Clinical measurements of fast neutron spectra using an organic liquid scintillator fast neutron spectrometer

The development of a system for measuring fast neutron spectra in tissue-equivalent water phantoms is described and some measurements of fast neutron spectra pertinent to radiotherapy and in-vivo neutron activation analysis (IVNAA) is reported. Two liquid scintillator detector units were assembled, and in one attention was directed to good spatial resolution and minimum neutron flux perturbation in a water phantom. The zero-crossing technique was used to discriminate between the scintillation pulse shapes produced by fast neutrons and gamma-rays. The pulse height response and pulse shape response of the two detectors to a range of monoenergetic neutron beams was studied. Results of a study of the effects of the presence and depths of a collimator, of different reflector and collimator aperture lining materials, and of field size on the neutron spectrum from a 238Pu-9Be neutron source in air and at depths in a phantom are given. Activation profiles for various body elements were derived from spectral measurements and the profile for the controversial 14N(n,2n)13N reaction agreed with an independent measurement and calculation. Kerma distributions with depth for the IVNAA facilities used were also derived. Suggestions were made to overcome the problems of expanding the range of neutron energies which could be accurately measured. (U.K.)