by Simon Levay
‘Whether there was true harm or not, [Johnson and Tudor’s] subjects were intruded on in a way that they shouldn’t have been,’ commented Schwartz by way of wrapping up the ethical issues. ‘They should have given this more thought, even given the mores of the time. Most importantly, it’s a useful thing today to teach both senior researchers like myself, and students, that you really have to think about these things. It’s very important, if anything, to err on the side of being cautious and more protective of human subjects and to be really good at perspective-taking: “What would this be like if this were my child, my relative, me, in the situation of being a subject; would this be OK?” And this is really at the heart of what an IRB tries to do.’
NUCLEAR CHEMISTRY: The Magic Island
IN AUGUST 1999, NUCLEAR chemists at the Lawrence Berkeley National Laboratory announced the creation of three atoms of a new ‘superheavy’ element, element 118. Two years later they had to retract their claim, and a firefight broke out that cost a star scientist his career and sullied the reputations of several others.
The University of California’s Berkeley campus had been a world leader in the discovery of new elements since 1940, when Edwin McMillan discovered element 93, neptunium. The university’s most famous element hunter was Glenn Seaborg, who discovered plutonium (element 94) in 1941 and followed it up with nine more elements, culminating in 1974 with the one that was named, in his honour, seaborgium (element 106). McMillan and Seaborg shared the 1951 Nobel Prize for Chemistry, but many other Berkeley scientists also played important roles in this work. These included Stanley Thompson, who helped discover most of the new elements up to element 101, and Albert Ghiorso, who shared credit for many of the elements discovered from the mid-1940s onward.
The use of the term ‘discover’ in this context is slightly odd. Darleane Hoffman (also a Berkeley Lab scientist since 1984) and others did discover minute amounts of plutonium and neptunium in natural uranium ores, but none of the other ‘transuranium’ elements exist in the natural world, unless perhaps in some distant supernova. Thus the process of discovery means creating them, not finding them as the term implies. In part, scientists use the term discover simply as a continuation of a tradition that started with the actual discovery of the lighter elements in nature. In addition, however, they probably use the term because they think of the transuranium elements as already existing in a Platonic universe to which their powerful instruments give them entry. They think this way because the properties of each element – even those that don’t exist – were fixed at the beginning of time, when the particles that make up atomic nuclei (the positively charged protons and uncharged neutrons) were endowed with their immutable characteristics.
To some extent, then, nuclear chemists can predict the properties of atomic nuclei that haven’t yet been discovered or created. The most basic theoretical formulation is this: the protons and neutrons are held tightly together by the ‘nuclear force’, which only acts over minute distances. Countering this attraction is the electrostatic repulsion between the protons’ positive charges, which tends to push them apart; this force acts over a much greater distance than the nuclear force. As one progresses to heavier and heavier nuclei, they become less and less stable, because the protons and neutrons cannot crowd closely enough together for the nuclear force to act at full strength between all the particles. Thus the electrostatic repulsion comes to dominate, causing the nucleus to break apart.
If this were the whole story, there would have to be an end to the periodic table of the elements, and that end would lie somewhere in the neighbourhood of element 106, the last element that Seaborg discovered. During the 1980s and 1990s, however, elements 107 to 112 were created – roughly in the sequence of their atomic numbers. Most of these discoveries were made by a group at the Institute for Heavy Ion Research (GSI) in Darmstadt, Germany. A Russian group, the Joint Institute for Nuclear Research in Dubna, near Moscow, was also a player.
The existence of these heavier elements had in fact been predicted by theorists who went beyond the simple model of the atomic nucleus just described. One of these theorists was Seaborg’s Polish-born colleague Wladyslaw Swiatecki, who joined the Berkeley group in 1957. Swiatecki and others believed that, within the nucleus, protons occupy a series of discrete energy levels that can be thought of as concentric shells. A nucleus whose outermost proton shell was completely filled would gain an extra measure of stability beyond that predicted by classical theory. Similarly, neutrons were thought to reside in their own shells and to confer extra stability on the nucleus when their outermost shell was filled. These nuclear shells are analogous to the better-known electron shells outside of the nucleus, which are filled in the inert gas elements helium, neon, and so on.
