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L-zing a broad beam of thermal neutrons from the graphite column of a nuclear reactor, the presence of the phantom is shown experimmtally to produce a tllsturhance of flux den*ity levels over distances a s lzrge a5 70 t o 100 cm from its surface. From the experimentally determined flux density levels inside the phantom, the level of flux density equivalent to 2.5 mrem hr-1 is derived as 91.5 n cm-2 sec-' when gonads and red bone-marrow are considered to he the critical organs. This is greater than the recommended level of 6 T O n cm-* sec-l, which may, thereforc, be needlessly restrictive. The significance of the results for thr calibration of instruments ancl dohemeters is dlacussed hriefly.