Gluttony at the Periodic Table
The Laboratory's earliest radiochemistry, apart from radiosodium, concerned nitrogen and oxygen. These elements lent themselves to experiment: they could be obtained very pure and deployed without fear of surface contamination; they have few
naturally occurring isotopes to confuse analysis; and they are easily excited by deuterons at 2 or 3 MeV. The experimental setup, which remained standard for gases, is indicated in figure 8.1. The first to work it were Lawrence, Henderson, and McMillan. They attacked the air and detected three groups of alpha particles and two of protons, which they assigned to reactions of nitrogen, but found no radioelements and did no chemistry. The ranges they measured for the alpha particles and protons did not agree with more careful determinations by Cockcroft; once again Lawrence had to remeasure and retract. McMillan persisted, substituted nitrogen for air and Livingston for Lawrence and Henderson, and uncovered a positron activity that lasted about two minutes. A little chemistry showed that the active substance formed water; a little reasoning ascribed the activity to a new radioelement, O15 , half-life 126 seconds, and to the reactions N14 (d,n)O15 , O15® N15 + e+ .[27] In a parallel investigation, Henry Newson, who came from and returned to the Chemistry Department at the University of Chicago, found that F17 (t = 1.16 m)
Fig. 8.1
Experimental arrangement for irradiation of gases.
Lawrence, McMillan, and Henderson,
PR, 47 (1935), 276.
could be made by (d,n) on O16 . He thereby recovered a known activity made by (a ,n) on N14 and raised his reputation in the Laboratory. (Lawrence had judged him to lack the pushiness needed to accomplish anything there.)[28]
Meanwhile McMillan and Lawrence worked on aluminum, which has but one natural isotope, and Henderson attacked magnesium, which has three. They used an apparatus similar to that of figure 8.1 with the target mounted in the beam. Protons, positrons, alpha particles, and neutrons came off aluminum, which could provide them all via the reactions Al27 (d,p)Al28 ® Si28 + e+ , Al27 (d,a )Mg25 , and Al27 (d,n)Si28 . The activity of Al28 (t = 156 sec), the only one studied, was scarcely fresh, having been prepared in France the natural way, by (a ,n) on phosphorus, and in Italy the Italian way, by (n,g ) on aluminum. By placing a series of foils in a line, McMillan and Lawrence measured the "excitation function," the yield of radioaluminum as a function of the energy of the incident deuterons. That brought nothing new either: the excitation function agreed with Gamow's theory.[29]
With a little more energy—3MeV—Lawrence and McMillan, now joined by Thornton, got results that diverged from theory. Oppenheimer became interested, calculated, and concluded that disintegration via (d,p) followed the Oppenheimer-Phillips process at the higher bombarding energies. Meanwhile Henderson was getting two different radioactive products from the heaviest magnesium isotope, Mg26 , that is, Mg27 (t = 10 m) via (d,p) and Lawrence's Na24 via (d,a ). Although both activities were known (Fermi had made Mg27 by neutron capture), Henderson could claim the first case in which two different products resulted from the bombardment of a single isotope by the same charged particle. He determined the excitation functions for both, and consulted Oppenheimer. The sage authorized the conclusion that the Na24 came into existence by Gamow capture and the Mg27 by the mechanism of Oppenheimer and Phillips.[30]
The Laboratory's appetite at its first sitting at the periodic table appears from the menu of the meeting of the American Physical Society held in Berkeley at the end of December 1935. Cyclotroneers gave twelve talks, only one of which concerned machinery. Otherwise the subjects were elements excited by deuterons: copper, nitrogen, and oxygen, whose excitation functions Newson followed to energies above the nuclear potential barrier, where the Gamow curve no longer holds; phosphorus, argon, nickel, cobalt, zinc, and arsenic, made radioactive by Paxton, Snell, Thornton, and Livingood; nitrogen, fluorine, sodium, aluminum, silicon, phosphorus, chlorine, argon, and potassium, whose beta and gamma emissions gave employment