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Almost thirty years ago the criteria that are currently used to verify claims for the discovery of a new element were set down by the comprehensive work of a Transfermium Working Group, TWG, jointly established by IUPAC and IUPAP. The recent completion of the naming of the 118 elements in the first seven periods of the Periodic Table of the Elements was considered as an opportunity for a review of these criteria in the light of the experimental and theoretical advances in the field. In late 2016 the Unions decided to establish a new Joint Working Group, JWG, consisting of six members determined by the Unions. A first meeting of the JWG was in May 2017. One year later this report was finished. In a first part the works and conclusions of the TWG and the Joint Working Parties, JWP, deciding on the discovery of the now named elements are summarized. Possible experimental developments for production and identification of new elements beyond the presently known ones are estimated. Criteria and guidelines for establishing priority of discovery of these potential new elements are presented. Special emphasis is given to a description for the application of the criteria and the limits for their applicability.

The search for new decay chains of oganesson isotopes is presented. The experiment utilized the Dubna Gas Filled Recoil Separator and a highly segmented recoil-decay detection system. The signals from all detectors were analyzed in parallel by digital and analog data acquisition systems. For the first time, a target of mixed californium (51% Cf249, 13% Cf250, and 36% Cf251) recovered from decayed Cf252 sources was produced and irradiated with an intense Ca48 beam. The observation of a new decay chain of Og294 is reported. The prospects for reaching new isotopes Og295,296 are discussed.

Almost thirty years ago the criteria that are currently used to verify claims for the discovery of a new element were set down by the comprehensive work of a Transfermium Working Group, TWG, jointly established by IUPAC and IUPAP. The recent completion of the naming of the 118 elements in the first seven periods of the Periodic Table of the Elements was considered as an opportunity for a review of these criteria in the light of the experimental and theoretical advances in the field. In late 2016 the Unions decided to establish a new Joint Working Group, JWG, consisting of six members determined by the Unions. A first meeting of the JWG was in May 2017. One year later this report was finished. In a first part the works and conclusions of the TWG and the Joint Working Parties, JWP, deciding on the discovery of the now named elements are summarized. Possible experimental developments for production and identification of new elements beyond the presently known ones are estimated. Criteria and guidelines for establishing priority of discovery of these potential new elements are presented. Special emphasis is given to a description for the application of the criteria and the limits for their applicability.

Since 2000, six new super-heavy elements with atomic numbers 113 through 118 have been synthesized in hot fusion reactions of 48Ca beams on actinide targets. These target materials, including 242Pu, 244Pu, 243Am, 245Cm, 248Cm, 249Cf, and 249Bk, are available in very limited quantities and require specialized production and processing facilities resident in only a few research centers worldwide. This report describes the production and chemical processing of heavy actinide materials for super-heavy element research, current availabilities of these materials, and related target fabrication techniques. The impact of actinide materials in super-heavy element discovery is reviewed, and strategies for enhancing the production of rare actinides including 249Bk, 251Cf, and 254Es are described.

The heaviest elements are synthesized in heavy-ion induced hot fusion reactions with various actinide targets. Because the actinide material is often available only in very limited amounts, a deposition method with high yields (~90 %) is needed. We report on the production of 244Pu, 243Am, 248Cm, 249Bk, and 249Cf targets on thin Ti backings by molecular plating. Different chemical purification steps using ion chromatographic techniques were applied for the purification of 249Cf and 244Pu. The deposition procedure applied for the production of ~0.4–0.8 mg/cm2 thick targets is described. The deposition yield was determined either by α-particle or γ-ray spectroscopy. Furthermore, neutron activation analysis has been applied in the case of 244Pu, 243Am, and 248Cm. Information about the spatial distribution and homogeneity of the target layer was obtained by radiographic imaging.

