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When conducting uniformity measurements of a wide area reference source with a detector having a window of a size similar to that of a gridded individual portion area on the source, it is important to carefully consider neighbor effects on measuring emission rates of the individual target portion resulting from the gap between the source and detector window. Optimization of the uniformity measurement conditions was studied for beta-emitting wide area reference sources in this study. A measurement system consisting of a PR-gas (Ar: 90%+CH 4 : 10%) flow type windowed proportional counter and a motorized XY stage was installed. This system is adapted to the uniformity measurement of two different types of 36 Cl sources made by different manufacturers. Uniformity measurement of a 100 mm×100 mm source divided into 16 portions of 6.25 cm 2 (25 mm squared) each could be conducted using our system under the present conditions with a neighbor effect of around 15% or less. The measurement results by use of this system were also compared with those using the imaging plate technique. [ABSTRACT FROM AUTHOR]