Mass yield features of (gamma,pi(+)) and (gamma,pi(-)xn) reactions for x=0-9 on complex nuclei in the Delta region

Radiochemical measurements of the product yields from (gamma, pi(+) ) and ( gamma, pi(-)xn) reactions for x = 0 - 9 were performed on targets of Al-27, Cl-37, K-41, V-51, Co-59, Cu-65, As-75, Rb-87, Sr-88, Ag-109, In-115, Cs-1 33, Ba-138, La-139, and Ta-181, and of Li-7, B-11, N-14, Ca-44, V-51, Fe-56 , Co-59, Cu-63, Cu-65, As-75, Sr-88, Y-89, Ag-109, In-115, I-127, Te-130, C s-133, La-139, Lu-175, Au-197, and Bi-209, respectively, over bremsstrahlun g end-point energies (E-0) of 30-1200 MeV in steps of E-0 = 50 MeV or less. Yield variations as a function of the number of the emitted neutrons (x) f or each target at E(0)greater than or equal to 400MeV were found to be typi cal of (3,3) resonance. The reactions for neutron multiplicities as large a s x greater than or equal to 6 are notable for targets of mass A(t)greater than or equal to 100, while only the reactions for smaller x are measurable for the lighter targets. The yields for both (gamma, pi(-) ) and (gamma, p i(+)) are A(t)-independent for targets heavier than A(t)>30-40, while much smaller yields are reported for targets with A(t) = 7 - 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 high er than that expected from the neutron-to-proton ratio for the entire nucle us. The observed yields for individual (gamma,pi(-)xn) reactions having equ al x were also found to be a smoothly varying function of the neutron-to-pr oton ratio of the target, (N/Z)(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 (N/Z)(t ) = 1.30-1.40, corresponding to A(t) = 100- 130; this implies higher excita tion energies due to progressively larger medium effects in nuclei with A(t )>100. The results are compared with theoretical calculations made using th e photon-induced intranuclear cascade and evaporation analysis program by G abriel and Alsmiller. [S0556-2813(99)05703-9].