quasifission dynamics in the formation of superheavy elements · 2020. 5. 19. · doubly-magic...

Quasifission Dynamics in the Formation of Superheavy Elements

D.J. Hinde1,�, M. Dasgupta1, D.Y. Jeung1, G. Mohanto1,��, E. Prasad1,��, C. Simenel1, E. Williams1, I.P. Carter1,K.J. Cook1, Sunil Kalkal1, D.C. Rafferty1, E.C. Simpson1, H.M. David2, Ch.E. Düllmann2,3,4, and J. Khuyagbaatar2,3

1Department of Nuclear Physics, Research School of Physics and Engineering,Australian National University, Canberra, ACT 2601, Australia2GSI Helmholtzzentrum für Schwerionenforschung, D-64291 Darmstadt, Germany3Helmholtz Institute Mainz, D-55099 Mainz, Germany4Johannes Gutenberg Universität Mainz, D-55099 Mainz, Germany

Abstract. Superheavy elements are created through the fusion of two heavy nuclei. The large Coulomb en-ergy that makes superheavy elements unstable also makes fusion forming a compact compound nucleus veryunlikely. Instead, after sticking together for a short time, the two nuclei usually come apart, in a process calledquasifission. Mass-angle distributions give the most direct information on the characteristics and time scales ofquasifission. A systematic study of carefully chosen mass-angle distributions has provided information on theglobal trends of quasifission. Large deviations from these systematics at beam energies near the capture barrierreveal the major role played by the nuclear structure of the two colliding nuclei in determining the reactionoutcome, and thus implicitly in hindering or favouring superheavy element synthesis.

1 Introduction

Superheavy elements (SHE) are formed by heavy-ion fu-sion reactions. Fusion cross sections can be considerablysuppressed [1] by quasifission [2]. This non-equilibriumprocess results when the combined di-nuclear system,formed as the two nuclear surfaces stick together, subse-quently separates into two (fission-like) fragments, withthe initial kinetic energy largely or completely damped.Quasifission can occur very rapidly, typically in lessthan 10−20s, before a compact compound nucleus can bereached [2–5]. The probability of quasifission (PQF) canbe very large, thus the complementary probability of com-pound nucleus formation (PCN = 1 - PQF) can be small,quite likely lower than 10−3 in reactions forming super-heavy elements. Understanding the competition betweenquasifission and fusion is thus very important in predictingthe optimal fusion reactions to use to form new elementsand isotopes in the superheavy mass region.

A key characteristic, important for superheavy elementformation, is the “sticking time” following contact of thetwo nuclear surfaces [6]. It is expected that the stickingtime is correlated with PCN : where the sticking time islonger, then PCN would be expected to be larger (morefavourable for SHE synthesis). The average sticking timecan be extracted from measurements of quasifission an-gular distributions. The two colliding nuclei always ap-

�e-mail: [email protected]��Current address: BARC, Mumbai, India��Permanent address: Department of Physics, School of Mathematicaland Physical Sciences, Central University of Kerala, Kasaragod 671314,India.

proach each other along the beam axis, and after contactrotate with angular velocities that can be calculated. Mea-surement of the rotation angle thus allows estimation ofthe sticking time. As the system rotates, mass flow alsooccurs between the two nuclei. Measurement of the ve-locity vectors of both fragments gives direct informationon the centre-of-mass angle and mass-ratio of the frag-ments at scission, defined as MR = M1/(M1 + M2), aswell as providing excellent discrimination against fissionevents resulting from peripheral (transfer-induced) pro-cesses [4, 7]. The mass-ratio (or mass) plotted as a func-tion of the centre-of-mass angle is referred to as a mass-angle distribution, or MAD. This gives direct informationon the dynamical time scales, as long as the system under-goes less than a full rotation (taking ∼10−20s). This is usu-ally the case for collisions of heavy nuclei, as shown firstby measurements at GSI [2, 8], and by later results fromANU [3–5, 9–15]. Inter-relationships of different mea-surements, and background to the results presented hereare given in recent conference proceedings [16–20].

