Neutron flux determination for BNCT by means of neutron activation analysis

In April 1996 Phase One in the construction of a Boron Neutron Capture Therapy (BNCT) facility at the TRIGA research reactor at VTT, Otaniemi, Espoo, was completed. The idea of BNCT is to introduce 10B into the tumour cells, expose the patient's head to a neutron flux and let the reaction products of the 10B neutron capture reaction destroy the tumour. The neutron capture cross-section of 10B is inversely proportional to the neutron velocity. Thus, thermal neutrons are most likely to cause the desired effect in the tissue. The flux reaching the patient's head from the outside of the skull should be epithermal in order to penetrate the patient's skull, and at the same time, slow down to thermal energies. To evaluate the performance of the epithermal field for BNCT the desired epithermal (0.5 cV - 10 keV) neutron flux and the undesired fast and thermal fluxes had to be determined. The method used for neutron flux characterisation was neutron activation analysis. The basis for this analysis method is the variability of the neutron capture cross-sections of different elements as a function of neutron energy. The product of the microscopic cross-section and the flux, integrated over energy, gives the reaction rate, which equals the probability per second of a target atom to absorb a neutron in the flux in question. When the elements used as detectors are properly chosen, comprehensive information about the neutron flux can be obtained. Neutron activation analysis is a semiempirical method: the reaction rates of several samples are both measured and calculated according to flux simulations and the final results are obtained by adjusting the calculated neutron spectrum by means of the measurement results. The chosen activation foils were irradiated in the neutron beam and their reaction rates were determined from activity measurements. The measured reaction rates were used to validate the neutron transport calculations (DORT). The generalised least-squares code LSL-M2 was used to adjust the neutron spectra. A 47-group energy structure was used in the calculations and in the adjustment procedure. The beam is suitable for BNCT: the fast and epithermal parts of the neutron flux at the outer surface of the moderator meet the requirements set for the treatment beam in