Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic 208Pb. This has generally been attributed to the enhanced binding energy of 208Pb causing a valley in the potential energy surface, attracting quasifission trajectories. To investigate this interpretation, binary quasifission mass spectra and cross-sections have been measured at near-barrier energies for reactions of 50Ti with actinide nuclides from 238U to 249Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from ∼70% of calculated capture cross-sections for 238U to only ∼40% for 249Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the 208Pb peak originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above 208Pb. The important role of sequential fission calls for re-interpretation of quasifission characteristics and dynamics in superheavy element synthesis reactions.

Background: Mass and angle distribution measurements have illuminated many aspects of the physical variables controlling quasifission. However, mapping from detection angle to reaction time is clouded by the wide range of contributing angular momenta Lh over bar , ranging from 0 h over bar to the maximum of the reaction (greater than or similar to 100 h over bar ), which complicates the mapping, and thus limits our understanding of the reaction dynamics. Purpose: To investigate the angular momentum dependence of the reaction dynamics in quasifission and determine the fission fragment mass evolutions in the reaction. Method: The mass and angular distributions of products of the reactions Cr-52 + Pt-198 and Cr-54 + Pt-196 were measured. The distributions were compared with quasifission simulation results. Results: Mass angle distributions are reproduced by utilizing a new quasifission mass evolution model, and including a mass-symmetric component associated with low L. The latter increases in yield at higher beam energy. Conclusions: The symmetric component represents the total of slow quasifission and fusion-fission. The increase in contribution with energy above the Coulomb barrier suggests that an extra-push energy is required to achieve fusion.

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.

Flerovium (Fl, element 114) is the heaviest element chemically studied so far. To date, its interaction with gold was investigated in two gas-solid chromatography experiments, which reported two different types of interaction, however, each based on the level of a few registered atoms only. Whereas noble-gas-like properties were suggested from the first experiment, the second one pointed at a volatile-metal-like character. Here, we present further experimental data on adsorption studies of Fl on silicon oxide and gold surfaces, accounting for the inhomogeneous nature of the surface, as it was used in the experiment and analyzed as part of the reported studies. We confirm that Fl is highly volatile and the least reactive member of group 14. Our experimental observations suggest that Fl exhibits lower reactivity towards Au than the volatile metal Hg, but higher reactivity than the noble gas Rn.

This workshop focuses on methods, tools, and techniques to design and develop Trustworthy Autonomous Systems (TAS). TAS is an emerging area of interactive systems that is expanding the scope and remit of engineering. At every scale, making autonomous systems trustworthy is a collective task that requires a multidisciplinary team to work together to understand trust design requirements and provide effective and creative solutions. TAS introduce unique challenges in the design and development of interactive systems because they may have the capacity to learn and evolve, they may need to make decisions or take actions independently with little or no human oversight, and they will be deployed in quite different cultural and regulatory environments. TAS engineers need robust design methods, tools, and techniques to meet diverse TAS requirements and objectives. Our prior research argued for TAS engineers to develop skills in three core areas: soft, strategic, and technical [1]. However, little has been done to flesh out the specific methods, tools, and techniques that TAS engineers should draw on. This workshop intends to invite interactive systems experts to contribute promising design methods, tools, and techniques – particularly in the area of user/actor and design requirements modelling. The workshop aims to present innovative modelling techniques, test these approaches through discussion, think about the main challenges, refine TAS required skills and steer the overarching strategy in this new field for the future.

Intelligent highly-automated systems (HASs) are increasingly being created and deployed at scale with a broad range of purposes and operational environments. In uncertain or safety-critical environments, HASs are frequently designed to seamlessly co-operate with humans, thus, forming human-machine teams (HMTs) to achieve collective goals. Trust plays an important role in this dynamic: humans need to be able to develop an appropriate level of trust in their HAS teammate(s) to form an HMT capable of safely and effectively working towards goal completion. Using Autonomous Ground Vehicles (AGVs) as an example of an HAS used in dynamic social contexts, we explore interdependent teaming and communication between humans and AGVs in different contexts and examine the role of trust and communication in these teams. Drawing on lessons from the AGV example for the design of an HAS used for an HMT more broadly, we argue that trust is experienced and built differently in different contexts, necessitating context-specific approaches to designing for trust in such systems.

Mass and angle distributions for the ^{52}Cr+^{198}Pt and ^{54}Cr+^{196}Pt reactions (both forming ^{250}No) were measured and subtracted, giving new information on fast quasifission mass evolution, and the first direct determination of the dependence of sticking times on angular momentum. TDHF calculations showed good agreement with average experimental values, but experimental mass distributions unexpectedly extended to symmetric splits while the peak yield remained close to the initial masses. This implies a strong role of fluctuations in mass division early in the collision, giving insights into the transition from fast energy dissipative deep-inelastic collisions to quasifission.

