本論文探討醫用加速器－包含電子直線加速器與質子迴旋加速器，在運轉過程中可能發生的中子污染，並建立輻射屏蔽的評估模式與數據資料。

醫用電子直線加速器中，高於8 MeV的加馬射線與機頭組件碰撞後，可能產生光中子。本文利用蒙地卡羅計算機程式FLUKA，詳盡模擬10 MV直線加速器機頭產生的洩漏中子，在治療室與迷道的遷移情形，並與He-3比例計數器與氣泡式偵檢器的測量結果進行比較。研究發現：10 MV模式下的機頭中子產量隨次要準直儀的照野縮小而增加。然而，接近射束出口的準直儀可阻隔上方組件產生的部份中子，使照野10×10 cm2以下，射束出口下方的洩漏中子通量，隨照野縮小反而降低。對加速器輸出的每監控單位而言，中子通量並未因多葉式準直儀在三度空間順形治療與強度調控放射治療的使用而明顯增加。加速器機頭治療角度的不同，使迷道出口的中子計數率測量值發生變化，經FLUKA程式計算結果驗證後發現，差異最大可達20%。中子劑量隨迷道長度的衰減情形，可由FLUKA程式與實驗測量獲得，亦證實簡單的Kersey經驗公式，可保守地適用於10 MV直線加速器的中子污染評估。

醫用迴旋加速器主要用以加速重荷電粒子（如質子，氘子等），藉由高速粒子與靶材發生的核子反應，生產放射性同位素供醫療使用。本文第二部分即計算醫用迴旋加速器能量範圍內（10-100 MeV），質子分別撞擊厚靶（碳、氮、鋁、鐵、銅與鎢）產生的中子射源項，及該中子射源經混凝土屏蔽後的劑量衰減。中子射源項可利用ICRU-63報告的能量及角度雙微分中子產生截面，並考量質子於靶中的連續能失計算而得。混凝土中的劑量衰減則沿用FLUKA程式進行模擬。藉由射源劑量項與劑量衰減長度的資料建立，可簡易計算相對於質子入射方向上，各角度產生的中子穿透混凝土屏蔽後的有效劑量。對能量低於30 MeV的質子加速器而言，劑量衰減長度隨靶材及中子發射角度的變化可以忽略，其值可以單一數值29.57 g cm-2表示。對於能量高於30MeV的質子加速器而言，入射質子能量愈高，中子在小角度上造成的劑量衰減長度愈長。此外，射源劑量項的角度分布在低原子序的靶材傾向較強的前向性，在高原子序靶材則接近均向性。中子射源項計算結果同時應用於實際案例－慈濟醫院迴旋加速器屏蔽設計的改善研究。

This study investigated the neutron contaminations from medical accelerators, including electron linear accelerators and proton cyclotrons. The evaluation methods and data for neutron shielding were also provided for practical uses.

For medical electron linear accelerators, photoneutrons produced by photonuclear interaction between X-ray above 8 MeV and gantry materials. In this study, photoneutron production from a 10 MV LINAC gantry head and transport in accelerator facility were simulated using the FLUKA Monte Carlo code, and compared with the measurement by using He-3 proportional counter and neutron bubble detectors. The results indicate that the neutron yields increase with the decreasing filed size collimated by the secondary collimators. Neutrons produced from upper gantry components will be attenuated to some extent by the lower collimators, the leakage neutron fluence under beam outlet decrease with the field size reducing from 10×10 cm2. Neutron fluence per unit monitor unit does not increase significantly due to the using of multi-leaf collimator in 3-D conformal therapy and intensity modulated radiation therapy. The measured variation up to 20% of the neutron leakage as a function of gantry position at the maze exit was reproduced by the calculation. The neutron dose and/or counting rates for detectors along the maze path were compared between measurements and Monte Carlo calculations as well as Kersey empirical method.

