醫用迴旋加速器中子輻射之度量

近年來因正子造影(Positron Emission Tomography，PET)檢查方式普及，各大醫院紛紛設立PET中心，且隨著醫用迴旋加速器(cyclotron)的設立，工作人員亦增加。迴旋加速器運轉過程中18O(p,n)18F核反應會產生額外中子，中子更會直接或間接造成工作人員及民眾的體外輻射劑量。本論文以(1)德國FAG公司出產之 FHT 751中子偵檢器度量中子劑量率，及(2)活化箔片法測量中子通率對設立於中山醫學大學附醫具自體屏蔽RDS-111迴旋加速器。結果發現，在加速器自體屏蔽接縫地面處得到的中子劑量率及中子通率均最高，並隨著距離的增加而減少，中子劑量率最高由接縫處191±0.15μSv/h降至門口2.48±0.01μSv/h，熱中子通率最高由接縫處地面上的2057±3.80 n/cm2 s1降至門口9.23±1.45 n/cm2 s1，這表示在迴旋加速器地面上的屏蔽較弱。為配合衛生署規定自民國九十五年一月一日實施「斷層掃描用正子放射同位素調製作業要點」，本中心於民國九十四年七月進行改建。加建短迷宮牆後，加速器室之中子通率降了1.52~4.02倍。研究結果顯示本加速器之自體屏蔽可有效阻擋由靶位釋出之中子，對人員造成的輻射劑量也相當低，而位於控制台等工作人員運轉時所在之處，測得之中子劑量率所造成體外輻射劑量更遠低於法規限值。由本研究可得知，在中山醫學大學附設醫院迴旋加速器中心工作，是相當安全的。

Accelerated growth in the use of positron emission tomography (PET) had been resulted in a steady increase in the number of PET centers in Taiwan. The neutrons that were emitted from (p, n) reactions directly or indirectly caused hazard to the health of medical personnel and the public. In this work, (1) FHT-751 neutron detector made by FAG was to monitor the neutron dose rate during the operating period; (2) neutron flux of cyclotron room was mapped using foil activation method at self-shielded RDS-111 cyclotron of Chung Shan Medical University Hospital (CSMUH). Results indicated that neutron dose rate and neutron flux were the highest one on the floor of the interlocking block, (191±0.15μSv h-1, 2057±3.80 n cm-2 s-1) and were reduced down to the door. (2.48±0.01μSv h-1, 9.23±1.45 n cm-2 s-1) The neutron dose rate decreased as the distance from the block increased, that clearly indicated the weakness of the self-shielded design of the lower part of the RDS-111 cyclotron. To meet the Guideline of production of PET published by Department of Health, the new design of self-shield CTI RDS-111 was improved by adding maze and wall. The neutron fluxes presented reducing factors of 1.52~4.02 due to this design. Annual “extra” neutron DR for medical personnel was measured and evaluated far below the recommended ICRP 60 limit. The results revealed that the shell-shielded system adequately protected well the laboratory personnel. Thus the medical professional staff and the general public in CSMUH were safe.