To investigate a proximity effect of ducts on shield performance against γ radiation, an experiment was performed at JRR-4 by entering the γ-ray beam into a concrete shield wall of 100 cm-thickness with 3 or 5 straight cylindrical ducts of radius of 4.45 cm placed in a straight line or crosswise at interval of 8.9 cm. The dose rates were measured using digital dosimeters on a horizontal line 20 cm apart from the rear of the wall with 0, 1, 3 and 5 ducts, and with the incident angles of 0°, 7°, 14° and 20°, respectively. The dose rate distributions depended on the number of ducts and the incident angle, and the dose rate ratios of with-three-ducts to no-duct distributed within 3.612, 1.35.0 and 1.14.3, for the incident angles of 7°, 14° and 20°, while those of with-single-duct to no-duct within 1.27.1, 1.12.7 and 1.01.9, respectively. The experiment was analyzed using a multigroup single scattering code G33YSN able to deal with the geometry of the ducts exactly. For each incident angle, the calculation agreed with the experiment within a factor of 2.
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