Irradiations of $^{239}\mathrm{Pu}$ and $^{240}\mathrm{Pu}$ targets with $^{48}\mathrm{Ca}$ beams aimed at the synthesis of $Z=114$ flerovium isotopes were performed at the Dubna Gas Filled Recoil Separator. A new spontaneously fissioning (SF) isotope $^{284}\mathrm{Fl}$ was produced for the first time in the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ (250 MeV) and $^{239}\mathrm{Pu}+^{48}\mathrm{Ca}$ (245 MeV) reactions. The cross section of the $^{239}\mathrm{Pu}(^{48}\mathrm{Ca},3n)^{284}\mathrm{Fl}$ reaction channel was about 20 times lower than predicted by theoretical models and about 50 times lower than the maximum fusion-evaporation cross section for the $3n$ and $4n$ channels measured in the $^{244}\mathrm{Pu}+^{48}\mathrm{Ca}$ reaction. In the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ experiment, performed at 245 MeV in order to maximize the $3n$-evaporation channel, three decay chains of $^{285}\mathrm{Fl}$ were detected. The $\ensuremath{\alpha}$-decay energy of $^{285}\mathrm{Fl}$ was measured for the first time and decay properties of its descendants $^{281}\mathrm{Cn}, ^{277}\mathrm{Ds}, ^{273}\mathrm{Hs}, ^{269}\mathrm{Sg}$, and $^{265}\mathrm{Rf}$ were determined with higher accuracy. The assignment of SF events observed during the irradiation of the $^{240}\mathrm{Pu}$ target with a 250 MeV $^{48}\mathrm{Ca}$ beam to $^{284}\mathrm{Fl}$ decay is presented and discussed. The cross sections at both $^{48}\mathrm{Ca}$ energies are similar and exceed that observed in the reaction with the lighter isotope $^{239}\mathrm{Pu}$ by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate a rapid decrease of stability of superheavy nuclei as the neutron number decreases from the predicted magic neutron number $N=184$.

The reaction of 249 Bk with 48 Ca have been reinvestigated to provide new evidence for the discovery of element 117 on a larger number of events. The experiments were performed at five projectile energies and with a total beam dose of 48 Ca of about 4.6×10 19 . Two isotopes 293,294 117 were synthesized in the 249 Bk+ 48 Ca reaction, providing excitation functions and α-decay spectra of the produced isotopes that establishes these nuclei to be the products of the 4n- and 3n-evaporation channels, respectively. Decay properties of 293,294 117 and of all the daughter products agree with the data of the experiment in which these nuclei were synthesized for the first time in 2010. The new 289 115 events, populated by α decay of 293 117, demonstrate the same decay properties as those observed for 289 115 produced in the 243 Am( 48 Ca,2n) reaction thus providing cross- bombardment evidence. In addition, a single decay of 294 118 was observed from the reaction with 249 Cf - a result of the in-growth of 249 Cf in the 249 Bk target.

Langmuir probe measurements, photography, and monitoring of downstream microwave propagation were used to characterize argon plasmas obtained with a mirror field electron cyclotron resonance source. For certain system operating parameters, abrupt changes in plasma characteristics were observed. These were consistent with a transition in the mirror region plasma density from overdense (plasma density>cutoff density) to underdense. At higher powers and pressures, the visible light emission from the central region of the plasma diminished. Ionization of a large fraction of injected atoms before they reach the center is a likely contributor to this phenomenon. The high ionization efficiency also contributes to substantial differences between mirror and downstream pressure measurements. As a result, downstream pressure is the preferred reference for system pressure control. Finally, particular operating conditions were found to result in source chamber sputtering due to plasma potentials in the mirror region i...

