Type testing and routine calibration of neutron personal dosemeters: Phantoms and phantom backscatter

It is proposed that personal dosemeters be type tested and routinely calibrated in terms of the ICRU quantities H(p)(10) and H(p)(0.07): these quantities are defined as the dose equivalent at 10 mm and 0.07 mm respectively, in a 300 x 300 x 150 mm³ slab of ICRU 4-element tissue. In practice ICRU tissue cannot be fabricated so phantoms of various surrogate materials are used; for calibration and routine performance tests irradiation is sometimes performed free-in-air to facilitate the simultaneous exposure of large numbers of dosemeters to the same fluence. It may then be necessary to apply dosemeter-specific corrections to take into account the difference between the backscattered radiation field in the practical situation and that for which the calibration quantity is defined. It has been suggested that if a standard phantom of dimensions 300 x 300 x 150 mm³ composed of poly-methyl methacrylate (PMMA) is used, then no corrections for differences in backscatter are required. Results are presented for the differences in fluence response of routinely used neutron personal dosemeters irradiated on 300 x 300 x 150 mm³ phantoms composed of a tissue substitute material (MS20), PMMA and polyethylene. These are compared with free-air irradiations corrected for room scatter by the shadow cone technique. The contribution to the response due to backscatter from the phantom is thus derived and is discussed in terms of the appropriateness of phantom selection.