Superheavy

by Kit Chapman

  A day earlier, the Berkeley–Livermore team had attempted to make element 116. When the scientists had checked their catcher foils, they had seen a thin layer of black crud. Deciding to play it safe, the crud had been analysed along with everything else. Placing it in the detectors, it soon started pinging with spontaneous fission. They hadn’t just made element 116, they had chemically isolated it too. Excitement gripped the lab. It was, potentially, the greatest discovery since fission.

  Ghiorso picked up the phone and called Stanley Thompson to tell him the news. Aged only 64, Thompson was on his deathbed with cancer. The great chemist, a man who had come up with the technique to isolate plutonium and who had overseen the first atomic bomb’s production in Hanford, was too ill to reply. ‘[I] talked to him,’ Ghiorso later wrote in The Transuranium People, ‘and was certain that Stan understood what had been accomplished.’

  Before he settled for the night, Ghiorso checked his results. Something was nagging him. Taking a midnight stroll to the lab, he checked the filter paper, which should have been clean. It was giving off radioactivity too. Puzzled, he decided to get some sleep. The nightmare was to follow soon after.

  In the morning, during his shower, Ghiorso realised what had happened. The Berkeley team hadn’t seen a superheavy element at all. The radioactivity was just from fission, while the mysterious crud was the charred remains of glue used to cement the foils in place. It was a false alarm. But it was too late to tell Thompson. Shortly after his phone call with Ghiorso, he had passed away.1

  In his eulogy, Thompson’s son-in-law Kenneth Lincoln gave him a Lakota name: Cante Ksapa – ‘Wise Heart’. ‘He was a man to be liked and respected – a man of old values, the essential and simple ways of living … a man of good will, with many friends from all walks of life.’ Glenn Seaborg’s own tribute was just as potent. ‘His radiochemical research during the Second World War rivals in importance the isolation of radium by Marie and Pierre Curie, and his leadership in the discovery of five transuranium elements must rank as among the leading chemical accomplishments of his time […] chemistry lost an extraordinary practitioner, and I lost a lifelong friend.’

  Stanley Thompson was the first of the transuranic element giants to fall. But a new generation of researchers were emerging in Germany – and a host of new elements were about to follow.

  * * *

  Wixhausen is an unfortunate name for a borough. When it was founded a little over 800 years ago, Wickenhusen just meant ‘houses on the pond’. Gradually, the name has shifted and German slang has evolved. Today, to the embarrassment or amusement of the locals, ‘wix’ sounds like the word for … uhm … to pleasure oneself. No wonder the local laboratory is largely referred to by the nearest city instead: Darmstadt.

  The GSI Helmholtz Centre for Heavy Ion Research (GSI for short) is on the edge of town. Heading out to the lab means cutting past car dealerships and auto body shops and then through open fields and thick woodland. Gradually, things begin to get stranger. Trees give way to lavish, glass-fronted conference centres. Road signs warn you of migrating hordes of toads. Parks are decorated with scattered remains of particle accelerators lovingly preserved as art installations. It’s only then you realise you’ve arrived at one of the most advanced laboratories in Europe. Despite its apparent isolation, the locals know all about this jewel in the crown of German science; the last time it had an open day, 11,000 people turned up at the gates. Out back, still a building site with only a few of the tools in place, GSI is building FAIR – the Facility for Antiproton and Ion Research – that will see scientists from 50 countries partnering to smash things together, annihilate matter and try to discover the very origins of the universe. In another part of the lab, nuclear technicians pioneer targeted therapy beams for destroying cancers, their machines able to set the beam intensity to pass harmlessly through tissue and bone before irradiating a tumour with complete precision. GSI unlocks worlds and saves lives.

  The lab is part of the West German economic miracle, evidence of the staggering speed with which the country rebuilt itself after the Second World War. By 1969 nuclear physics departments across Germany had been building research accelerators at such a rate that there were 20 small-to-medium machines across the country, all in competition. This was such a duplication of effort that the universities in Darmstadt, Frankfurt and Marburg decided to pool their resources together; later, universities in Giessen, Heidelberg and Mainz joined the project. The result was GSI – the first lab that could not only compete with Berkeley and Dubna, but beat them.

