Effect of Phantom Size and Composition on Neutron Dosemeter Reading

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Unmoderated and D 2 O-moderated 252 Cf neutron sources were used to irradiate several types of personal neutron dosemeters. The dosemeters were irradiated on polymethylmethacrylate (PMMA) phantoms with frontal dimensions 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 the ratios of dose equivalents at the surfaces of these phantoms were performed using the MCNP computer code. Additional calculations were performed for a 241 Am-Be neutron source and for phantoms composed of Intemational Commission on Radiation Units and Measurements (ICRU) tissue and of water ; the water phantom used the design specified recently by the Intemational Organization for Standardization (ISO). The calculations indicated negligible differences in the effects that either of the PMMA phantoms produced on the neutron energy spectra studied. However, 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 water phantom and the ICRU tissue phantom by about 4%. Dosemeters irradiated on the two different sized PMMA phantoms, using unmoderated and D 2 O moderated 252 Cf 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 dosimetry accreditation programmes.