Above-barrier capture cross-sections for heavy ion-fusion are systematically lower than predictions, with the discrepancy rapidly growing with increasing charge product of the colliding nuclei. It has been proposed that this can be explained by the onset of energy dissipation <italic>outside</italic> the barrier. Here, we present measurements of both capture cross-sections and the reflected flux in the 32S+ 208Pb reaction to quantitatively correlate signatures of energy dissipation with the reduction of capture. The measurements of the reflected flux show that multinucleon transfer begins outside the barrier leading to high excitation energies and effective energy dissipation. We deduce the resulting change in kinetic energy relative to the new potential, and combine these with coupled channels calculations. This demonstrates that discrepancies between calculations and experimental cross-sections and barrier distributions can be quantitatively related to the observed multinucleon transfer.

United States

Australia

K.J. Cook

E.C. Simpson

D.C. Rafferty

D.J. Hinde

M. Dasgupta

J. Buete

S.L. Hayles

Italy

L. Corradi

M. Evers

E. Fioretto

D.H. Luong

Croatia

T. Mijatović

G. Montagnoli

A.M. Stefanini

S. Szilner

Quantitatively relating multinucleon transfer and fusion

We present the results of a broad, systematic study of heavy-ion induced fission mass distributions for every even-Z compound nucleus ( <math altimg='si1.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>CN</mi></mrow></msub></math>) from 144Gd to 212Th. We find systematic evidence of shell-driven structure in <italic>every</italic> fission mass distribution. The change in shape of the mass distributions with <math altimg='si1.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>CN</mi></mrow></msub></math> is consistent with the results of quantitative simultaneous fitting in mass and total kinetic energy, demonstrating that fragment proton shell gaps at <math altimg='si3.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>FF</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>34</mn><mo>,</mo><mn>36</mn></math> and <math altimg='si4.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>FF</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>44</mn><mo>,</mo><mn>46</mn></math> are <italic>both</italic> major drivers of fission mass distributions below the actinide region. The mass distributions show enhanced yields at mass symmetry for values of <math altimg='si1.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>CN</mi></mrow></msub></math> equal to two times these favoured <math altimg='si5.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>FF</mi></mrow></msub></math> values. Thus, the same shell gaps that are drivers of mass-asymmetric fission also affect mass distributions at and near mass-symmetry. For all systems a second, more mass-asymmetric, fission mode is required to fit the fission mass distributions. If driven by a single shell gap, it appears to be in the light fragment around <math altimg='si6.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mi mathvariant='normal'>FF</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>28</mn><mo>,</mo><mn>30</mn></math>.

B.M.A. Swinton-Bland

A.C. Berriman

D.Y. Jeung

K. Banerjee

L.T. Bezzina

I.P. Carter

C. Sengupta

C. Simenel

Universality of shell effects in fusion-fission mass distributions

The observation of mass-asymmetric fission in neutron-deficient $^{180}$Hg 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 $^{178}$Pt 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 $^{178}$Pt 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.

T. Tanaka

M.A. Stoyer

Multi-modal mass-asymmetric fission of Pt from simultaneous mass-kinetic energy fitting

Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic $^{208}$Pb. This has generally been attributed to the enhanced binding energy of $^{208}$Pb 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 $^{50}$Ti with actinide nuclides from $^{238}$U to $^{249}$Cf. 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 $^{238}$U to only ∼40% for $^{249}$Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the $^{208}$Pb 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 $^{208}$Pb. The important role of sequential fission calls for re-interpretation of quasifission characteristics and dynamics in superheavy element synthesis reactions.