2 Experimental Setup

The detector configuration used at the Australian NationalUniversity (ANU) to measure MAD is shown in Fig.1. Itconsists of two or three large area multi-wire proportionalcounters (MWPC). They detect reaction products from theinteraction of the pulsed beam with targets typically 50-200 µg/cm2 in thickness, oriented with the normal to thetarget face at 60◦ to the beam. This eliminates shadow-ing of the detectors by the target frame. The average en-

EPJ Web of Conferences 163, 00023 (2017) DOI: 10.1051/epjconf/201716300023FUSION17

© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).

TargetPulsed beam

MWPC2 28 x 36 cm t,X,Y


19.5 cm V1

V2

V1cm

V2cm

~ 1 ns FWHM

θlab

φlab

MWPC3 14 x 36 cm t,X,YKinematic coincidence:

Determine (binary) mass-ratio: MR = M1/(M1+M2) = V2cm/(V1cm+V2cm)

Figure 1. Enhanced ANU experimental setup to measure fission mass-angle distributions over the widest achievable angular range.The angular coverage of the MWPC1 and MWPC3 detectors, for events in coincidence with MWPC2, are shown in scattering angle θand azimuthal angle φ.

ergy loss of the beam and the fragments in the target ma-terial is corrected for in the analysis. The wide coverageof MWPC1 and MWPC3 in scattering angle (θ) and az-imuthal angle (φ) for coincidence events with MWPC2 isdemonstrated in Fig.1. From the good resolution of thedetectors in position (∼1 mm) and time (< 500 ps), andknowledge of the interaction time of the beam pulse withthe target, the velocity vectors of the detected particlesare determined. From the deduced velocity vectors in thecentre-of-mass frame, for each binary event the centre-of-mass angle and the mass-ratio are determined, thus gener-ating the MAD.

Figure 2. The symbol indicates the classification of MAD ob-served, shown as a function of the charge product of the col-liding nuclei ZpZt and the atomic number of the compound nu-cleus ZCN=Zp+Zt. The numbers refer to the specific reactionin Ref.[4]. The diagonal full blue line represents the empiricalboundary between reactions with no mass-angle correlation (left)and those that have (right). The diagonal red dashed line indi-cates the boundary of reactions which no longer exhibit a peak atsymmetry in the angle-integrated fission mass distribution. Thethin purple line represents the locus of reactions with Pb. Ex-amples of each type of MAD are shown in the panels above,with their reaction number. The purple circles and arrow referto Cr+Pb measurements discussed towards the end of the paper.

3 MAD systematics

Experimental MAD have been divided into three cate-gories [4] having: (i) a mass-angle correlation with aminimum yield at mass-symmetry - associated with shortsticking times (MAD1); (ii) a mass-angle correlation withpeak yield at mass-symmetry - resulting from intermedi-ate sticking times (MAD2); and (iii) no significant mass-angle correlation and a narrow mass-distribution – asso-ciated with long sticking times, including fission follow-ing fusion (MAD3). Examples of each type of MAD areshown in the upper panels of Fig.2. The systematic trendsof MAD characteristics with the identity of the two col-liding nuclei was studied [duRietz13], to determine globaltrends of quasifission dynamics. This is in analogy withthe evaluation of the smooth liquid drop model depen-dence of nuclear masses on N and Z, where deviationshighlight the effects of nuclear structure. Choosing bom-barding energies E well-above the mean capture barrierB (around E/B=1.08), nuclear structure effects were min-imised. It was found [duRietz13] at these bombarding en-ergies that the MADs are indeed strongly correlated withglobal variables. The simplest variables are the Coulombrepulsion in the entrance channel (related to the productof the proton numbers of the projectile and target nucleiZpZt), and the compound nucleus atomic number ZCN , asillustrated in Fig.2. However, for particular cases, it hasbeen found that the nuclear structure of the nuclei in theentrance channel is extremely important in determiningthe sticking times and MAD characteristics. This relates todoubly-magic neutron-rich nuclei such as 48Ca and 208Pb,and prolate deformed actinide nuclei, all used in SHE for-mation reactions, as described below.