The numbers of protons and neutrons that conferred stability were said to be ‘magic’, and a nucleus that contained magic numbers of both protons and neutrons were ‘doubly magic’. These might exist in sizes far beyond the limits set by classical theory. In other words, even element 112 might not be the end of the road.
Not everyone agreed on exactly what these magic numbers of protons and neutrons were, or even whether they were meaningful concepts at all. Still, this was the conceptual framework that guided research in the 1990s. And what it meant was that simply going for the next-heaviest undiscovered element on the list might not be the best approach: some elements well beyond the presently achieved limits might actually be more stable and easier to create.
Also, this approach meant that both the number of protons (which defines which element we are talking about) and the number of neutrons (which defines which isotope of that element we are talking about) needed to be considered when thinking about creating superheavy elements. To illustrate this, Seaborg, Swiatecki and others used a chart that plotted the proton number (on the vertical axis) and the neutron number (on the horizontal axis) of all known atomic nuclei. On this chart, the already-known nuclei formed a long, narrow cluster running from the bottom left (a hydrogen nucleus) toward the top right (the currently heaviest element). The cluster resembled the image of the Outer Hebrides as seen on a map. Outside this cluster lay a ‘sea of instability’ in which nuclei could not exist, or not for long enough to be detected. Yet across this sea in a direction farther upward and to the right (corresponding, say, to the location of Shetland), might lie ‘islands of stability’ or ‘magic islands’ – the homes of yet-undiscovered superheavy nuclei with sets of protons and neutrons close to doubly magic numbers. The rumoured existence of these islands offered as powerful a lure to nuclear chemists as the fabled Spice Islands did to the explorers of old.
There seem to have been some differences of opinion within the Berkeley Lab concerning these ideas. Darleane Hoffman and Albert Ghiorso clearly believed in the idea of islands of stability or magic islands, because in a 2000 book they frequently used these phrases when describing their laboratory’s goals. I got a different story from Walter Loveland, a somewhat younger nuclear chemist from Oregon State University who joined the Berkeley Lab for the 1998-1999 year, and who played an important role in the ill-fated search for element 118. ‘I would disabuse you of this idea of the “island of stability”,’ he said in a 2006 interview. ‘Those predictions were made in the 1960s when it was thought that there would be a group of elements with half-lives that were long even relative to the age of the universe, and that they’d form this island. We don’t believe in that anymore – that’s not right. What we know is that there may be nuclei whose half-lives are longer than their neighbours’, but they seem to be connected to the mainland of lighter elements by a peninsula. They are not islands in a sea of instability.’
By 1998, when the effort to detect element 118 began, Berkeley’s glory days of element hunting were long over. Stanley Thompson had died in 1976*. Seaborg was 86, and in August of 1998 he suffered a devastating stroke that led to his death six months later. Ghiorso was 83; Swiatecki and Hoffman, the youngsters, were 72 and 71. Room 307 of Berkeley’s
Gilman Hall, where Seaborg identified plutonium, had been a US National Historic Landmark for 32 years. And though much other good work had been done at the Berkeley Lab, not a single new element had been discovered there in more than two decades.
The Berkeley Lab did have the tools to produce superheavy elements, however. One essential tool was the 88-inch Cyclotron – the giant descendant of the hand-held ‘proton merry-go-round’ invented by Ernest Lawrence in the 1920s. The Cyclotron accelerates nuclei of a chosen isotope (let’s say 48Ca, which are calcium nuclei with 20 protons and 28 neutrons, yielding a total mass number of 48) to speeds that can exceed 1,000 kilometres per second, giving them tremendous kinetic energy.
