The search for transuranium elements initiated by Fermi continued unabated. In the spring of 1940, Edward McMillan and Philip Abelson, working at the University of California Berkeley (UCB), exposed a natural uranium target to 12 Mev neutrons produced by bombarding Be with cyclotron-accelerated deuterons (2H1).
238U92 + 1n0 --> 239U92 + g
239 U92 t ½ = 23.5 min.--> 239Np93 + b -
(The distinction between discovery and isolation is significant. Discovery refers to the first nuclear and chemical proof of the existence of atoms of a new element, while isolation is the procurement of the first weighable amount in pure form. There was often a considerable amount of time between the discovery and isolation of the transuranium elements.)
(Courtesy of the University of California, Berkeley)
In the summer of 1940, Glen Seaborg, Arthur Wahl, and Joseph Kennedy, a group of chemists at Berkeley, began a search for the next transuranium element, 94, which they thought to be a decay product (daughter) of Np-239.
239 Np93 t ½ = 2.3 days -1ß - --> 239? 94
Continuing the search for element 94 in the winter of 1941, they bombarded uranium oxide with 16 Mev deuterons from the Berkeley cyclotron. They chemically identified another isotope of neptunium, Np-238, which decayed by beta emission to an isotope of element 94 (plutonium) that then emitted alpha particles.
238 U92 + 2H1 --> 238Np93 + 21n 0
238 Np93 t 1/2 = 2.12 days --> 238Pu94 + b -
238 Pu94 t1/2 = 90 years --> a 2+ + 236U 92
The Pu alpha particle emitter was separated chemically from U, Np, and other reaction products and oxidized with potassium peroxydisulfate, K2S2O8, to a fluoride-soluble oxidation state. The other products of this neutron bombardment did not undergo oxidation with K2S2O8 and thus remained insoluble. The alpha activity (400 counts per minute) used to trace Pu was concentrated in the resulting solution. The Pu in solution was then reduced to a lower oxidation state with SO2 and precipitated as a fluoride using Ce3+ and La3+ as carriers. This chemistry, based on oxidation, reduction, and precipitation reactions, would later prove the basis for the large-scale production of plutonium. The alpha particle energy and activity was unique and thus indicated a new element. At this time, Seaborg and Wahl could not identify which Pu isotope produced the alpha activity. It was later identified as Pu-238.
Seaborg remarked, "During this time, a great deal was learned about the chemistry of plutonium. It was established that plutonium in its higher oxidation state was not carried by lanthanum fluoride or cerium fluoride, in contrast to plutonium in the lower state, which was quantitatively coprecipitated with these compounds. The lower state could be oxidized to the higher state with oxidizing agents such as persulfate, dichromate, permanganate, or periodate ions, and then reduced by treatment with sulfur dioxide or bromide ion to the lower oxidation state."
Later in the spring of 1941, another more important isotope of plutonium, Pu-239, was produced using neutrons from the Berkeley cyclotron to target a uranium compound surrounded by paraffin. As Fermi’s group had discovered, the paraffin acted as a moderator to slow the neutrons and thus increase the chances of interaction with the target. This new Pu isotope, an alpha emitter with a half life of about 24,000 years, was separated from other reaction products using the same chemistry as that used to isolate Pu-238. However, the longer half-life of Pu-239 reduced its activity, making it more difficult to detect than Pu-238.
238 U92 + 1n0 --> 239U92 + g
239 U92 t 1/2 = 23.5 min. --> 239Np93 + b -
239 Np93 t 1/2 = 2.35 days --> 239Pu94 + b -
239 Pu93 t 1/2 = 24,110 yrs. --> 235U92 + a
In March 1941, Seaborg’s group irradiated a sample estimated to contain 0.25 mg of Pu-239 surrounded by paraffin with neutrons produced in the cyclotron. Under these conditions, this isotope appeared to undergo fission. When the Pu was replaced with a sample containing approximately 0.5 mg U-235, the other known fissionable material, neutron-induced fission was also observed, but at a rate approximately half that of Pu-239. This discovery raised the possibility of using a controlled chain reaction to produce quantities of Pu-239 sufficient for nuclear weapons. The product Pu-239 would have to be separated from the unreacted uranium and fission products by chemical means. It now became important to investigate the chemistry of plutonium to develop large-scale separation procedures.
Although the Pu-239 isotope had the potential to be fissionable material for bombs or power generation, realization of this potential required larger amounts of this isotope. Large-scale production of Pu-239 required a controlled nuclear chain reaction of uranium, a feat that would soon be achieved by Enrico Fermi and Leo Szilard in Chicago.
Complete Bibliography on Plutonium from the ALSOS Digital Library for Nuclear Issues
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