4 Effect of spherical closed shells

To investigate in detail the effect of closed shells inthe entrance channel on quasifission probabilities andcharacteristics, measurements [13] of MADs were madefor 40,44,48Ca projectiles bombarding targets of 208,204Pb(forming 248,252,256No with ZCN =102), and for 48Ti bom-barding 200Hg (248No) and 208Pb (forming 256Db with ZCN


2

TargetPulsed beam



19.5 cm V1

V2

V1cm

V2cm

~ 1 ns FWHM

θlab

φlab

MWPC3 14 x 36 cm t,X,YKinematic coincidence:

Determine (binary) mass-ratio: MR = M1/(M1+M2) = V2cm/(V1cm+V2cm)

Figure 1. Enhanced ANU experimental setup to measure fission mass-angle distributions over the widest achievable angular range.The angular coverage of the MWPC1 and MWPC3 detectors, for events in coincidence with MWPC2, are shown in scattering angle θand azimuthal angle φ.

ergy loss of the beam and the fragments in the target ma-terial is corrected for in the analysis. The wide coverageof MWPC1 and MWPC3 in scattering angle (θ) and az-imuthal angle (φ) for coincidence events with MWPC2 isdemonstrated in Fig.1. From the good resolution of thedetectors in position (∼1 mm) and time (< 500 ps), andknowledge of the interaction time of the beam pulse withthe target, the velocity vectors of the detected particlesare determined. From the deduced velocity vectors in thecentre-of-mass frame, for each binary event the centre-of-mass angle and the mass-ratio are determined, thus gener-ating the MAD.

Figure 2. The symbol indicates the classification of MAD ob-served, shown as a function of the charge product of the col-liding nuclei ZpZt and the atomic number of the compound nu-cleus ZCN=Zp+Zt. The numbers refer to the specific reactionin Ref.[4]. The diagonal full blue line represents the empiricalboundary between reactions with no mass-angle correlation (left)and those that have (right). The diagonal red dashed line indi-cates the boundary of reactions which no longer exhibit a peak atsymmetry in the angle-integrated fission mass distribution. Thethin purple line represents the locus of reactions with Pb. Ex-amples of each type of MAD are shown in the panels above,with their reaction number. The purple circles and arrow referto Cr+Pb measurements discussed towards the end of the paper.

3 MAD systematics

Experimental MAD have been divided into three cate-gories [4] having: (i) a mass-angle correlation with aminimum yield at mass-symmetry - associated with shortsticking times (MAD1); (ii) a mass-angle correlation withpeak yield at mass-symmetry - resulting from intermedi-ate sticking times (MAD2); and (iii) no significant mass-angle correlation and a narrow mass-distribution – asso-ciated with long sticking times, including fission follow-ing fusion (MAD3). Examples of each type of MAD areshown in the upper panels of Fig.2. The systematic trendsof MAD characteristics with the identity of the two col-liding nuclei was studied [duRietz13], to determine globaltrends of quasifission dynamics. This is in analogy withthe evaluation of the smooth liquid drop model depen-dence of nuclear masses on N and Z, where deviationshighlight the effects of nuclear structure. Choosing bom-barding energies E well-above the mean capture barrierB (around E/B=1.08), nuclear structure effects were min-imised. It was found [duRietz13] at these bombarding en-ergies that the MADs are indeed strongly correlated withglobal variables. The simplest variables are the Coulombrepulsion in the entrance channel (related to the productof the proton numbers of the projectile and target nucleiZpZt), and the compound nucleus atomic number ZCN , asillustrated in Fig.2. However, for particular cases, it hasbeen found that the nuclear structure of the nuclei in theentrance channel is extremely important in determiningthe sticking times and MAD characteristics. This relates todoubly-magic neutron-rich nuclei such as 48Ca and 208Pb,and prolate deformed actinide nuclei, all used in SHE for-mation reactions, as described below.

