Effect of collective enhancement in level density in the fission of pre-actinides

Fission fragment angular distributions for three reactions, ${}^{19}\mathrm{F}+{}^{182}\mathrm{W},{}^{19}\mathrm{F}+{}^{187}\mathrm{Re}$, and ${}^{19}\mathrm{F}+{}^{193}\mathrm{Ir}$, are measured in the laboratory energy range of 82–120 MeV. Extracted fission cross sections of the present systems as well as those of three others from literature $({}^{19}\mathrm{F}+{}^{192}\mathrm{Os},{}^{19}\mathrm{F}+{}^{194}\mathrm{Pt}$, and ${}^{19}\mathrm{F}+{}^{197}\mathrm{Au})$ are compared with the predictions of a statistical model which takes into account the effects of shell, orientation degree of freedom, and collective enhancement in level density (CELD). In all the cases, the standard statistical model predictions overestimate the measured fission cross section, indicating the presence of some amount of dynamical effects in the exit channel. A dissipation strength of $2\times {10}^{21}{\mathrm{s}}^{-1}$ is found to be sufficient to reproduce the data of all the reactions. No scaling of fission barrier height to fit the data is required.

Figure 1

Measured fission fragment angular distributions along with the best fit to data for the reaction ${}^{19}\mathrm{F}+{}^{182}\mathrm{W}$.

Figure 2

Experimental (square box) and theoretical ${\sigma }_{\mathrm{cap}}$ for (a) ${}^{19}\mathrm{F}+{}^{182}\mathrm{W}$, (b) ${}^{19}\mathrm{F}+{}^{192}\mathrm{Os}$, (c) ${}^{19}\mathrm{F}+{}^{194}\mathrm{Pt}$, and (d) ${}^{19}\mathrm{F}+{}^{197}\mathrm{Au}$ reactions. The vertical arrow in each panel indicates position of the Coulomb barrier. Experimental data for the different systems are taken from (a) fission from present study plus ER from Ref. [24], (b) from Ref. [15], (c) from Ref. [16], and (d) from Ref. [19].

Figure 3

$\frac{A_{\exp }-1}{A_{\mathrm{cal}}-1}$ as a function of $\frac{E_{\mathrm{c}.\mathrm{m}.}}{V_{\mathrm{B}}}$ for reactions involving ${}^{19}\mathrm{F}$ projectile on various targets. Here $V_{\mathrm{B}}$ is the Coulomb barrier. Data for the systems other than the present one are obtained from the literature (see text).

Figure 4

Measured and calculated ${\sigma }_{\mathrm{fis}}$ for (a) ${}^{19}\mathrm{F}+{}^{182}\mathrm{W}$, (b) ${}^{19}\mathrm{F}+{}^{187}\mathrm{Re}$, (c) ${}^{19}\mathrm{F}+{}^{192}\mathrm{Os}$, (d) ${}^{19}\mathrm{F}+{}^{193}\mathrm{Ir}$, (e) ${}^{19}\mathrm{F}+{}^{194}\mathrm{Pt}$, and (f) ${}^{19}\mathrm{F}+{}^{197}\mathrm{Au}$. Data points from the present work are shown with solid yellow squares. Data for the systems other than the present ones: Solid green squares in panel (c) are from Ref. [15], solid gray squares and solid cyan diamonds in panel (e) from Refs. [16] and [17], respectively, and hollow diamonds, solid green circles, and solid orange triangles in panel (f) are from Refs. [18], [19] and [20], respectively. The dashed blue lines are the predictions including only shell (Sh) and $K$-orientation $\left(K_{\mathrm{or}}\right)$ effects. The continuous magenta lines represent vecstat predictions including the effects of shell, $K$ orientation, and CELD $\left(K_{\mathrm{coll}}\right)$. The dotted maroon lines represent vecstat predictions including the effects of shell, $K$ orientation, CELD, and a suitable strength of dissipation. The vertical arrow in each panel indicates position of the Coulomb barrier.