Probing fusion-fission dynamics in ${}^{203}\text{Bi}$

Background: Complete fusion between two massive nuclei after capture inside the potential barrier is inhibited by competing fission-like processes. The target-projectile composite system may reseparate after capture without proceeding towards formation of the compound nucleus (CN), which is equilibrated in all degrees of freedom. The nature of these non-CN fission (NCNF) processes and factors that affect them are not completely known yet.

Purpose: The nuclear mass regions from where NCNF processes begin to manifest themselves are not clearly demarcated. This work aims to study the onset of NCNF, if any, in the mass region $\sim 200$.

Methods: Fission fragment (FF) mass and angular distribution (MAD) and pre-scission and post-scission neutron multiplicities were measured for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$ at a laboratory energy $\left(E_{\mathrm{lab}}\right)$ range of 84–125 MeV. The measurements were carried out using two multiwire proportional counters (MWPC) to detect the FFs in coincidence and four neutron detectors to measure neutron time of flight (TOF). Statistical model (SM) calculation was performed.

Results: No significant mass-angle correlation was observed in the MAD plots. Extracted mass ratio distributions were single-peaked and of Gaussian shape. Measured pre-scission neutron multiplicity values indicated dissipative nature of CN decay for this reaction.

Conclusions: No clear signatures of NCNF were observed in the studied reaction, indicating that the target-projectile composite system predominantly proceeds towards formation of the CN after capture.

Figure 1

Experimental setup (top view) used for measurement of neutron multiplicity in coincidence with FFs for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$. Two monitor detectors (M1 and M2), two fission detectors (MWPC1 and MWPC2), and four neutron detectors (N30, N60, N90, and N120) were used in the experiment. The primary beam was dumped on an adequately shielded Faraday cup placed $\sim 3$ m downstream from the target.

Figure 2

Anode $\left(\mathrm{\Delta }t\right)$ versus cathode $\left(\mathrm{\Delta }E\right)$ plots from (a) MWPC1 and (b) MWPC2 for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$ at $E_{\mathrm{lab}}=99.8$ MeV. While FFs and elastically scattered projectiles were separated in MWPC1, additional coincidence condition had to be invoked to obtain a clear separation between the two groups in MWPC2.

Figure 3

PSD versus TOF spectrum for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$, measured at $E_{\text{lab}}=99.8$ MeV with the N30 detector. Neutron events are shown within the rectangular gate.

Figure 4

Scatter plots of (a)–(d) $\frac{v_{\parallel }}{v_{\mathrm{c}.\mathrm{m}.}}$ versus $v_{\perp }$, (e)–(h) $M_{\mathrm{R}}$ versus $\frac{v_{\parallel }}{v_{\mathrm{c}.\mathrm{m}.}}$, (i)–(l) ${\theta }_{1_{\mathrm{lab}}}+{\theta }_{2_{\mathrm{lab}}}$ versus ${\varphi }_{1_{\mathrm{lab}}}+{\varphi }_{2_{\mathrm{lab}}}$, (m)–(p) $M_{\mathrm{R}}$ versus RTKE, and (q)–(t) $M_{\mathrm{R}}$ versus ${\theta }_{\mathrm{c}.\mathrm{m}.}$ for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$ at $E_{\mathrm{lab}}=89.8$, 99.8, 109.8 and 119.8 MeV. See text for details.

Figure 5

Extracted FF mass ratio distributions for the system ${}^{19}\text{F}+{}^{184}\text{W}$ at different $E_{\mathrm{lab}}$. The area under each curve has been normalized to unity. The distributions are symmetric, single-peaked, and have Gaussian shapes with the mean at ${\mathrm{M}}_{\mathrm{R}}=0.5$. The fit for the lowest projectile energy $(E_{\mathrm{lab}}=84.8$ MeV) is not shown because of poor statistics.

Figure 6

Standard deviation of FF mass ratio distributions for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$ as a function of $E_{\mathrm{c}.\mathrm{m}.}$. Uncertainties in the Gaussian (shown in Fig. 5) fitting procedure are only shown here. The thick line is a least-squares linear fit to the data points.

Figure 7

Double differential cross sections for evaporated neutrons for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$ at $E_{\mathrm{lab}}=99.8$ MeV. ${\psi }_{1_{\mathrm{lab}}}$ and ${\psi }_{2_{\mathrm{lab}}}$ are the angles between a neutron detector and MWPC1 and MWPC2, respectively (see Table 1). Data are shown by hollow circles whereas the thick continuous (red) lines denote the fit using Eq. (5). Dashed (green) lines represent pre-scission components. Post-scission components are shown by dash-dotted (blue) and dash-dash-dotted (orange) lines.

Figure 8

Measured $M_{\mathrm{pre}}$ for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$ along with the SM predictions.

Figure 9

$M_{\mathrm{total}}$ for fission as a function of $E_{\mathrm{x}}$ (see text) for the reaction ${}^{19}\text{F}+{}^{184}\text{W}$. The horizontal error bars in experimental data points and the shaded region indicate uncertainties in calculating $E_{\mathrm{x}}$.