Rutherford could now also claim to have been present at the artificial conception of atomic disintegration. The method he had pioneered with natural radiation to chip off sections of atomic matter had been modernised and amplified to fully disintegrate atoms through human means. All that was needed was for Cockcroft and Walton to ascertain the scientific certainty of their breakthrough, while keeping the influx of well-wishers and the morbidly curious at bay.
Working late into the subsequent evenings, the pair studied the results of more collisions and confirmed that their accelerated proton beam had transmuted the light metal lithium’s atoms, each with three protons in its nucleus, into beryllium, which has four protons. This new element existed but for fractions of a second before it disintegrated into a pair of nuclear fragments, both containing a helium nucleus. As Rutherford well knew, alpha particles are ionised helium atoms. It was these helium atoms that caused the scintillations they witnessed, their impact with the screen carrying far greater energy than that of the bombarding protons that had set them in motion.
The researchers knew the precise energies of the accelerated protons, as well as the atomic mass of both the lithium and the helium atoms. By measuring the decrease in mass that occurred when the transmutation occurred, and by observing the behaviour of the particles striking the scintillation screen, they calculated the kinetic energy liberated through this process to be 17.2 million electron volts. This number corresponded with Einstein’s equation linking mass with energy: the first time E = mc2 had been definitively proven through experiment.
The ‘big science’ era of high-energy particle accelerators had arrived, at the expense of Rutherford’s beloved benchtop experiments. But in applying the same principle he attached to laboratory apparatus, the professor conceded that the only way to fully understand the inner workings of the atom was to take it to pieces.
* * *
In the space of several months, not only had researchers at the Cavendish discovered the neutral particle that would allow far deeper penetration of the atoms of heavy elements, but they had also proven the methodology that would ultimately deliver it. The vast stores of untapped energy within the very essence of matter appeared ripe for harvest, if a means could be found to sustain and contain their release. German Hans Bethe, the 1967 Nobel laureate for physics, considered that everything learned before 1932 comprised ‘the pre-history of nuclear physics, and from 1932 on [was] the history of nuclear physics’.14
There was now no doubt, however, as to the Cavendish’s pre-eminence in the field of experimental physics. Under Rutherford’s stewardship, its resolute focus on questions pertaining to the structure and secrets of the atomic nucleus was unprecedented. As Chadwick later noted: ‘This was perhaps the first time that a great laboratory had concentrated so large a part of its effort on one particular problem.’15
* * *
The photograph of Cavendish Laboratory’s class of 1932 depicts nine eventual Nobel Prize recipients, eight of them seated in the front row. Immediately behind one of those – the grand old man J.J. Thomson – stands Mark Oliphant, his right ear tilted tellingly towards the camera.
As was the case for Rutherford upon landing at the Cavendish in 1895 – just as the discovery of x-rays and radiation altered the direction of physics research forever – Oliphant knew he now stood squarely at the epicentre of historic events, which were unfolding in a hurry at a monumental time. He just needed to find the appropriate vehicle to carry him to the heart of the action.
Rutherford’s own research path had been redefined more than thirty years earlier when Thomson recognised that the young man would be best deployed in pursuing the big questions that occupied the foremost minds. Rutherford would perpetuate that precedent, by setting his surrogate son onto a problem integral to science’s future trajectory – in the process, transforming Oliphant from talented experimentalist to nuclear physicist.
11
FUSION
North Wales and Cambridge, 1932 to 1933
It was early spring of that auspicious year, as the foxgloves blazed purple and pink against Celyn’s whitewashed walls, and the air warmed with staccato squawks from newborn lambs, that Rutherford opened the door for Mark Oliphant to join the atom smashers.
Until that holiday weekend – filled with morning walks along the Watkin Path and beside streams of rushing snow-melt overhung by willows’ canopies, as well as lazy afternoons surveying the valley from Celyn’s terrace – Oliphant’s work had centred on the radioactivity of potassium, and the acceleration of low-energy beams of positive ions through gas discharges. Now he would be let loose on the heavy weaponry.
