Radiochemical measurements of the product yields from $(\gamma ,{\pi }^{+})$ and $(\gamma ,{\pi }^{-}xn)$ reactions for $x=0\mathrm{}–9$ were performed on targets of ${}^{27}\mathrm{Al},$ ${}^{37}\mathrm{Cl},$ ${}^{41}\mathrm{K},$ ${}^{51}\mathrm{V},$ ${}^{59}\mathrm{Co},$ ${}^{65}\mathrm{Cu},$ ${}^{75}\mathrm{As},$ ${}^{87}\mathrm{Rb},$ ${}^{88}\mathrm{Sr},$ ${}^{109}\mathrm{Ag},$ ${}^{115}\mathrm{In},$ ${}^{133}\mathrm{Cs},$ ${}^{138}\mathrm{Ba},$ ${}^{139}\mathrm{La},$ and ${}^{181}\mathrm{Ta},$ and of ${}^7\mathrm{Li},$ ${}^{11}\mathrm{B},$ ${}^{14}\mathrm{N},$ ${}^{44}\mathrm{Ca},$ ${}^{51}\mathrm{V},$ ${}^{56}\mathrm{Fe},$ ${}^{59}\mathrm{Co},$ ${}^{63}\mathrm{Cu},$ ${}^{65}\mathrm{Cu},$ ${}^{75}\mathrm{As},$ ${}^{88}\mathrm{Sr},$ ${}^{89}\mathrm{Y},$ ${}^{109}\mathrm{Ag},$ ${}^{115}\mathrm{In},$ ${}^{127}\mathrm{I},$ ${}^{130}\mathrm{Te},$ ${}^{133}\mathrm{Cs},$ ${}^{139}\mathrm{La},$ ${}^{175}\mathrm{Lu},$ ${}^{197}\mathrm{Au},$ and ${}^{209}\mathrm{Bi},$ respectively, over bremsstrahlung end-point energies ${(E}_0)$ of 30–1200 MeV in steps of $E_0=50\mathrm{}\mathrm{MeV}$ or less. Yield variations as a function of the number of the emitted neutrons (x) for each target at $E_0>~400\mathrm{MeV}$ were found to be typical of (3,3) resonance. The reactions for neutron multiplicities as large as $x>~6$ are notable for targets of mass $A_t>~100,$ while only the reactions for smaller x are measurable for the lighter targets. The yields for both $(\gamma ,{\pi }^{-})$ and $(\gamma ,{\pi }^{+})$ are $A_t$-independent for targets heavier than $A_t>30\mathrm{}–40,$ while much smaller yields are reported for targets with $A_t=7\mathrm{}–14.$ The yield ratio $(\gamma ,{\pi }^{-})/(\gamma ,{\pi }^{+})$ becomes as high as 5.5 and such a high value suggests that the neutron density in the surface region of nucleus is higher than that expected from the neutron-to-proton ratio for the entire nucleus. The observed yields for individual $(\gamma ,{\pi }^{-}xn)$ reactions having equal x were also found to be a smoothly varying function of the neutron-to-proton ratio of the target, ${\mathrm{}(N/Z)\mathrm{}}_t,$ not of the target mass, $A_t$ or number of target neutrons, $N_t.$ This implies that the reactions are initiated via competitive photoabsorptions by neutrons and protons in the target nucleus. The smooth variation of the profile changes its characteristics at ${\mathrm{}(N/Z)\mathrm{}}_t=1.30\mathrm{}–1.40,$ corresponding to $A_t=100\mathrm{}–130;$ this implies higher excitation energies due to progressively larger medium effects in nuclei with $A_t>100.\mathrm{}$ The results are compared with theoretical calculations made using the photon-induced intranuclear cascade and evaporation analysis program by Gabriel and Alsmiller.

• Received 26 October 1998

DOI:https://doi.org/10.1103/PhysRevC.59.1497