Background: At energies above the capture barrier, coupled-channels (CC) calculations with a standard nuclear potential diffuseness (0.65 fm) increasingly overestimate experimental capture cross section as the charge product (${Z}_{1}{Z}_{2}$) of the colliding nuclei increases. It has been suggested this may be linked to energy dissipation outside the capture barrier.Purpose: We investigate quantitatively the role of energy dissipation in suppressing capture in heavy ion fusion reactions.Method: The yields of sequential fission, including that resulting from deep inelastic collisions, and of fission following capture were determined simultaneously for collisions of $^{18}\mathrm{O}, ^{30}\mathrm{Si}, ^{34}\mathrm{S}$, and $^{40}\mathrm{Ca}+^{232}\mathrm{Th}$ at a range of energies around the respective capture barriers.Results: The ratio of experimental to CC capture cross sections was found to decrease with increasing ${Z}_{1}{Z}_{2}$. Conversely, the ratio of sequential fission to capture-fission increased with increasing ${Z}_{1}{Z}_{2}$. The sum of sequential and capture fission agrees quite well with the CC cross sections.Conclusions: The experimental capture fission and sequential fission cross sections, and their comparison with CC calculations, give a consistent picture that the increase in density overlap at the capture barrier with increasing ${Z}_{1}{Z}_{2}$ of the colliding nuclei is correlated with increasing energy dissipative processes. These compete increasingly strongly with capture as the ${Z}_{1}{Z}_{2}$ of the reaction increases. For the $^{40}\mathrm{Ca}$ reaction, the total fission yield exceeds expectations from capture model calculations, indicating that deep inelastic processes occur both from trajectories that would have led to capture and also from more peripheral trajectories.

Measurements of mass and angular distributions have been made for fission-like outcomes in reactions forming isotopes of flerovium (Z=114), using 48Ca, 50Ti, and 54Cr projectiles. The dominant fast quasifission process, which masks the presence of fusion-fission, has minimum yield at the most backward angles, where the sensitivity to fusion-fission is thus highest. In fitting the backward angle mass spectra, only weak evidence for a component of super-asymmetric fission was found, but a near-symmetric fission component was consistently required for the 48Ca + 244Pu reaction, giving upper limit to the fusion probabilities PCN of ∼10−2, ∼5 times lower than previous results. PCN for the 50Ti reaction was lower than 48Ca, whilst no evidence of fusion-fission was found for the 54Cr reaction.

The synthesis of new superheavy elements beyond oganesson (Z=118) requires fusion reactions with projectile nuclei with proton numbers larger than that of 48Ca (Z=20), which has been successfully employed for the synthesis of elements with Z=112-118. In such reactions, fusion is drastically hindered by fast non-equilibrated dynamical processes. Attempts to produce nuclei with Z=120 using the 64Ni+238U, 58Fe+244Pu, 54Cr+248Cm, and 50Ti+249Cf reactions have been made, which all result in larger Coulomb forces than for 48Ca-induced reactions, but no discovery has been confirmed to date. In this work, mass and angle distributions of fission fragments from these reactions have been measured with large angular coverage to aid in selection of the most promising projectile-target combination that would favor fusion. The results yield information on reaction contact times, with the longest exhibited by 50Ti+249Cf.

The competition between the dominant mass-asymmetric and rarer narrow mass-symmetric fission modes in actinide nuclei are controlled by deformed and spherical shell effects. The low energy fission of 80180Hg was recently observed to be strongly mass-asymmetric, indicating that despite spherical shell gaps in fragments around 4090Zr, the system does not fission mass-symmetrically. Several theoretical approaches have been used to explain this unexpected result.To investigate the underlying mechanism, systematic measurements of fission mass distributions for isotopes of Os, Pt, Hg and Pb, formed in fusion reactions with p, 12C, 32S, 40,48Ca projectiles, have been made for excitation energies above the fission saddle-point (Eeff⁎) between 2.8 and 28.2 MeV. Evidence for mass-asymmetric fission is widespread, manifested as flat topped mass distributions or significant deviations from a single Gaussian shape. The systematic trends seen cannot be attributed to quasifission. Comparing two-Gaussian fits at a wide range of E⁎, it is concluded that the fit centroids reflect the low energy character of mass-asymmetric fission in the sub-lead region.Quantitative comparisons were made with microscopic calculations by Scamps and Simenel (2019) [33] of fission mass-asymmetries attributed to the influence of shell gaps in both neutrons (N=52, 56 for compact octuple deformations) and protons (Z=34 and Z=42, 44, 46 with large quadrupole deformations). For the predominant fission mode in the calculations, having one elongated and one compact fragment, the results are in extremely good agreement with all experimental values. This provides strong support for both the calculations, and the exploration of mass-asymmetric fission systematics through heavy ion fusion reactions. The total kinetic energy distributions for 176Pt and 180Pt do not show any evidence of a low TKE mass-symmetric fission mode, as had been reported for 178Pt by Tsekhanovich et al. (2019) [39].

