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In the summer of 1936, Segrè, then newly appointed professor of physics at the University of Palermo, visited Berkeley, to see the cyclotron, to escape the heat at his main place of sojourn, Columbia, and to survey possibilities of escaping from the heat in Italy should war threaten. He returned to Palermo "still dreaming of the cyclotron" and carrying some bits of copper strip that he had scavenged from the chamber of the 27-inch. He and his associates separated radioisotopes of copper, zinc, and perhaps manganese from the scrap. 'Twas but antipasto. "We would like very

much to have more copper," Segrè wrote. "I think you can send any substance in a letter."^[38] Meanwhile the cyclotron had been opened for repairs. Lawrence salvaged more copper and the molybdenum strip that protected the dee edge at the exit slot. He had it all cut up and sent in several letters. It was an act of headstrong generosity. Lawrence suspected that the molybdenum contained an activity of long life but of too little promise to add its investigation to the rigorous Berkeley routine. "We are all very busy here, but there is nothing very exciting at the time to report."^[39] There would have been some excitement had they kept the hot molybdenum.

If the long activity arose via (d,n) on molybdenum, it belonged to element 43. Number 43, alias davyum, lucium, nipponium, and masurium, had been nondiscovered several times. No trace of it had turned up in the surveys of x-ray spectra by H.G.J. Moseley and his successors; the only evidence for its existence when Lawrence sent Segrè his second installment of scrap was three faint x-ray lines observed by Walter and Ida Noddack in 1925 in the course of their successful detection of element 75 (rhenium). Segrè and Carlo Perrier, Palermo's professor of mineralogy and an accomplished analytical chemist, took the Laboratory's molybdenum apart. They separated a large amount of radiophosphorus, which they found to contaminate everything from Berkeley, and handed it to colleagues in physiology to administer to rats.^[40] They tried to carry the residual activity on molybdenum and the elements immediately below it, niobium and zirconium, but to no avail; its chemistry was closer to that of rhenium, the heavy homologue of "masurium." In April Segrè notified Lawrence: "All the activity is due to some substances which have all chemical characters one would expect to find in the element 43." Perrier and Segrè saw indications of three different radioisotopes of 43, to which the Palermo group soon attached half-lives of 90, 50, and 80 days, in order of relative abundance. As for stable isotopes of "masurium," Perrier and Segrè declared

them to be "absent," indeed, nonexistent; no known element could be invoked to act as a carrier for the new activity.^[41] Consequently, just after the war, when weighable amounts of element 43 were created in nuclear reactors, Perrier and Segrè annihilated "masurium" and named their element, the first made by man before its discovery in nature, "technetium."^[42]

"The cyclotron evidently proves to be a sort of hen laying golden eggs." So Segrè wrote Lawrence at the start of the analysis of the molybdenum strip. He publicly acknowledged his gratitude not only by the usual professional thanks, but also, what Lawrence no doubt preferred, by advertising the cyclotron. Perrier and Segrè concluded their presentation of the radiochemistry of element 43: "We hope also that this research carried on months after the end of the irradiation and thousands of miles from the cyclotron may help to show the tremendous possibilities of this instrument." Segrè asked for more active long-lived material and sent some purified uranium oxide for irradiation by slow neutrons in the hope of making more unnatural elements, perhaps alpha emitters from transurania. "I think that when you are producing neutrons every point near to the cyclotron gets a stronger irradiation than with the most powerful sources available in Europe."^[43]

Lawrence at first was more impressed by the salvage and application of P³² than by the news of element 43: "We are all amazed here to to see the amount of good work you have done with such a trivial amount of radiophosphorus." As for claims based on complex radiochemical analysis, he had reason to be wary: "Of course there are difficulties."^[44] By the fall of 1937, when sending P³² for Segrè's colleagues and more scrap for Segrè himself, he wrote, with his usual optimism: "We are only too glad to send you

material because you have accomplished so much with what little we have furnished." Indeed, although it was not known at the time, Segrè had accomplished precisely what Lawrence had prepared: the creation of new materials interesting in themselves and applicable to medicine. The element has been in clinical use since 1963. One isotope, Tc⁹⁹ , which gives a useful gamma ray, is an important agent in visualizing tumors and abscesses of the liver, in imaging the living skeleton, and in brain scanning. Its production by accelerators became the basis of a multi-million dollar industry.^[45]

