Systematic study of precompound neutron emission in $\alpha$-particle-induced reactions

Background: The precompound emission process in light-ion fusion interactions has been a topic of considerable interest to nuclear physicists for many years. Although widespread theoretical and experimental efforts have been devoted to understand the reaction dynamics of precompound emission, no systematic study has been carried out to determine the driving input parameters in such reactions.

Purpose: The importance of this study has large impact on reaction dynamics. In order to get parameters describing the precompound emission, excitation functions have been measured by using an $\alpha$-particle beam on ${}^{141}\mathrm{Pr}$ target nuclei. Further, a sensitive analysis of excitation functions has been performed to investigate the systematics of the precompound process with mass number of target nuclei.

Method: The off-line $\gamma$-ray-spectrometry-based activation technique has been used for the measurement of the excitation functions. The presently measured excitation functions on the system $\alpha +{}^{141}\mathrm{Pr}$ (and those experimental values taken from literature on the $\alpha +{}^{51}\mathrm{V}, \alpha +{}^{55}\mathrm{Mn}, \alpha +{}^{93}\mathrm{Nb}, \alpha +{}^{121}\mathrm{Sb}$, and $\alpha +{}^{123}\mathrm{Sb}$ systems) have been analyzed within the framework of compound and precompound emissions by using model code alice.

Results: The analysis performed by using code alice with the same set of precompound parameters indicates that the experimentally measured excitation functions could be reproduced only when the precompound emission, simulated theoretically, has been taken into account. The influence of various important precompound parameters of the code alice with their physically accepted values for these systems has been judged and a systematics on precompound emission process is achieved with mass number of the target nuclei.

Conclusions: The developed systematics for $\alpha$-induced reactions on target nuclei ${}^{51}\mathrm{V}, {}^{55}\mathrm{Mn}, {}^{93}\mathrm{Nb}, {}^{121}\mathrm{Sb}, {}^{123}\mathrm{Sb}$, and ${}^{141}\mathrm{Pr}$ indicates that the precompound process is governed by the excitation energy available to the nucleons at the surface the composite systems. Furthermore, mass number of the target nuclei may also play an important role in precompound process at low projectile energies.

Figure 1

A typical block diagram of $\gamma$-ray spectrometer setup along with source-detector arrangements.

Figure 2

The measured geometry-dependent efficiency as a function of $\gamma$-ray energies.

Figure 3

A typical $\gamma$-ray spectrum of the irradiated sample at 40 MeV.

Figure 4

(a) and (d) shows the effect of variation of parameter $K=8$, 9, and 10 on calculated EFs for reactions ${}^{141}\mathrm{Pr}(\alpha ,n){}^{144}\mathrm{Pm}$ and ${}^{51}\mathrm{V}(\alpha ,n){}^{54}\mathrm{Mn}$ by using code alice. The experimentally measured EFs are also shown in this figure. (b) and (e) shows the effect of variation of parameter $n_0=4$, 5, and 6 of the code alice on the calculated EFs for these reactions. (c) and (f) shows the effect of variation of parameter cost = 0 and cost= 2 on calculated EFs for these reactions. These parameters used in alice calculations are also discussed in the text.

Figure 5

The experimental and calculated EFs for reactions ${}^{55}\mathrm{Mn}(\alpha ,n){}^{58m+g}\mathrm{Co}, {}^{93}\mathrm{Nb}(\alpha ,n){}^{96}\mathrm{Tc}{}^{121}\mathrm{Sb}(\alpha ,n){}^{124}\mathrm{I}$, and ${}^{123}\mathrm{Sb}(\alpha ,n){}^{126}\mathrm{I}$. The parameters used in alice calculations for these reactions are same as in Fig. 1, and are discussed in the text.

Figure 6

The variation PCN fraction $F_{\mathrm{PCN}}$ as a function of center of mass energy $E_{\mathrm{CM}}$ for $\alpha +{}^{51}\mathrm{V}, \alpha +{}^{55}\mathrm{Mn}, \alpha +{}^{93}\mathrm{Nb}, \alpha +{}^{121}\mathrm{Sb}, \alpha +{}^{123}\mathrm{Sb}$, and $\alpha +{}^{141}\mathrm{Pr}$ systems, respectively.

Figure 7

The variation PCN fraction $F_{\mathrm{PCN}}$ as a function of excitation energy, $E^{*}$ for $\alpha +{}^{51}\mathrm{V}, \alpha +{}^{55}\mathrm{Mn}, \alpha +{}^{93}\mathrm{Nb}, \alpha +{}^{121}\mathrm{Sb}, \alpha +{}^{123}\mathrm{Sb}$, and $\alpha +{}^{141}\mathrm{Pr}$ systems, respectively.

Figure 8

The variation PCN fraction of $F_{\mathrm{PCN}}$ as a function of excitation energy per nucleon at the surface of the composite system, i.e., $E^{*}/{\mathrm{A}}^{2/3}$, for $\alpha +{}^{51}\mathrm{V}, \alpha +{}^{55}\mathrm{Mn}, \alpha +{}^{93}\mathrm{Nb}, \alpha +{}^{121}\mathrm{Sb}, \alpha +{}^{123}\mathrm{Sb}$, and $\alpha +{}^{141}\mathrm{Pr}$ systems, respectively.

Figure 9

A systematics developed by plotting PCN fraction $F_{\mathrm{PCN}}$ as a function for target mass number ($A$) at a fixed value of $E^{*}/A^{2/3}$ for $\alpha +{}^{51}\mathrm{V}, \alpha +{}^{55}\mathrm{Mn}, \alpha +{}^{93}\mathrm{Nb}, \alpha +{}^{121}\mathrm{Sb}, \alpha +{}^{123}\mathrm{Sb}$, and $\alpha +{}^{141}\mathrm{Pr}$ systems, respectively, (see text for details).