| Summary: | 博士 === 國立清華大學 === 原子科學研究所 === 79 === A coincidence gamma spectrometer has been used to measure the 134Cs activi
ty in the presence of intense interference from 137Cs and natural radioact
ivity in environmental sample. The coincidence spectrometer, with a large
BGO scintillator for gating purposes and a HPGe detector for taking the co
incident spectrum, is designed to enhance the coincidence photopeak countr
act of 134Cs and suppress the single photon countrate of 137Cs as well as
other natural radionuclidence while the interferring photopeaks is greatl
y suppressed. Absolute counting efficiency of the coincidence spectromete
r is calibrated for environmental sample with activity ratio of A(134Cs)/A
(137Cs) down to 5x10-5. The minimum detectable concentration of 134Cs in
beach sand is 0.233Bq/kg. Other applications and the flexibility of the sp
ectroscopic system as well as nuclear instrumentation using HPGe and BGO d
etectors are discussed.
本論文主要是以高純度鍺 (HPGe) 偵檢器與鍺酸鉍 (BGO)偵檢器組合的相符偵測系統
, 偵測極微量的銫-134核種. 利用銫-134之605kev與796kev相符加馬射線, 將BGO 偵
檢器的能窗設在796kev, 調整時間電路, 使時間電路訊號與放大器之能譜訊號同步進
入類比數值轉換器, 以相符模式經由多頻道分析儀記錄相符能譜. 本相符偵測系統對
於核爆落塵 (銫-134與銫-137之活性比為10-5) 與環測樣品
a銫-134與銫-137之活性比為0.5) , 均有優異的偵測能力, 特別是環測樣品含有極低
微銫-134的時候, 可以有效壓制 Bi-214 之609kev與Ac-228之795kev之自然背景, 減
低銫-134之605kev與796kev譜線干擾. 對於複雜核種存在的樣品, 亦能有效剔除非銫
-134核種的譜線, 使得微量銫-134易於偵測, 本相符系統岸砂銫-134之最低可測濃度
(MDC)為0.233Bq/kg.
本論文對HPGe偵檢器探討偵測大體積環測水樣的幾何最適化的研究, 亦有詳細的探討
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