Characteristics of neutron production and concrete activation in cyclotron vaults for self-shielded and non-self-shielded facilities

Abstract

Currently, 13 radioisotope production cyclotrons with proton energies of 9.6–30 MeV exist in Taiwan. Seven of these cyclotrons have self-shielding designs, and the remaining six cyclotrons are non-self-shielded types. The following (p,xn) nuclear reactions with various targets are involved in their operations: 18O(p,n)18F, 68Zn(p,n)68Ga, 68Zn(p,2n)67Ga, 69Ga(p,2n)68Ge, 100Mo(p,pn)99Mo, 100Mo(p,2n)99mTc, 89Y(p,n)89Zr, 112Cd(p,2n)111In, 203Tl(p,3n)201Pb, 124Xe(p,2n)123Cs, and 15N(p,n)15O. The secondary neutrons accompanying these reactions can induce material activation in the cyclotron components and shield structure to varying degrees. By using the FLUKA Monte Carlo code, the characteristics of neutron yields and the resulting spatial and energy distributions of neutrons in cyclotron vaults were evaluated for various targets in proton bombardment. A systematic assessment of residual long-lived radioactivity in forward and lateral concrete walls was conducted. The result for the 18O(p,n)18F reaction with 18-MeV protons was used as a reference for comparisons of the concrete activation induced by neutrons generated through various production routes. The similarities and differences in activation results for the self-shielded and non-self-shielded cyclotrons are presented and discussed.