Fission neutron/gamma irradiation of Bacillus thuringiensis bacteria at the Texas A&M University Nuclear Science Center Reactor

Abstract

The objective of this research is to fully characterize the effectiveness of the Texas A&M University Nuclear Science Center Reactor (TAMU NSCR) neutrons for bacterial sterilization, and to assess the secondary gamma flux produced when neutrons collide with nuclei in biological materials. Sterilization of bacteria by exposure to gamma rays and charged particles is fairly well understood. Exposure to neutrons and gamma rays from fission as a means of sterilization has not to date been adequately characterized. The lack of data on the relationship between biological detriment resulting from thermal or fast neutron exposures and absorbed doses as applied in countermeasures to weapons of mass destruction (WMD) is the primary motivation for this investigation of neutron doses to endospores. Bacillus thuringiensis (Bt) spores were irradiated after producing and sampling them using standard microbiological procedures. Irradiation was accomplished using neutrons and gamma rays from the 1-MW TRIGA reactor at the TAMU NSCR using a reactor power of 100 kilowatts (kW). The combination of neutron and gamma-ray absorbed dose provided an effective means of sterilization of these types of spores; it yielded a 100-percent kill for the first study. Survival curves have been developed, from subsequent experiments, for these energy dependent neutron interactions with biological materials using a combination of radiation dosimetry, microbiological culture techniques, and computer modeling (Monte Carlo Neutral Particle history modeling - MCNP). Survival curves indicate a D₃₇ value of 321.08 Gy. Additional work is needed to investigate the specific bacteria used in biological weapons in order to understand agent-specific radiation sensitivity. Once this is done, more effective and meaningful experiments can be conducted in order to tailor the neutron source strength to the robustness of the threat.