Impact of multichance fission on fragment-neutron correlations in ${}^{227}\mathrm{Pa}$

We investigate the influence of neutron emission in the fission of ${}^{227}\mathrm{Pa}$ populated by complete fusion of ${}^{19}\mathrm{F}$ with ${}^{208}\mathrm{Pb}$ at various excitation energies ($E^{*}$). Mass gated pre-scission neutron multiplicities (${\nu }_{\mathrm{pre}}$) were determined by fragment-neutron angular correlation and time of flight of fission fragments and neutrons using the National Array of Neutron Detectors facility. Obtained $\mathrm{Mass}-{\nu }_{\mathrm{pre}}$ correlation showed that, at lower $E^{*}=24.2$ and 32.4 MeV, larger ${\nu }_{\mathrm{pre}}$ is correlated with asymmetric mass division. On the other hand, at higher $E^{*}=46.1$ and 59.6 MeV, larger ${\nu }_{\mathrm{pre}}$ is correlated with symmetric mass division. The results were analyzed within the framework of the general description of fission observables GEneral description of Fission observables (GEF) model with multichance fission included. The analysis of fragment mass–total kinetic energy correlation for different chance fission clearly indicates a revival of shell effects at $E^{*}=24.2$ and 32.4 MeV as a consequence of sequential fission decay. At these energies, higher chance fission decreases the saddle point excitation energy considerably where shell effects are prominent. The interplay of shell mediated mass asymmetric fission and symmetric fission gives rise to an energy dependent $\mathrm{Mass}-{\nu }_{\mathrm{pre}}$ correlation. We have compared the experimental results with $\mathrm{Mass}-{\nu }_{\mathrm{pre}}$ correlation predicted by the GEF model. It is concluded that the correlation of larger ${\nu }_{\mathrm{pre}}$ with asymmetric mass at lower excitation energies is a signature of shell effects reinstated by sequential fission decay. At the two higher excitation energies, despite the multichance fission and consequent decrease in saddle point energy, the available excitation energy appears sufficient for the attenuation of shell effects.

Figure 1

Schematic view of the experiment setup used for measuring neutron multiplicity in coincidence with fission. ${\theta }_n, {\theta }_{nf1}$, and ${\theta }_{nf2}$ represent relative angles between various neutron sources [compound nucleus (CN), fragments $f1$ and $f2$] and neutron detectors. Neutron detectors (N1 to N16 of NAND array) in the reaction plane alone are displayed (see text for details).

Figure 2

(a) Time correlation of complementary events recorded in two MWPCs for the ${}^{19}\mathrm{F}+{}^{208}\mathrm{Pb}$ reaction at 105 MeV. Events corresponding to fission fragments and scattered beam particles are indicated. The inset shows the velocity correlation plot for fission events derived from time of fight. (b) Plot of zero-cross time versus time of flight recorded in a neutron detector.

Figure 3

(a)–(c) Examples of the three-moving-source fits to double differential neutron multiplicity spectra in the laboratory frame at various angles for the ${}^{19}\mathrm{F}+{}^{208}\mathrm{Pb}$ reaction at 105 MeV beam energy. (d) Angular correlation of neutron yield $dM/d\mathrm{\Omega }$ as a function of ${\theta }_n$ for detectors N1–N16. The contributions from individual neutron sources are indicated by curves; green dashed (fragment 1), blue dot-dashed (fragment 2), and pink dotted (CN source). The red solid line indicates total contributions from all sources and black solid circles denote data points with statistical errors. The dotted vertical line in (d) at ${\theta }_n$ = $0^{\circ }$ separates the data measured at $\mathrm{\Phi }$ = $0^{\circ }$ and ${180}^{\circ }$.

Figure 4

Pre-scission neutron multiplicity measured for ${}^{227}\mathrm{Pa}$ in the range of $E^{*}=24.2$–59.6 MeV. Present measurements are shown as filled circles and previous measured data of Schmitt $\mathit{et}\mathit{al}.$ [21] is denoted with a filled diamond. The GEF model prediction corresponding to each measurement is shown as filled squares.

Figure 5

Experimental and GEF predicted $\mathrm{Mass}-\mathrm{TKE}$ correlations for ${}^{227}\mathrm{Pa}$ at $E^{*}=59.6$, 46.1, 32.4, and 24.2 MeV. The upper panels (a)–(d) and (e)–(h) correspond to GEF produced $\mathrm{Mass}-\mathrm{TKE}$ spectra corresponding to last and second-to-last chance fissions, respectively. (i)–(l) represent GEF produced $\mathrm{Mass}-\mathrm{TKE}$ correlations with the contribution of all chance fission summed according to their probabilities. The solid lines represent the mean TKE as a function of fragment mass. Measured $\mathrm{Mass}-\mathrm{TKE}$ correlations are shown in (m)–(o). The filled circles (black) represent the average TKE for given mass bin. See the text for details.

Figure 6

Correlation between experimental ${\nu }_{\mathrm{pre}}$ and fragment mass compared with the GEF model calculation for ${}^{227}\mathrm{Pa}$ at $E^{*}=59.6$, 46.1, 32.4, and 24.2 MeV. Filled squares with error bars denote the ${\nu }_{\mathrm{pre}}$ extracted from symmetric and asymmetric mass cuts to total mass distribution. Data of $E^{*}=24.2$ MeV are from Schmitt $\mathit{et}\mathit{al}.$ [21].