EFFECT OF PHANTOM SIZE AND COMPOSITION ON NEUTRON DOSIMETER READING

Unmoderated and D2O-moderated Cf-252 neutron sources were used to irra diate several types of personal neutron dosemeters. The dosemeters wer e irradiated on polymethylmethacrylate (PMMA) phantoms with frontal di mensions of 40 X 40 cm(2) and 30 X 30 cm(2); both phantoms were 15 cm thick; Monte Carlo computer calculations of the neutron spectra and th e ratios of dose equivalents at the surfaces of these phantoms were pe rformed using the MCNP computer code. Additional calculations were per formed fore Am-241-Be neutron source and for phantoms composed of Inte rnational Commission on Radiation Units and Measurements (ICRU) tissue and of water; the water phantom used the design specified recently by the International Organization for Standardization (ISO). The calcula tions indicated negligible differences in the effects that either of t he PMMA phantoms produced on the neutron energy spectra studied. Howev er, the dose equivalent at the surface of the 30 X 30 X 15 cm(3) PMMA phantom differed from the dose equivalents at the surfaces of the wate r phantom and the ICRU tissue phantom by about 4%. Dosemeters irradiat ed on the two different sized PMMA phantoms, using unmoderated and D2O moderated Cf-252 neutron sources, gave nearly identical readings. All of the phantoms had similar effects on the neutron spectra, producing additional low energy neutrons. It was concluded that either size of PMMA phantom could be used for the irradiations required by the US dos imetry accreditation programmes.