A steady beam of these energetic nuclei emerges from the Cyclotron and enters another piece of equipment, the gas-filled separator. This instrument was built by a Berkeley group led by Ken Gregorich, who belongs to a younger generation; he was one of Seaborg’s last graduate students. Within the separator, the beam passes through a thin foil made from another isotope, such as 244Pu (plutonium with 94 protons and 150 neutrons). The hope is that a very occasional beam nucleus will strike a target nucleus just right. If so, the kinetic energy of the beam nucleus overcomes the electrostatic repulsion between the two negatively charged nuclei (known as the Coulomb barrier) and the two nuclei fuse, yielding a compound nucleus. In this example it has 114 protons (making it the as-yet-unnamed superheavy element 114) plus 178 neutrons.
Because of the kinetic energy of the incoming 48Ca missile, the compound nucleus is put in an excited state, like a drop of water brought nearly to a boil. Most of this excitation energy is carried off almost instantly by the ‘evaporation’ of a few neutrons. The remaining, slightly lighter isotope of element 114 flies onward through the instrument, and a series of magnets deflect it from the main beam and deposit it onto a silicon detector. This nucleus is itself unstable: it breaks down into lighter nuclei over some period of time that might range from microseconds to minutes. The detector identifies the time, location and energy of the particles produced in these sequential breakdown events, and from this data the superheavy nucleus that gave rise to them can be identified. Voila, a new element – element 114 in the case of this hypothetical example – albeit just a single atom and a very short-lived one at that.
That was the basic idea, but up until then it hadn’t worked. Either the energy of the incoming nucleus was too low to overcome the reticence posed by the target nucleus’s Coulomb barrier, or it was too high and the target nucleus simply disintegrated, like a toad struck by a flying princess. There didn’t seem to be any ‘just-right’ level of energy – the gentle kiss that would allow the long-hidden Prince Charming to step forth.
Things seemed to change in 1998. A young Polish theoretical physicist by the name of Robert Smolanczuk, who was then at the Berkeley Lab’s German rival, GSI, did new calculations of the expected probabilities for the creation of superheavy elements by various fusion reactions. He reported that if atoms of 280Pb (the commonest natural isotope of lead) were bombarded with a beam of 86Kr nuclei (an isotope of the inert gas krypton) that had been accelerated to exactly the right energy, there was a surprisingly good probability of scoring hits that would generate nuclei of element 118 – a superheavy element far beyond what had been created up until then. ‘What Smolanczuk picked up on,’ Gregorich told me, ‘was that you need to bombard at an energy level that’s just over the Coulomb barrier: this happens to be the correct energy needed to make the product.’
Nuclear chemists calculate the probability of such successful hits in units called barns, whose name derives not from some famous scientist named Barn or Barnovsky, but from the phrase ‘can’t hit the broad side of a barn’. Most theorists thought that the probability for such reactions fell off exponentially with increasing atomic number, so that by the time one got to element 118, it might be down in the femtobarn range (one quadrillionth of a barn) or less, making the reaction essentially unachievable even in a lifetime of trying. Smolanczuk, on the other hand, pegged the probability as being many orders of magnitude higher, at 670 picobarns – nearly one billionth of a barn. This was still a challenging proposition but within the realm of possibility.
According to Smolanczuk’s calculations, the compound nucleus formed by the fusion reaction would evaporate just one neutron. The reaction would be a ‘cold fusion’, contrasted with the ‘hot fusion’ reactions in which the compound nucleus was more highly excited and evaporated several neutrons, like the 48Ca/244Pu example described earlier. Thus the reaction would leave an atom of 293118 – a nucleus with 118 protons and 175 neutrons. On the basis of shell theory, this atom seemed to lie on the western shore of a magic island: it could probably exist for a very brief period – a fraction of a millisecond – but it would have too few neutrons to last for longer.
Darleane Hoffman, still a leader of the Berkeley group in spite of having been officially retired for seven years, invited Smolanczuk to join the lab, which he did in October of 1998. His ideas got a very enthusiastic response. Hoffman, along with Albert Ghiorso, urged the junior members of the team to put Smolanczuk’s reaction to the test, and quickly. Smolanczuk had told the German and Russian groups about his ideas, and so the quest for element 118 had suddenly become an international horse race – in Hoffman and Ghiorso’s minds, at least.
