The (${}^8\mathrm{Li}$, α) reaction at low energy: Direct ${}^4\mathrm{H}$ cluster transfer?

The (${}^8\mathrm{Li}$, α) reaction has been studied at E(${}^8\mathrm{Li})=27.7$ MeV on targets of ${}^{\mathrm{nat}}$C, ${}^{27}\mathrm{Al}$, and ${}^{208}\mathrm{Pb}$ using a secondary ${}^8\mathrm{Li}$ beam. The α-energy spectra for ${}^{27}\mathrm{Al}$ and ${}^{208}\mathrm{Pb}$ are generally nondistinct and resemble fragmentation spectra. In contrast, data for the ${}^{\mathrm{nat}}$C target exhibit enhanced (${}^8\mathrm{Li}$, α) cross sections in the region corresponding to population of low-lying levels in ${}^{16}\mathrm{N}$. Angular distributions were determined over a large angular range and exhibit a strong forward peaking for ${}^{27}\mathrm{Al}$ and some forward peaking for the ${}^{\mathrm{nat}}$C data. They are otherwise rather featureless but distinct from spectra expected from fusion evaporation. The data for ${}^{27}\mathrm{Al}$ are indicative of either direct or sequential fragmentation processes. Similar analysis of limited data obtained at $E({}^8\mathrm{Li})=27.7$ MeV for the ${}^{208}\mathrm{Pb}$ target, along with data previously obtained at higher ${}^8\mathrm{Li}$ energies, is suggestive of a sequential breakup mechanism. In the case of the ${}^{\mathrm{nat}}$C target, the data corresponding to low excitation energies in ${}^{16}\mathrm{N}$ exhibit (${}^8\mathrm{Li}$, α) angular distributions that appear to be most consistent with a ${}^4\mathrm{H}$ transfer mechanism. However, we cannot distinguish between a one-step ${}^4\mathrm{H}$ cluster transfer and a multistep ${}^4\mathrm{H}$ transfer (such as $n+{}^3\mathrm{H})$.

Figure 1

A sample $E\text{−}E$ Si detector telescope spectrum used for particle identification. The $\alpha$-particle band from (${}^8\mathrm{Li}$, $\alpha )$ is indicated together with the intense ${}^8{\mathrm{Li}}^{+++}$ scattered beam and the α group arising from ${}^4{\mathrm{He}}^{++}$ contamination of the secondary beam. The energy calibration is 60 keV/channel.

Figure 2

Forward-angle $\alpha$-particle energy spectra obtained for ${}^{\mathrm{nat}}$C. The ground state (g.s.) Q value for a direct (${}^8\mathrm{Li}$, $\alpha )$ reaction is indicated. The curves shown represent a fit (in the lab-frame) to the lower $\alpha$-energy portion of the spectra using a one- or two-source moving-source fragmentation model with parameters noted in Figs. 4 and 5.

Figure 3

Same as Fig. 2 but for an ${}^{27}\mathrm{Al}$ target.

Figure 4

Angular distributions for ${}^{\mathrm{nat}}$C(${}^8\mathrm{Li}$, $\alpha )$ data taken in 3 MeV $\alpha$-energy bins (labeled in terms of excitation energy in ${}^{16}\mathrm{N}$ for a direct two-body ${}^4\mathrm{H}$ transfer reaction). The calculations shown are for a two-source moving-source fragmentation model ($v_R/v_P=1.0,0.5,$ and $T_{\alpha }=2$ MeV, respectively). The calculations are based on the normalization to the $\alpha$-energy spectra shown in Fig. 2.

Figure 5

Same as Fig. 3 but for the ${}^{27}\mathrm{Al}$ target. A two-source fragmentation model calculation is shown ($v_R/v_P=1$.0, 0.5, and $T_{\alpha }=2$ MeV, respectively) normalized to the spectra as shown in Fig. 3 (see text). A distorted wave Born approximation (DWBA) calculation (arbitrarily normalized) simulating sequential breakup is also shown (dashed curve; see text).

Figure 6

Angular distributions for ${}^{\mathrm{nat}}$C(${}^8\mathrm{Li}$, $\alpha )$ data taken in 3 MeV $\alpha$-energy bins (labeled in terms of excitation energy in ${}^{16}\mathrm{N}$ for a direct two-body ${}^4\mathrm{H}$ transfer reaction). The calculations shown are typical FRDWBA ${}^4\mathrm{H}$ cluster-transfer calculations arbitrarily normalized to the data shown (solid curve, $J_f^{\pi }=3$${}^{-}$; dashed curve, $J_f^{\pi }=2$${}^{-}$). The calculations have been averaged over angular acceptance of the detector. Scattering angles and cross sections are given in the center of mass (c.m.) system for ${}^{12}\mathrm{C}$(${}^8\mathrm{Li}$, $\alpha ){}^{16}$N.

Figure 7

Same as Fig. 6, but with calculations performed at E(${}^8\mathrm{Li}$) = 14.8 MeV and compared with the corresponding ${}^{12}\mathrm{C}$(${}^8\mathrm{Li}$, α) data taken from Ref. 4.

Figure 8

A semiclassical sequential transfer calculation for ${}^{12}\mathrm{C}$(${}^8\mathrm{Li}$, $\alpha ){}^{16}\mathrm{N}$ compared with a one-step ${}^4\mathrm{H}$ transfer calculation (arbitrarily normalized) and the experimental data.

Figure 9

Energy spectrum of $\alpha$ particles observed from ${}^8\mathrm{Li}$ + ${}^{208}\mathrm{Pb}$ at $E_{c.m.}$(${}^8\mathrm{Li}$) = 34.4 MeV (Ref. 10; Prof. E. Aguilera, private communication).

Figure 10

Angular distribution of α particles from ${}^8\mathrm{Li}$ + ${}^{208}\mathrm{Pb}$ at $E_{c.m.}({}^8\mathrm{Li})=33.4$ MeV (Ref. 10) compared with a sequential-breakup calculation extrapolated from those shown in Ref. 23 for ${}^6\mathrm{Li}$ + ${}^{208}\mathrm{Pb}$ at a comparable energy. The latter has been slightly renormalized to fit the present data.