  ‘It wasn’t clear where to build it,’ remembers Gottfried Münzenberg, as we sit down in the cafe to relax with a coffee. ‘There was competition with Heidelberg, with Karlsruhe … the idea was to come here because Darmstadt gave the land, and it’s not far from Frankfurt Airport.’

  Münzenberg is easy to interview. He cuts a relaxed figure: warm, affable and friendly. White-haired and bearing a passing resemblance to David McCallum’s character Ducky from NCIS, he relishes in sharing his stories between pastry bites. Münzenberg hails from Wolfsburg – the home of Al Ghiorso’s Volkswagen Beetle – and learned his English in the 1960s when he came to the UK to study at another car manufacturing hub: Luton. He chuckles as I assure him it hasn’t changed much.

  Once GSI’s site was decided, the Germans had to build their accelerator. ‘The idea was to create an accelerator that could accelerate all the elements,’ Münzenberg recalls. ‘It wasn’t clear what to shoot … all that was clear was that the Berkeley recipes wouldn’t work! So, we had to build an accelerator, and we had to build a spectrometer that was capable of detecting everything.’

  It was ambition on a scale unmatched in nuclear science. Once again, it was a team starting from scratch and inventing their own way. Münzenberg was there from the start as an expert in ion optics, a fresh-faced postdoc with boundless energy who would do whatever was asked. Günter Herrmann, a physicist from the University of Mainz and another member of the GSI team, called in favours to get special permission to use radioactive materials for research (‘You can’t do that these days, it’s forbidden,’ Münzenberg says, ‘but once we had permission, we had permission…’). Others worked on the target and the interactions with matter. The ion source, in a surprising twist, was given to the Germans by Georgy Flerov.

  The German team applied themselves, but it wasn’t all work and no play. Part of preparing the machine required the condenser to be cleaned with alcohol. Münzenberg and another colleague doctored the solution with a few ‘organic contaminants’. It wasn’t enough to ruin the clean-up, just enough to make the whole lab smell of whiskey. ‘We had a lot of fun,’ Münzenberg winks.

  The man put in charge overall was Peter Armbruster. Handsome, dark-haired and sporting long sideburns, Armbruster had grown up in Dachau, under the shadow of its concentration camp. In the 1950s he had studied physics at the technical universities of Stuttgart and Munich and had become increasingly interested in heavy ion fission. As GSI’s senior scientist, he had complete authority to decide what the German lab would investigate. ‘Armbruster was the person,’ Münzenberg recalls. ‘It was the perfect team. Armbruster was the boss – never the group leader, always a director. He chose the [equipment]. No committee, no discussion. He prepared the whole thing … we did presentations, he wrote them up overnight and submitted them the next day.’

  It was a sign of how fast things at GSI would move. The lab went from breaking ground to building the most advanced accelerator in the world – the Universal Linear Accelerator (UNILAC) – in five years. ‘It was very exciting,’ Münzenberg laughs. ‘We started one year, made the design the next, the year after we built it … everyone said “what you do will never work” … but it was very fast. And we were lucky. Nature was good to us.’

  Münzenberg is being modest. Under his leadership, the Germans were about to double the number of superheavy elements.

  * * *

  ‘So, this is the birthplace of our heavy
elements.’ Michael Block is the current superheavy element physics lead at GSI. He’s giving me a walking tour, starting with the staggering vastness of UNILAC. We’re in a long concrete tunnel, straight as an arrow, with a giant purple tube towering next to us as it runs the entire 120m (400ft) to the target. Its bulk is large enough for several people to climb inside and walk upright. At key points, latched into the gargantuan accelerator like suction cups on an udder, metal cylinders prod out. These are connected to thick black wires that eventually coil out into the wall.

  Figure 10 Maintaining the interior of the UNILAC linear accelerator, GSI Darmstadt.

