The programme of the United States National Bureau of Standards in dosimetry standards for neutron radiation therapy

This report discusses two aspects of the neutron dosimetry programme at the United States National Bureau of Standards (NBS), namely the plans and progress towards establishing dosimetry standards for neutron radiation therapy, and an investigation of the neutron and gamma-ray tissue kerma rates from a 252Cf source. Neutron radiation therapy is being clinically tested at a number of centres in the world. To maximize the chances of success of this radiation therapy modality, good physical dosimetry is needed. To facilitate exchange of therapy experience between institutions, the United States dosimetry standards base must be accurate and consistent with the international standards system. The purpose of the NBS programme is to improve the accuracy and consistency of measurements of absorbed dose for neutron radiation therapy by providing national dosimetry standards and improved data on neutron interactions with tissue and tissue-equivalent materials. A longer-term goal is to develop a calibration facility at NBS where neutron dosimeters can be calibrated and their energy dependence studied. As an initial step in establishing the neutron dosimetry standards programme, it was decided to make neutron and gamma-ray measurements of a 252Cf source at NBS, whose neutron emission rate had been measured and was known with an uncertainty of 1%. Neutrons emitted by spontaneous fission in 252Cf sources are used for interstitial and intracavitary applications to neutron therapy, and for calibrating various dosimeters used for radiation protection. Besides being convenient sources of fission neutrons, such sources also emit copious amounts of gamma rays. Both radiation components must be accurately assessed for proper application. (author)