Germany

H.M. Albers

Ch.E. Düllmann

J. Khuyagbaatar

B. Kindler

N. Lobanov

B. Lommel

C. Mokry

E. Prasad

J. Runke

J.F. Smith

P. Thörle-Pospiech

N. Trautmann

K. Vo-Phuoc

J. Walshe

E. Williams

A. Yakushev

Sequential fission and the influence of Pb closed shells on the dynamics of superheavy element synthesis reactions

Measurements of mass and angular distributions have been made for fission-like outcomes in reactions forming isotopes of flerovium (Z=114), using $^{48}$Ca, $^{50}$Ti, and $^{54}$Cr 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 $^{48}$Ca + $^{244}$Pu reaction, giving upper limit to the fusion probabilities <math><msub><mrow><mi>P</mi></mrow><mrow><mi>C</mi><mi>N</mi></mrow></msub></math> of ∼10$^{−2}$, ∼5 times lower than previous results. <math><msub><mrow><mi>P</mi></mrow><mrow><mi>C</mi><mi>N</mi></mrow></msub></math> for the $^{50}$Ti reaction was lower than $^{48}$Ca, whilst no evidence of fusion-fission was found for the $^{54}$Cr reaction.

India

J. Sadhukhan

N. Schunck

Sensitive search for near-symmetric and super-asymmetric fusion-fission of the superheavy element Flerovium (Z=114)

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 <math><msubsup><mrow></mrow><mrow><mspace width="0.25em"></mspace><mn>80</mn></mrow><mrow><mn>180</mn></mrow></msubsup></math>Hg was recently observed to be strongly mass-asymmetric, indicating that despite spherical shell gaps in fragments around <math><msubsup><mrow></mrow><mrow><mn>40</mn></mrow><mrow><mn>90</mn></mrow></msubsup></math>Zr, 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, $^{12}$C, $^{32}$S, $^{40,48}$Ca projectiles, have been made for excitation energies above the fission saddle-point (E<math><msubsup><mrow></mrow><mrow><mi>eff</mi></mrow><mrow><mo>⁎</mo></mrow></msubsup></math>) 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 $^{176}$Pt and $^{180}$Pt do not show any evidence of a low TKE mass-symmetric fission mode, as had been reported for $^{178}$Pt by Tsekhanovich et al. (2019) [39].

R. Bernard

Systematics of the mass-asymmetric fission of excited nuclei from Os to Pb

The synthesis of new superheavy elements beyond oganesson (Z=118) requires fusion reactions with projectile nuclei with proton numbers larger than that of $^{48}$Ca (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 $^{64}$Ni+$^{238}$U, $^{58}$Fe+$^{244}$Pu, $^{54}$Cr+$^{248}$Cm, and $^{50}$Ti+$^{249}$Cf reactions have been made, which all result in larger Coulomb forces than for $^{48}$Ca-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 $^{50}$Ti+$^{249}$Cf.

K. Eberhardt

S. Kalkal

N.R. Lobanov

Japan

K. Sekizawa

Zeptosecond contact times for element Z=120 synthesis

The new neutron-deficient nuclei 240 Es and 236 Bk were synthesised at the gas-filled recoil separator RITU. They were identified by their radioactive decay chains starting from 240 Es produced in the fusion–evaporation reaction 209 Bi( 34 S,3n) 240 Es. Half-lives of 6(2)s and 22−6+13s were obtained for 240 Es and 236 Bk, respectively. Two groups of α particles with energies Eα=8.19(3)MeV and 8.09(3)MeV were unambiguously assigned to 240 Es. Electron-capture delayed fission branches with probabilities of 0.16(6) and 0.04(2) were measured for 240 Es and 236 Bk, respectively. These new data show a continuation of the exponential increase of ECDF probabilities in more neutron-deficient isotopes.

Finland

J. Konki

J. Uusitalo

P.T. Greenlees

K. Auranen

H. Badran

M. Block

France

R. Briselet

United Kingdom

D.M. Cox

A. Di Nitto

T. Grahn

K. Hauschild

A. Herzán

R.D. Herzberg

F.P. Heßberger

R. Julin

S. Juutinen

E. Jäger

J. Krier

M. Leino

A. Lopez-Martens

M. Mallaburn

K. Nishio

J. Pakarinen

P. Papadakis

J. Partanen

P. Peura

P. Rahkila

K. Rezynkina

P. Ruotsalainen

M. Sandzelius

J. Sarén

C. Scholey

J. Sorri

S. Stolze

B. Sulignano

Ch. Theisen

A. Ward

V. Yakusheva

Towards saturation of the electron-capture delayed fission probability: The new isotopes 240 Es and 236 Bk