4 Effect of spherical closed shells

To investigate in detail the effect of closed shells inthe entrance channel on quasifission probabilities andcharacteristics, measurements [13] of MADs were madefor 40,44,48Ca projectiles bombarding targets of 208,204Pb(forming 248,252,256No with ZCN =102), and for 48Ti bom-barding 200Hg (248No) and 208Pb (forming 256Db with ZCN

Coun

tsθ

c.m

.

X 0.1 X 0.1 X 0.267 X 0.01 X 0.05

0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8

4500

3000

1500

0

45

90

135

180

MR

16O + 238UX 0.4 X 0.2 X 0.4 X 0.1

48Ti + 200Hg 48Ti + 208Pb 44Ca + 204Pb 48Ca + 204Pb 40Ca + 208Pb

254 Fm 248 No 248 No 248 No252 No256 Db48Ca + 208Pb

256 No100 102102102102102 104

1

101

102

103

104 Counts/pixel

0.0 0.2 0.4 0.6 0.8 1.0

X 0.1

NMagic 2 0 2 2 3 4 4 ∆(N/Z) 0.57 0.32 0.35 0.29 0.09 0.54 0.14

0.0 0.2 0.4 0.6 0.8

σMR 0.081 0.237 0.120 0.114 0.084 0.126 0.068

X 0.1

Figure 3. Measured mass-angle distributions for the indicated reactions at E/B∼0.98 (upper panels), compared to 16O+238U (left)measured at an above-barrier energy, where fast quasifission would be expected to be negligible. The MAD for 48Ca+208Pb shows a dipin coverage at 30◦ and 150◦ due to the gap between detectors shown in Fig.1. In the projected mass ratio spectra for 45◦ < θc.m.

Counts/pixel

260Sg 262Sg (Z=106) 258Sg

NMagic 1 2 2 2 3∆(N/Z) 0.24 0.45 0.35 0.29 0.37

258Sg 262Sg 260Sg

Figure 5. MAD and projected mass-ratio distributions for backward angles from 90◦ to 135◦ (as indicated in the MAD plots), forreactions of 50,52,54Cr isotopes with 204,206,208Pb. Sub-barrier beam energies (denoted by E/B), resulted in the low excitation energiesE∗. As in Fig.4, the total number of magic numbers of the projectile and target nuclei NMagic, and the difference ∆(N/Z) betweenthe projectile and target nucleus N/Z ratios are shown for each reaction. The reaction outcome changes from a minimum in yield atmass-symmetry (left) to a narrow peak at symmetry, which shows no evidence of a mass-angle correlation.

ber of magic numbers in the entrance channel NMagic,and then by the difference ∆(N/Z) between the N/Z val-ues of the target and projectile nuclei. The left-most re-action has only a single magic number in the entrancechannel, and shows a U-shaped mass distribution, consis-tent with MAD1, as expected from systematics (right-handpurple circle in Fig.2). With two magic numbers, the re-actions better matched in N/Z (smaller values of ∆(N/Z))show a peak at mass-symmetry, associated with an angle-independent ridge in the MAD. With three magic numbers,but less favourable ∆(N/Z), a similar result is observed.

Fits were performed to the mass-ratio spectra, in-cluding an asymmetric U-shaped background from fast

quasifission (derived from the reactions without a mass-symmetric peak) and a mass-symmetric Gaussian peak.As the beam energy for the Cr+Pb reactions is increased,Fig.6(a) shows that the mass-symmetric peak progres-sively becomes a smaller fraction of the total fission yieldwithin the range 0.3< MR

ZCN=102 ZCN=118ZCN=106 ZCN=106

0.0 0.2 0.4 0.6 0.8 1.0MR

0.0 0.2 0.4 0.6 0.8 1.0

MR

Counts/pixel

Counts/pixel

48Ca + 208Pb 34S + 232Th 50Ti + 248CmC

ount

s180

135

90

45

0

θ c.m

.