As Aristotle said, the road from Athens to Corinth runs also from Corinth to Athens. In the case of technetium, Europeans detected a new element in material made in Berkeley. In the case of the isobars of mass three, the Laboratory made a discovery prepared in Europe. Soon after Rutherford and his collaborators had identified the d-d reactions, the Cavendish and also Fritz Paneth and G.P. Thomson at Imperial College, London, sought to produce enough H³ and He³ in Cockcroft-Walton accelerators to investigate their physical and chemical properties. All failed. Rutherford supposed that the elusive isobars combined with stray protons and neutrons to form alpha particles, of which, however, no trace could be found.^[46] Efforts to detect He³ by the spectroscope also failed, except in passing and at Princeton, where physicists saw and then did not see lines in the spectra of the products of d-d reactions ascribable to a light helium isotope.^[47] By 1935 the case of the uncollectible isobars was attracting attention on both sides of the Atlantic.

The most promising route appeared to be the isolation of "triterium," as Rutherford called it, from heavy water. This quest rested on the assumption that H³ is stable. Weighing in its favor was a report by Tuve's group of the presence of tritium in the

heavy water made by Urey and F.G. Brickwedde and the apparent slight excess in mass of lightest helium over heaviest hydrogen.^[48] But Tuve's result had not been duplicated and the argument from the masses was far from secure. Although the Cavendish's values of the energies and masses entering into the reaction H² (d,p)H³ were regarded as so reliable that the mass of H³ determined from them stood as one of the most certain of nuclear constants, the mass of He³ , deduced from the reaction H² (d,n)He³ , stumbled over the difficulty of measuring the kinetic energy of the liberated neutron. Rutherford's group made He³ just slightly heavier than H³ ; analysis by the meticulous T.W. Bonner and W.M. Brubaker, who also used the reaction Li⁶ (d,a )He³ , made the masses the same within experimental error; and Hans Bethe and R.F. Bacher, scrutinizing it all in the spring of 1936, awarded He³ the tenuous excess of about two ten-thousandths of a mass unit.^[49] Excess in the atom, as in the human, may be a sign of instability. Already in 1934, the Cavendish physicists conjectured that He³ might transmute into stable H³ by emission of a positron, in the manner then just made fashionable by the studies of Joliot and Curie.^[50]

The first in the field were Walker Bleakney and his associates at Princeton, who had electrolyzed 75 tons of ordinary water down to its heaviest cubic centimeter before learning of the Cavendish evidence for the existence of H³ . They set their precious material free in Bleakney's mass-spectrometer, found traces of particles of mass five that they declared to be molecules of H² H³ , and inferred that H³ constitutes about one part in a billion of ordinary water. Their colleagues at Princeton, G.P. Harnwell and H.D. Smyth, corroborated their finding by running the product of a gas discharge in deuterium into the spectrograph; and, in a further dividend, the Princeton mass-spectroscopists identified He³ in the d-d product as well. A year later Tuve's group again reported

"stable hydrogen atoms of mass 3 in numerous electrolytic deuterium samples" put through their magnetic analyzer.^[51]

The British were unable to duplicate the feats at Princeton and Washington. From Norsk Hydro, which by the mid 1930s had become the world's largest producer of deuterium, the Cavendish received 11 grams of the heaviest and most expensive remains of the electrolysis of 13,000 tons of water. (In 1935 Norsk Hydro-Elektrisk Kvaelstofaktieselskab sold almost pure heavy water for $1.25/g in quantities over 50 grams; in 1938, the price had declined to 75 cents/g for lots of 25 grams.)^[52] Aston could not find a drop of heaviest water in the Norwegian stock. Meanwhile, Mark Oliphant and Fritz Paneth and G.P. Thomson had failed to confirm Harnwell and Smyth, and guessed that the Princetonians, who had begun to doubt themselves, had been misled by the release of helium that had been dissolved into the glass walls of their discharge tube.^[53] By mid 1937 a consensus of sorts had been reached. "The claims of the Americans . . . were ill-founded," Thomson wrote Rutherford after visiting Princeton. "I am glad that you are coming to a similar conclusion." The next move appeared to Thomson to be to persuade Lawrence to devote a large amount of cyclotron time to irradiating a bucket of heavy water with deuterons in order to search for the spectrum of He³ .^[54] Nothing seems to have come of his proposal. The Laboratory had a more certain manufacture than the elusive isobars of mass three.

