The chemical behavior of superheavy elements (SHEs, Z > 103) remains poorly understood. Their chemical properties are expected to deviate from established trends, challenging the predictive power of the periodic table. To investigate these elements experimentally, they must first be synthesized through nuclear reactions and then quickly subjected to chemical studies before they decay. Given the low production rates of these reactions and the need for measurements on an atom-at-a-time basis, innovative techniques are needed. To address these challenges, a novel gas-phase chemistry method has been developed at Lawrence Berkeley National Laboratory, utilizing the Berkeley Gas-filled Separator and FIONA. This technique enables the production, identification, and study of molecular species formed by SHEs. As a proof of concept, we present measurements on the formation and identification of 151,152HoO+ molecules, demonstrating the capability to study the production of radioactive molecules under controlled conditions and directly identify them via their mass-to-charge ratio. These measurements validate the effectiveness of this technique for low-statistics SHE studies, highlighting the potential of this approach to ignite the next generation of experimental SHE chemistry research, offering a path to re-evaluate SHE placement on the periodic table.

An extensive dataset of cumulative fission product yields has been generated under a joint collaboration between Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), and the Triangle Universities Nuclear Laboratory (TUNL). The energy dependence of the cumulative yield for a select number of high -yield fission products has been measured using quasimonoenergetic neutrons with energies between 5.5 and 11.0 MeV. This is in addition to previously published data covering 0.5-4.5 and 14.8 MeV. The absolute number of fissions was determined during the irradiation period using dual -fission ionization chambers, and the fission products were measured postactivation by whole target gamma -ray spectroscopy. This paper presents the absolute cumulative fission product yields as a function of incident neutron energy from the neutron -induced fission of 235U, 238U, and 239Pu isotopes at four incident energies in the second -chance fission region and compares them with existing literature values. Corrections relevant to this collaboration's previously published data are also discussed.

The ^{244}Pu(^{50}Ti,xn)^{294-x}Lv reaction was investigated at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron. The experiment was aimed at the production of a superheavy element with Z≥114 by irradiating an actinide target with a beam heavier than ^{48}Ca. Produced Lv ions were separated from the unwanted beam and nuclear reaction products using the Berkeley Gas-filled Separator and implanted into a newly commissioned focal-plane detector system. Two decay chains were observed and assigned to the decay of ^{290}Lv. The production cross section was measured to be σ_{prod}=0.44(_{-0.28}^{+0.58})  pb at a center-of-target center-of-mass energy of 220(3) MeV. This represents the first published measurement of the production of a superheavy element near the "island of stability," with a beam of ^{50}Ti and is an essential precursor in the pursuit of searching for new elements beyond Z=118.

For many fission products, the $\ensuremath{\gamma}$ rays emitted following $\ensuremath{\beta}$ decay provide an easily detectable signature that can be used to identify their quantities and distributions in a sample. As a result, $\ensuremath{\gamma}$-ray spectroscopy is often exploited to study fission-product yields, provided sufficiently accurate information on the $\ensuremath{\gamma}$-ray intensities is available. In many cases, the uncertainties in the existing nuclear data are large enough that they compromise the precision achievable for modern experiments and applications. In this paper, we present high-precision results for the absolute $\ensuremath{\gamma}$-ray emission intensities for the most intense transitions in the $\ensuremath{\beta}$ decays of $^{144}\mathrm{Ce}$ and $^{147}\mathrm{Nd}$. We measured these intensities to $\ensuremath{\lesssim}1%$ accuracy by producing radiopure samples with fission-product beams at CARIBU and detecting the emitted radiation with a $4\ensuremath{\pi}\ensuremath{\beta}$ counter and a meticulously efficiency-calibrated high purity germanium detector at Texas A University.

For many fission products, the gamma rays emitted following beta decay provide an easily detectable signature that can be used to identify their quantities and distributions in a sample. As a result, gamma-ray spectroscopy is often exploited to study fission-product yields, provided sufficiently accurate information on the gamma-ray intensities is available. In many cases, the uncertainties in the existing nuclear data are large enough that they compromise the precision achievable for modern experiments and applications. In this paper, we present high-precision results for the absolute gamma-ray emission intensities for the most intense transitions in the beta decays of Ce-144 and (147) Nd. We measured these intensities to less than or similar to 1% accuracy by producing radiopure samples with fission-product beams at CARIBU and detecting the emitted radiation with a 4 pi beta counter and a meticulously efficiency-calibrated high purity germanium detector at Texas A&M University.

Experiments conducted at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron Facility aimed to produce and study the decay of the previously unobserved isotope (255 )Db. This isotope was produced in the Pb-206 ( V-51 ,2 n ) (255 )Db reaction, separated from unreacted beam material and reaction by-products with the Berkeley Gas-filled Separator, and then implanted into a double-sided silicon-strip detector at the BGS focal plane. Decay properties of 255 Db were determined from the analysis of evaporation residue (EVR) fission and EVR-alpha-alpha correlations. The properties of this new isotope of dubnium differ dramatically from those of its neighboring Db isotopes. (255 )Db was found to decay primarily by spontaneous fission (SF) with a small alpha-decay branch, where the average half-life of the observed decays was t (1 / 2) = 2.6 (+ 0.4)(- 0.3) ms. Theoretical calculations were performed using the Wentzel-Kramers-Brillouin approximation, with parameters calculated within a self-consistent microscopic approach, to see if these unique properties could be reproduced. A SF half-life estimate is obtained that closely matches the measured value, while simultaneously pointing out the sensitivities that need to be further constrained in future work.

