Measurements of ${}^{17}\mathrm{F}$ scattering by ${}^{208}\mathrm{Pb}$ with a new type of large solid angle detector array

A new pixel-structure detector array with a large solid angle coverage has been used for the first time to study the elastic scattering of exotic ${}^{17}\mathrm{F}$ nuclei from a ${}^{208}\mathrm{Pb}$ target at $90.4\mathrm{MeV}$. The experimental data have been analyzed in the framework of the optical model potential and the real and imaginary strong absorption radii have been evaluated. These quantities have been compared with those obtained for the system ${}^{19}\mathrm{F}+{}^{208}\mathrm{Pb}$ at the same energy in the center of mass frame. The ${}^{17}\mathrm{F}+{}^{208}\mathrm{Pb}$ reaction cross section is more similar to those of the systems ${}^{16,17}\mathrm{O}+{}^{208}\mathrm{Pb}$ rather than to the one of the system ${}^{19}\mathrm{F}+{}^{208}\mathrm{Pb}$ at similar energies: this indicates that in the energy range around the Coulomb barrier the breakup channel is still weak. The exclusive breakup cross section ${}^{17}\mathrm{F}\to {}^{16}\mathrm{O}+p$ has been measured for the first time at energy below the Coulomb barrier.

Figure 1

Photo of two detectors of an EXODET telescope before their assembling.

Figure 2

Schematic of the electronic circuit for one of the EXODET detectors.

Figure 3

(a) $\Delta E$ spectrum collected from the backward detector of the EXODET apparatus; (b) $\Delta E$ spectrum gated by $\mathrm{JT}=10$ and $\mathrm{TOT}=6$; (c) $\Delta E$ spectrum gated by $\mathrm{JT}=10$ and TOT ranging from 2 to 4. See text for more details on JT and TOT.

Figure 4

(a) Jitter time (JT) spectrum of the backward $\Delta E$ detector. Lower panel: Time over threshold (TOT) spectra of the events collected by the $\Delta E$ detector at backward angles with: (b) $\mathrm{JT}=10$ and energy loss greater than $59\mathrm{MeV}$ (corresponding to the ${}^{17}\mathrm{F}$ peak), (c) $\mathrm{JT}=10$ and energy loss smaller than $10\mathrm{MeV}$ (light particle range).

Figure 5

${}^{17}\mathrm{F}\left({}^{19}\mathrm{F}\right)+{}^{208}\mathrm{Pb}$ quasielastic scattering angular distributions at $90.4\left(91\right)$- and $98\text{−}\mathrm{MeV}$ beam energy. The continuous lines are the results of the optical model best fits for the ${}^{17}\mathrm{F}$ system. The parameters used at $90.4\mathrm{MeV}$ for the system ${}^{17}\mathrm{F}+{}^{208}\mathrm{Pb}$ are listed in Table , while for the data at $98\mathrm{MeV}$ we used the same sets, but the imaginary depth $\left(W_0=12.8\mathrm{MeV}\right)$, in order to minimize the ${\chi }^2$. The quoted errors for the data at $90.4\mathrm{MeV}$ are statistical, whereas those at $98\mathrm{MeV}$ include both statistical and systematical uncertainty.

Figure 6

Behavior of real and imaginary potential fitting the ${}^{17}\mathrm{F}$ elastic scattering data at $90.4\mathrm{MeV}$ for the different values of diffuseness and radius listed in Table .

Figure 7

Comparison of the optical model analysis of the experimental elastic scattering angular distributions for four different systems: ${}^{16,17}\mathrm{O}+{}^{208}\mathrm{Pb}$ at $78\text{−}\mathrm{MeV}$, ${}^{17}\mathrm{F}+{}^{208}\mathrm{Pb}$ at $90.4–\mathrm{MeV}$, and ${}^{19}\mathrm{F}+{}^{208}\mathrm{Pb}$ at $91–\mathrm{MeV}$ beam energy. The potential parameters used are indicated in Table . For the reaction involving ${}^{19}\mathrm{F}$, we adopted the standard Woods-Saxon already used for ${}^{17}\mathrm{F}$, adding the excitation to the second excited state at $E_x=0.197\mathrm{MeV}$ from a DWBA calculation (the first level at $E_x=0.110\mathrm{MeV}$ was not included, since its coupling to the ground state is very weak). The curve labeled “elastic $+2\mathrm{nd}$ state inelastic scattering best fit” is the result of this calculation. From this analysis, it was also possible to extract the pure elastic data (the curve labeled “optical model elastic scattering analysis”). Experimental data for the ${}^{17}\mathrm{F}+{}^{208}\mathrm{Pb}$ system studied in this paper are also shown for completeness.