Observation of a Large Reaction Cross Section in the Drip-Line Nucleus ${}^{22}\mathbf{C}$

Reaction cross sections (${\sigma }_R$) for ${}^{19}\mathrm{C}$, ${}^{20}\mathrm{C}$ and the drip-line nucleus ${}^{22}\mathrm{C}$ on a liquid hydrogen target have been measured at around $40A\mathrm{MeV}$ by a transmission method. A large enhancement of ${\sigma }_R$ for ${}^{22}\mathrm{C}$ compared to those for neighboring C isotopes was observed. Using a finite-range Glauber calculation under an optical-limit approximation the rms matter radius of ${}^{22}\mathrm{C}$ was deduced to be $5.4\pm 0.9\mathrm{fm}$. It does not follow the systematic behavior of radii in carbon isotopes with $N\le 14$, suggesting a neutron halo. It was found by an analysis based on a few-body Glauber calculation that the two-valence neutrons in ${}^{22}\mathrm{C}$ preferentially occupy the $1s_{1/2}$ orbital.

Figure 1

(a) Two-dimensional plot of $Z$ versus $A/Q$ in front of the reaction target. (b) $Z$ projection of Fig. 1a. The solid line indicates a Gaussian fit to the $Z=6$ peak, yielding a $\mathit{\Delta }Z=0.24$ in FWHM. (c) $A/Q$-projection spectrum for the $Z=6$ particles. The solid line indicates a Gaussian fit to the ${}^{22}\mathrm{C}$ peak, yielding a $\mathit{\Delta }A=0.12$ in FWHM.

Figure 2

The ${\tilde{r}}_m$ as a function of the neutron number of C isotopes. The filled square and circles show the present result and those determined at GSI [14], respectively, while open symbols are the result of the calculation [22]. The lines connect the open circles. The inset shows ${\rho }_p(r)$ (solid line) and ${\rho }_n(r)$ (dotted line) of ${}^{22}\mathrm{C}$ for the determined parameter. See text.

Figure 3

The ${\sigma }_R$ for $f=1.0$ (red triangles) and that for $f=0.0$ (blue triangles), with $S_{2n}=420\mathrm{keV}$ (open symbols) and $S_{2n}=10\mathrm{keV}$ (closed symbols), respectively. The lines are to guide the eye. The experimental data (solid circles) as a function of the mass number of C isotopes are also plotted.