Project 9 Advancement of integrated system for dose estimation in BNCT (R2P9)

概要

INTRODUCTION: Development in accelerator-based irradiation systems for BNCT is underway. In the near future, BNCT using these newly developed systems may be carried out at multiple facilities across the world. Con- sidering this situation, it is important that the estimations for dose quantity and quality are performed consistently among several irradiation fields, and that the equivalency of BNCT is guaranteed, within and across BNCT systems. Then, we are establishing QA/QC system for BNCT.
As part of the QA/QC system, we are developing estima- tion method for neutron energy spectrum using Bonner sphere [1]. For our spectrometer using Bonner sphere, liquid such as pure water and/or boric acid solution is used as the moderator. A multi-layer concentric-sphere case with several sphere shells is prepared. The modera- tor and its diameter are changeable without entering the irradiation room, by the remote supply and drainage of liquid moderator in the several layers. For the detector, activation foils are remotely changed, or online meas- urement is performed using SOF detector, etc.

MATERIALS AND METHODS: In the neutron en- ergy spectrometry by Bonner-sphere, the combinations of the moderator material and diameter should be previously decided and prepared. Of course, the more information can be obtained as the more moderators and detectors are prepared. However, the information number from those measured data is less than the combination number, be- cause of the overlapped regions among the combinations. The selection is important, in which the more information number is obtained for the combination number.
The combination of moderator and detector is decided, for that the response functions cannot be approximated by the linear functions of the other response functions. The accuracy and precision for the spectrometry can be higher, because the independent information can be ob- tained from the measurement by the respective combina- tions. We were developed the selection method, High Independence Selection (HIS) [2].
On the assumption of the application in the standard epi-thermal neutron irradiation mode of Heavy Water Neutron Irradiation Facility (HWNIF), the combination of the moderators for boron-10 concentration and diame- ter was optimized by HIS. Based on this optimization, a prototype of RBS was made. Some experiments were performed for the characteristic verification of the proto- type RBS at HWNIF.

RESULTS: The configuration of the RBS was decided as follows. A five-layer concentric spherical acrylic shell is used as a container. Each acrylic wall is 1 mm in thickness. The moderator injection part is 9 mm in thick- ness for each layer. Pure water and 0.14-wt% boric acid water for boron-10 were used as liquid moderators. LiCaF scintillation detector and gold wire were used as the detectors. Figure 1 shows the combination of moder- ator injection patterns for RBS, used in the experiments. In both cases of LiCaF scintillation detector and gold wire, the measured values were larger than the calculated values. The concentration of boric acid was assumed to be 0.14 wt% in the calculation, but the actual concentra- tion was 0.12 wt%. This is considered to be one of the reasons for the disagreement. The other possible reasons are the differences in sizes between the actually prepared spectrometer, field size, beam directionality, etc..
Figure 2 shows the comparison between the nominal spectrum and unfolded spectrum. Unfolding was per- formed by GRAVEL using the response function of each Bonner sphere corrected by multiplying the ratio for measured/calculated values. The nominal spectrum of the epi-thermal neutron irradiation mode was input as an initial guess. The spectrum obtained by the unfolding reproduced the nominal spectrum relatively well, but the absolute value was overestimated about two times.

CONCLUSION: We have plan to perform (1) simula- tion with boric acid concentration set to 0.12 wt%, and (2) preparation of a Bonner sphere spectrometer includ- ing the remote mechanism for the supply and drainage of the liquid moderators.

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