Nuclear dissipation at high excitation energy and angular momenta in reaction forming ${}^{227}\mathrm{Np}$

Neutron multiplicity excitation function has been measured for the ${}^{30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction populating the ${}^{227}\mathrm{Np}$ compound nucleus at excitation energies in the range 44.1–78.8 MeV using the National Array of Neutron Detector facility of Inter University Accelerator Centre, New Delhi. Measured pre-scission neutron multiplicity values are analyzed using a statistical model incorporating Krammer's fission width due to the dissipative drag in nuclear fission, shell corrections in fission barrier and level density, collective enhancement of level density, and $K$-orientation effect. The present work demonstrates that a strong fission hindrance is essential to reproduce the experimental pre-scission neutrons, whereas the temperature dependent dissipation coefficient as observed in a few recent measurements is not required to reproduce the experimental ${\nu }_{\mathrm{pre}}$ data. No substantial effect of collective enhancement of nuclear level density and tilting away effect of compound nucleus spin on neutron emission prior to the scission configuration was observed unlike fission of preactinides.

Figure 1

Schematic of the detector setup of NAND array used in the present study. The neutron detectors are earmarked to indicate their position in the array specified by their respective polar azimuthal angles. See Table 1 for details.

Figure 2

Time correlation spectra of complementary fragments detected in the MWPC's kept at folding angle direction.

Figure 3

Pulse shape discrimination (PSD) versus time of flight (TOF) spectra of one of the neutron detectors at 173.4 MeV beam energy.

Figure 4

Experimental double differential neutron multiplicity spectra (solid circles) from ${}^{227}\mathrm{Np}$ CN at an excitation energy of 62.4 MeV for 16 neutron detectors in the reaction plane. The multiple moving source fits for the pre-scission (dashed lines) and post-scission contribution from one fragment (dotted-dash lines) and that from the other (dotted-dotted-dash lines) are also shown. The total contribution from all the three sources are indicated by the solid line.

Figure 5

Experimental neutron angular distribution (solid square) along with the simulated results (black solid line) at different energies. Angular distribution of neutrons emitted from three sources are also shown. Dashed line represents contribution from the CN, long dashed line and dot dashed line indicate the contributions from fission fragments, respectively.

Figure 6

Experimental ${\nu }_{\mathrm{pre}}$ for ${}^{227}\mathrm{Np}$ at different excitation energies. The ${\nu }_{\mathrm{pre}}$ reported for the ${}^{229}\mathrm{Np}$ [39] is also shown in the same plot.

Figure 7

Variation of ${\nu }_{\mathrm{pre}}$ for the ${}^{30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction for different dissipation strength considering the CELD and $K$-orientation effect in the statistical model calculations is shown in panel (a). Solid squares represent the experimental ${\nu }_{\mathrm{pre}}$ for the same system. Similar estimates of ${\nu }_{\mathrm{sad}-\mathrm{sciss}}$ and ${\nu }_{\mathrm{pre}-\mathrm{sad}}$ for different $\beta$ values are also shown in (b) and (c), respectively.

Figure 8

Influence of CELD and $K$-orientation effect in ${\nu }_{\mathrm{pre}}, {\nu }_{\mathrm{sad}-\mathrm{sciss}}$, and ${\nu }_{\mathrm{pre}-\mathrm{sad}}$ for the ${}^{30}\mathrm{Si}+{}^{197}\mathrm{Au}$ reaction for $\beta =10{\mathrm{zs}}^{-1}$ is demonstrated in (a), (b), and (c).