Element 117 yields long-lived decay products

Researchers in Russia and the US have collaborated to synthesize a new element with atomic number 117. The project was led by Yuri Oganessian, ¹ of the Joint Institute for Nuclear Research in Dubna, Russia, who also spearheaded the discoveries of elements 113-116 and 118. He and his Dubna colleagues worked with US researchers from Oak Ridge National Laboratory, Lawrence Livermore National Laboratory, Vanderbilt University, and the University of Nevada, Las Vegas.

The synthesis was done with an eye toward the eagerly anticipated island of stability, a region just off the chart of known nuclides and centered around the next proton number Z and neutron number N that correspond to filled nucleon shells. Theorists agree that the target value of N is 184 but disagree on the value of Z: It could be 114, 120, or 126.

The isotopes of element 117, with lifetimes measured in milliseconds, were not especially stable. That was to be expected, though, since they had too few neutrons to hold together their many protons. But as the atoms spat out one alpha particle after another—losing two protons and two neutrons at a time—they formed chains of never-before-seen isotopes with higher N/Z ratios, and thus greater stability.

Smashing

Superheavy atoms are made by fusing two lighter atoms together. But not just any lighter atoms will do. Their ground-state energies should be low, and their N/Z ratios high. Elements up to Z = 112 can be made by bombarding a target of lead or bismuth with lighter ions. Lead-208 is “doubly magic”—its protons and its neutrons both form filled shells—and thus especially stable, and ²⁰⁹Bi is singly magic. But as the projectile ions’ atomic number increases, the reaction cross section goes down: The method would yield atoms of element 113 at an impractical rate of just one per year.

In the late 1990s, the Dubna researchers began using a beam of calcium-48 with a target of an actinide element. ² Calcium-48 is doubly magic, has unusually many neutrons for its relatively low Z, and makes up about 0.2% of natural Ca samples. To make element 117, the target material had to be berkelium. That made the experiment tricky and was part of the reason why element 118 was discovered first: The only isotope that can be made in quantity, ²⁴⁹Bk, has a half-life of just 330 days. Californium-249, used to make element 118, has a half-life of more than 350 years.

The Bk was made at Oak Ridge’s High Flux Isotope Reactor, where researchers create heavy actinides by bathing lighter, naturally occurring actinides with neutrons. Atoms take on neutrons one by one until they become unstable to beta decay and their Z goes up by one. The Oak Ridge researchers are also experts in actinide purification. From a sample of Cf and other elements, they isolated 22 mg of ²⁴⁹Bk and shipped the sample to Dubna.

Carried away

In the Dubna lab, a beam of ⁴⁸Ca ions was accelerated in the U-400 cyclotron—400 refers to the diameter in centimeters—and directed at the Bk target. Most of the ions passed straight through, but a few struck the Bk nuclei, overcame the Coulomb barrier, and reacted. The fusion products formed with a great deal of excess internal energy. Before the nuclei could settle into bound states, some of that energy was carried away by neutron evaporation. The number of neutrons lost depends on the starting excitation energy, which depends in turn on the ⁴⁸Ca impact energy. The researchers used two different ⁴⁸Ca beam energies, so they made two isotopes of element 117—one with three neutrons lost and one with four—as shown in figure 1.

Momentum from the ⁴⁸Ca beam carried the newly formed nuclei out of the target. To separate the reaction products from the unreacted ⁴⁸Ca atoms, the researchers used the 4-meter-long Dubna gas-filled recoil separator, shown in figure 2. Atoms emerging from the target found themselves in a chamber filled with hydrogen gas and surrounded by powerful magnets.

Repeated collisions with hydrogen molecules kept each atom in a steady ionized state, causing it to be deflected by the magnetic field. Thanks to their different masses and speeds, the Ca ions went one way and the fusion products another. To help ensure that atoms reaching the detector were the desired element 117 rather than other, unwanted fusion products, the researchers calibrated the separator so that one in three element-117 atoms, and almost no others, made it through.

Atoms that survived the separator then passed through time-of-flight detectors and became embedded in position-sensitive silicon strip detectors. There they remained while the surrounding strip detectors and side detectors caught any alpha particles they emitted or fission products they produced. By recording the timings and energies of the successive decays and comparing them to theoretical predictions, the researchers could deduce the identities of the original nuclei.

Long life

Over the 150 days the experiment ran, a total of six Z = 117 atoms were observed: five of mass 293 and one of mass 294. Both isotopes underwent a series of alpha decays followed by a spontaneous fission. In addition to the two 117 isotopes, the decay chains contained 9 new isotopes of old elements, all of them with more neutrons than the previously synthesized isotopes. For elements 115, 113, and roentgenium (Z = 111), higher N meant a clear increase in stability, evidenced by lower alpha-particle energies and longer alpha-decay half-lives, as shown in figure 3. That trend is in agreement with predictions for the island of stability: All the nuclei formed are on the neutron-poor side of the island, so increasing N brings them closer to the island’s shore.

Longer-lived isotopes of the oddnumbered superheavy elements, especially element 113, may allow studies of the elements’ electronic—that is, chemical—properties. Those studies are important because relativistic effects, which become stronger with increasing Z, change the energies of electronic orbitals. For the superheavy elements, relativity may even change the order in which the orbitals are filled and thus the structure of the periodic table. The way the table is drawn, element 113 sits below thallium. But its chemistry may be very different from thallium’s.

It’s not easy to study the chemistry of an element whose atoms live for just a few seconds and can be produced only one at a time. Researchers do it by measuring the atoms’ affinity for surfaces. In earlier experiments on copernicium (then known merely as element 112, or ununbium), researchers from Dubna, the Paul Scherrer Institute and the University of Bern in Switzerland, and the Institute of Electron Technology in Warsaw, Poland, swept the atoms of interest through a narrow channel between two gold-plated detector arrays. ³ The near end of the channel was kept relatively warm, and the far end was kept cold. The researchers wondered if Cn would be an inert gas like radon rather than a metal like mercury, which sits above it in the periodic table. Atoms of a metal would cling to the warm surfaces, whereas atoms of a gas would fly through to the colder surfaces. Two Cn atoms were detected, both at the warm end; the researchers concluded that Cn was more like Hg than like Rn.

Elemental

The future of the ⁴⁸Ca technique depends on the stability and ease of making the target materials. Synthesis of element 119 using a ⁴⁸Ca beam would require an einsteinium target, probably ²⁵⁴Es, with a half-life of 276 days. But ²⁵⁴Es has been produced only in microgram quantities; synthesis of the required milligram amounts would be a multimillion-dollar effort. Fermium, the next actinide in line, has a shorter lifetime still, and only picograms of it have been made. It’s likely, therefore, that future element discoveries will be made using a higher-Z beam. That doesn’t mean that ⁴⁸Ca is finished. Heavier isotopes of Cf, for example, could be used to make heavier atoms of element 118 and its decay products.