Distribution of local 137Cs anomalies on the seafloor near the Fukushima Dai-ichi Nuclear Power Plant.

An estimated 3.5±0.7×10(15) Bq of (137)Cs is thought to have been discharged into the ocean following the melt down at Fukushima Dai-ichi Nuclear Power Plant (F1NPP). While efforts have been made to monitor seafloor radiation levels, the sampling techniques used cannot capture the continuous distribution of radionuclides. In this work, we apply in situ measurement techniques using a towed gamma ray spectrometer to map the continuous distribution of (137)Cs on the seafloor within 20 km of the F1NPP. The results reveal the existence of local (137)Cs anomalies, with levels of (137)Cs an order of magnitude higher than the surrounding seafloors. The sizes of the anomalies mapped in this work range from a few meters to a few hundreds of meters in length, and it is demonstrated that the distribution of these anomalies is strongly influenced by meter scale features of the terrain.

Neutron dosimetry with 6LiF-rich TL sheet.

A (6)LiF-rich thermoluminescent sheet-type dosemeter ((6)LiF-rich NTL sheet) was developed for neutron 2D dosimetry. The dosemeter utilises the (6)Li(n, alpha)(3)H reaction to detect thermal neutrons. Responses of the (6)LiF-rich NTL sheet to neutrons were measured at the neutron beam irradiation facility for BNCT in JRR-4 Research Reactor at the Japan Atomic Energy Research Institute. Placement of a multi-leaf collimator at the output port of the neutron (beam) irradiation facility, produced either stripe- or round-shaped neutron distributions; the spatial distribution was measured using the developed NTL sheets. Direct measurements of neutron attenuation in water were also carried out using the developed NTL sheet, submersed in a water phantom. In each experiment, NTL sheets having natural abundance (7.9%) of LiF, and (6)LiF-enriched NTL (18.94%) sheet were irradiated under the same conditions. The ratio of thermoluminescence intensities of the (6)LiF-rich NTL sheet to that of the normal NTL sheet was compared to a theoretically calculated value. The experimental measurements are shown to be in good agreement with the calculations.

Energy response of a two-dimensional sheet-type LiF:Mg,Cu,P TL dosemeter to photons.

A 2-D tissue-equivalent sheet-type dosemeter (NTL sheet) was developed using thermoluminescent material of LiF:Mg,Cu,P (NTL-250). The energy responses of the NTL sheet and NTL-250 powder were measured with 10-150 keV monoenergetic photons from synchrotron radiation at SPring-8. The sample was irradiated by a rotational method for the uniform irradiation with the narrow beam. Linearity of the NTL-250 was confirmed up to 2 Gy. Energy responses of the NTL sheet and NTL-250 powder were close to that of soft tissue. On the other hand, the BaSO(4) sheet, which has been used practically, showed the response that the sensitivity approximately 60 keV was 100 times higher than that for (60)Co gamma rays. Therefore the NTL sheet can be said to have excellent properties for dose measurements.