Elastic scattering and $\alpha$-particle production in ${}^6$He $+$ ${}^{208}$Pb collisions at 22 MeV

Experimental results of the elastic scattering of ${}^6$He on ${}^{208}$Pb at $E_{\mathrm{LAB}}=22$ MeV, measured at the CRC facility (Louvain-la-Neuve, Belgium), are presented, including results on the ${}^4$He production channel. These data were taken with full angular coverage and high angular resolution. Both experimental cross sections are compared with continuum discretized coupled channels and distorted-wave Born approximation calculations, where direct breakup and transfer to the continuum processes are considered. The elastic data confirm the absence of the Coulomb rainbow, while the distribution of $\alpha$ particles indicates that such production is mostly generated by transfer to the continuum.

Figure 1

Sketch of the experimental setup. The LAB scattering angles covered by each telescope are indicated.

Figure 2

Typical particle identification spectrum $\Delta E$-$E_t$ of one strip from the CD-forward telescope, obtained with the ${}^6$He beam at $E_{\mathrm{LAB}}=22$ MeV. The solid line ellipsoid encloses events corresponding to elastically scattered ${}^6$He. The dashed polygon encloses ${}^4$He fragments. The dot-dashed polygon encloses ${}^6$He scattered in some elements of the setup other than the ${}^{208}$Pb target (see text for details).

Figure 3

Typical particle identification spectrum $\Delta E$-$E_t$ of one strip from the CD-backward telescope, obtained with the ${}^6$He beam at $E_{\mathrm{LAB}}=22$ MeV. The solid line ellipsoid encloses events corresponding to elastically scattered ${}^6$He. The dashed polygon encloses ${}^4$He fragments (see text for details).

Figure 4

Typical particle identification spectra $\Delta E$-$E_t$ of two strips from the square telescopes, obtained with the ${}^6$He beam at $E_{\mathrm{LAB}}=22$ MeV: (a) square-forward telescope; (b) square-backward telescope. In both histograms the solid line ellipsoid encloses elastically scattered ${}^6$He, while dashed and dot-dashed polygons enclose ${}^4$He fragments (see text for details).

Figure 5

${}^4$He yield as a function of the total energy (at ${\theta }_{\mathrm{LAB}}={81}^{\circ }$). The low-energy region is not related to the studied reaction. The high-energy region shows the $\alpha$-particle yield produced in ${}^6\text{He}+{}^{208}\text{Pb}$ collisions (see text for details).

Figure 6

Angular distributions, relative to the Rutherford cross section, for the elastic scattering of ${}^6$He on ${}^{208}$Pb at $E_{\mathrm{LAB}}$ $=$ 22 MeV. The present data (circles) are compared with previous data (triangles) from Ref. [6] as well as three calculations (see text for details).

Figure 7

Top: Angular distribution of $\alpha$-particle production relative to elastically scattered ${}^6$He in collisions with ${}^{208}$Pb at 22 MeV. The data (circles) are compared with previous data (triangles) from Ref. [7]. A DWBA calculation is also included in the plot (solid line). Bottom: The differential cross section of $\alpha$-particle production as a function of ${\theta }_{\mathrm{LAB}}$. The values for this observable resulting from the same DWBA calculation and data from Ref. [7] are also shown. See text for details.

Figure 8

Experimental energy distribution of the produced ${}^4$He (bars) for the angular range ${80}^{\circ }<{\theta }_{\mathrm{LAB}}<{90}^{\circ }$ compared with a DWBA calculation where transfer to the continuum is considered (solid line). The arrows indicate the expected energy of the scattered ${}^4$He at ${\theta }_{\mathrm{LAB}}={85}^{\circ }$, assuming (i) $1n$-transfer (dotted line corresponding to $Q_{gg}$ and dot-dashed to zero $Q$ value); (ii) $2n$-transfer (dashed line corresponding to $Q_{gg}$, long dashed line to zero $Q$ value, and solid line to $Q=-4$ MeV).