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.80.0 0.2 0.4 0.6 0.8 1.0

Cou

nts

34S + 232Th

0.0 0.2 0.4 0.6 0.8 1.0

E < VB E > VB

E < VB E > VBE < VB

E < VB

E > VB

MR MR

180

135

90

0

θ c.m

.

0.0 0.2 0.4 0.6 0.8MRMR

(a)

(b)

(c)

(d)

(e)

(f)

Counts/pixel(g)

(h)

MR vpar – vCN (cm/ns)

45

0.5

0.0

-0.5

V per

p(c

m/n

s)

0.0 0.4 0.8-0.4-0.8

1.0

-1.0

Figure 8. MAD and projected mass distributions for the four reactions indicated (see text). For the 50Ti+248Cm reaction, rather thanthe mass distribution, panel (h) shows the source velocity deduced assuming binary kinematics. The x-axis shows the componentparallel to the beam (expressed as the difference between the measured velocity and the calculated centre-of-mass velocity), whilst they-axis shows the component perpendicular both to the fission plane and the beam axis [7]. For a binary event originating from capture(corresponding to full momentum transfer from the projectile), the events should be close to the centre of the plot, around (0,0). Thegroup of events at (-VCN ,0) correspond to spontaneous fission of the 248Cm target. The MAD for 50Ti+248Cm, gated as shown in (h), wasmeasured at an energy above the expected barrier energy. It most closely resembles that for the 34S+232Th reaction at the sub-barrierenergy shown in (c).

Fig.8(e) and (f) show data for the same reaction at anabove-barrier energy. The mass-asymmetric componentnow makes up a small fraction of the total fission-likeevents. Its mass-centroid is essentially unchanged [5], asindicated by the dashed lines, but the group has movedfrom backward to more forward angles (both character-istics indicated by the arrows). This can be associatedwith increased mean angular momentum at this above-barrier energy, allowing a greater rotation angle (in a giventime) before scission. The predominance of the group ofevents around mass-symmetry (but having a clear mass-angle correlation) is associated with the higher probabilityof equatorial collisions. The three dimensional geometry(resulting in a sinθ probability weighting [7]) means thatfor prolate nuclei, the probability of equatorial collisionsat energies well-above the average barrier energy is muchhigher than that of axial (deformation aligned) collisions.The mass-angle correlation shows, despite the compactcontact configuration in equatorial collisions, that thesecollisions predominantly result in quasifission, in con-trast with the sub-barrier 48Ca+208Pb data, which showsno mass-angle correlation. Although equatorial collisionswith a prolate deformed nucleus do seem to result in longersticking times and increased PCN compared with an equiv-alent spherical nucleus, the comparison of MADs (a) and(e) suggests that the enhancement is not as great as the ef-fect of colliding two doubly-magic neutron-rich nuclei at asub-barrier energy. More detailed analysis of a larger num-ber of measurements is needed to reach a more quantitativeconclusion about this factor in SHE formation reactions.

For all reactions with actinide target nuclei, there isa non-negligible probability of sequential fission of thetarget-like nucleus after the transfer of nucleons between

the colliding nuclei early in the reaction. The analy-sis method allowing clean separation of the full momen-tum transfer fission following capture is illustrated forthe 50Ti+248Cm reaction in Fig.8(h). The tightly groupedevents in the centre of the graph (0,0) correspond to bi-nary events [4, 7], originating from a source with the ve-locity of the centre-of-mass in the collision, with only asmall momentum spread due to neutron evaporation. Thesurrounding broad semi-circular distribution of events cor-responds to three-body events, most likely of fission ofvarious target-like nuclei resulting from nucleon transfers,with the projectile-like nuclei recoiling with various anglesand energies giving a broad distribution of velocities to thetarget-like nuclei. The tight group of events at (-VCN ,0)correspond to spontaneous fission of the 248Cm target nu-clei, stationary in the laboratory frame. These show theexpected asymmetric-peaked fission mass distribution. Atypical gate on FMT events, as used to generate the MADin Fig.8(g), is shown by the black circle.