The energy dependence of high-yield fission products has been measured using quasi-monoenergetic neutron beams at energies between 5.5 and 11.0 MeV. The absolute number of fissions during the irradiation period was determined via dual-fission ionization chambers, while the fission products were measured via direct g-ray spectroscopy. This paper presents absolute fission product yields from neutron-induced fission of 235 U, 238 U, and 239 Pu isotopes for five incident energies in the second chance fission region.

Experiments were performed at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron Facility to study the decays of neutron-deficient dubnium isotopes. These isotopes were produced in the 206 Pb( 51 V, xn ) 255,256Db Db reaction, and excitation functions were measured. This article reports on the observed properties of the 256 Db decay chain. The produced 256 Db nuclei were separated from unreacted-beam material and reaction byproducts with the Berkeley Gas-filled Separator (BGS) before being implanted into a double-sided silicon strip detector at the BGS focal plane. Decay properties of 256 Db and its daughters were then extracted from the analysis of correlations between implanted Db nuclei with alpha decay chains and spontaneous fission (SF) events. In total, 86 decay chains and 38 SF events were observed, giving increased statistics as compared to previous studies. Improved decay data are presented for 256 Db and its daughter isotopes 252 Lr, 252 No, 248 Md, 248 Fm, 244 Es, and 244 Cf.

Received 16 September 2022Revised 30 January 2023DOI:https://doi.org/10.1103/PhysRevC.107.029902©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levels & level densitiesFissionNuclear structure & decaysProperties90 ≤ A ≤ 149Nuclear Physics

We present cumulative fission product yields from the 238U(n, f ) reaction at an incident energy of 4.6 MeV for fission products with half-lives from less than one second to several hours. We employed direct gamma-ray spectroscopy to measure unseparated fission products with a conventional neutron activation analysis and a cyclic activation technique using the RApid Belt-driven Irradiated Target Transfer System (RABITTS). Yields were determined for 78 isotopes and isomers, covering the mass range 84 A 148 and the charge range from 33(As) Z 58(Ce). Our experimental cumulative yields are compared to the literature and evaluated data at En approximate to 0.5 MeV from ENDF/B-VIII.0 and JEFF-3.3 libraries and with theoretical models using the GEF and FREYA codes at En = 4.6 MeV.

The observation of mass-asymmetric fission in neutron-deficient 180Hg dramatically expanded the region of mass-asymmetric fission found across the nuclide chart, and has led to intense experimental and theoretical investigations into the fission of sub-lead nuclei. In particular, two major questions have been raised: how many fission modes are present in the fission of sub-lead nuclides, and which shells dictate these modes?Notably, investigations of the fission modes of 178Pt have led to contrasting results. To solve this disparity, new high-statistics data have been measured at the lowest excitation energy to-date using the CUBE fission spectrometer at The Australian National University. A new fitting procedure was developed to fit the high-statistics two-dimensional mass-kinetic energy distribution without external constraints.The fission of 178Pt can best be described by three fission modes: one mass-symmetric and two mass-asymmetric. Comparisons to previous analyses highlight the necessity of fitting the two-dimensional mass-kinetic energy distribution, rather than fitting slices of individual one-dimensional projections of the full distribution. Systematic studies of high-statistics measurements, combined with a rigorous statistical analysis offer the best chance to determine the shell effects responsible for multi-modal mass-asymmetric fission in this region of the nuclide chart.

We present cumulative fission product yields from the $^{238}\mathrm{U}(n,f)$ reaction at an incident energy of 4.6 MeV for fission products with half-lives from less than one second to several hours. We employed direct $\ensuremath{\gamma}$-ray spectroscopy to measure unseparated fission products with a conventional neutron activation analysis and a cyclic activation technique using the RApid Belt-driven Irradiated Target Transfer System (RABITTS). Yields were determined for 78 isotopes and isomers, covering the mass range $84\ensuremath{\le}A\ensuremath{\le}148$ and the charge range from $33\text{(As)}\ensuremath{\le}Z\ensuremath{\le}58\text{(Ce)}$. Our experimental cumulative yields are compared to the literature and evaluated data at ${E}_{n}\ensuremath{\approx}0.5$ MeV from ENDF/B-VIII.0 and JEFF-3.3 libraries and with theoretical models using the gef and freya codes at ${E}_{n}=4.6$ MeV.

Background: The predominant mass-asymmetric fission of actinide nuclides occurs mainly through the so-called standard I and standard II modes. Though understood to be caused by shape-dependent shell structures encountered between the fission barrier deformation and scission, the most relevant shell gaps are still not firmly established. The standard I mode had been associated with the spherical doubly magic Sn-132, and thus the Z = 50 proton shell, but recently it has been proposed that standard I and standard II are associated with quadrupole and octupole deformed gaps at Z = 52 and 56, respectively. Purpose: We investigate how the relative probabilities of the standard I and standard II modes vary with excitation energy near threshold, probing where the two modes bifurcate. Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission mass distributions for the p + Th-232 reaction (forming Pa-233) at closely spaced bombarding energy intervals from 6.5 to 28 MeV. Results: A model-independent analysis of the energy dependence of the shape of the mass-asymmetric peak shows a strong dependence of the standard I and standard II relative probability on excitation energy near threshold. Conclusions: The results are consistent with the standard II mode having a lower fission barrier than standard I in Pa-233, with the latter increasing continually in relative probability above its barrier energy. It is concluded that multichance fission, in particular last chance fission, plays a strong role in determining the observed energy dependence of all fission modes.

Received 27 September 2022DOI:https://doi.org/10.1103/PhysRevC.106.049905©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsEnergy levels & level densitiesIsomer decaysNuclear structure & decaysProperties90 ≤ A ≤ 149Nuclear Physics