An electron cyclotron resonance (ECR) multicusp plasmatron has been developed by feeding a multicusp bucket arc chamber with a compact ECR plasma source. This novel source produces large (about 25 cm diam), uniform (to within ±10%), dense (>1011 cm−3) plasmas of argon, helium, hydrogen, and oxygen. It has been operated to produce an oxygen plasma for etching 12.7 cm (5 in.) positive photoresist‐coated silicon wafers with uniformity within ±8%. Results and potential applications of this new ECR plasma source for plasma processing of thin films are discussed.An electron cyclotron resonance (ECR) multicusp plasmatron has been developed by feeding a multicusp bucket arc chamber with a compact ECR plasma source. This novel source produces large (about 25 cm diam), uniform (to within ±10%), dense (>1011 cm−3) plasmas of argon, helium, hydrogen, and oxygen. It has been operated to produce an oxygen plasma for etching 12.7 cm (5 in.) positive photoresist‐coated silicon wafers with uniformity within ±8%. Results and potential applications of this new ECR plasma source for plasma processing of thin films are discussed.

The reaction of 249Bk with 48Ca has been investigated with an aim of synthesizing and studying the decay properties of isotopes of the new element 117. The experiments were performed at five projectile energies (in two runs, in 2009-2010 and 2012) and with a total beam dose of 48Ca ions of about 9x1019 The experiments yielded data on a-decay characteristics and excitation functions of the produced nuclei that establish these to be 293117 and 294117 – the products of the 4n- and 3n-evaporation channels, respectively. In total, we have observed 20 decay chains of Z=117 nuclides. The cross sections were measured to be 1.1 pb for the 3n and 2.4 pb for the 4n-reaction channel. The new 289115 events, populated by α decay of 117, demonstrate the same decay properties as those observed for 115 produced in the 243Am(48Ca,2n) reaction thus providing cross-bombardment evidence. In addition, a single decay of 294118 was observed from the reaction with 249Cf – a result of the in-growth of 249Cf in the 249Bk target. The observed decay chain of 294118 is in good agreement with decay properties obtained in 2002-2005 in the experiments with the reaction 249Cf(48Ca,3n)294118. The energies and half-lives of the odd-Z isotopes observed in the 117 decay chains together with the results obtained for lower-Z superheavy nuclei demonstrate enhancement of nuclear stability with increasing neutron number towards the predicted new magic number N=184.

The reaction 243Am + 48Ca has been reinvestigated to provide new evidence for the discovery of elements 113, 115. Twenty eight new 288115 decay chains were detected in this reaction to increase from three to 31 the number of 288115 atoms observed. In addition, four new decay chains were observed for the first time and assigned to the decay of 289115. These new 289115 events have the same properties for their decay chains as those observed for 289115 populated in the alpha decay of 293117 produced in the 249Bk + 48Ca reaction to provide cross-bombardment evidence. These new high statistics data sets and the cross-bombardment agreement provide definitive evidence for the discoveries of the new elements with Z = 113, 115, 117.

The synthesis of the new chemical element with atomic number Z=117 is presented. The isotopes 293117 and 294117were produced in fusion reactions between 48Ca and 249Bk. The 249Bk was produced in the High Flux Isotope Reactor and chemically separated at Oak Ridge. Decay chains involving eleven new nuclei were identified by means of the Dubna Gas Filled Recoil Separator. The measured decay properties show a strong rise of stability for super-heavy nuclei toward N=184.

Results of recent plasma edge experiments using surface collection probes and laser‐induced fluorescence in the ISX‐B tokamak and EBT‐S bumpy torus are summarized. Data are presented for ion temperatures and fluxes, neutral fluxes and energies, impurity fluxes and introduction mechanisms, and erosion and redeposition rates. Comparison is made where possible with results from other plasma diagnostics. It is found that the edge ion temperature is ∼20 eV in these devices (comparable to the edge electron temperature) and that wall sputtering (by charge exchange neutrals in ISX‐B and self‐ions in EBT‐S) is the dominant heavy impurity introduction mechanism.

An experiment to test beryllium as a limiter material has been performed in the ISX-B tokamak. The effect of the plasma on the limiter and the effect of the limiter on the plasma were studied in detail. Heat and particle fluxes to the limiter were measured, and limiter damage by melting was documented as a function of power flux. Strong melting and evaporation of the limiter caused beryllium gettering of the vacuum vessel. Postmortem analysis of the limiter was performed to document the amount of retained hydrogen and the erosion and impurity deposition on the limiter. The effect of the limiter on the plasma performance was studied in terms of parameter space, impurity content, and confinement for the ungettered and gettered cases. Operational experience with beryllium in a fusion experiment is discussed.