Rutherford mistrusted the new, ever-bigger laboratory machinery, but he understood its necessity – particularly if Cockcroft and Walton’s apparatus could be refined to deliver a greater concentration of protons within beams aimed at various metal targets. So, as the Rutherfords and Oliphants enjoyed a weekend in Snowdonia, Rutherford suggested that his most gifted experimental engineer join him in formal collaboration to further the artificial disintegration of nuclei.
Oliphant scarcely needed to mull over such an offer. This was, after all, what he had envisaged when first he set his sights on working with the world’s foremost physicist.
The brief, as it was explained during those lazy couple of days at Celyn, was fairly fluid. Oliphant would utilise his flair for design and methodology to improve Cockcroft and Walton’s accelerator, in the hope of gaining more accurate measurements of the energies released in the transformation process. Rutherford would then bring to bear his innate understanding of sub-atomic structures, and the implications of their interplay, to interpret those findings. The professor’s faith in his protégé was underscored when Oliphant learned he had a free hand to design and execute the research.
Back at Cambridge, Oliphant set himself up in the cool, claustrophobic, stone-floored basement of the Cavendish’s renowned Rayleigh Wing. In the adjoining room, a brass plaque denoted the place where Lord Rayleigh had determined the standard unit of electrical measurement subsequently announced as the ohm. That was the same experimental space where J.J. Thomson had discovered the electron thirty-six years earlier, and where Rutherford and Chadwick had then achieved artificial transmutation of elements for the first time.
However, while Oliphant’s commission came with Rutherford’s full blessing, it was also subject to his intractable budgetary restrictions. The canal-ray (anode-ray) tube through which the protons would hurtle was fitted horizontally, in contrast to the lofty vertical set-up of the Cockcroft–Walton device, because the basement’s low ceiling would not accommodate an alternative configuration. Each of the linear amplifier’s six segments was housed within a separate biscuit tin. The design was a triumph of oddments made fit for purpose, with seals rendered airtight through strategic placement of plasticine, and the transformers that generated the required voltages mostly salvaged from discarded x-ray machines.
‘Like all Cavendish equipment up to that time,’ Oliphant later recalled, ‘ours was hastily assembled from whatever bits and pieces were available, so that it often gave trouble. Rutherford was very irritated by delays of this kind, but was singularly uninterested in finding the money to buy more reliable components.’1
By devising a machine that could fire a greater concentration of protons within a narrower beam, Oliphant was able to operate at less than a third of the voltage generated by Cockcroft and Walton. Had he replicated those earlier currents, it would have been his mortal soul, rather than the soles of his shoes, seared into the Cavendish’s stone floors, and its legend, when he strayed too close to the generator.
Not that anyone involved blithely dismissed the danger of their endeavours. In the new accelerator’s early iteration, its motor-driven generator and accompanying rectifiers, transformers and condensers were constructed atop a bare pinewood table. The table’s legs were strategically placed within oil-filled jars, to negate the electricity’s earthward charge. This
ploy was revised after the table heated so quickly it caught fire, when the voltage had reached barely half its required output.
However, the breakthrough that allowed Oliphant to make real progress was made not in the acknowledged heart of physics research at Cambridge, but amid the rapidly growing research capabilities across the Atlantic Ocean.
* * *
It was during Rutherford’s now-famous 1920 Bakerian Lecture, when he had so presciently floated his theory of the neutron, that he had also wondered aloud about the prospects of a heavy hydrogen atom. This, he foresaw, would be formed with a nuclear mass of two units – a proton and a neutral particle – rather than simply the one, and accompanied by a single electron.
A dozen years later, in that same annus mirabilis of 1932, Harold Urey confirmed the theory in his laboratory at Columbia University in New York. He named the isotope ‘deuterium’ and its atypical nucleus a ‘deuteron’.
