Photoexcitation of isomers by Bremsstrahlung of 4 KeV electrons

Se-77 , Br- 79 ,Kr- 83, Sr- 87, Y- 89, Tc- 99, Rh -103, Ag-107, Ag-109, Cd-111, In-113, In-115, Sn-117, Xe-129, Xe-131, Ba-135, Ba-137, Er-167, Lu-176, Hf -179, W - 183, Os-189, Ir-191, Ir-193, Pt- 195, Au-197 isomers were excited by ( gamma , gamma ) reaction, using Bremsstrahlung from a 4 MeV, 25 micro A electron linear accelerator. Induced radioactivities were determined by Ge and well - type NaI (Tl) gamma - spectrometers. A pneumatic transport system was used for successive repetition of irradiation and measurement cycles in the study of short - lived ( order of second ) isomers. Thick - target Bremsstrahlung spectrum shape was calculated while the amplitude was determined by total flux measurements carried out by TL and chemical dosemeters as flux monitors. From the measured yields and calculated flux densities overall (mean) cross section and integrated cross section values were deduced for the energy region from threshold up the endpoint energy. The results demonstrate that isomer excitation is a multi parametric process, depending on level density, spin difference between isomeric and ground states, energy of the isomeric level etc., which leads to a large variety in the magnitude of the excitation cross section among individual nuclides. Comparison of the results with earlier ones from experiments by high activity Sc- 46, Co- 60, Pr-142 and Na- 24 irradiating sources seems to be instructive, indicating particularly strong excitations in the rare -earth region, starting from relatively low energies already, in spite of large delta I-s in some cases. 1 tab.; 6 refs.(author)