Role of target shell structure in direct reactions involving weakly bound ${}^7\mathrm{Li}$

The effect of the shell structure of the target nuclei (${}^{89}\mathrm{Y}$ and ${}^{93}\mathrm{Nb}$) on reaction mechanisms involving the weakly bound ${}^7\mathrm{Li}$ projectile has been investigated via a comparative study of different processes. Measurements of direct breakup, nucleon transfer leading to the ejectile in bound as well as unbound states, and elastic scattering have been carried out for the ${}^7\mathrm{Li}+{}^{89}\mathrm{Y}$ system at three different energies near the Coulomb barrier. An extensive analysis of this comprehensive data set was carried out using coupled channel calculations and compared with a previous similar study for the ${}^7\mathrm{Li}+{}^{93}\mathrm{Nb}$ system. The observed cross sections at low target excitations for nucleon transfer reactions correlate with the vacancies in the valance orbitals.

Figure 1

Typical particle identification spectrum of $\mathrm{\Delta }E$ vs $E_{\mathrm{tot}}$ for the ${}^7\mathrm{Li}+{}^{89}\mathrm{Y}$ system at $E_{\mathrm{lab}}=27.7\mathrm{MeV}$, ${\theta }_{\mathrm{lab}}={60}^{\circ }$.

Figure 2

Measured energy correlations of fragments in breakup of (a) ${}^8\mathrm{Be}^{*}$ ($1p$ pickup), (b) ${}^6\mathrm{Li}^{*}$ ($1n$ stripping), and (c) ${}^7\mathrm{Li}^{*}$ at $E_{\mathrm{lab}}=27.7\mathrm{MeV}$, ${\theta }_{\mathrm{ejectile}}={55}^{\circ }\text{–}{65}^{\circ }$, corresponding to ${\theta }_{\mathrm{rel}}^{\alpha \alpha }=3^{\circ }$, ${\theta }_{\mathrm{rel}}^{\alpha d}={10}^{\circ }$, and ${\theta }_{\mathrm{rel}}^{\alpha t}={15}^{\circ }$, respectively. The arrow on the $x$ axis indicates the detection threshold. The kinematical curves obtained from the Monte Carlo simulation are shown by gray shading. The projections of the relative energy distribution are shown in the corresponding right panels: (d), (e), and (f).

Figure 3

Measured excitation energy spectra of target-like products for the ${}^7\mathrm{Li}+{}^{89}\mathrm{Y},{}^{93}\mathrm{Nb}$ reactions at $E_{\mathrm{lab}}=27.7\mathrm{MeV}$. The upward arrows indicate the position of $E^{*}=Q_{\mathrm{gg}}-Q_{\mathrm{opt}}$. States included in the coupled channel calculations are indicated. Details are given in the text.

Figure 4

The measured angular distributions of (a) elastic scattering and (b) inclusive $\alpha$ yield for the ${}^7\mathrm{Li}+{}^{89}\mathrm{Y}$ system. The lines in (a) and (b) correspond to CDCC calculations and Gaussian fits, respectively.

Figure 5

The measured cross sections for prompt and resonant (from the ${\frac{7}{2}}^{-}$ state) breakup of ${}^7\mathrm{Li}$ compared with CDCC calculations for the ${}^{89}\mathrm{Y}$ target at $E_{\mathrm{lab}}=27.7\mathrm{MeV}$.

Figure 6

Experimental absolute angular distributions of the $1n$-stripping ${}^{89}\mathrm{Y}({}^7\mathrm{Li},{}^6\mathrm{Li}^{*}){}^{90}\mathrm{Y}^{*}$ reaction at 22.6, 27.7, and 29.7 MeV for (a) $E^{*}\left({}^{90}\mathrm{Y}\right)\le 1\mathrm{MeV}$ and (b) $E^{*}\left({}^{90}\mathrm{Y}\right)\le 4\mathrm{MeV}$ compared with CCBA calculations.

Figure 7

Measured angular distributions of the ${}^{89}\mathrm{Y}({}^7\mathrm{Li},{}^8\mathrm{Be}){}^{88}\mathrm{Sr}^{*}$ ($1p$-pickup) channel for (a) ground state, (b) first excited state, and (c) 2.5 $\le E^{*}\le$ 5.0 MeV of ${}^{88}\mathrm{Sr}$. CCBA calculations are represented by lines.

Figure 8

The cross sections of $1n$ stripping leading to ${}^6\mathrm{Li}$(g.s.) and ${}^6\mathrm{Li}\left(3^{+}\right)$, respectively, for ${}^7\mathrm{Li}$ induced reactions with ${}^{89}\mathrm{Y}$ and ${}^{93}\mathrm{Nb}$ targets are compared. The cross sections corresponding to the ranges of excitation energy of target-like nuclei $E^{*}\le 1\mathrm{MeV}$ and $E^{*}\le 4\mathrm{MeV}$ are presented in (a),(c) and (b),(d), respectively. The dashed and solid lines represent the results of CCBA calculations for ${}^{89}\mathrm{Y}$ and ${}^{93}\mathrm{Nb}$ targets, respectively.

Figure 9

The cross sections of $1p$ pickup leading to the ${}^8\mathrm{Be}_{\mathrm{g}.\mathrm{s}.}$ for both the ${}^{89}\mathrm{Y}$ and ${}^{93}\mathrm{Nb}$ targets for three different excitation energy ranges of the target-like nuclei as labeled in the figure. The CCBA calculations are shown in dashed and solid lines for ${}^{89}\mathrm{Y}$ and ${}^{93}\mathrm{Nb}$ targets, respectively.

Figure 10

The measured angle integrated cross sections of (a) the $1p$ stripping leading to ${}^6\mathrm{He}_{\mathrm{g}.\mathrm{s}.}$ and (b) direct breakup of ${}^7\mathrm{Li}$ for ${}^{89}\mathrm{Y}$ and ${}^{93}\mathrm{Nb}$ targets are compared. The line represents the calculations for the ${}^7\mathrm{Li}+{}^{89}\mathrm{Y}$ system.