Soon after this, Urey recognised that it might be possible to concentrate deuterium through the electrolysis of water. This rigorous process was successfully attempted the following year, and the resulting molecule dubbed ‘heavy water’. This substance exists naturally, with around one in every 6000 water molecules carrying the additional neutron that lends it double the mass of everyday H2O. Within a decade, its role as a neutron moderator used to slow the runaway process of uranium fission would make it integral to atomic energy production. But to artificially produce this elusive substance, American chemist G.N. Lewis constructed a twentymetre-tall separation plant in his laboratory at Berkeley near San Francisco. At peak operation, it gave up less than a teaspoon of heavy water per day. Given the huge amount of energy required to achieve that minimal result, it was deemed useful for little other than experimental value.
Rutherford, however, quickly saw the benefits that might be gained from using these deuterons in disintegration experiments, given that they consisted of a proton as well as the neutron he had already employed to circumvent the electrical forces of atomic nuclei. When Lewis visited the Cavendish in the summer of 1933, he presented Rutherford with a minute pair of sealed glass ampules, inside each of which a single drop of heavy water glistened.
Rutherford entrusted them directly to Oliphant, who, once he had successfully extracted the deuterium to release the prized hydrogen atoms, saw his experimental work gather fresh momentum, and momentous significance.
‘Straight away, a new world was opened to us,’ Oliphant would recall of the heavy hydrogen’s effect on his experimentation. ‘There were a new set of explosions, atomic explosions, which were terrific in their intensity, and in the number that took place. It was like entering a new realm of star-watching . . . when looking at these scintillations.’2
Trained in a beam and mixed with helium, these particles proved highly efficient in disintegrating the nuclei of light metals, but produced some puzzling results. Regardless of the target material into which the beam was fired, the outcome was curiously consistent. The new electronic counting mechanism, which had replaced zinc sulfide screens, detected an array of debris – protons and neutrons – all carrying similar amounts of energy. And if the beam was maintained for a long period, the emanations were increased rather than exhausted.
Finally, with Rutherford’s input, Oliphant deduced that whatever element was employed as the target for the accelerated particles was itself being covered with a coating of heavy hydrogen. Therefore, the scintillations being recorded came from deuterons bombarding deuterium.
To corroborate this thesis, Oliphant’s colleague Paul Harteck, an Austrian physical chemist, prepared small quantities of compounds containing heavy hydrogen, which were then used as targets. The first of these was ‘heavy’ ammonium chloride, effectively a form of smelling salts. When the beams containing deuterons struck the deuterons embedded in the targets, the pattern from the earlier collisions was repeated as long-range particles were liberated by violent collisions.
Identifying the matter being spat out by this disintegration required detailed measurement of the energy that each emanation carried. This was achieved by calculating the distance they could travel through a range of impossibly thin mica screens, some so sheer they replicated the stopping capability of less than one centimetre of air.
Using these tools, Oliphant found that one group of particles produced by the disintegration could barely make it beyond the mica screen and into the electronic counting chamber. These short-range particles were found to be ions of hydrogen that carried a single charge but also weighed three times as much as ordinary hydrogen.
What Oliphant had artificially produced was a previously unknown radioactive isotope of hydrogen that appears in minute quantities in water; in proportion to natural hydrogen, its presence is 1:10-18. Its increased mass was the result of a nucleus that contained a proton plus a pair of neutrons. Because it was triple the weight and structure of ordinary hydrogen, Rutherford, Oliphant and Harteck christened the new entity ‘tritium’, and the particles they had discovered ‘tritons’.
The finding meant there were now three known types of hydrogen, the universe’s most abundant element: ordinary hydrogen with a solitary proton; ‘heavy’ hydrogen, which also carried a deuteron; and the new tritium, or hydrogen-3. Closer examination revealed that the transformation came about when the high-speed collision of deuterons with deuterium liberated large numbers of protons and neutrons, as well as the proton–double neutron combination at the heart of hydrogen-3.