  A cyclotron is impressive because it’s basically an alien disc lodged under the largest magnet you’ve ever seen. A linear accelerator is even more awesome. Watching it stretch out into the distance, it looks and feels like a sci-fi space cannon. GSI believes that it’s also the better option for element hunting. ‘You need a high-intensity beam,’ Block says. ‘And if you want high intensity, then you better go straight.’

  We walk the tunnel, side by side next to the giant ion gun barrel, occasionally stopping to see where a part has been removed for maintenance. At these points the accelerator’s hollow insides are revealed. Each section is a segmented chamber coated with gleaming copper. Inside, a line of metal rings like thick doughnuts are suspended by rods in the centre of the pipe’s maw. The beam passes through these doughnut holes using the carrot-and-stick method perfected by 80 years of particle bombardment. At the start of the accelerator, the beam element (let’s say uranium) is placed in a gas-filled chamber. A voltage is applied, tearing off a couple of the atom’s electrons, before an electric field pulls the uranium ions into the accelerator. Here, pushed and pulled by changing the electric voltage at exactly the right moment, they hit their target at 30,000km (18,600 miles) a second – about 10 per cent the speed of light. The whole trip takes 10 microseconds.

  GSI’s system is designed to run several different experiments at once. Rather than firing a continuous stream of ions, the beam comes in 5-millisecond pulses every 20 milliseconds. About 1 per cent of ions are diverted to GSI’s synchrotron, a closed ring where the ions continue to build up speed, flying around in circles hundreds of thousands of times before eventually hitting 270,000km (170,000 miles) a second. At that speed, you go from the Earth to the Sun in about nine minutes. Once FAIR is operational, its 1.1km (0.7 mile) accelerator ring will be able to speed up 500 billion uranium atoms to 95 per cent the speed of light in a single burst.

  The other 99 per cent of the ions go on to the bitter end, eventually slamming into a piece of target that’s thinner than kitchen foil. Block picks up an example, a strip of lead-208 that’s already been bombarded. Its skin has flared deep yellow. ‘See this different-coloured part? We don’t use just lead – we use lead sulfide. It can take more beam before it’s used up. Usually, it’s good for running for a couple of weeks.’ As lead is in easy supply (as is bismuth, the other possible target), GSI buys in bulk and mounts several targets at once on a rapidly spinning wheel: that way, the beam gets shared around rather than just hitting the same spot endlessly.

  ‘In the good old days, we could run up to 6,000 hours of beam a year,’ Block says. ‘Nowadays we have three or four months, with maintenance breaks in between. A lack of manpower restricts us because of the construction of FAIR.’

  We head away from the accelerator and into a control room that looks like the bridge of the starship Enterprise. Consoles with little lights, endless dials, readouts and gauges, all set in panels painted a potent burned orange that hasn’t been in style since the 1970s. ‘It’s a bit like Star Trek, y’know?’ Block grins. ‘This was how we used to have things set up. Nowadays it’s had a major makeover. We can control everything far more easily: switching between elements, ions, beam lines, changing the pattern of the pulses, changing the intensity of the beam and changing the energy.’ Everything is simpler too. When GSI first formed, the laboratory had an entire electronics shack for the experiments that was filled with equipment. Today, advances in computing and digital measurement means a single panel the size of a desktop computer can run the whole experiment.

  But UNILAC is only half of the reason for GSI’s success. It’s all very well being able to create an element – you still have to spot it too. When GSI was formed, the reaction cross sections were so low that it was virtually impossible to find your creations among all the background noise of ions pinging everywhere. GSI’s answer was the Separator for Heavy Ion Reaction Products – SHIP. The name wasn’t an accident; remembering the sea of instability, the Germans were convinced they could sail off to find the new elements before their superpower competition.

  Block leads me through a maze of walkways, crawlways, narrow gaps and locked doorways. We’re heading into the very belly of the beast – the home of the separator. SHIP is a complex piece of kit, but the idea behind it is relatively simple. If a fusion product is created (by the ions hitting the target), the laws of physics mean it’ll be travelling at a much slower velocity than the ions in the beam. With a complex system of magnets and electric fields, the high-velocity ions can be steered away into a dead end, allowing only the slower particles to head into the detectors. It’s a technique called in-flight recoil separation.

