COMPARING TWO MEASUREMENTS OF THE SAME COSMIC-RAY NEUTRON SPECTRUM USING STANDARD BONNER SPHERES AND HIGH-SENSITIVITY BONNER CYLINDERS

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Abstract

Ground-level cosmic-ray neutron spectra in Taiwan were measured using two neutron spectrometers, standard Bonner spheres and homemade high-sensitivity Bonner cylinders. The Bonner sphere system consisted of 12 polyethylene spheres of various diameters and four extended-range spheres that contained embedded metal shells. A set of 16 similarly designed Bonner cylinders was assembled based on a large cylindrical 3He proportional counter, showing an efficiency ˜18 times higher than that of the central probe of the Bonner spheres. However, these Bonner cylinders were not fully symmetrical and exhibited angular dependence in their responses to incoming neutrons. Focusing on the low-intensity neutron background, this study presents a systematic comparison between Bonner spheres and cylinders in terms of their response characteristics, counting statistics, and unfolded neutron spectra. The comparison indicated that the neutron probe of the Bonner spheres had a small but non-negligible counting noise that required subtraction from the recorded data. According to the response functions calculated for isotropic neutron incidence, the neutron spectrum determined using the high-sensitivity Bonner cylinders agreed well with that measured using the Bonner spheres. The annual effective doses from cosmic-ray neutrons at the location were determined to be 30.5 and 30.8 μSv by using the Bonner sphere and cylinder spectrometers, respectively, which corresponded to respective total neutron fluxes of 5.39 × 10−3 and 5.43 × 10−3 cm−2s−1.

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