The 50Ti+248Cm reaction forms the compound nu-cleus 298Og (Z=118). The MAD shown in Fig.8(h)was measured at the Australian National University, us-ing a target from Mainz/GSI, at a beam energy abovethe predicted [29] average capture barrier energy, whereequatorial collisions having the longest sticking timeswould be expected to be dominant. However, the MADmore closely resembles the sub-barrier measurement for34S+232Th shown in Fig.8(c), rather than the above-barriermeasurement shown in Fig.8(e). The relative yield ofmass-symmetric events is smaller than for the sub-barrier34S+232Th reaction, but the fast quasifission events stillshow a considerable mass flow towards mass-symmetry.As in panels (c) and (e), the dashed lines show the ex-


6

ZCN=102 ZCN=118ZCN=106 ZCN=106

0.0 0.2 0.4 0.6 0.8 1.0MR

0.0 0.2 0.4 0.6 0.8 1.0

MR

Counts/pixel

Counts/pixel

48Ca + 208Pb 34S + 232Th 50Ti + 248Cm

Cou

nts

180

135

90

45

0

θ c.m

.

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.80.0 0.2 0.4 0.6 0.8 1.0

Cou

nts

34S + 232Th

0.0 0.2 0.4 0.6 0.8 1.0

E < VB E > VB

E < VB E > VBE < VB

E < VB

E > VB

MR MR

180

135

90

0

θ c.m

.

0.0 0.2 0.4 0.6 0.8MRMR

(a)

(b)

(c)

(d)

(e)

(f)

Counts/pixel

(g)

(h)

MR vpar – vCN (cm/ns)

45

0.5

0.0

-0.5

V per

p(c

m/n

s)

0.0 0.4 0.8-0.4-0.8

1.0

-1.0

Figure 8. MAD and projected mass distributions for the four reactions indicated (see text). For the 50Ti+248Cm reaction, rather thanthe mass distribution, panel (h) shows the source velocity deduced assuming binary kinematics. The x-axis shows the componentparallel to the beam (expressed as the difference between the measured velocity and the calculated centre-of-mass velocity), whilst they-axis shows the component perpendicular both to the fission plane and the beam axis [7]. For a binary event originating from capture(corresponding to full momentum transfer from the projectile), the events should be close to the centre of the plot, around (0,0). Thegroup of events at (-VCN ,0) correspond to spontaneous fission of the 248Cm target. The MAD for 50Ti+248Cm, gated as shown in (h), wasmeasured at an energy above the expected barrier energy. It most closely resembles that for the 34S+232Th reaction at the sub-barrierenergy shown in (c).

Fig.8(e) and (f) show data for the same reaction at anabove-barrier energy. The mass-asymmetric componentnow makes up a small fraction of the total fission-likeevents. Its mass-centroid is essentially unchanged [5], asindicated by the dashed lines, but the group has movedfrom backward to more forward angles (both character-istics indicated by the arrows). This can be associatedwith increased mean angular momentum at this above-barrier energy, allowing a greater rotation angle (in a giventime) before scission. The predominance of the group ofevents around mass-symmetry (but having a clear mass-angle correlation) is associated with the higher probabilityof equatorial collisions. The three dimensional geometry(resulting in a sinθ probability weighting [7]) means thatfor prolate nuclei, the probability of equatorial collisionsat energies well-above the average barrier energy is muchhigher than that of axial (deformation aligned) collisions.The mass-angle correlation shows, despite the compactcontact configuration in equatorial collisions, that thesecollisions predominantly result in quasifission, in con-trast with the sub-barrier 48Ca+208Pb data, which showsno mass-angle correlation. Although equatorial collisionswith a prolate deformed nucleus do seem to result in longersticking times and increased PCN compared with an equiv-alent spherical nucleus, the comparison of MADs (a) and(e) suggests that the enhancement is not as great as the ef-fect of colliding two doubly-magic neutron-rich nuclei at asub-barrier energy. More detailed analysis of a larger num-ber of measurements is needed to reach a more quantitativeconclusion about this factor in SHE formation reactions.

