-
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
MWPC1 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
EPJ Web of Conferences 163, 00023 (2017) DOI:
10.1051/epjconf/201716300023FUSION17
2
-
TargetPulsed beam
MWPC2 28 x 36 cm t,X,Y
MWPC1 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
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
-
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-
EPJ Web of Conferences 163, 00023 (2017) DOI:
10.1051/epjconf/201716300023FUSION17
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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