Measurement of mass-gated neutron multiplicity for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction at 57.4 MeV excitation energy

The neutron emission in coincidence with the fragments have been investigated for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction populating the near superheavy compound nucleus ${}^{256}\mathrm{Rf}$ at an excitation energy of 57.4 MeV. The National Array of Neutron Detectors facility is used for a precise determination of the prescission ($M_n^{\mathrm{pre}}$) and postscission ($M_n^{\mathrm{post}}$) neutron multiplicities as a function of fission observables. A moving source fitting procedure has been adopted to deduce $M_n^{\mathrm{pre}}$ and $M_n^{\mathrm{post}}$. The variation of $M_n^{\mathrm{pre}}$ with the mass-split and total kinetic energy (TKE) of the fission fragments have been studied to understand the fission dynamics of ${}^{256}\mathrm{Rf}$. It is observed that $M_n^{\mathrm{pre}}$ increases from the value of 1.66 $\pm$ 0.07 to 2.23 $\pm$ 0.07 with transition from the asymmetric to the symmetric mass region. $M_n^{\mathrm{pre}}$ is also found to increase with the decrease in TKE, which is probably due to the neutron emission during the acceleration time of the fission fragments in this heavy system. The experimental results for neutron multiplicity have also been compared with the theoretical predictions from the statistical model calculations. From this comparison, the value of reduced dissipation strength for the ${}^{256}\mathrm{Rf}$ nucleus is found to be (13.0 $\pm$ 1.0) $\times {10}^{21} {\mathrm{s}}^{-1}$ and a fission delay time of (${67.3}_{-3.9}^{+5.3}$) $\times {10}^{-21}$ s has also been estimated. For the spontaneous fission of ${}^{256}\mathrm{Rf}$, the extracted average neutron multiplicity $M_n^{sf}$ is found to be 4.4 $\pm$ 1.0 which is in good agreement with the recently reported value for the ${}^{258,260}\mathrm{Rf}$ isotopes.

Figure 1

The pictorial view of the NAND array at IUAC.

Figure 2

Two-dimensional plot of TOF vs PSD used to separate the neutrons from the $\gamma$'s. The neutron lobe is outlined using a black solid line.

Figure 3

Comparison of the experimentally deduced neutron efficiency (filled square) with the one obtained using Monte Carlo simulation code fluka (solid line) at 120 keV electron equivalent detection threshold.

Figure 4

(a) The scatter plot of mass and TKE of fission fragments and color palette (on top) represents the counts and (b) the variation of $\langle \mathrm{TKE}\rangle$ with the fragment mass for the gated region (black gate shown in the upper panel) for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction.

Figure 5

Double differential neutron multiplicity spectra for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction at an excitation energy of 57.4 MeV for the different neutron detectors corresponding to the symmetric mass cut ($A_{\mathrm{FF}}$ = $\frac{A_{\mathrm{CN}}}{2} \pm$ 20). The fits for the prescission (dashed lines) and postscission contributions from the fragment F1 (dotted lines) and those from the fragment F2 (dash-dotted lines) are indicated. The solid lines correspond to the total neutron contribution. The relative angles between the fragments and the detected neutron are also quoted.

Figure 6

The multiple-source fitting (solid lines) to the experimental neutron ${\theta }_n$ (in the laboratory frame) distributions (solid circles) for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction for the different ${\varphi }_n$ corresponding to the symmetric mass cut. The dashed lines correspond to the prescission contribution while the dotted and dash-dotted lines indicate the individual postscission contributions from fragments F1 and F2 respectively.

Figure 7

Variation of excitation energy of the fission fragments as a function of their mass for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction (see text).

Figure 8

(a) The four RTKE cuts applied on the MED matrix of the fission fragments as explained in the text. (b) Total projection of TKE in the considered mass window together with the extracted values of prescission (solid upward-pointing triangles), postscission (solid downward-pointing triangles), and total (solid circles) neutron multiplicities plotted at the mean TKE values corresponding to the four RTKE cuts shown in the top panel.

Figure 9

Variation of the prescission neutron multiplicity ($M_n^{\mathrm{pre}}$) with (a) the reduced dissipation coefficient ($\beta$) and (b) delay time (${\tau }_{\mathrm{delay}}$) for the ${}^{48}\mathrm{Ti}+{}^{208}\mathrm{Pb}$ reaction at an excitation energy of 57.4 MeV.