Medical cyclotrons accelerate heavy charged particles （proton, deuteron, etc.） to produce radiation isotopes used for clinical applications. In the second part of this study, the characteristics of neutron sources and their attenuation in concrete were investigated for 10-100 MeV protons striking on target materials of C, N, Al, Fe, Cu and W. Thick-target double differential neutron yields were first calculated from the（p, xn）cross sections recommended in the ICRU Report 63 considering continue slowing down of proton in the target. Transport simulations of those neutrons in concrete were performed by the FLUKA code. The results of source term and the corresponding attenuation lengths provide for simply estimation of effective dose behind concrete shielding at the direction of neutron emission. For proton energies below ~30 MeV the variation of the attenuation length is very small and could be treated as a constant value（29.57 g cm-2）. For proton energies over ~30 MeV the attenuation length become more and more forward peaked due to the increase in proton energy. The source term was also found to be more or less isotropic for high-Z materials. The result of the neutron source term was also applied in a practical case – the refined shielding design for the cyclotron room of the Buddhist Tzu Chi general hospital.

[1]National Council on Radiation Protection and Measurements. “Neutron contamination from medical electron accelerators”, NCRP Report No. 79 (Bethesda, MD: NCRP) (1984).
[2]X.S. Mao, K.R. Kase, J.C. Liu, W.R. Nelson, J.H. Kleck, S. Johnsen, “Neutron source in the Varian CLINAC 2100C/2300C medical accelerator calculated by the EGS4 code”, Health Phys. 72(4), 524-529 (1997).
[3]K.R. Kase, X.S. Mao, W.R. Nelson, J.C. Liu, J.H. Kleck, M. Elsalim, “Neutron fluence and energy spectra around the Varian CLINAC 2100C/2300C medical accelerator”, Health Phys. 74(1), 38-47 (1998).
[4]W.R. Nelson, H. Hirayama, D.W.O. Rogers, “The EGS4 code system”, SLAC-256, Stanford Linear Accelerator Center (Stanford, CA) (1985).
[5]M.B. Emmett, “The MORSE Monte Carlo radiation transport code system”, ORNL-4972, (Oak-Ridge, TN: Oak Ridge National Laboratory) (1984).
[6]E.J. Waller, T.J. Jamieson, D. Cole, T. Cousins, R.B. Jammal, “Experimental and computational determination of neutron dose equivalent around radiotherapy accelerators”, Radiat. Prot. Dosim. 107(4), 225-232 (2003).
[7]E. J. Waller, “Neutron production associated with radiotherapy linear accelerators using intensity modulated radiation therapy mode”, Health Phys. 85(Supplement 2), S75-S77 (2003).
[8]R.C. McCall, T.M. Jenkins, R.A. Shore, “Transport of accelerator produced neutrons in a concrete room”, IEEE trans. Nucl. Sci., NS-26(1) 1593-1597 (1979).
[9]R.W. Kersey, “Estimation of neutron and gamma radiation dose in the entrance maze of SL 75-20 linear accelerator”, Medicamundi, 24(3) 151-155 (1979).
[10]R.L. French, M.B. Wells, “An angle-dependent albedo for fast-neutron reflection calculations”, Nucl. Sci. Eng. 19 441-448 (1964).
[11]National Council on Radiation Protection and Measurements. “Radiation protection design guidelines for 0.1-100 MeV particle accelerator facilities”, NCRP Report No. 51 (Bethesda, MD: NCRP) (1977).
[12]International Atomic Energy Agency, “Charged particle cross-section database for medical radioisotope production: diagnostic radioisotopes and monitor reactions”, Report IAEA-TECDOC-1211 (Vienna: IAEA) (2001).
[13]S. Agosteo, A. Fassò, A. Ferrari, P.R. Sala, M. Silari, P. Tabarelli de Fatis, “Double differential distributions and attenuation in concrete for neutron produced by 100-400 MeV protons on iron and tissue targets”, Nucl. Instr. and Meth. in Phys. Res. B 114, 70-80 (1996).
