Computational design and preliminary measurements of a mixed-field irradiation facility

A boron neutron capture therapy (BNCT) facility was built at the R2-0 research reactor at Studsvik (Sweden). This facility was characterized by the presence of two different neutron filter/moderator assemblies: one optimized for clinical irradiation of patients affected by glioblastoma multiforme, a tumor of the central nervous system; the other conceived to provide a well-characterized mixed radiation field with variable ratios of high- and low-LET components. In this paper the design of the large irradiation cavity located at the end of the second beam will be described. The possibility to change the thickness of the heavy water moderator inside the filter makes it possible to vary the neutron spectrum inside the irradiation cavity to achieve different mixed-field conditions. The main goal of this work was to design the irradiation cavity to achieve a pure thermal neutron field when using the maximum thickness of D2O. Preliminary measurements performed inside the modified irradiation cavity confirmed the calculated results. The thermal neutron flux measured at the entrance of the irradiation cavity for a D2O thickness of 45 cm was equal to 2.63 × 109 cm−2 s−1 (2.0%, 1σ) in comparison with an estimated value of 2.71 × 109 cm−2 s−1 (5.1%, 1σ). In the case of 15 cm of D2O the measured thermal neutron flux was 5.18 × 109 cm−2 s−1 (2.4%, 1σ) in comparison with an estimated value of 5.25 × 109 cm−2 s−1 (5.0%, 1σ). According to the calculation the photon contamination inside the irradiation cavity is very low, however, no preliminary measurements were performed to confirm this data. Although both the nuclear research reactors located at Studsvik have been definitely shutdown, information on the design and on the results obtained are still relevant and could be useful to plan or compare similar installations elsewhere.