•The last generation large solid angle magnetic spectrometers for very heavy ions.•Transfer of multiple pairs, providing valuable information on nucleon-nucleon correlations.•The study of the ...properties of the heavy binary partner via transfer reactions.
Significant advances have been achieved in the last years in the field of multinucleon transfer reactions. The advent of the last generation large solid angle magnetic spectrometers pushed the detection efficiency more than an order of magnitude above previous limits, with a significant gain in mass resolution for very heavy ions. Further, the coupling of these spectrometers to large gamma arrays allowed to perform gamma-particle coincidences. One can thus detect the transfer strength to the lowest excited levels of binary products and perform gamma spectroscopy for nuclei moderately far from stability, especially in the neutron-rich region. Via transfer of multiple pairs valuable information on nucleon-nucleon correlations can also be derived, especially from measurements performed below the Coulomb barrier. There is growing interest in the study of the properties of the heavy binary partner, since the transfer mechanism may allow the production of (moderately) neutron rich nuclei in the Pb and in the actinides regions, crucial also for astrophysics. Present studies are relevant for future studies with radioactive beams.
We present some of the recent experimental results in heavy-ion transfer reactions obtained with the large solid angle magnetic spectrometer PRISMA at energies close to the Coulomb barrier. We focus ...on a series of experiments that have been carried out to study the nucleon-nucleon correlations for closed shell and superfluid systems. They are discussed together with the newest results concerning the proton transfer channels above and below the Coulomb barrier. The second set of the experiments was performed to study the production mechanism of heavy neutron-rich nuclei and the related effects of secondary processes.
58Ni +64Ni is the first case where the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced, in a very well known experiment by Beckerman et al., by comparing with the ...two systems 58Ni +58Ni and 64Ni +64Ni. Subsequent measurements on 64Ni +64Ni showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for 58Ni +64Ni down to the measured level of 0.1 mb. In the present experiment the excitation function has been extended by two orders of magnitude downward. The cross sections for 58Ni + 64Ni continue decreasing very smoothly below the barrier, down to ≃1 μb. The logarithmic slope of the excitation function increases slowly, showing a tendency to saturate at the lowest energies. No maximum of the astrophysical S-factor is observed. Coupled-channels (CC) calculations using a Woods-Saxon potential and including inelastic excitations only, underestimate the sub-barrier cross sections by a large amount. Good agreement is found by adding two-neutron transfer couplings to a schematical level. This behaviour is quite different from what already observed for 64Ni+64Ni (no positive Q-value transfer channels available), where a clear low-energy maximum of the S-factor appears, and whose excitation function is overestimated by a standard Woods-Saxon CC calculation. No hindrance effect is observed in 58Ni+64Ni in the measured energy range. This trend at deep sub-barrier energies reinforces the recent suggestion that the availability of several states following transfer with Q >0, effectively counterbalances the Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier.
For the first time in an application to nuclear astrophysics, a process induced by the unstable
5
He = (
4
He-n) nucleus, the
3
He+
5
He
→
2
α
reaction, has been studied through the Trojan Horse ...Method (THM). For that purpose, the quasi-free (QF) contribution of the
9
Be(
3
He,
α
α
)
4
He reaction was selected at
E
3
He
=
4
MeV incident energy. The reaction was studied in a kinematically complete experiment following a recent publication (Spitaleri et al. in Eur Phys J A 56:18, 2020), where for the quasi free contribution the momentum distribution between
α
and
5
He particle cluster in the
9
Be nucleus in the ground state have been extracted. The angular distribution of the QF
3
He+
5
He
→
2
α
reaction was measured at
θ
cm
= 78
∘
–115
∘
. The energy dependence of the differential cross section of the
3
He+
5
He
→
2
α
virtual reaction was extracted in the energy range
E
cm
= 0–650 keV. The total cross section obtained from the Trojan-horse method was normalized to absolute cross sections from a theoretical calculation in the energy range
E
cm
=300–620 keV.
.
The response function of the magnetic spectrometer PRISMA is studied via a Monte Carlo simulation that employs a ray tracing code to determine the trajectories of individual rays through the ...electromagnetic fields. The calculated response is tested on angular and energy distributions provided by theoretical calculations for the
48
Ca +
64
Ni multinucleon transfer reaction and applied to the corresponding experimental data.
A new study of the quasi-free contribution to the
3
He
+
9
Be
→
3
α
reaction (
Q
value = 19.004 MeV) at low
E
3
He
energy is presented. The reaction was studied in a kinematically complete experiment ...at beam energy of 4 MeV. To clarify the presence of the quasi-free mechanism, the
4
He
–
5
He
momentum distribution of the
9
Be
ground state was extracted. Standard tests were also carried out to confirm the presence of the quasi-free contribution and are reported in this work. The full width at half maximum of the
4
He
–
5
He
inter-cluster momentum distribution inside
9
Be
was measured with improved accuracy, by adopting several different approaches that all led to consistent results. These preliminary investigations on the reaction mechanism are fundamental for future studies employing the Trojan Horse Method, with the
5
He
unbound nucleus as a virtual projectile.
The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behaviour of heavy-ion fusion at extreme ...sub-barrier energies, including reactions involving lighter systems, e.g. reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the S-factor vs. energy. Whether there is an S-factor maximum at very low energies for systems with a positive fusion Q-value is an experimentally challenging question. Our aim has been to search evidence for fusion hindrance in 12C + 24Mg which is a medium-light systems with positive Q-value for fusion, besides the heavier cases where hindrance is recognised to be a general phenomenon. The experiment has been performed at the XTU Tandem accelerator of LNL by directly detecting the fusion evaporation residues at very forward angles. The excitation function has been extended down to ≃10μb, i.e. 4 orders of magnitude lower than previous measurements and we observe that the S-factor develops a clear maximum vs. energy. Coupled-Channels calculations using a Woods-Saxon potential give a good account of the data near and above the barrier but over predict the cross sections at very low energies. Therefore the hindrance phenomenon is clearly recognised in 12 C + 24 Mg with an energy threshold that nicely fits the systematics in several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (as=1.6mb) is indeed found for this system. It may be possible to extend the measurements further down in energy.
.
Sub-barrier fusion of the two near-by systems
36
S +
50
Ti,
51
V has been measured at the Laboratori Nazionali di Legnaro (INFN). Motivation for the experiment came from the possible effect of the ...non-zero spin of the ground state of the
51
V nucleus on the sub-barrier excitation function, and in particular on the shape of the barrier distribution. No previous data were available for these two systems near the barrier. Our results show that the two measured excitation functions are very similar down to the level of 20-30μb. The same is true for the two barrier distributions. Coupled-channel calculations have been performed including the low energy excitations of both projectile and the two targets. We have indication that the low-lying levels in
51
V can be interpreted in the weak-coupling scheme, that is,
51
V (
I
) =
50
Ti(2
+
) ⊗
p
(1
f
7/2
) and that the extra proton in the
1
f
7
/
2
shell does not have a significant influence on sub-barrier fusion.
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