Absolute fluence measurement for a prototype neutron radiotherapy source.

An inexpensive recoil-proton counter telescope has been designed for absolute fluence measurements of a gas-target neutron source for radiation therapy. The detector has an absolute efficiency of 1.1 times 10-minus 9 at 20 cm from an isotropic source and is useful for production rates of 10-9 minus 10-13 neutrons per second. The telescope consists of a thin hydrogenous irradiator foil and a surface-barrier detector to count recoil protons within a defined solid angle. The telescope provides n-gamma discrimination as well as discrimination against scattered neutrons. Initial tests of the counter telescope were performed using the DD reaction employed in development stages of the gas target. A clear separation of full-energy recoil protons from background and scattered neutron events was evident in the pulse-height spectra without the use of coincidence gating techniques.