Mechanisms in Knockout Reactions

We report the first detailed study of the relative importance of the stripping and diffraction mechanisms involved in nucleon knockout reactions, by the use of a coincidence measurement of the residue and fast proton following one-proton knockout reactions. The measurements used the S800 spectrograph in combination with the HiRA detector array at the NSCL. Results for the reactions ${}^9\mathrm{Be}({}^9\mathrm{C},{}^8\mathrm{B}+X)Y$ and ${}^9\mathrm{Be}({}^8\mathrm{B},{}^7\mathrm{Be}+X)Y$ are presented and compared with theoretical predictions for the two reaction mechanisms calculated using the eikonal model. The data show a clear distinction between the stripping and diffraction mechanisms and the measured relative proportions are very well reproduced by the reaction theory. This agreement adds support to the results of knockout reaction analyses and their applications to the spectroscopy of rare isotopes.

Figure 1

Two dimensional spectra of the energy of protons and of the heavy residue in one-proton knockout reactions from ${}^9\mathrm{C}$ (top) and ${}^8\mathrm{B}$ (bottom) projectiles, respectively. The narrow bands of constant energy sum correspond to elastic breakup whereas other events are associated with inelastic breakup (see text). The small fluctuations in number of counts are due to the way the spectra were produced, by adding the data from the various magnetic rigidity settings used during the experiment.

Figure 2

Energy sum spectra of the one-proton knockout residue and the light particles detected in coincidence in the HiRA detector array for ${}^9\mathrm{C}$ (top) and ${}^8\mathrm{B}$ (bottom) projectiles. The sharp peak corresponding to elastic breakup is visible in proton coincidence events, whereas it disappears for deuteron and other inelastic coincidence events (see text). The inelastic and elastic components of the fit are shown, as well as the location of the cut indicated by the double arrow. The tail of the inelastic component leaking into the elastic peak amounts to 33% and 25% for the ${}^9\mathrm{C}$ and ${}^8\mathrm{B}$ breakups, respectively. The amount of elastic component leaking into the inelastic peak is negligible, due to the narrow width of the elastic peak.

Figure 3

Proton scattering angular distributions obtained for ${}^9\mathrm{C}$ (top) and ${}^8\mathrm{B}$ (bottom) elastic breakup events, after subtraction of the inelastic contamination using the fitted energy sum spectra (see Fig. 2). Also shown are the results from a CDCC calculation of the same quantity. The CDCC distributions are used to deduce the amount of unobserved cross section due to the lack of angular coverage between 0° and 10°, after (minor) normalization to the data.