[14]A. Fassò, A.Ferrari, P.R. Sala, “Electron-photon transport in FLUKA: status”, Proc. Monte Carlo 2000 Conf., Lisbon, October 23-26 2000. (Eds) Kling, A., Barão, F., Nakagawa, M., Távora, L. and Vaz, P. (Berlin: Springer-Verlag), 159-164 (2001).
[15]A. Fassò, A Ferrari., J. Ranft, P.R. Sala, “FLUKA: status and prospective for hadronic applications”, Proc. Monte Carlo 2000 Conf., Lisbon, October 23-26 2000. (Eds) Kling, A., Barão, F., Nakagawa, M., Távora, L. and Vaz, P. (Berlin: Springer-Verlag), 955-960 (2001).
[16]J.F. Briesmeister (Ed.), “MCNP- A general Monte Carlo N-particle transport code, Version 4C”, Los Alamos National Laboratory, LANL Report LA-13709-M, (Los Alamos, NM: LANL) (2000).
[17]K.A. Van Riper, “Sabrina user’s guide”, White Rock Science, (White Rock, NM) (1997)
[18]H. Ing, R.A. Noulty, T.D. McLean, “Bubble detectors - A maturing technology”, Radiat. Meas. 27 (1) 1-11 (1997).
[19]National Council on Radiation Protection and Measurements. “Protection against neutron radiation”, NCRP Report No. 38 (Washington, DC: NCRP) (1971).
[20]Varian Oncology System, “Monte Carlo project”, Confidential information to National Tsing Hua University, (1999).
[21]International Atomic Energy Agency, “Handbook on photonuclear data for applications – cross section and spectra”, Report IAEA-TECDOC-Draft No. 3 (Vienna: IAEA) (2000).
[22]Varian Medical System, “CLINAC 1800C, 2100C(/D), 2300C/D, 2100SC, 21EX and 23 Ex radiation leakage data”, http://www.varian.com/shared/oncy/pdf/12000.pdf, (1998).
[23]American Nuclear Society, “American national standard for calculation and measurement of direct and scattered gamma radiation from LWR nuclear power planets”, ANSI/ANS-6.6.1-1987 Revision of ANSI/ANS-6.6.1-1979 (1987).
[24]International Commission on Radiological Protection, “Conversion coefficients for use in radiological protection against external radiation”, ICRP Publication 74. (Ann. ICRP, 26(3-4)) (Oxford, UK: ICRP) (1996).
[25]Varian Medical System, “Neutron door for high energy accelerator”, http://www.varian.com/shared/oncy/pdf/12006.pdf, (2003).
[26]International Commission on Radiation Units and Measurements. “Nuclear data for neutron and proton radiotherapy and for radiation protection”, ICRU Report 63 (Bethesda, MD: ICRU) (2000).
[27]International Commission on Radiation Units and Measurements. “Stopping powers and rangers for protons and alpha particles”, ICRU Report 49 (Bethesda, MD: ICRU) (1993).
[28]A. Ferrari, M. Pelliccioni, M. Pillon, “Fluence to effective dose and effective dose equivalent conversion coefficients for photons from 50 keV to 10 GeV”, Radiat. Prot. Dosim. 67(4), 245-251 (1996).
[29]慈濟綜合醫院，”財團法人佛教慈濟綜合醫院迴旋加速器安全評估報告（修訂三版□定稿）”，（花蓮，中華民國）(2003).
[30]GE Healthcare, “PETtrace-Shielded Machine: Summary of Source Terms, Radiation Fields and Radwaste Production”, RP001182, Rev. 0.2 (2004).
[31]Haghighat, A. and Wagner J.C, “Monte Carlo variance reduction with deterministic importance functions”, Progress in Nuclear Energy 42(1), 25-53 (2003).
[32]Rhoades, W.A. and Simpson, D.B, “The TORT three-dimensional discrete ordinates neutron/photon transport code (TORT Version 3)”, Oak Ridge National Laboratory. ORNL/TM-13221, (Oak Ridge, TN: ORNL) (1997).
[33]R.J. Sheu, R.D. Sheu, S.H. Jiang, C. H. Kao, “Adjoint acceleration of Monte Carlo simulations using TORT/MCNP coupling approach: a case study on the shielding improvement for the cyclotron room of the Buddhist Tzu Chi general hospital”, Radiat. Prot. Dosim. 113(2), 140-151 (2005).