With one puzzle solved, others immediately emerged – among them, the identity of other particles released in this explosion of sub-atomic fury. That hunt called for technical wizardry that surpassed even Oliphant’s.
George Crowe, who served as Rutherford’s laboratory steward for so long he lost the tips of several fingers due to the highly radioactive alpha particle source elements he regularly handled, was able to split a mica screen of such gossamer fragility it exerted stopping power the equivalent of just 1.5 millimetres of air. It was only by being fixed to a fine mesh of brass that this barely there filter was able to withstand the atmospheric pressure.
The delicate screen allowed Oliphant to study low-range particles that flew a very short distance from the target. This in turn brought another discovery – in circumstances that Oliphant would delightedly recount to scientific conferences, at public addresses, in media interviews and during fireside chats for the next half a century.
We found a group of particles which clearly carried a double charge and appeared to be alpha particles, in numbers equal to the protons and tritons. The observation produced consternation among us. The equality of fluxes suggested that all three groups of charged particles originated in the same process. Rutherford produced hypothesis after hypothesis, going back to the records again and again, and doing abortive arithmetic throughout the afternoon. Finally, we gave up and went home to think about it.
I went over all the afternoon’s work again, telephoned Cockcroft who had no new ideas to offer, and went to bed tired out. At 3am the telephone rang. Fearing bad news, for a call at that time is always ominous, my wife, who wakens instantly, answered it and came back to tell me that ‘the Professor’ [Rutherford] wanted to speak to me.
Still drugged with sleep, I hear an apologetic voice express sorrow for waking me, then excitedly say: ‘I’ve got it. Those short-range particles are helium mass three.’ Shocked into attention, I asked on what possible grounds could he conclude that this was so, as no possible combination of twice two could give particles of mass three and one of mass unity.
Rutherford roared: ‘Reasons! Reasons! I feel it in my water!’ He then told me that he believed the helium particle of mass three to be the companion of a neutron, produced in an alternative reaction which just happened to occur with the same probability as the reaction producing protons and tritons.
I went back to bed, but not to sleep. I called in to see Rutherford at Newnham Cottage after breakfast, and went through his approximate calculations with
him. We agreed that the way to clinch the conclusion was to measure, as accurately as we could, the range of the doubly charged group of particles, and the energy of the neutrons . . . Of course, Rutherford was right.
By the end of the morning we had satisfied ourselves that an alternative reaction of two deuterons, produced a neutron and a helium particle of mass three, the energy released being close to that in the other reaction. The mass of helium three worked out to be a little less than that of tritium.3
* * *
This series of elegantly brilliant experiments would stand as the pinnacle of Oliphant’s hands-on research career. He later described that time of discovery that culminated in the northern spring of 1934 as ‘the most thrilling’ among all those in which he was directly involved at the Cavendish.
What neither he, nor those working with him, comprehended at the time was the enormity of the secret his improvised accelerator and restive inquisitiveness had uncorked.
The deuterium nuclei that Oliphant and Harteck (under Rutherford’s guidance) fired at a deuterium target carried sufficiently high energy to break through electrical repulsion forces within the particles and fuse them together. This coupling of two deuterium nuclei formed the nucleus of the next-lightest element on the periodic table (helium) and released vast levels of heat and gamma radiation in the process.
The heat (in the form of particle acceleration) that was needed to overcome the electrical repulsion and achieve this result meant the process became known as ‘thermonuclear fusion’. What had been successfully replicated, for the first time, at the Cavendish was the method by which the sun produces energy to sustain life on earth. It is, essentially, a giant nuclear reactor that uses hydrogen gas as fuel and huge pressure at its core to fuse around 300 million tons of hydrogen into helium each day. The energy released through this transformation creates a temperature at the sun’s centre estimated to be about 20 million degrees centigrade.
The Basis of Everything Page 18