For all reactions with actinide target nuclei, there isa non-negligible probability of sequential fission of thetarget-like nucleus after the transfer of nucleons between

the colliding nuclei early in the reaction. The analy-sis method allowing clean separation of the full momen-tum transfer fission following capture is illustrated forthe 50Ti+248Cm reaction in Fig.8(h). The tightly groupedevents in the centre of the graph (0,0) correspond to bi-nary events [4, 7], originating from a source with the ve-locity of the centre-of-mass in the collision, with only asmall momentum spread due to neutron evaporation. Thesurrounding broad semi-circular distribution of events cor-responds to three-body events, most likely of fission ofvarious target-like nuclei resulting from nucleon transfers,with the projectile-like nuclei recoiling with various anglesand energies giving a broad distribution of velocities to thetarget-like nuclei. The tight group of events at (-VCN ,0)correspond to spontaneous fission of the 248Cm target nu-clei, stationary in the laboratory frame. These show theexpected asymmetric-peaked fission mass distribution. Atypical gate on FMT events, as used to generate the MADin Fig.8(g), is shown by the black circle.

The 50Ti+248Cm reaction forms the compound nu-cleus 298Og (Z=118). The MAD shown in Fig.8(h)was measured at the Australian National University, us-ing a target from Mainz/GSI, at a beam energy abovethe predicted [29] average capture barrier energy, whereequatorial collisions having the longest sticking timeswould be expected to be dominant. However, the MADmore closely resembles the sub-barrier measurement for34S+232Th shown in Fig.8(c), rather than the above-barriermeasurement shown in Fig.8(e). The relative yield ofmass-symmetric events is smaller than for the sub-barrier34S+232Th reaction, but the fast quasifission events stillshow a considerable mass flow towards mass-symmetry.As in panels (c) and (e), the dashed lines show the ex-

pected mass-ratio if one of the quasifission fragments were208Pb.

The role of 208Pb in quasifission mass distributions wasinvestigated in the 40Ca+238U reaction, both experimen-tally and through TDHF calculations [15]. It was con-cluded that the closed shells centred on 208Pb play a strongrole in determining the mass-splits, but for that reaction,only for axial (tip) collisions. In the 50Ti+248Cm reac-tion, the yield around 208Pb seems to be associated withall orientations. Various possible contributions to the ob-served behaviour need to be investigated. These includethe possible effect of shells in the lighter fragment, se-quential fission of heavy target-like nuclei, deviation ofthe orientation-dependent effective capture barriers fromexpectations, and changes in dynamics resulting from theheavier nuclei involved in the collision.

A program of measurements of quasifission (includ-ing MAD over a wide angular range) for projectiles from48Ca to 64Ni, bombarding targets from 208Pb to 249Cf, isin progress at the ANU. Analysis and interpretation of thisbody of data will give more quantitative insights into SHEsynthesis reactions.

6 ConclusionsMass-angle distributions give the most direct informationon the characteristics and time scales of quasifission. Asystematic study of carefully chosen mass-angle distri-butions has provided information on the global trends ofquasifission. Large deviations from these systematics re-veal the major role played by the nuclear structure of thetwo colliding nuclei in determining the quasifission prob-ability and time scale, and thus implicitly in hindering orfavouring superheavy element synthesis.

The observation of rapid changes in quasifission out-comes, depending on magicity, neutron number, and beamenergy is a severe challenge for models of quasifission andSHE formation to reproduce. And yet this level of sen-sitivity of reaction dynamics to nuclear structure is whatmodels must attempt to reproduce, in order to map out theoptimum experimental opportunities to create new super-heavy elements and isotopes in the future.

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