to Kurie, Richardson, Paxton, Cork, and their cloud chamber. For most of this work, deuteron energies ran about 3.5 MeV and the elements studied were no heavier than arsenic (atomic number, Z , = 33). This was a little tame and routine for the boss. Lawrence's name appeared on two papers at the APS meeting. In one, with Cooksey and Kurie, he described improvements in the cyclotron that resulted in 6 MeV deuterons; in the other, with James Cork, he announced the discovery that platinum nuclei (Z = 78!) "resonated" when hit by such rapid particles. This response from the tough platinum nucleus, which he thought he had "transmuted" to gold, was most gratifying. As Lawrence wrote the Macy Foundation in October 1935, six months earlier he would not have thought such alchemy possible with energies attainable in the Laboratory.[31]
During 1936 the Laboratory worked its way forward from iron, using the faster deuterons then available and trusting in the efficiency of the Oppenheimer-Phillips process apparently so potent in aluminum. (In fact, as Bethe later showed in an elaborate, but approximate, calculation, the Oppenheimer-Phillips process would not have been detectable below Z = 30.)[32] At the
meetings of the spring and early summer of 1936, Van Voorhis introduced a duplicitous copper, which, having imbibed a neutron perhaps in the manner of Oppenheimer-Phillips, decays by either a positron to nickel or an electron to zinc; he thereby found much, but missed more, since the predominant mode of decay of Cu64 is via a process, K-electron capture, then undetected. Livingood reported on the unseparated messes he made with 5 MeV deuterons on several metals and also on his attempt, fleetingly successful, to make the first artificial-natural radioelement (Bi210 , alias RaE) via the Oppenheimer-Phillips process Bi209 (d,p)Bi210 . He thought he glimpsed the faint beta decay of RaE, and also alpha particles of the right range to arise from RaE's descendent Po210 .[33] The man who first identified RaE, Rutherford, was delighted to know that Lawrence could make a, or perhaps any, link of a naturally occurring radioactive series. "[It is] a great triumph for your apparatus."[34]
Another triumph seemed in the offing. Cork had continued with the experiments on platinum. Together he and Lawrence identified four activities, two arising (according to them) from platinum isotopes excited by an unknown process more powerful than Oppenheimer-Phillips and two from irridium isotopes produced by an unlikely (d,a ) reaction. They disclosed further that the excitation function of platinum did not increase monotonically with energy, but showed several bumps or resonances.[35] Oppenheimer developed a new theory to account for it all. At this point, in April 1937, Niels Bohr passed through Berkeley on his way to Japan.
Bohr had just put the finishing touches on his theory likening the nucleus to a liquid drop with modes of excitation incompa-
tible with Lawrence's platinum resonances. At a seminar arranged especially to discuss the latest curiosity of Berkeley experiment and theory, Lawrence announced that his measurements conflicted with Bohr's ideas, but agreed perfectly with Oppenheimer's, and Oppenheimer gave what Kamen remembered as "a typically stupefyingly brilliant exposition of its theoretical consequences." Bohr declared that the data had to be wrong. When he left, the resonances became sharper, the experiments more convincing.[36]
Lawrence did not wish to repeat the saga of the disintegrating deuteron. He summoned McMillan, who had once traced an apparent activity of platinum under deuteron bombardment to radioactive nitrogen driven by recoil into the surface of the metal. McMillan realized that he needed chemical advice (the separations on which Lawrence and Cork had relied were hurriedly done by Newson during his last days at the Laboratory). He called on Kamen and a new graduate student in chemistry, Samuel Ruben. It took them over three months of strenuous chemical work—eighteen hours at a time—to separate the activities and to trace the exotic "resonances" to just plain dirt. By rubbing Laboratory grime into platinum foils before bombarding them, Kamen was able to reproduce most of Cork's measurements.[37]