Anomalous behavior of the level density parameter in neutron and charged particle evaporation

The compound nucleus ${}^{76}\mathrm{Kr}*$ was populated at the excitation energy of $75\mathrm{MeV}$ and angular momentum of $39\hslash$ in fusion reactions with two complementary, mass-symmetric $({}^{31}\mathrm{P}+{}^{45}\mathrm{Sc})$ and mass-asymmetric $({}^{12}\mathrm{C}+{}^{64}\mathrm{Zn})$ entrance channels. The neutron evaporation spectra were measured and compared with the predictions of statistical model calculations using the transmission coefficients for the spherical nuclei in the inverse absorption channel, the rotating liquid drop model moment of inertia, and the normal systematic value of $A∕8$ for the level density parameter $a$. The results for the mass-asymmetric reaction are found to be consistent with the predictions of the statistical model calculations. However, for the mass-symmetric reaction $({}^{31}\mathrm{P}+{}^{45}\mathrm{Sc})$, the experimental spectra are found to be harder than the theoretical neutron spectra and the statistical model calculations require a lower value of $A∕10$ for the parameter $a$ to reproduce the shape of the experimental spectra, indicating the neutron to be evaporated at higher temperature for the same excitation energy and angular momentum in symmetric system. According to the dynamical model, the formation time $(37\times {10}^{-22}\sec )$ of the compound nucleus for the symmetric ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ system is significantly higher than that $(29\times {10}^{-22}\sec )$ for the asymmetric ${}^{12}\mathrm{C}+{}^{64}\mathrm{Zn}$ system. This may probably lead to the formation of a temperature-equilibrated dinuclear complex that may be responsible for neutron emission at higher temperature in the case of the symmetric system.

Figure 1

Comparison of the experimental neutron spectra (dots) with the statistical model (solid line) using $r_0=1.25$ and $a=A∕8$ for the asymmetric reaction ${}^{12}\mathrm{C}+{}^{64}\mathrm{Zn}$ with $l_{\mathit{max}}=39\hslash$ and $E*=75\mathrm{MeV}$ at $E_{\mathit{lab}}=85\mathrm{MeV}$.

Figure 2

Comparison of the experimental neutron spectra (dots) with the statistical model (solid line) using $a=A∕8$ and $r_0=1.25$ for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=39\hslash$ and $E*=70\mathrm{MeV}$ at $E_{\mathit{lab}}=112\mathrm{MeV}$.

Figure 3

Comparison of the experimental neutron spectra (dots) with the statistical model (solid line) using $a=A∕8$ and $r_0=1.25$ for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=43\hslash$ and $E*=75\mathrm{MeV}$ at $E_{\mathit{lab}}=120\mathrm{MeV}$.

Figure 4

Comparison of the experimental neutron spectra (dots) with the statistical model (solid line) using $a=A∕10$ and $r_0=1.25$ for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=39\hslash$ and $E*=70\mathrm{MeV}$ at $E_{\mathit{lab}}=112\mathrm{MeV}$.

Figure 5

Comparison of the experimental neutron spectra (dots) with the statistical model (solid line) using $a=A∕10$ and $r_0=1.25$ for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=43\hslash$ and $E*=75\mathrm{MeV}$ at $E_{\mathit{lab}}=120\mathrm{Mev}$.

Figure 6

Comparison of the experimental neutron spectra for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ (triangle) and for the asymmetric system ${}^{12}\mathrm{C}+{}^{64}\mathrm{Zn}$ (dots), in the center-of-mass system.

Figure 7

Comparison of the experimental neutron spectra in the center-of-mass system (circles) with statistical model using $a=A∕8$ and $r_0=1.25$ (dashed line) and using $a=A∕10$ and $r_0=1.25$ (solid line) for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=43\hslash$ and $E*=75\mathrm{MeV}$ at $E_{\mathit{lab}}=120\mathrm{MeV}$.

Figure 8

Comparison of the experimental alpha spectra (dots) with the statistical model (solid line) using the transmission coefficients for the spherical nuclei and the RLDM moment of inertia for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=43\hslash$ and $E*=75\mathrm{MeV}$ at $E_{\mathit{lab}}=120\mathrm{MeV}$.

Figure 9

Comparison of the experimental proton spectra (dots) with the statistical model (solid line) using the transmission coefficients for the spherical nuclei and the RLDM moment of inertia for the symmetric reaction ${}^{31}\mathrm{P}+{}^{45}\mathrm{Sc}$ with $l_{\mathit{max}}=43\hslash$ and $E*=75\mathrm{MeV}$ at $E_{\mathit{lab}}=120\mathrm{MeV}$.