H. Obayashi

K. Hayashi

K. Shin

National Institute for Fusion Science (NIFS-Repository)

§2. Design Study of High Quality Neutron Do-simeter Shin, K. (Dept. Nucl. Eng., Kyoto Univ.) Hayashi, K. (Hitachi Eng., Co., Ltd.) Obayashi, H. The objective of the study is to make design study of a high accuracy neutron dose monitor which can be utilized in the experimental room during the D-plasma discharge as a head of the monitoring system. To develop a high efficiency detector, we adopted Andersson- Braun type detector, which was com-posed of a 3 He detector and a polyethylene mod-erator. The design was carried out based on the adjoint transport calculation by ANISN code, as-suming the reaction cross section of 3 He as an ad-joint source in the 3 He detector region. The re-sponse of the monitor for the beam neutrons was estimated. The group constants utilized in the calculations were taken from the DLC-119 /HILO which were collapsed to 23 groups. ' Calculations were made varing the polyethy-lene thickness, with assuming a thin B4 C neu-tron absorber layer in the moderator. The re-sults for each thickness moderator obtained with and without the absorber were summed up with an adjusted weighting factor so that the summed curve may best agreed with the 1-cm dose equiv-alence curve given by ICRP-51. As a result of ~ = C:::to- 1 = 0 "" ....= cu Polyethylene 12cm, Lead 3cm ··-·--···· B4C Omm ------- B4C 2mm -- Combined ICRPSl 0 • ICRU Sphere 30cm Thick Plane the above survey calculations, the most adequate polyethelene thickness which gave the best fit of the response to the ICRP 1-cm dose in the whole energy range was 12"" 15 em. The location of the absorber layer affected the response only slightly. Therefore we selected the 12-cm thickness for the polyethylene moderator and the absorber was located at the center in the moderator. The remaining problem in the response was in the higher energy range above several MeV, where the response curve decreased with increasing the neutron energy and underestimated largely the 1-cm dose curve. To improve the response at these energies, a neutron multiplier zone of lead was added outside the moderator. The survey cal-culation clarified that a 2""3 em thick lead layer was enough to improve the response of the higher energy part. The response curve of the monitor which was composed of the best materials configuration thus determined is compared with the 1-cm-dose curve of ICRP-51. It is seen that the designed moni-tor has an enough accuracy for the neutron dose monitoring up to 100 MeV. Fig.1 Comparison of the estimated detector re-sponse with ICRP 1-cm dose curve. ..... _ ...... .: •.. i f_.r···,.-·., .. .;··~ .. ;··; ,.. ,., r--· • •1t ;I;-=,:··-.1 o ~-, r-l · • • :--r--... _.. _J-~-··--···-···J···-····-;···-···.;··-····-·.: = 2to- 2 r-··u-···; 0 ____ ::--I I I I I ~·._ ____ , ······-···-····-····-····r·-··-····· ~--.! ______ J r __ o_i r _____ ;-____ 9---------6·----r-~r---- 0' --------------J 10 -J Neutron Energy (MeV) Fig.1 Comparison of the estimated detector response with ICRP 1-cm dose curve. 291 

Design Study of High Quality Neutron Dosimeter

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