The production of superheavy elements through the fusion of two heavy nuclei is severely hindered by the quasifission process, which results in the fission of heavy systems before an equilibrated compound nucleus (CN) can be formed. The heaviest elements have been synthesised using 48Ca as the projectile nucleus. However, the use of 48Ca in the formation of new superheavy elements has been exhausted, thus a detailed understanding of the properties that made 48Ca so successful is required. Measurements of mass-angle distributions allow fission fragment mass distribution widths to be determined. The effect of the orientation of prolate deformed target nuclei is presented. Closed shells in the entrance channel are also shown to be more important than the stability of the formed CN in reducing the quasifission component, with reduced mass widths for reactions with the closed shell target nuclei 144Sm and 208Pb. Comparison to mass widths for 48Ti-induced reactions show a significant increase in the mass width compared to 48Ca-induced reactions, highlighting the difficulty faced in forming new superheavy elements using projectiles with higher atomic number than 48Ca.

Background: Formation of a fully equilibrated compound nucleus is a critical step in the heavy-ion fusion reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. In particular, the impact of changing the neutron richness on the quasifission process is not well known. A previous study of Cr + W reactions at a constant 13% above the Coulomb barrier concluded that an increase in neutron richness leads to a decrease in the prominence of the quasifission reaction channel. Purpose: The dynamics of quasifission for reactions with varying neutron richness was explored at a constant excitation energy, closer to the interaction barrier than the previous work, to see if the correlation between neutron richness and quasifission is valid at lower energies. Methods: Mass distributions were measured at the Australian National University for eight different combinations of Cr + W reactions, using the kinematic coincidence method. To eliminate the effect of differing excitation energies, measurements were made at beam energies chosen to give 52 MeV of excitation energy in all the compound nuclei. Results: A curvature parameter, describing the shape of the mass distributions, was determined for the fission-like fragment mass distributions for each reaction, and compared to various reaction parameters known to influence quasifission. Conclusions: The present work demonstrates that, at energies near the interaction barrier, the beam energy with respect to the barrier is as important as neutron-richness effects in determining the quasifission characteristics in these Cr + W reactions involving statically deformed target nuclei, and both are important considerations for future heavy and superheavy element production reactions.

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of ^{48}Ca, ^{50}Ti, and ^{54}Cr with ^{208}Pb. Moving from ^{48}Ca to ^{54}Cr leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity P_{CN} (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for ^{50}Ti and 15 for ^{54}Cr in comparison to ^{48}Ca. The energy dependence of P_{CN} indicates that cold fusion reactions (involving ^{208}Pb) are not driven by a diffusion process.

The authors acknowledge the support of Australian Research Council research grants DE140100784, DP140101337, DP160101254,DP170102318, DP170102423, German Academic ExchangeService (DAAD) via funds of the German Federal Ministryof Education and Research (BMBF). The Australian FederalGovernment NCRIS program is acknowledged for support ofoperations of the ANU Heavy Ion Accelerator Facility.

Background: Quasifission (QF) has gained tremendous importance in heavy-ion nuclear physics research because of its strong influence on superheavy-element synthesis. Collisions involving closed-shell nuclei in the entrance channel are found to affect the QF reaction mechanism. Hence, it is important to improve the understanding of their effect on QF. Apart from that, some recent studies show that the difference in N/Z of reaction partners influences the reaction dynamics. Since heavier doubly magic nuclei have different N/Z than lighter doubly magic nuclei, it is important to understand the effect of N/Z mismatch as well as the effect of shell closures. Purpose: To investigate the effect of entrance-channel shell closures and N/Z asymmetry on QF. The reactions were chosen to decouple these effects from the contributions of other entrance-channel parameters. Method: Fission fragment mass-angle distributions were measured using the CUBE fission spectrometer, consisting of two large area position-sensitive multi-wire proportional counters (MWPCs), for five reactions, namely, Cr-50 (208)pb, Cr-52 (206,208)pb, Cr-54 (204,208)pb. Result: Two components were observed in the measured fragment mass angle distribution, a fast mass-asymmetric quasifission and a slow mass-symmetric component having a less significant mass-angle correlation. The ratio of these components was found to depend on spherical closed shells in the entrance channel nuclei and the magnitude of the N/Z mismatch between the two reaction partners, as well as the beam energy. Conclusions: Entrance-channel spherical closed shells can enhance compound nucleus formation provided the N/Z asymmetry is small. Increase in the N/Z asymmetry is expected to destroy the effect of entrance-channel spherical closed shells, through nucleon transfer reactions.