Working gas composition and edge impurity levels have been monitored during H-D changeover for non-gettered, ohmic discharges in the ISX-B tokamak using optical spectroscopy, mass spectrometry, and surface probe analysis. The isotopic exchange in the plasma is ~80% complete within the first 15 ms of the first discharge in the new isotope, but requires ~10 discharges to reach 98% completion. Fe edge densities increase by a factor of ~3 for D as compared to H plasmas, whereas 0 levels remain comparable for D and H operation. The changeover results indicate that the plasma isotopic composition in ISX-B is dominated by the fill gas, and that most of the smaller contribution from isotopic exchange with exposed surfaces is completed early in the discharge. The impurity data suggest a hydrogen sputtering mechanism for heavy impurity introduction and a chemical process for 0 release.

The enhanced sputtering of graphite at temperatures above 1000 K has been investigated for a variety of incident ion species and energies. Papyex graphite strips were irradiated with 35 to 150 keV H, C, O, and Ar ions in the temperature range from 300 to 1800 K, and with 0.13 to 8 keV H ions at 1800 K. Sputtering yields were determined by weight change measurements, and were compared in many cases with yields determined by in-situ ion beam analysis of collector probes. For temperatures above 1000 K, enhanced sputtering yields are found which increase with temperature to 6 to 20 times room temperature values at 1800 K. At a given temperature, measured yields for all species and energy combinations scale with the nuclear deposited energy at the surface and do not correlate with electronic losses. The low energy H measurements indicate a transferred energy threshold of ∼ 5 eV for the enhanced erosion process, significantly lower than the displacement threshold energy in graphite of 30 eV. The enhanced erosion is characterized by an activation energy which varies from 0.5 to 1.1 eV with decreasing incident particle mass. The overall results, together with recently reported velocity and mass spectra of the sputtered particles, suggest a radiation-enhanced sublimation mechanism.

The possible synergistic effects which can contribute to plasma-surface interaction phenomena in fusion devices are reviewed. These effects include the influence of reactive ions, surface modification, temperature, radiation damage, and external forces and fields on erosion yields, hydrogen retention and release, and other surface processes. The important synergistic effects are described in terms of surface and edge conditions encountered in present fusion devices and expected in future reactors. Priority data needs include the chemical erosion of graphite at high particle fluxes, melt-layer stability under disruption-induced eddy-current forces, the influence of bulk neutron damage on hydrogen retention, and in-situ measurements of synergistic effects in operating fusion devices.

We present the results of a combined x-ray and neutron diffraction study of the temperature dependence of the "forbidden" (222) reflection in germanium. Integrated intensities were measured from room temperature to 850\ifmmode^\circ\else\textdegree\fi{}C with x rays and to 904\ifmmode^\circ\else\textdegree\fi{}C with neutrons. In addition, an earlier x-ray study of the silicon (222) reflection has been improved and extended to 800\ifmmode^\circ\else\textdegree\fi{}C and a correction has been made in the corresponding neutron results. The germanium and silicon data are interpreted in terms of anharmonic atomic vibrations and anticentrosymmetric valence-charge distributions. Contrary to our previous reports, the temperature dependence of the scattering from the bonding electrons is found to be similar to that from the core electrons.

The density and velocity distribution of neutral Fe atoms in the plasma-edge of the ISX-B tokamak have been measured by laser fluorescence spectroscopy. Fe neutrals were excited at 302 nm using a flashlamp-pumped dye laser. The fluorescence light was detected at 382 nm with an interference filter and photomultiplier system. The frequency-doubled laser output (700 W) and pulse length (500 ns) allowed saturation pumping of the Fe transition thereby enabling absolute measurement of densities. Insertion of an additional Fabry-Perot etalon into the laser cavity narrowed the laser bandwidth to 0.6 pm corresponding to a velocity resolution of 600 m/s (0.1 eV for Fe). Neutral densities were measured for two locations in the plasma-edge as a function of discharge time. Results show a detection sensitivity of 1012 atoms/m3 with a neutral Fe density of a few 1012 atoms/m3 in the first 20 ms of the discharge and 1014 atoms/m3 30 ms after the discharge. The velocity distribution of the neutral Fe at the end of the discharge is thermal (400–1000 K) while the smaller peak at the beginning of the discharge shows evidence of higher velocities.

