Beam Collimator Design for the Epithermal Neutron Beam at the Tsing Hua Open-Pool Reactor (THOR)

Abstract

In Taiwan, a program of epithermal neutron beam installation has been progressed since 1995. The thermal column of the Tsing Hua Open-pool Reactor (THOR) is selected as the site to be modified as an epithermal neutron beam. THOR is a 1 MW, light water moderated and cooled reactor of the TRIGA conversion type. By replacing the removable portion of the graphite blocks in the thermal column with a properly designed filters, the modification of the epithermal neutron beam was completed in 1997. Figure 1 shows the horizontal cross-section of the THOR epithermal neutron beam facility. The purpose of the lead and poly-B used at the end of the thermal column was to lower the background dose of the irradiation room. The filters were composed of Al and A1F3 for extracting the epithermal neutron while the Bi was used for reducing the gamma ray component. The distance between the Bi face to the irradiation port is 16 cm. This space was left for beam collimator design with a cross-section of 40 × 40 cm². Before progressing to the animal experiments and clinical trials by using this epithermal neutron beam, a beam collimator has to be installed to provide a suitable beam size, and to lower the background dose of the irradiation room from different components of radiation. The purpose of this research is aimed at providing a convenient beam collimator and to lower the background dose without reducing the epithermal neutron flux at the irradiation port.