Probing transfer to unbound states of the ejectile with weakly bound ${}^7\mathrm{Li}$ on ${}^{93}\mathrm{Nb}$

The two-step process of transfer followed by breakup is explored by measuring a rather complete set of exclusive data for reaction channels populating states in the ejectile continua of the ${}^7\mathrm{Li}+{}^{93}\mathrm{Nb}$ system at energies close to the Coulomb barrier. The cross sections for $\alpha +\alpha$ events from one proton pickup were found to be smaller than those for $\alpha +d$ events from one neutron stripping and $\alpha +t$ events from direct breakup of ${}^7\mathrm{Li}$. Coupled-channels Born approximation and continuum-discretized coupled-channels calculations describe the data well and support the conclusion that the $\alpha +d$ and $\alpha +\alpha$ events are produced by direct transfer to unbound states of the ejectile.

Figure 1

Measured energy correlation spectra of breakup fragments for ${}^7\mathrm{Li}$ on ${}^{93}\mathrm{Nb}$ at $E_{\mathrm{beam}}=28$ MeV and ${\theta }_{\mathrm{lab}}$ = ${60}^{\circ }$. (a), (c), and (e): $E_{\alpha }$ vs the relative energy $E_{\alpha \alpha }, E_{\alpha \mathit{d}}$, and $E_{\alpha \mathit{t}}$, 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 shaded distributions in (b), (d), and (f) correspond to projections of the $\alpha$-particle energy for the data in (a), (c), and (e), respectively. The arrow on the $x$ axis indicates the detection threshold. The kinematical curves plotted as dashed, solid, and dotted lines correspond to (a) $E^{☆}$(${}^{92}\mathrm{Zr}$) = 1.0, 3.0, and 5.0 MeV (c) $E^{☆}$(${}^{94}\mathrm{Nb}$) = 0.0, 0.5, and 1.0 MeV, and (e) $E^{☆}$(${}^{93}\mathrm{Nb}$) = 0.0, 1.0, and 2.0 MeV. $E_{\alpha }$ resulting from Monte Carlo simulations are shown as solid lines in (b), (d), and (f) for fragments originating from ${}^8\mathrm{Be}$(g.s.), ${}^6\mathrm{Li}$($3^{+}$), and ${}^7\mathrm{Li}$($7/2^{-}$), respectively (see text for details).

Figure 2

Measured inclusive and exclusive cross sections for the ${}^7\mathrm{Li}+{}^{93}\mathrm{Nb}$ system at 28 MeV. (a) Elastic scattering data and the CDCC calculation. The inclusive cross section for $\alpha$ production is shown in the inset. (b) Prompt and resonant (from the $7/2^{-}$ state) breakup of ${}^7\mathrm{Li}$, shown as asterisks and filled circles, respectively. The CDCC results for prompt and resonant breakup are denoted by dot-dot-dashed and dashed lines, respectively. (c) Exclusive data for $1p$ pickup to ${}^8\mathrm{Be}$($0_1{}^{+}$) and $1n$ stripping to ${}^6\mathrm{Li}$($3_1{}^{+}$) are presented as filled circles and asterisks, respectively. The CCBA calculations for $1p$ pickup and $1n$ stripping are denoted by dot-dashed and solid lines, respectively.

Figure 3

$1n$ stripping and $1p$ pickup cross sections for the ${}^7\mathrm{Li}+{}^{93}\mathrm{Nb}$ system at 24, 28, and 30 MeV. (a) The $\alpha +d$ cross sections for the $[{}^7\mathrm{Li},{}^6\mathrm{Li}\left(3_1{}^{+}\right)]$ reaction. (b) The $\alpha +\alpha$ cross sections for the $[{}^7\mathrm{Li},{}^8\mathrm{Be}\left(0_1{}^{+}\right)]$ reaction. In both panels the dotted, solid, and dashed lines correspond to the CCBA calculations at 24, 28, and 30 MeV, respectively.