Physical Review Letters

With its 93 protons, neptunium is the first transuranium element, located just to the right of uranium on the Periodic Table. Neptunium was first synthesized by Edwin McMillan and Philip H. Abelson by activating a uranium trioxide target with neutrons from Berkeley’s 60-inch cyclotron.

One of the half-lives observed in the activated uranium target was 23 minutes, matching that of ²³⁹U. But the activated target also presented a new half-life of 2.3 days of unknown origin.

McMillan and Abelson demonstrated that a decrease in 23 minute activity resulted in an increase in the 2.3 day activity, concluding that the unknown source of the 2.3 day activity must be a daughter of ²³⁹U. Along with prior observations that the unknown source differed chemically from all known elements, McMillan and Abelson convincingly showed that a new element had been created.

The discovery of neptunium was reported in a Letter to the Editor in the Physical Review in 1940, just prior to the U.S. entry into World War II.

Plutonium (Z=94), americium (Z=95), and curium (Z=96) were all intentionally synthesized at Berkeley’s 60-inch cyclotron by bombarding neutrons, deuterons, or alpha particles on targets of uranium or plutonium. The new elements were chemically identified at the Metallurgical Laboratory at the University of Chicago, the same site which produced the first controlled nuclear chain reaction. Due to wartime security concerns Physical Review delayed publishing the 1941 discovery of plutonium until 1946. The method of production for elements 95 and 96 can be found in US patents 3156523 and 3161462, respectively.

Berkelium (Z=97) and californium (Z=98) were also produced at the University of California. The target material consisted of milligram-sized quantities of americium and microgram-sized quantities of curium. In both cases the targets were bombarded with helium ions produced by Berkeley’s cyclotron. Isolation of the new elements required “tedious chemical procedures.” These experiments served as important stepping stones for the discovery of even heavier elements because both berkelium and californium were the target materials used in the hot-fusion experiments that took place fifty years later.

The elements einsteinium (Z=99) and fermium (Z=100) were both produced in samples of uranium exposed to the extreme neutron flux of the 1952 “Mike” thermonuclear explosion. Cold war tensions kept these results secret until their publication in Physical Review in 1955. Physical Review published two papers in 1954 that reported on the production of elements 99 and 100 using traditional accelerators. One of the Letters to the Editor hinted at the prior discovery in its penultimate paragraph: “There is unpublished information relevant to element 99 at the University of California, Argonne National Laboratory, and Los Alamos Scientific Laboratory. Until this information is published, the question of the first preparation should not be prejudged on the basis of this paper or the preceding one.”

With the advent of new heavy-ion sources ranging from helium to neon, the elements 101–106 were discovered at Lawrence Berkeley National Laboratory (LBNL) and the Flerov Laboratory of Nuclear Reactions (FLNR). The use of heavy-ion-induced nuclear fusion reactions was a challenging prospect as the energy of the projectile had to be tuned precisely. Too low, and the projectile couldn’t overcome the Coulomb barrier of the target nucleus. Too high, and the newly created compound nucleus would undergo fission before settling into its ground state. These discoveries were reported in several journals.

The low survival rates of heavy nuclei produced in heavy-ion induced reactions made the production of new elements beyond seaborgium (Z=106) untenable with previously used methods.

GSI in Darmstadt used a new method, dubbed cold fusion, to discover the elements 107–112. Cold fusion reactions use beam and target nuclei that are closer in mass (for example, a beam of calcium ions on a lead or bismuth target). The resulting compound nucleus is produced at lower excitation energy thus increasing its survival probability considerably. The emission of none or one neutron along with some photons is enough to bring the new element into its ground state.

These endeavours were aided by development of separator technology such as SHIP, Separator for Heavy Ion reaction Products (shown in the image), which allowed for the separation of new species from unwanted reaction products. These studies were published in Zeitschrift für Physik A Atoms and Nuclei and the Journal of the Physical Society of Japan.

Elements 114–118 were synthesized by a method called “hot fusion” in which a high-energy projectile (such as calcium) bombards a heavier human-made actinide target. The resulting compound nucleus is created at a high excitation energy and must emit several neutrons before decaying to its ground state.

The targets necessary for these reactions can only be synthesized at high-flux nuclear reactors such as the one at Oak Ridge National Laboratory; three milligrams of highly purified berkelium requires about 18 months to produce.