The erosion of carbon and the deposition of carbon and impurities have been determined for graphite samples exposed at the wall of JET during the six months of operation in 1985. Graphite erosion was determined using SIMS analysis of the depth change of implanted 13C marker layers in the samples. Impurity deposition was measured using SIMS and Rutherford backscattering. The samples showed high erosion ranging from 0.2 to 0.6 μm at various locations, with significant poloidal variation in both erosion and deposition. The deposition of substantial quantities of metal impurities and carbon was related to a probe accident and a deliberate heavy carbonization procedure during the operation period. The highest observed erosion exceeds the expected sputter erosion due to charge exchange neutrals by more than an order of magnitude, unless neutral fluxes are enhanced at the inner wall due to ions arriving along open field lines during current ramp-down.

An isotopic marker technique has been developed and used to measure near-surface erosion and carbon and impurity deposition on graphite samples in the ASDEX divertor chamber. Papyex graphite strips were enriched to approx. 20% /sup 13/C over the first 1000 A by ion implantation. The implanted /sup 13/C can be clearly distinguished from /sup 12/C of the host by Rutherford backscattering using 2.7 MeV /sup 4/He. Carbon erosion and deposition were determined from changes in the depth distribution of the /sup 13/C, and impurity accumulation was also monitored in the backscattering spectrum. The /sup 13/C profiles remained sharp in spite of high heat fluxes to the samples (sufficient to melt stainless steel), and the results showed clear evidence of simultaneous erosion and deposition of carbon and impurities. During 2.4 MW neutral injection, net carbon erosion exceeded 250 A/s in the divertor throat. High carbon erosion was also observed during periods of substantial Fe and Ti impurity accumulation on the samples. The results demonstrate the applicability of the /sup 13/C technique for measuring erosion/deposition at high-heat-flux surfaces such as limiters or divertor plates.

Quantitative radial profiles have been determined for deuterium and heavy impurities ( A ⩾ 12 ) trapped on deposition probes in the plasma edge of ISX-B. Silicon samples were exposed to 1, 4 and 12 tokamak discharges at distances of 10 to 50 mm behind the limiter radius for both ohmic and beam-heated plasmas. The species, quantities and saturation behavior of the principal impurities were determined by ion backscattering and channeling analysis, while nuclear reaction D( 3 He, p) 4 He and damage analyses were used for the detection and depth profiling of deuterium. The components of stainless steel (Fe, Cr, Ni) were found to increase linearly with the number of discharges, while radial profiles for oxygen and D showed saturation effects in a single discharge. Estimates of D energies and both impurity and D fluxes in the plasma-edge are given for ohmically heated and 350 kW neutral beam injected discharges.

Plasma-material interactions in the scrapeoff region of a tokamak nave important effects on the overall performance of the machine. In order to prudently select the most appropriate materials for walls, limiters and armor plate, it is necessary to characterize the plasma that interacts with these surfaces and to understand what effect different modes of operation of the tokamak have on plasma characteristics. We have made a series of measurements on the ISX tokamak using deposition probe techniques to identify and quantify the impurities (Z ≥ 8) in the limiter shadow and to determine the temporal behavior of both impurity and plasma particles in this region. These measurements have been made under a variety of tokamak operating conditions, including doth ohmic and neutral beam heated discharges. The results are interpreted in terms of edge conditions, impurity introduction, gas puffing and the relative importance of wall and limiter contributions.