Ken Gregorich, the designer of the gas-filled detector, and Walter Loveland, the visitor from Oregon State, took the lead in setting up the experimental apparatus. For the data analysis, they turned to Victor Ninov.
Ninov was born in Bulgaria, but had obtained his doctorate at GSI before coming to Berkeley. While at the German lab he took part in the research that led to the discovery of elements 107 to 112. Darleane Hoffman and Ken Gregorich hired Ninov away from GSI in 1996. Because of his achievements at the German lab, the move was thought to be quite a coup for the Berkeley group – an acquisition that would greatly increase their chances of finding a superheavy element of their own.
At Berkeley, Ninov worked closely with Gregorich on the construction of the gas-filled separator and its associated instruments. His most valuable sphere of expertise, however, was in developing and using the software that analysed the output of the detectors. This software had to search the instruments’ output files, which were binary files recorded on magnetic tape. The software looked for events whose time, location and magnitude corresponded to what would be expected for the breakdown of the nuclei that were being sought.
In the case of element 118, the original 293118 nucleus was expected to decay by giving off a sequence of alpha particles, each of which consisted of two protons and two neutrons. These alpha emissions were what the detector actually detected. The first alpha emission would mark the breakdown from 293118 to 289116 – itself an undiscovered element. The next would mark the breakdown of that nucleus to 285114, and so on all the way down to 269106, which is seaborgium. The entire cascade was expected to take about two seconds. The software was designed to pick this characteristic chain of alpha emissions out of millions of irrelevant events.
Ninov was clearly under a great deal of pressure in the last few months of 1996. ‘We… convinced Victor Ninov that the reaction should be run as soon as possible,’ wrote Ghiorso and Hoffman later, ‘as we greatly feared GSI or Dubna might do it first.’ Whether this pressure came only from Ghiorso and Hoffman, or also from Gregorich and Loveland is not clear. When I talked to the latter two men in 2006, both of them suggested that the experiment was planned more as a shakedown cruise for the new equipment than as a confident attempt to find a new chemical element.
For several months, technical difficulties with the apparatus delayed them. The general level of motivation jumped considerably in January of 1999, however, when a report came in that the Dubna lab had created a single atom of element 114, using the calcium-plutonium reaction described earlier.
The Russians had run through more than a million dollars’ worth of 48Ca to achieve that one seemingly successful
strike. If superheavy nuclei are defined as those with an atomic number greater than 112 – the usage favoured by Hoffman and Ghiorso – then this was the first superheavy nucleus ever created. What’s more, the nucleus stayed intact for all of half a minute before breaking down. If this finding was genuine, the Russians had already landed on or near an island of stability. ‘[W]e felt happy that at last the Magic Island had been found,’ wrote Ghiorso and Hoffman in their 2000 book, ‘and we redoubled our efforts to get our own experiment under way.’
On April 8, 1999, the Berkeley experiment finally began. Over a period of four days, the lead foil target was bombarded with 700,000,000,000,000,000 krypton nuclei, each of them boosted to an energy of nearly half a million electron volts. When Ninov applied his software to the resulting data, he found two alpha-decay chains. The energies of the alpha emissions and the time intervals between them were remarkably close to the values predicted by Robert Smolanczuk. It seemed clear that the run had produced two atoms of element 118, which had decayed into another never-before-seen element element – 116 – and then into element 114 and even lighter elements.
To be sure of the result, the group ran another experiment a couple of weeks later, in which they hurled more than twice as many krypton atoms at the target as they had done during the first run. The researchers were therefore expecting that they might see four or more alpha chains. In fact they only saw one, but it was a beauty, again confirming Smolanczuk’s predictions. So the research team assumed that the lower yield on the second run was just a statistical fluctuation, and they added the one sighting to the previous two, meaning that three atoms of element 118 had now been created.