Fission time scale from pre-scission neutron and $\alpha$ multiplicities in the ${}^{16}\mathrm{O}$ + ${}^{194}\mathrm{Pt}$ reaction

Pre- and post-scission $\alpha$-particle multiplicities have been measured for the reaction ${}^{16}O+{}^{194}Pt$ at 98.4 MeV forming ${}^{210}Rn$ compound nucleus. $\alpha$ particles were measured at various angles in coincidence with the fission fragments. Moving source technique was used to extract the pre- and post-scission contributions to the particle multiplicity. Study of the fission mechanism using the different probes are helpful in understanding the detailed reaction dynamics. The neutron multiplicities for this reaction have been reported earlier. The multiplicities of neutrons and $\alpha$ particles were reproduced using standard statistical model code joanne2 by varying the transient (${\tau }_{tr}$) and saddle to scission (${\tau }_{ssc}$) times. This code includes deformation dependent-particle transmission coefficients, binding energies and level densities. Fission time scales of the order of 50–65 $\times {10}^{-21}$ s are required to reproduce the neutron and $\alpha$-particle multiplicities.

Figure 1

Schematic diagram of experimental setup representing fission detectors as MWPC 1 and 2 kept at folding angle and charged particle detectors CsI(Tl) kept at ${45}^{\circ }$, ${85}^{\circ }$, ${115}^{\circ }$, and ${135}^{\circ }$

Figure 2

Two-dimensional plot of long shaping time (${\tau }_L$) vs short shaping time (${\tau }_S$), representing different bands corresponding to different particles. $\gamma$ from photodiode are seen in the top band, Second and third band represents protons and deuterons, respectively, whereas the bottom band represents $\alpha$ particles.

Figure 3

Particle identification plot with PID on $Y$ axis and long shaping time (${\tau }_L$) on $X$ axis. Black-marked band represents the $\alpha$-particles band

Figure 4

Fission fragments mass ratio distribution at $E_{\mathrm{lab}}=98.4$ MeV. The distribution is symmetric and centered at 0.5. Solid line is the Gaussian fit to the experimental mass ratio distribution.

Figure 5

Normalized $\alpha$-particle multiplicity spectra along with fits of moving source formula. These spectra are shown at various relative angles between the CsI detectors and the fission detectors in laboratory frame. Various contributions are represented in different colors. Green (dotted) and blue (dash-dotted) line represents the contribution from two fragments or post-scission segment whereas the brown (dashed) line is representing the compound nucleus contributions or pre-scission segment and the neck contribution is represented by red (dash-dot-dot) line or near scission emission. The error bars represent the statistical errors only.

Figure 6

Dissipation energy as a function of time for various values of angular momentum.

Figure 7

Dinucleus elongation as a function of time for various values of angular momentum.

Figure 8

Potential energy diagram ${}^{210}\text{Rn}$, representing the deformation energy as a function of elongation for various angular momentum values.

Figure 9

Deformed liquid drop model predictions of the deviation of binding energies from spherical nucleus for $\nu$-, $\pi$-, and $\alpha$-particle emission

Figure 10

${\tau }_{tr}\text{−}{\tau }_{ssc}$ plot from joanne2 for various $Z_{ssc}$ deformation values required to reproduce experimental ${\nu }_{\mathrm{pre}}$, and ${\alpha }_{\mathrm{pre}}$ are shown in black and red, respectively. (a) $Z_{ssc}=2.0$, (b) $Z_{ssc}=2.17$, (c) $Z_{ssc}=2.3$, and (d) $Z_{ssc}=2.4$, and corresponding particle emission.

Figure 11

The variation of ${\nu }_{\mathrm{pre}}$ and ${\alpha }_{\mathrm{pre}}$ as a function ${\tau }_{ssc}$ in (a) and (c) and as a function of $Z_{ssc}$ in (b) and (d) for ${}^{16}\text{O}+{}^{194}\text{Pt}$. The shaded region corresponds to value of ${\nu }_{\mathrm{pre}}$ and ${\alpha }_{\mathrm{pre}}$.