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Four New Elements Are Added to the Periodic Table:
A Space in the Periodic Table is Saved
for an Element Made in Asia

KOSUKE MORITA
DEPARTMENT OF PHYSICS, KYUSHU UNIVERSITY, JAPAN

INTRODUCTION

Early in the morning, at 5:50 a.m. (JST), on the last day of 2015, I received an e-mail from the president of IUPAC (International Union of Pure and Applied Chemistry) Division II, Professor Jan Reedijk. He wrote, "May I first of all congratulate you and all of your colleagues in the Riken collaboration on the fact that the discovery of the element with Atomic Numbers of 113 has been assigned to work that you and your collaborating team has carried out." It was the moment where it truly hit me that our group had become the very first Asian scientific research group to discover a new element.

THE EVOLVING PERIODIC TABLE

Reports on the discovery of the elements with atomic numbers 113, 115, 117, and 118 were published in two separate articles as IUPAC Technical Reports [1, 2] by the fourth IUPAC/IUPAP Joint Working Party (JWP). The priority for the discovery of element 113 was assigned to the RIKEN collaboration team. The element with atomic numbers 115 and 117 were discovered by a collaborative effort among the Joint Institute for Nuclear Research (JINR) in Dubna, Russia; Lawrence Livermore National Laboratory (LLNR) in California; and Oak-Ridge National Laboratory in Tennessee, USA. The element 118 was discovered through the collaboration of JINR and LLNR.

I can imagine the long and hard discussions held among the JWP members to reach the final assessments, and deeply appreciate JWP's hard work. Furthermore, I have high respect for the many long years of collaborative effort by the Russian and American research groups to achieve outstanding scientific results.

The elements, currently only known by temporary names as ununtrium (Uut), ununpentium (Uup), ununseptium (Uus), and ununoctium (Uuo), will soon be given their permanent names. With the discoveries confirmed, the "7th period of the periodic table of elements is completed," according to the IUPAC. The periodic table now looks perfect, or should I say, in "very good shape." The periodic table is still evolving, however, and it is our wish as researchers to continue to "destroy (enhance) the beauty" of the periodic table by creating new elements 119, 120, and so on, thus filling the very new 8th period.

CREATING NEW SUPERHEAVY ELEMENTS

The elements with atomic numbers greater than 92 are called transuranium elements. All transuranium elements were initially discovered by artificial productions; the 93rd element neptunium (Np) and the 94th element plutonium (Pu) were subsequently discovered in nature after they were artificially produced. Formerly, in order to search for new elements, target materials (with large atomic numbers available at the time)were bombarded with neutron flux and/or 'light' charged particle beams such as deuteron and helium beams, to produce the heaviest elements. Subsequently, all elements with atomic numbers greater than 101 have been created with the use of heavy ion beams. We call the elements with atomic numbers greater or equal to 104, superheavy elements.

At present, a complete fusion reaction between two heavy atomic nuclei, beam and target is being used to produce the atomic nucleus of the superheavy elements. Generally speaking, the probability of fusion of two atomic nuclei with atomic numbers Z1 and Z2 becomes smaller with an increase of the product of the two atomic numbers Z1×Z2. Therefore, the probability of producing the nuclei of superheavy elements is extremely small.

Researchers whose goal is to catch an atom of a superheavy element must overcome this problem. They will need a high intensity beam, a highly efficient recoil ion separator with high performance regarding background-reduction, and a highly segmented position sensitive detector to enhance sensitivity and selectivity.

OVERCOMING YEARS OF CHALLENGES AT RIKEN

We started an experiment aimed at producing an isotope of the 113th element on September 5, 2003. The experiment concluded on October 1, 2012, with several intermissions during the span of a little over nine years. A zinc (Z = 30) with atomic mass number of 70, 70Zn beam was irradiated on the metallic foil of a bismuth (209Bi) target. The atom (ion) of our interest, 278Uut with the atomic number 113 and the mass number 278, recoiledout of the target and went into the same direction as the beam. The ion was separated in-flight from the high intensity beam particles and other unwanted charged particles by a gas-filled type recoil ion separator (GARIS), and guided to the focal plane of GARIS, where a position sensitive semiconductor detector was set. The 278Uut ion was then implanted into the detector, and a series of disintegrations of the 278Uut atom took place at the same spot where the atom was implanted. We could verify the existence of 278Uut and the subsequent decays from electrical signals from the detector.


Fig. 1: The author stands by the gas-filled recoil ion separator GARIS.

Three atoms of the isotope 278Uut were detected in total [3 -6] during the net irradiation time of 575 days. Two of them disintegrated by four consecutive alpha-decays followed by a spontaneous fission, while one of them disintegrated by six consecutive alpha decays. According to the analysis, fourth and fifth decays were unambiguously attributed to the well-known sequential decays of 266Bh (Z = 107) and 262Db (Z = 105). By tracing back the three preceding alpha decays (→ 270Mt (Z = 109) → 274Rg (Z = 111) → 278Uut), these three decay chains consequently were attributed to the ones originating from the isotope 278Uut. The observed decay chains are shown in Figure 2.


Fig. 2: Decay chains observed in the experiment.

NAMING THE NEW ELEMENT

I received an e-mail from Professor Jan Reedijk again on January 30, 2016, officially inviting us to propose a permanent name and a two-letter symbol for the 113th element. The deadline for submission was tentatively set for April 1, 2016. We have discussed the prospective names for element 113 among the collaborators and will propose our final decision by that time. It would be such a great honor for our research group to name the new element.

WHAT'S NEXT?

Several attempts at producing isotopes of the 8th period of theperiodic elements have already been performed by research groups (e.g. in Germany and in Russia). As the first step, we are planning to measure the probability of fusion for a 248Cm (Z = 96) + 50Ti (Z = 22) reaction to produce the heaviest isotope of the 118th element. After we determine the probability, we will proceed to the next step in our search for new elements again.

Acknowledgements: A key to our success can no doubt be attributed to the stable operation of the experimental facilities throughout an extended period of time. The author would like to thank all the collaborators, especially the members of the Superheavy Element Laboratory, including those who have already left RIKEN. They conducted extraordinarily long, enjoyable, and at times difficult and painstaking experiments together with the author. The experiments were performed at the RI Beam Factory operated by the RIKEN Nishina Center and CNS, University of Tokyo. The author is grateful to the accelerator staff members for their cooperation and assistance during the experiments. This research was partially supported by a Grant-in-Aid for Specially Promoted Research, 19002005, 2007, provided by the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Our research would not have been possible without the help and support of the RIKEN personnel, from the top executives to the assistants. The author would also like to thank many of those not acknowledged here who have contributed in making our work successful and enjoyable.

References

[1] P. Karol et al., Pure Allp. Chem. 88, 139, (2016).
[2] P. Karol et al., Pure Allp. Chem. 88, 154, (2016).
[3] K. Morita et al., J. Phys. Soc. Jpn. 73, 1738, (2004).
[4] K. Morita et al., J. Phys. Soc. Jpn. 78, 064201, (2007).
[5] K. Morita et al., J. Phys. Soc. Jpn. 76, 043201, (2007).
[6] K. Morita et al., J. Phys. Soc. Jpn. 81, 103201, (2012).

Kosuke Morita is a professor at the Department of Physics, Kyushu University and group director at the Research Group for Superheavy Elements, RIKEN Nishina Center, RIKEN (contract employee). His research field is experimental nuclear physics. He has worked at RIKEN since 1984. He has been a professor at Kyushu University since 2013.