ELECTROMAGNETIC DISSOCIATION OF RELATIVISTIC SI-28 INTO P+AL-27

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of Si-28 into p + Al-27 at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p-Al-27 invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in Si-28 and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszacker-Williams approximation is combined with experimental data on the photonuclear reaction Si-28(gamma, p)Al-27, and slight differences are observed only at low final-state energy.