The present invention provides an -particle-emitting radioisotope chelation complex comprising ²²⁵Actinium (²²⁵Ac) and 1,4,7,10,13,16-hexaazacyclohexadecane-N,N,N,N,N,N-hexaacetic acid (HEHA) (²²⁵Ac-HEHA). Also provided is a bifunctional HEHA, and a bifunctional ²²⁵Ac-HEHA. The bifunctional HEHA and the bifunctional ²²⁵Ac-HEHA can be conjugated to a targeting agent. In view of the above, the present invention provides a method of making HEHA and methods of making a bifunctional HEHA, including a conjugate thereof. Also provided are a method of treating disease, a method of treating cancer, a method of decontaminating a sample from ²²⁵Ac, a method of decontaminating a person who has been externally contaminated with ²²⁵Ac, and a method of detoxifying a person who has internalized ²²⁵Ac.