The ${}^8$Li($p,\alpha$)${}^5$He reaction at low energies, and ${}^9$Be spectroscopy around the proton threshold

We present a direct measurement of the low-energy ${}^8\mathrm{Li}{(p,\alpha )}^5\mathrm{He}$ cross section, using a radioactive ${}^8$Li beam impinging on a thick target. With four beam energies, we cover the energy range between $E_{\mathrm{c}.\mathrm{m}.}=0.2$ and 2.1 MeV. An $R$-matrix analysis of the data is performed and suggests the existence of two broad overlapping resonances ($5/2^{+}$ at $E_{\mathrm{c}.\mathrm{m}.}=1.69$ MeV and $7/2^{+}$ at $E_{\mathrm{c}.\mathrm{m}.}=1.76$ MeV). At low energies our data are sensitive to the properties of a subthreshold state ($E_x=16.67$ MeV) and of two resonances above threshold. These resonances were observed in previous experiments. The $R$-matrix fit confirms spin assignments, and provides partial widths. We propose a new ${}^8\mathrm{Li}{(p,\alpha )}^5\mathrm{He}$ reaction rate and briefly discuss its influence in nuclear astrophysics.

Figure 1

Bidimensional energy spectra obtained at $E_{\mathrm{lab}}{(}^8\mathrm{Li})=19.0$ MeV and ${\theta }_{\mathrm{lab}}=13.5^{\circ }\pm 2.6^{\circ }$, with the scattered ejectiles of all beams focused by the first solenoid, respectively, on the gold (a) or on the (CH${}_2$)${}_n$ (b) secondary targets. The abscissa is the total energy in MeV and the ordinate is the energy loss in the $\Delta E$ detector in MeV. (c) The projection of the $\alpha$-strip on the total energy axis.

Figure 2

Energy centroids of the peak at $E_{\alpha }=27$ MeV as a function of the laboratory detection angles (dots) measured at $E{(}^8\mathrm{Li})=19.0$ MeV. The angular uncertainty is $\pm 2.6^{\circ }$. The solid line describes the kinematic variation of the reaction ${}^1$H(${}^8$$\mathrm{Li},\alpha$)${}^5$He at $E{(}^8\mathrm{Li})=19.0$ MeV. The dashed line corresponds to the energy of the ${}^8$Be from the ${}^1$H(${}^8$Li,${}^8$Be)$n$ reaction at $E{(}^8\mathrm{Li})=19.0$ MeV; the $\alpha$ particles originating from the decay of ${}^8$Be have energy typically about half of the ${}^8$Be energy. The dotted line corresponds to the ${}^1$H(${}^7$$\mathrm{Li},\alpha$)${}^4$He reaction at $E{(}^7{\mathrm{Li}}^{2+})=9.66$ MeV.

Figure 3

Energy spectra of the $\alpha$ particles measured with a ${}^8$Li beam (and contaminants) on the (CH${}_2$)${}_n$ thick target at ${\theta }_{\mathrm{lab}}=13.5^{\circ }\pm 2.6^{\circ }$ and $E_{\mathrm{inc}}{(}^8\mathrm{Li})=13.2$, 14.5, 17.0, and 19.0 MeV. Some arrows indicate peaks, which have a rapid variation with the incident energy and cannot be related to resonances in the compound systems; others indicate the maximum energy of $\alpha$ particles from the ${}^1$H(${}^8\mathrm{Li},\alpha$) reaction, with the energy loss in the target taken into account.

Figure 4

(a) Comparison of three different excitation functions measured at 13.2 MeV incident energy, one measured with (CH${}_2$)${}_n$ and the other two with a natural carbon target. The excitation functions were normalized to superimpose the peak at $E_{\alpha }=13.5$ MeV, which corresponds to the ${}^{12}$C(${}^7$$\mathrm{Li},\alpha$)${}^{15}$N${}_{\mathrm{gs}}$ reaction, present in all three excitation functions. (b) Comparison of the cross sections of the (${}^8$$\mathrm{Li},\alpha$) reaction produced on (CH${}_2$)${}_n$ and on natural carbon targets and the result of the subtraction.

Figure 5

The experimental cross sections measured at $E{(}^8\mathrm{Li})=19.0$ MeV (line with squares) compared to the continuous energy distribution of the $\alpha$ particles resulting from the three-body breakup (solid line) and to the energy spectra of the $\alpha$ particles resulting, respectively, from the decay of ${}^5$He${}_{\mathrm{g}.\mathrm{s}.}$ (dashed line) and of ${}^5$He${}^{*}$ (dotted line). The simulation assumes a Breit-Wigner resonance situated at $E{(}^8\mathrm{Li})=15.3$ MeV with $\Gamma =0.7$ MeV (dash-dotted line). The cross section obtained after the subtraction of all three background contributions is represented by circles. The error bars are omitted for the sake of clarity.

Figure 6

Complete excitation function at ${\theta }_{\mathrm{lab}}=13.5^{\circ }$ without an averaging procedure (a) and with an averaging procedure (b). See text for details.

Figure 7

${}^8\mathrm{Li}{(p,\alpha )}^5\mathrm{He}$ differential cross sections at ${\theta }_{\mathrm{lab}}=13.5^{\circ }$, with the $R$-matrix fit (solid line). The data are presented as in Fig. 6.

Figure 8

Integrated cross section (upper panel) and $S$ factor (lower panel) obtained with the $R$-matrix parameters of Table .

Figure 9

${}^8\mathrm{Li}{(p,\alpha )}^5\mathrm{He}$ and ${}^8\mathrm{Li}{(\alpha ,n)}^{11}\mathrm{B}$ reaction rates multiplied by the proton and $\alpha$ mass fractions (see text).