Attenuation and activation characteristics of steel and tungsten and the suitability of these materials for use in a fast neutron multileaf collimator

A computer controlled multileaf collimator (MLC) is being designed to repla ce the multirod collimator (MRC) at present used to shape the d(48.5)+Be ne utron beam from the Harper Hospital superconducting cyclotron. The computer controlled MLC will improve efficiency and allow for the future developmen t of intensity modulated radiation therapy with neutrons. The existing MRC uses tungsten rods, while the new MLC will use steel as the leaf material. In the current study the attenuation and activation characteristics of stee l are compared with these of tungsten to ensure that (a) the attenuation ac hieved in the MLC is at least equivalent to that of the existing MRC, and ( b) that the activation of the steel will not result in a significant change in the activation levels within the treatment room. The latter point is im portant since personnel exposure (particularly to the radiation therapy tec hnologists) from induced radioactivity must be minimized. Measurement of th e neutron beam attenuation in a broad beam geometry showed that a 30 cm thi ck steel leaf yielded 2.5% transmission. This compared favorably with the 4 % transmission obtained with the existing MRC. Irradiation of steel and tun gsten samples at different depths in a 30 cm steel block indicated that the activation of steel should be no worse than that of tungsten. (C) 2001 Ame rican Association of Physicists in Medicine.