Integral cross section of the⁹⁹Tc(γ,γ′)^99mTc reaction at 4 MeV

^99mTc production was studied with the aid of photoexcitation by a 4 MeV endpoint energy bremsstrahlung from the LINAC of the Institute of Isotopes, Budapest. The intensity of the γ-flux was monitored by disc-shaped natural indium plates, placed in front of and behind the small cylindrical aluminium holders containing TcO₂ samples in a powder form. Isomeric activities were measured through the 140 keV γ-line by a Ge spectrometer. The integral cross section at 4 MeV was found to be 63.3±7.1 μbMeV, which can be considered reasonable compared to the corresponding value of 55.3 μbMeV established for the¹¹⁵In (γ,γ′)^115mIn reaction. We also attempted the photoexcitation of^99mTc by irradiation with γ-rays from a 1.5×10¹⁵ Bq⁶⁰Co source, but no isomeric activity could be observed. This places the first activation level between 1.33 and 4 MeV.