Background: The formation of superheavy elements (SHEs) by fusion of two massive nuclei is severely inhibited by the competing quasifission process. Lowexcitation energies favor SHE survival against fusion-fission competition. In "cold" fusion with spherical target nuclei near Pb-208, SHE yields are largest at beam energies significantly below the average capture barrier. In "hot" fusion with statically deformed actinide nuclei, this is not the case. Here the elongated deformation-aligned configurations in sub-barrier capture reactions inhibits fusion (formation of a compact compound nucleus), instead favoring rapid reseparation through quasifission. Purpose: To determine the probabilities of fast and slow quasifission in reactions with prolate statically deformed actinide nuclei, through measurement and quantitative analysis of the dependence of quasifission characteristics at beam energies spanning the average capture barrier energy. Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission and quasifission mass and angle distributions for reactions with projectiles from C to S, bombarding Th and U target nuclei. Results: Mass-asymmetric quasifission occurring on a fast time scale, associated with collisions with the tips of the prolate actinide nuclei, shows a rapid increase in probability with increasing projectile charge, the transition being centered around projectile atomic number ZP = 14. For mass-symmetric fission events, deviations of angular anisotropies from expectations for fusion fission, indicating a component of slower quasifission, suggest a similar transition, but centered around ZP similar to 8. Conclusions: Collisions with the tips of statically deformed prolate actinide nuclei show evidence for two distinct quasifission processes of different time scales. Their probabilities both increase rapidly with the projectile charge. The probability of fusion can be severely suppressed by these two quasifission processes, since the sub-barrier heavy element yield is likely to be determined by the product of the probabilities of surviving each quasifission process.

Superheavy elements are primarily formed through heavy ion fusion reactions(1-3). Formation of a fully equilibrated compound nucleus is a critical step in this reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration(1'4-8). To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. The interplay between the fusion-fission and quasifission reaction channels was explored by measuring fission mass distributions in eight different combinations of Cr+W reactions, with varying neutron-richness, at the Australian National University. The reactions were measured in two energy regimes: one at 13% above the Bass fusion barrier(9) and one at 52.0 MeV of excitation energy in the compound nucleus, E-CN*. For the systems measured at E-c.m./V-Bass = 1.13 the dependence on the neutron-richness is clear(10). However, for the reactions at E-CN* = 52.0 MeV, the dependence is less clear and additional factors are shown to play a vital role, especially the influence of deformation on the effective fusion barrier. The present work demonstrates that quasifission is an important process in competition with heavy-ion fusion in reactions with intermediate mass projectiles, particularly with more neutron-rich systems.

Background: Fusion of heavy nuclei requires energy dissipation to trap the system inside the capture barrier. At high angular momentum, the centrifugal potential causes the barrier radius to reduce, which may lead to energy dissipation outside the barrier, affecting the fusion angular momentum distributions and thus the capture cross sections. Purpose: To investigate the sensitivity of fusion-fission cross sections as a probe of fusion dynamics at high angular momentum. Method: Fission of the compound nucleus Yb-164 formed by three different fusion reactions, namely, O-16 + Sm-148, Si-28 + Ba-136, and Ca-40 + Sn-124, was measured at four beam energies well above their respective capture barriers. Fission cross sections were extracted from the measured fission fragment angular distributions and compared with model calculations of fusion and subsequent fission. Fusion and evaporation residue cross sections available in the literature for the same or similar reactions were used to guide model calculations and obtain the fusion angular momentum distributions. Results: The measured fission characteristics were found to be consistent with fusion-fission, as expected, justifying the use of the statistical model to calculate fission cross sections for each reaction. Significantly different fission cross-section predictions were obtained from calculations using angular momentum distributions corresponding to different coupling schemes and different diffuseness parameter of the nuclear potential. A large diffuseness parameter (>>0.65 fm) of the nuclear potential was observed to give the best reproduction of both the experimental fusion as well as fission cross sections. Conclusions: Experimental fission cross sections provide a stringent constraint to the fusion model calculations and thus prove to be a sensitive probe for understanding fusion dynamics at high angular momentum. This is shown in the present work by a simultaneous analysis of the fusion and fission cross sections for systems where fission cross sections form a small fraction of the fusion cross section and where noncompound nuclear processes are not a dominant competing channel. Observations from this work also suggest the requirement of evaporation residue as well as fission cross sections of higher precision than those generally available for drawing quantitative conclusions.