  Free from the beam, any fusion products pass through detectors without all the noise. Now you can measure everything you need to prove an element has been created. First, an atomic-sized speed trap – two foils about 30cm (12in) apart – records the time of flight (usually about one to two milliseconds). Then, an array of detectors waits to pick up any signs of alpha radiation. ‘Alpha decay goes in all directions,’ Block says. ‘So, if you only had one detector, at most you’d see half of it. We send the particles into a box covered with detectors. Most particles hit the sides of the box or the stop detector. It gives you a coverage of 80 to 90 per cent.’

  I blink and try and get my head around things. Block is playing with one of the magnets, holding up a spanner a few inches away and letting go, watching it catapult in mid-air and attach to the side. Fortunately, he’s also spotted my confusion. ‘So, production of a new element. New atom recoils out, goes through the filter [to remove the beam]. It implants into the wall. Then you have a chance to register alpha decays.’ Simple, right?

  This isn’t the only thing GSI can do. Gone are the guessing games about what you’ve made that are associated with spontaneous fission and alpha decay: GSI’s equipment can weigh individual atoms as they fly past. As each isotope has a unique weight, this allows you to tell exactly what you’ve created. More importantly, mass measurement allows you to detect if you’ve made a single atom of something within the island of stability. While these atoms are unlikely to alpha decay or fission in the detector (that should take millions of years, remember), with mass measurements you can still spot if something has been created.

  ‘How sensitive is your mass measurement?’ I ask.

  Block thinks for a moment, staring up at the ceiling and sucking in a breath as he comes up with an answer. ‘Imagine a giant commercial airliner,’ he finally decides. ‘An Airbus A380. Our weight changes are so sensitive I could detect if you left a 1 cent coin on a seat in first class.’

  All of these different measurements make the sensitivity of GSI’s machine astonishing. ‘We’ve hit a cross section of 90 femtometers.’ Block says. That’s 10-41m2. I try and keep in my head that cross section is a measure of probability, rather than size. At that level, even the most improbable reactions – the slimmest chances of fusion occurring – will eventually happen just through chance. All you need to do is keep your machine running long enough.

  I think about the elements yet undiscovered. ‘So, ignoring how much it would cost, if you kept everything running non-stop for 10 years, you might find something?’

  Block shrugs. ‘You can find anything if you have enough time.’

  ‘And money,’ I add.

  Block nods ruefully. ‘And money.’
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br />   * * *

  Sigurd Hofmann is a man who comes prepared. Some interviewees sit down at a table and remember the good times; others tell you their own perspective then dare you to challenge their views. Hofmann has brought along a slide presentation and 26 peer-reviewed scientific papers. In the comfort of GSI’s modern office block, we sit down over tea and biscuits and begin to delve into the world of element-making.

  Hofmann was another early arrival at GSI. He was born in 1944 in what was then the Sudetenland of Germany, where his father worked making Bohemian glass for lamps. When he was 16 months old, the Russian army pushed into the region and the victorious Czechs forced his family to flee, first to Thuringia, then to Darmstadt. Here they settled, inadvertently placing the young Sigurd in one of West Germany’s leading science hubs. Electing to stay close to home, he studied physics at the city’s technical university, where he gained experience in nuclear reactions and the emerging science of computer programming. ‘I had one of the first computers,’ he recalls. ‘It had 8k of memory, and I wrote a program so I could analyse gamma spectra.’ When GSI was created nearby, the team needed a computer expert for SHIP. Hofmann was the perfect candidate.

  The accelerator at GSI started up for the first time in 1976. It was the only lab in the world capable of accelerating uranium, and so for five years mostly shot uranium beams (why waste the best accelerator in the world on something others could do?). But the team yearned to try to get involved in element discovery. In 1976/77, the team managed to muster five and a half days of beam time to spend hunting for superheavies, aiming to create elements as high as 122. They had about as much luck as every other lab. ‘I no longer remember in detail our great disappointment,’ Hofmann wrote in his book On Beyond Uranium. ‘Disappointments are easy to repress. But the records do not refer to any bottles of champagne.’