RESIDUAL LONG-LIVED RADIOACTIVITY DISTRIBUTION IN THE INNER CONCRETE WALL OF A CYCLOTRON VAULT

We measured the depth distribution of residual long-lived radioactivit y in the inner concrete wall of a cyclotron vault by assaying concrete cores and we estimated the neutron flux distribution in the inner con crete wan by means of activation detectors. Nine long-lived radioactiv e nuclides (Sc-46, Fe-59, Co-60, Zn-65, Cs-134, Eu-152, Eu-154, Na-22, and Mn-54) were identified from the gamma-ray spectra measured in the concrete samples. It was confirmed that the radionuclides induced by thermal neutrons through the (n, gamma) reaction are dominant, and tha t the induced activity by thermal neutrons is greatest at a depth of 5 to 10 cm rather than at the surface of the concrete and decreases exp onentially beyond a depth of about 20 cm. By comparing the radioactivi ty and neutron flux distributions, we can estimate the induced long-li ved radioactivity in concrete after a long period of operation from th e short-term activation measurement.