The density and the velocity distribution of neutral Al released from the vacuum wall in the EBT-S fusion device have been measured using laser-induced fluorescence. Al I was excited at the 308 nm resonance transition using a frequency-doubled dye laser. Absolute Al-densities (~ 108/cm3) were determined at distances between 5 cm and 14 cm from the wall and for various plasma conditions, by comparison of the intensity of induced fluorescence with that obtained from Rayleigh scattering. Al-velocity distributions perpendicular to the wall were determined by scanning the laser through the Doppler-broadened absorption line. The influence of geometry and ionization on the shape of the distributions is given special attention to. From these distributions mean velocities were derived (~ 2.6 × 105cm/s), and along with the measured densities Al-fluxes were calculated. The results indicate sputtering, the possible mechanisms of which are discussed.

Laser-induced fluorescence has been used to measure the density and ionization length of neutral metal atoms released from a reference limiter during plasma discharges in the Impurity Study Experiment (ISX-B) tokamak. A tunable, pulsed laser beam is introduced into the tokamak at the midplane along a major radius and passes in front of a stainless steel reference limiter positioned between the wall and the main limiter radius. Neutral iron atoms released from the limiter are detected by saturation pumping of the ground-state transition at 302 nm, followed by off resonance detection of the resulting fluorescence radiation. The sampling rate is 15 Hz; additional time resolution is provided by overlaying similar discharges with different time synchronization. In ohmic discharges in ISX-B, a nearly constant impurity release rate of 8 × 1014 atom cm−2 s−1 occurs during the steady-state part of the discharge, with substantial peaks during the breakdown, rampup, and termination of the shot. Measured ionization lengths are suggestive of particle velocities typical of sputtering. These results are presented with similar data from neutral-beam-heated shots.

A description is given of an experiment that has been designed for the ISX-B tokamak at the Oak Ridge National Laboratory using beryllium as the primary plasma limiter.

Normal incidence sputtering yields have been measured for 200 – 2000 eV hydrogen ions on Au, Ni and 304 stainless steel films using a Rutherford backscattering technique. The measured yields agree well with recent results obtained at higher ion doses using the weight-loss method as well as with model calculations based on the binary collision approximation.

The weight changes of Be tiles which functioned as a rail limiter in ISX‐B for more than 3500 beam‐heated discharges have been determined. The net weight loss for the limiter was 2.0 g, with the central tiles losing a total of 3.2 g, and the inboard tiles gaining 1.2 g. The weight loss is attributed primarily to the release of Be droplets as a result of limiter surface melting. The weight gains resulted from an inward flow of molten material along the limiter surface. The results indicate high erosion (melt loss) with incomplete and nonuniform redeposition (melt flow) of limiter material during periods of limiter melting.

Measurements have been made of the absolute quantities of deuterium retained on samples exposed in the plasma edge region of both the A and B versions of the ISX tokamak. Results have been obtained as functions of both radial position behind the limiter and exposure direction relative to the toroidal field lines. Work of this type employing deposition probes has been carried out by various groups on other tokamaks so that comparison of conditions in different machines is now possible. (MOW)

The process determining the plasma surface interaction in today's high temperature plasma experiments are investigated following several lines. First, in plasma devices, the particle and energy fluxes to the different first wall areas the fluxes from the walls back into the plasma are measured and the boundary plasma parameters are determined. The surface composition and structure of the walls, limiters and divertor plates are analyzed following exposure to many discharges. Secondly, the different surface processes which are expected to contribute to the plasma surface interaction (particularly to hydrogen particle balance and impurity introduction) are studied in simulation experiments using well defined particle beams.

Proton beams have been used to generate Kossel lines in single crystals of copper by the excitation of characteristic radiation at atomic sites inside the crystals. The observed lines exhibit a fine structure similar to lines previously generated using incident electrons and x rays. A key advantage of the positive−ion technique is that the bremsstrahlung background associated with the electron generation of Kossel lines is eliminated. The possible application of the positive−ion technique to impurity lattice location is discussed.