Spectrum measurements and calculations in the epithermal neutron beam at the FiR 1 BNCT facility

After a period of intense planning, building and scientific activity by a multi-disciplinary team from VTT Chemical Technology, Helsinki University, Helsinki University Central Hospital and the Radiation and Nuclear Safety Authority (STUK) an epithermal Boron Neutron Capture Therapy (BNCT) treatment facility for malignant brain tumours has been established at the FiR 1 TRIGA Mark H reactor in Espoo, Finland [1]. A crosssection of the epithermal beam, including the reactor core and the FLUENTALtm moderator, is shown in Fig. 1. In the first stage (1996) the thickness of the moderator was 75 cm. Calculations and measurements with this configuration showed that the neutron field was of very good quality with a very small contamination of fast neutrons [2, 9]. However, in order to increase the intensity and reduce treatment times the thickness of the moderator was reduced to 63 cm in the final configuration (November 1997, shown in Fig. 1), despite a somewhat higher anticipated fast flux component. The aperture diameter is adjustable from 8 cm to 20 cm in 3 cm steps by adding and removing Li-poly plates. Naturally, the neutron field in the treatment beam must be characterised as well as possible. An accurate knowledge of the source neutron spectrum in the BNCT beam is essential for treatment planning and for in-phantom calculations and measurements. For that purpose extensive calculations and measurements have been performed at several locations, both for a true "free beam" configuration and with a water phantom at the exit aperture.