In July 1939 owing to the idleness of the 60-inch cyclotron as it awaited adequate shielding, the matter was reopened in Berkeley. Luis Alvarez thought to fuse deuterons in the 37-inch and feed the product into the 60-inch, which he would use as a giant mass-

spectrograph. He apparently accepted Bethe's revaluation of the mass data in 1938, which corrected in the wrong direction made He³ definitely heavier than H³ and allotted light helium a period of 5,000 years. "This would mean that He³ cannot be found in nature."^[55] This was to ignore the recomputation made by Bethe and Livingston in 1937, using revised values of the relevant parameters, which lowered the mass difference by a factor of ten; and also the reconsiderations of Bonner, who found that neutrons carried off more energy in H² (d,n)He³ than he had thought, and lowered the mass of He³ below that of H³ .^[56] Rutherford thought it safest to assume the stability of both isobars. Holding with Bethe, Alvarez was alert to any anomaly that indicated that He³ does not have a period of five millennia.^[57]

When Alvarez and a graduate student, Robert Cornog, began their search, the magnetic field of the 60-inch cyclotron was set to accelerate alpha particles. As a preliminary check on the background radiation through the machine, Alvarez watched an oscilloscope that monitored the current to the target while the operating crew decreased the field. The current through the cyclotron fell to zero, as expected, when the field no longer held the alpha particles in phase with the radio frequency potential on the dees. At the end of the test, the crew turned off the field after readjusting it for accelerating alpha particles. As the rapidly changing field passed through the setting for mass three, a sudden burst of particles registered on the oscilloscope. The general phenomenon—a momentary spike in a cyclotron beam as the magnet current sweeps rapidly through the resonance point—had been noticed by M.C. Henderson and Milton White, who explained that the changing magnetic flux set up eddy currents in the pole faces that acted like shims.

Alvarez did not expect to see a spike. It made his day ("one of the finest moments of my scientific life"). It indicated particles of mass three in the cyclotron's source. The source was natural

helium from a deep well in Texas, where it had lain for geologic ages. Evidently He³ had a half-life greater than 5,000 years. In fact, as Alvarez proclaimed, it is stable.^[58] He and Cornog shimmed the 60-inch to give a He³ beam and estimated the relative abundance of the light and heavy isotopes as 10^–7 for atmospheric, and 10^–8 for well helium. (These numbers are low by a factor of ten; the higher percentage in the atmosphere arises from creation of He³ by cosmic rays.) That solved half the old problem. They then looked for radioactive H³ in the product of d-d reactions passed into an ionization chamber. A new long activity with chemical properties of hydrogen rewarded their search.^[59]

The most obvious next step was to determine the half-life of the radioactive hydrogen. A first estimate, made by Thanksgiving day, 1939, was 230 days, later diminished to 150 days, and then raised to perhaps ten years. The lower numbers may stand as a warning to unwary experimenters: as Cornog discovered to his chagrin, they measured the rate, not of decay, but of the leak of hydrogen through a rubber tube used in the apparatus.^[60] This last number came from McMillan, who had measured the half-life of H³ without discovering it. He had guessed that an activity he had noticed in 1936 and reckoned at ten years belonged to a suppositious Be¹⁰ made by (d,p) along with the B¹⁰ made by (d,n) in the usual cyclotron irradiation of beryllium. After the disclosure of the activity of H³ , physicists at the University of Chicago got a nice radioactive gas on dissolving a specimen similar to McMillan's. They supposed they dealt with the product of the reaction Be⁹ (d,H³ )Be⁸ and that McMillan's half-life characterized tritium.^[61] The period is, in fact, 12.6 years. Tritium is constantly

made in the atmosphere by cosmic rays. The heavy Norwegian rainwater examined by the Cavendish consequently contained radioactive hydrogen, which lived long enough to reveal itself to a Geiger counter after the war.^[62]

Lawrence was delighted with the identification of the isobars of mass three. He gave it pride of place in his report to the Research Corporation on the Laboratory's work for 1939. When the report was submitted early in 1940, negotiations with the Rockefeller Foundation over the 184-inch cyclotron had reached their critical phase. Lawrence gave the result a gloss that he doubtless expected Poillon to pass on to the Foundation. "Radioactively labelled hydrogen opens up a tremendously wide and fruitful field of investigation in all biology and chemistry."^[63]