Measurement of $gamma$ rays from the giant resonances excited by $^{12}$C$(p,p)$ reaction at 392 MeV and 0$^{circ}$

We measured both the differential cross section ($sigma_{p,p^prime}$ $=d^2sigma/dOmega dE_{x}$) and the $gamma$-ray emission probability ($R_gamma(E_x)$ $=sigma_{p,p^primegamma}$/$sigma_{p,p^prime}$) from the giant resonances excited by $rm^{12}C$(textit{p,p}$^prime$) reaction at 392 MeV and 0$^circ$, using a magnetic spectrometer and an array of NaI(Tl) counters. The absolute value of $R_gamma(E_x)$ was calibrated by using the well-known $gamma$-ray emission probability from $rm^{12}C^* ( 15.11$ MeV, $ 1^+$, $T=1$) and $rm^{16}O^* ( 6.9$ MeV, $2^+$, $T=0$) states within 5% uncertainty. We found that $R_gamma(E_x)$ starts from zero at $E_x=16$ MeV, increases to a maximum of 53.3$pm$0.4$pm$3.9% at $E_x=27$ MeV and then decreases. We also compared the measured values of $R_gamma(E_x)$ with statistical model calculation based on the Hauser-Feshbach formalism in the energy region $E_x=$ 16-32 MeV and discussed the features of $gamma$-ray emission probability quantitatively.