © 2011 Macmillan Publishers Limited.
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