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.

Australia

D.Y. Jeung

D.J. Hinde

M. Dasgupta

C. Simenel

E.C. Simpson

K.J. Cook

Germany

H.M. Albers

J. Buete

I.P. Carter

Ch.E. Düllmann

J. Khuyagbaatar

B. Kindler

N. Lobanov

B. Lommel

C. Mokry

E. Prasad

J. Runke

C. Sengupta

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.

K. Banerjee

India

J. Sadhukhan

B.M.A. Swinton-Bland

L.T. Bezzina

United States

N. Schunck

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

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

D.C. Rafferty

Japan

K. Sekizawa

Zeptosecond contact times for element Z=120 synthesis

The isotopic distribution of nuclei produced in the 50Ti + 249Cf reaction has been studied at the gas-filled recoil separator TASCA at GSI Darmstadt, which separates ions according to differences in magnetic rigidity. The bombardment was performed at an energy around the Bass barrier and with the TASCA magnetic fields set for collecting fusion-evaporation reaction products. Fifty-three isotopes located “north-east” of 208Pb were identified as recoiling products formed in non-fusion channels of the reaction. These recoils were implanted with energies in two distinct ranges; besides one with higher energy, a significant low-energy contribution was identified. The latter observation was not expected to occur according to kinematics of the known types of reactions, namely quasi-elastic, multi-nucleon transfer, deep-inelastic collisions or quasifission. The present observations are discussed within the framework of two-body kinematics passing through the formation of a composite system.

A. Di Nitto

D. Ackermann

United Kingdom

L.L. Andersson

E. Badura

M. Block

H. Brand

I. Conrad

D.M. Cox

J. Dvorak

P.A. Ellison

N.E. Esker

J. Even

Sweden

C. Fahlander

U. Forsberg

J.M. Gates

P. Golubev

O. Gothe

K.E. Gregorich

W. Hartmann

R.D. Herzberg

F.P. Heßberger

J. Hoffmann

R. Hollinger

A. Hübner

E. Jäger

S. Klein

I. Kojouharov

J.V. Kratz

J. Krier

N. Kurz

S. Lahiri

M. Maiti

R. Mändl

E. Merchán

S. Minami

A.K. Mistry

H. Nitsche

Norway

J.P. Omtvedt

G.K. Pang

D. Renisch

D. Rudolph

Colombia

L.G. Sarmiento

M. Schädel

H. Schaffner

B. Schausten

A. Semchenkov

J. Steiner

Switzerland

A. Türler

Finland

J. Uusitalo

D. Ward

Poland

M. Wegrzecki

P. Wieczorek

N. Wiehl

V. Yakusheva

Study of non-fusion products in the Reaction

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.

J. Konki

P.T. Greenlees

K. Auranen

H. Badran

France

R. Briselet

T. Grahn

K. Hauschild

A. Herzán

R. Julin

S. Juutinen

M. Leino

A. Lopez-Martens

D.H. Luong

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

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

A novel rigorous statistical treatment is applied to available data (May 9, 2016) from search and spectroscopy experiments on the elements with atomic numbers Z=115 and Z=117 . The present analysis implies that the hitherto proposed cross-reaction link between α -decay chains associated with the isotopes 293 117 and 289 115 is highly improbable.

S. Åberg

A new assessment of the alleged link between element 115 and element 117 decay chains

Mass distributions of fission fragments from the compound nuclei 180 Hg and 190 Hg formed in fusion reactions 36 Ar + 144 Sm and 36 Ar + 154 Sm, respectively, were measured at initial excitation energies of E⁎(Hg180)=33–66 MeV and E⁎(Hg190)=48–71 MeV . In the fission of 180 Hg, the mass spectra were well reproduced by assuming only an asymmetric-mass division, with most probable light and heavy fragment masses A¯L/A¯H=79/101 . The mass asymmetry for 180 Hg agrees well with that obtained in the low-energy β+/EC -delayed fission of 180 Tl, from our earlier ISOLDE(CERN) experiment. Fission of 190 Hg is found to proceed in a similar way, delivering the mass asymmetry of A¯L/A¯H=83/107 , throughout the measured excitation energy range. The persistence as a function of excitation energy of the mass-asymmetric fission for both proton-rich Hg isotopes gives strong evidence for the survival of microscopic effects up to effective excitation energies of compound nuclei as high as 40 MeV. This behavior is different from fission of actinide nuclei and heavier mercury isotope 198 Hg.

A.N. Andreyev

R. Chapman

X. Derkx

Belgium

L. Ghys

K. Hirose

H. Ikezoe

H. Makii

I. Nishinaka

T. Ohtsuki

S.D. Pain

JINR

R. Sagaidak

I. Tsekhanovich

M. Venhart

Y. Wakabayashi

China

S. Yan

Excitation energy dependence of fragment-mass distributions from fission of 180,190 Hg formed in fusion reactions of 36 Ar + 144,154 Sm