Measurement of dynamical dipole $\gamma$-ray emission in the $N/Z$-asymmetric fusion reaction ${}^{16}\mathrm{O}+{}^{116}\mathrm{Sn}$ at 12 MeV/nucleon

A new measurement of the dynamical dipole emission was performed in the system ${}^{16}\mathrm{O}+{}^{116}\mathrm{Sn}$ at 12 MeV/nucleon. These data, together with those measured at 8.1 MeV/nucleon and 15.6 MeV/nucleon on the same system, provide the dependence of the dynamical dipole total emission yield on the beam energy. The energy removed by preequilibrium charged particles emission was directly measured and this made possible the direct estimation of the compound nucleus excitation energy. The experimental results show a weak increase of the dynamical dipole total yield with beam energy and they are in agreement both in trend and in absolute values with the predictions of the theoretical model based on the Boltzmann-Nordheim-Vlasov approach. The measured γ-ray angular distribution has a dipole character but with a strong quenching probably owing to the rotation of the dipolar axis during the fusion and thermalization processes.

Figure 1

The measured Phoswich ToF vs deposited energy in the first scintillator layer of the Phoswich detectors (indicated as GA). The black circle is the gate used to select the evaporation residues. The matrix has the requirement of a coincident high-energy γ ray ($E$ > 5 MeV).

Figure 2

The measured high-energy γ-ray spectra for the system ${}^{16}$O + ${}^{116}$Sn at 12 MeV/nucleon without (black squares) and with the experimental conditions on γ ToF (red dots) and ToF and residues (blue triangles).

Figure 3

The α-particle energy spectra measured at different angles by the Garfield array. The continuous blue line indicates the results of a moving source fit, where the spectra are fitted by a combination of two distributions: The first one (continuous red line) describes the pre-equilibrium emission, while the second one (dashed black line) describes the particle statistical emission.

Figure 4

The neutron preequilibrium multiplicity obtained from the model of Ref. [36] for the same reaction at different beam energies (black points) is compared to the data available in literature [30] (red squares).

Figure 5

The comparison between the total spectrum (triangles) measured in the reaction ${}^{16}$O + ${}^{116}$Sn at 12 MeV/nucleon and the corresponding statistical model calculations (line). The calculations were folded with the detector response function and normalized at 7–8 MeV.

Figure 6

The calculated DD total yield for the reaction ${}^{16}$O + ${}^{116}$Sn at 8.1 MeV/nucleon as a function of the impact parameter. The black squares refer to the stiff case while red dots correspond to the soft parametrization of the symmetry term in the EOS.

Figure 7

The DD yield as a function of the beam energy. The experimental data (blue diamonds) are compared to the theoretical predictions of the BNV model for the stiff (black squares) and the soft (red dots) parametrizations. The data at 8.1 and 15.6 MeV/nucleon are from Ref. [18].

Figure 8

(a) The measured DD γ emission angular distribution in the laboratory system (black squares). Data can be fitted by a dipole distribution quenched by a factor $a_2$ = −0.36 (black line). The theoretical model prediction for soft parametrization is shown as a dotted line and for the stiff one as a dashed line. The dot-dashed line represents a pure dipole angular distribution. (b) The measured DD angular distribution compared with the theoretical expectation, for impact parameter $b$ = 2 and $b$ = 4 fm, obtained in the soft case.

Figure 9

Integral emission probability β($t$) calculated at $b$ = 2 fm for the three different beam energies for the stiff parametrization of the symmetry term of the EOS. The curves for the impact factors 0, 4, and 6 fm, not shown, are very similar.