利用間接中子造影法進行硼中子捕獲治療中子射束之特性研究

Characterization study of a BNCT neutron beam using indirect neutron radiography

蔡碧恩(Pi-En Tsai)

指導教授 ： 江祥輝

國立清華大學/原子科學院/工程與系統科學系/碩士(2009年)

摘要

本研究旨在測量清華水池式反應器硼中子捕獲治療超熱中子射束之中子射束特性，研究方法乃利用中子活化分析與間接中子造影法在空氣與壓克力假體中分別測定中子強度與其二維空間分佈。此外，為因應頭頸部腫瘤治療，除原先中子束設計外，尚需外掛一材質為聚乙烯之準直器，因此，本研究亦針對有無加裝聚乙烯準直器進行中子射束之特性比較。
在中子活化法中，本研究採用金鋁、純銅、錳鎳三種金屬箔片組成的箔片組作為活化偵檢器，測量箔片反應率；此外，本研究亦利用純銅片與鎘片之不同組合方式，分別測得熱中子與超熱中子強度，並同時探討室散射中子對箔片反應率之貢獻。而在間接中子造影法中，本研究則利用影像數位板作為二維輻射偵檢器、銅片作為活化轉換材以及鎘片作為濾材進行中子二維空間分佈測量。
由實驗結果顯示，超熱中子射束之角度分佈較具前向性，而熱中子之角度分佈則接近均向分佈，與原始中子束射計吻合。至於室散射中子之影響，在距離射束出口20公分之範圍內，其對箔片反應率僅有數個百分比之貢獻。此外，當聚乙烯準直器外加於原中子束出口時，由於中子易於聚乙烯內發生散射並因而減速，使得準直器出口之熱中子強度較原射束出口為強；此一影響可提高熱中子在腫瘤表面相較於最大劑量處之通量比例，將有利於某些頭頸部腫瘤病例之治療。
本研究之實驗結果提供了中子射源調整與驗證最基本的依據，另外，此研究也顯示：利用中子活化分析結合間接中子造影法可迅速、有效進行中子射束之強度與二維空間分佈之測定。

關鍵字

硼中子捕獲治療 ； 間接中子造影法 ； 中子活化分析 ； 影像數位板

並列摘要

This study aims to well characterize the BNCT epithermal neutron beam at THOR through measurements. The foil activation method and the indirect neutron radiography were applied to measure the neutron intensity and the 2D spatial distributions. The measurements were not only performed free-in-air but also inside the PMMA phantoms. In addition to the original beam design for the brain tumor treatment, an extended PE collimator was recently designed for the head and neck tumor. Hence, the comparison of the neutron characteristics including intensity and 2D spatial distribution between these two conditions, with and without the extended collimator, was also carried out by measurements.
The triple-foil sets consisting of gold, copper, and manganese as well as the foil sets with different arrangements of copper and cadmium were utilized as the activation detectors to determine the reaction rates. Regarding the neutron flux mapping, three major components were utilized in the indirect neutron radiography: IP as a radiation area detector, copper foil as activation converters, and cadmium plates as filters.
According to the results of measurements, the angular distribution for epithermal neutrons is quite straight-forward, while the angular distribution for thermal neutrons is more close to isotropic, which is consistent with the original beam design. The contribution of the room-scattered neutrons was found to be only a few percent inside the irradiation field within 20 cm away from the beam exit. Once the PE extended collimator is mounted on the beam, owing to the scattering effect, the thermal neutron intensity will be built up and hence increase the ratio of the delivered neutron fluence at phantom surface to the maximum value, which benefit the treatment for head and neck tumors.
It is also demonstrated that, for a beam characterization, the indirect neutron radiography coupling with the foil activation method can precisely determine the neutron intensity and the 2D spatial distributions in an effort-saving way. Furthermore, the measurements can provide essential information for the source validation.

並列關鍵字

BNCT ； Indirect neutron radiography ； Neutron activation analysis ； Image plate

參考文獻

[1] Yuan-Hao Liu, Chun-Kai Huang, Pi-En Tsai, Ang-Yu Chen, Hong-Ming Liu, Shih-Chung Lee, and Shiang-Huei Jiang, BNCT Epithermal Neutron Beam Mapping by using Indirect Neutron Radiography. Nuclear Technology, to be published.

[2] Yen-Wan Hsueh Liu, Tai-Ting Huang, Shiang-Huei Jiang, and Hong-Ming Liu, Renovation of epithermal neutron beam for BNCT at THOR. Applied Radiation and Isotopes 61, 1039-1043 (2004).

[3] Yuan-Hao Liu, Sander Nievaart, Pi-En Tsai, Hong-Ming Liu, Ray Moss, and Shiang-Huei Jiang, Coarse-scaling adjustment of fine-group neutron spectra for epithermal neutron beams in BNCT using multiple activation detectors, Nuclear Instruments and Methods in Physics Research A 598, 764-773 (2009).

[4] Yuan-Hao Liu, Shiang-Huei Jiang, Yen-Wan Hsueh Liu, and Hong-Ming Liu, On-line Neutron Monitoring System of Epithermal Neutron Beam for BNCT at THOR, ICNCT-12, Takamazu, Kagawa, Japan (2006).

[5] Glenn F. Knoll. Radiation detection and measurement, 3rd ed (2000), p746-749.